Schematic Reference

The indicator shows when the mains is present at its output by a continuous glow of a neon bulb, La1, and when the fuse is blown by flashing of the neon bulb. When the fuse is intact, capacitor C2 acts as the series resistance for the neon bulb, so that this glows continuously. When the fuse has blow, the mains voltage across diode D1 is applied as a pulsating direct voltage to network R1-C1. Capacitor C1 charges slowly and when the voltage across it reaches 80–100 V, the neon bulb comes on. Capacitor C1 is then discharged slowly via diode D2 and the bulb.

When the voltage across it has dropped sufficiently, the bulb goes out, whereupon C1 slowly charges again. This process repeats itself, so that, provided the values of R1 and C1 are right, the bulb flashes visibly. The potential across capacitor C2 is a ramp with a peak value of 30 V (which is, of course, applied to the load). Note that the neon bulb used for this purpose must not be a type that has a built-in series resistor.

A circuit for monitoring supply voltages of ±5 V and ±12 V is readily constructed as shown in the diagram. It is appreciably simpler than the usual monitors that use comparators, and AND gates. The circuit is not intended to indicate the level of the inputs. In normal operation, transistors T1 and T3 must be seen as current sources. The drop across resistors R1 and R2 is 6.3 V (12 –5 –0.7). This means that the current is 6.3mA and this flows through diode D1 when all four voltages are present. However, if for instance, the –5 V line fails, transistor T3 remains on but the base-emitter junction of T2 is no longer biased, so that this transistor is cut off. When this happens, there is no current through D which then goes out.

The Type LM35 temperature sensor from National Semiconductor is very popular for two reasons: it produces an output voltage that is directly proportional to the measured temperature in degrees Celsius, and it enables temperatures below zero to be measured. A drawback of the device is, however, that in its standard application circuit it needs to be connected to the actual measuring circuit via a three-wire link. This drawback is neatly negated by the present circuit. When the LM35 is connected as shown, a two-wire link for the measurement range of –5 °C to +40 °C becomes possible.

Actually, the circuit shown is a temperature-dependent current source, since it uses the variation of the quiescent current with changes in temperature. The values of resistors R3 and R4 are calculated to give an output voltage of 10mV °C–1. Where good accuracy is desirable or necessary, 1% resistors should be used. In this context, note that a loss resistance in the link between sensor and measuring circuit may cause a measurement error of about 1 °C for every 5 ohms of resistance. Capacitor C1 eliminates undesired interference and noise signals. At an ambient temperature of 25 °C, the circuit draws a current of about 2mA.

The sensor/monitor shown in the diagram ‘wakes up’ the host system on detection of infra-red (IR) signals. It draws so little supply current that it can remain on continuously in a notebook computer or PDA device. Its ultra-low current drain (4µA maximum, 2.5µA typical) is primarily that of the comparator/reference device, IC1. The circuit is intended for the non-carrier systems common in infra-red Data Association (IrDA) applications. It also operates with carrier protocols such as those of TV remote controllers and Newton/Sharp ASK (an amplitude shift keying protocol developed by Sharp and used in the Apple Newton).

The range for 115,000-baud IrDA is limited to about 6 in (15 cm), but for 2400-baud IrDA, it improves to more than 12 in (30 cm). Immunity to ambient light is very good, although bright flashes usually cause false triggers. To handle such triggers, the system simply looks for IR activity after waking and then returns to sleep mode if none is present. The sensor shown, D1, a relatively large-area photo-diode packaged in an IR-filter material, produces about 60µA when exposed to heavy illumination, and 400mV when open-circuited. Most photo-diodes may be used. Operation is in the photovoltaic mode without applied bias.

This mode is slow and not generally used in photo-diode circuits, but speed is not essential here. The photovoltaic mode simplifies the circuit and saves a significant amount of power. In a more conventional configuration, for instance, photo-conductive, photo currents caused by ambient light and sourced by the bias network would increase the quiescent current about ten times.

Many modern circuits tend to work from a single supply voltage of 3V. But often they need a virtual earth at half the supply voltage for efficient operation. The splitter shown in the diagram bisects the supply voltage with a high-resistance potential divider, R1-R2, and buffers the resulting 1.5 V line with an op amp. Since the op amp used is not a fast type, the output is decoupled by capacitive divider C2-C3. This ensures that the impedance of the virtual earth point remains low over a wide frequency band. Because the potential at the junction C2-C3-R3 is fed back to the inverting input of IC1, the circuit becomes a standard voltage follower.

Resistor R3 ensures that the regulation remains stable. The circuit can regulate ±2mA without any difficulties. Because of the low current drawn by IC1, and the high resistance of R1 and R2, the overall current drain is low. In the absence of a load, it was 13µA in the prototype, of which 1.5µA flows through R1-R2. Finally, since IC1 can operate from a voltage as low as 1.6V, the splitter will remain fully operational when the battery nears the end of its charge or life.

The circuit in the diagram enables a negative voltage to be derived without the use of integrated circuits. Instead, it uses five n-p-n transistors that are driven by a 1 kHz (approx) TTL clock. When the clock input is high, transistors T1 and T2 link capacitor C1 to the supply voltage, UIN, which typically is 5 V. During this process, transistor T5 conducts so that T3 and T4 are off. When the clock input is low, T5 is cut off, whereupon transistors T3 and T4 are switched on via pull-up resistor R6 and either R4 or R5.

This results in the charge on C1 being shared between this capacitor and C2 Since the +ve terminal of C2 is at ground potential, its –ve terminal must become negative w.r.t. earth. The high level at the clock input must be of the same order as the positive input voltage, UIN, otherwise T1 cannot be switched on. The clock frequency should be around 1 kHz to ensure a duty cycle ratio of 1:1. Altering the ratio results in a different level of negative output voltage, but this is always smaller than that with a ratio of 1:1.

The title of this article naturally raises the question of why we think that the generous selection of fully integrated voltage regulators needs to be extended with a version constructed using discrete components. In other words, what does this circuit offer that the well-known ‘three-leggers’ don’t have? To start with, we can point out that this circuit is refreshingly simple for a discrete version. Three semiconductors, three resistors, a capacitor and a diode are all it needs. Of course, that’s still more components than an integrated regulator, so what exactly are the advantages of this circuit?

They are to be found in three areas: voltage range, bandwidth and current rating. The last of these is the primary strength of this circuit, since the maximum current depends only on the specifications of the output transistor. With the BD680, as used here, a current of 4 A can be delivered at a collect-emitter voltage of 10 V with adequate cooling (Rth = 3.12 K/W). The peak current is even 6 A. Try matching that with an integrated voltage regulator! The maximum input voltage is 30 V with the illustrated version of the circuit (UDSmax of T1), but this can easily be increased by using special high-voltage transistors.

The same applies to the bandwidth, which can be extended as desired, without any modifications to the circuit, by using high-speed transistors. Generally speaking, wide bandwidth is also not one of the strong points of integrated voltage regulators. As noted, the circuit is basically very simple. A zener diode (D1) fed with a constant current of around 1mA by a JFET current source (T1) provides the reference potential. C1 is connected in parallel with D1 to provide well-behaved startup behaviour (soft start). This capacitor also provides additional buffering and decouples noise and other disturbances. The startup time is around three seconds.

The only additional item that is needed for the voltage regulator is an output buffer for the reference potential. This takes the form of a sort of super-Darlington using T2 and T3. This works very well, but has the disadvantage that the output voltage is a bit lower (one diode drop) than the Zener voltage. P1 can be added to correct this, but this does reduce the regulation of the circuit. If the voltage difference is not important, it is thus better to replace P1 with a wire jumper. The main specifications of the voltage regulator are listed in Table 1.

This article should satisfy those who might want to build a low power FM transmitter. It is designed to use an input from another sound source (such as a guitar or microphone), and transmits on the commercial FM band - it is actually quite powerful, so make sure that you don't use it to transmit anything sensitive - it could easily be picked up from several hundred metres away. The FM band is 88 to 108MHz, and although it is getting fairly crowded nearly everywhere, you should still be able to find a blank spot on the dial.

NOTE: A few people have had trouble with this circuit. The biggest problem is not knowing if it is even oscillating, since the frequency is outside the range of most simple oscilloscopes. See Project 74 for a simple RF probe that will (or should) tell you that you have a useful signal at the antenna. If so, then you know it oscillates, and just have to find out at what frequency. This may require the use of an RF frequency counter if you just cannot locate the FM band.

Description

The circuit of the transmitter is shown in Figure 1, and as you can see it is quite simple. The first stage is the oscillator, and is tuned with the variable capacitor. Select an unused frequency, and carefully adjust C3 until the background noise stops (you have to disable the FM receiver's mute circuit to hear this).


Because the trimmer cap is very sensitive, make the final frequency adjustment on the receiver. When assembling the circuit, make sure the rotor of C3 is connected to the +9V supply. This ensures that there will be minimal frequency disturbance when the screwdriver touches the adjustment shaft. You can use a small piece of non copper-clad circuit board to make a screwdriver - this will not alter the frequency.

The frequency stability is improved considerably by adding a capacitor from the base of Q1 to ground. This ensures that the transistor operates in true common base at RF. A value of 1nF (ceramic) as shown is suitable, and will also limit the HF response to 15 kHz - this is a benefit for a simple circuit like this, and even commercial FM is usually limited to a 15kHz bandwidth.

Capacitors
All capacitors must be ceramic (with the exception of C1, see below), with C2 and C6 preferably being N750 (Negative temperature coefficient, 750 parts per million per degree Celsius). The others should be NPO types, since temperature correction is not needed (nor is it desirable). If you cannot get N750 caps, don't worry too much, the frequency stability of the circuit is not that good anyway (as with all simple transmitters).

How It Works
Q1 is the oscillator, and is a conventional Colpitts design. L1 and C3 (in parallel with C2) tunes the circuit to the desired frequency, and the output (from the emitter of Q1) is fed to the buffer and amplifier Q2. This isolates the antenna from the oscillator giving much better frequency stability, as well as providing considerable extra gain. L2 and C6 form a tuned collector load, and C7 helps to further isolate the circuit from the antenna, as well as preventing any possibility of short circuits should the antenna contact the grounded metal case that would normally be used for the complete transmitter.

The audio signal applied to the base of Q1 causes the frequency to change, as the transistor's collector current is modulated by the audio. This provides the frequency modulation (FM) that can be received on any standard FM band receiver. The audio input must be kept to a maximum of about 100mV, although this will vary somewhat from one unit to the next. Higher levels will cause the deviation (the maximum frequency shift) to exceed the limits in the receiver - usually ±75kHz.

With the value shown for C1, this limits the lower frequency response to about 50Hz (based only on R1, which is somewhat pessimistic) - if you need to go lower than this, then use a 1uF cap instead, which will allow a response down to at least 15Hz. C1 may be polyester or mylar, or a 1uF electrolytic may be used, either bipolar or polarised. If polarised, the positive terminal must connect to the 10k resistor.

Inductors
The inductors are nominally 10 turns (actually 9.5) of 1mm diameter enamelled copper wire. They are close wound on a 3mm diameter former, which is removed after the coils are wound. Carefully scrape away the enamel where the coil ends will go through the board - all the enamel must be removed to ensure good contact. Figure 2 shows a detail drawing of a coil. The coils should be mounted about 2mm above the board.

For those still stuck in the dark ages with imperial measurements (grin), 1mm is about 0.04" (0.0394") or 5/127 inch (chuckle) - you will have to work out what gauge that is, depending on which wire gauge system you use (there are several). You can see the benefits of metric already, can't you? To work out the other measurements, 1" = 25.4mm

NOTE: The inductors are critical, and must be wound exactly as described, or the frequency will be wrong.


The nominal (and very approximate) inductance for the coils is about 130nH.This is calculated according to the formula ...

L = N² * r² / (228r + 254l)

... where L = inductance in microhenries (uH), N = number of turns, r = average coil radius (2.0mm for the coil as shown), and l = coil length. All dimensions are in millimetres.

Pre-Emphasis

It is normal with FM transmission that "pre-emphasis" is used, and there is a corresponding amount of de-emphasis at the receiver. There are two standards (of course) - most of the world uses a 50us time constant, and the US uses 75us. These time constants represent a frequency of 3183Hz and 2122Hz respectively. This is the 3dB point of a simple filter that boosts the high frequencies on transmission and cuts the same highs again on reception, restoring the frequency response to normal, and reducing noise.

The simple transmitter above does not have this built in, so it can be added to the microphone preamp or line stage buffer circuit. These are both shown in Figure 3, and are of much higher quality than the standard offerings in most other designs.


Rather than a simple single transistor amp, using a TL061 opamp gives much better distortion figures, and a more predictable output impedance to the transmitter. If you want to use a dynamic microphone, leave out R1 (5.6k) since this is only needed to power an electret mic insert. The gain control (for either circuit) can be an internal preset, or a normal pot to allow adjustment to the maximum level without distortion with different signal sources. The 100nF bypass capacitors must be ceramic types, because of the frequency. Note that although a TL072 might work, they are not designed to operate at the low supply voltage used. The TL061 is specifically designed for low power operation.

The mic preamp has a maximum gain of 22, giving a microphone sensitivity of around 5mV. The line preamp has a gain of unity, so maximum input sensitivity is 100mV. Select the appropriate capacitor value for pre-emphasis as shown in Figure 3 depending on where you live. The pre-emphasis is not especially accurate, but will be quite good enough for the sorts of uses that a low power FM transmitter will be put to. Needless to say, this does not include "bugging" of rooms, as this is illegal almost everywhere.

I would advise that the preamp be in its own small sub-enclosure to prevent RF from entering the opamp input. This does not need to be anything fancy, and you could even just wrap some insulation around the preamp then just wrap the entire preamp unit in aluminum foil. Remember to make a good earth connection to the foil, or the shielding will serve no purpose.

Some older Strand dimmer units used a zero to -10V control signal, and the standard analogue control voltage is zero to +10V. This project allows the easy conversion from one standard to another. This is a very simple project, but may turn out to be a lifesaver for small theatre groups and the like. It has come to my attention that there are still a great many old Strand dimmers very much in use. The problem is that they are just too reliable, and won't go away ... but, they use a zero to -10V control signal, so are incompatible with the dimmer unit in these project pages, and with any new commercial analogue control console.

In addition, there are no doubt quite a few old lighting consoles that use this standard, which means that they can't drive modern dimmer packs. As it turns out, a simple opamp inverter will convert either standard to the other. This is shown in Figure 1.


There is really nothing to it. Use as many circuits as needed, and a simple power supply (such as that in Project 05) will drive as many of these inverters as are likely to be required in any lighting setup. The above circuit has two channels, and may be simply repeated as many times as you need to get the required number of channels. The 100 ohm resistors on each output are there to prevent the opamps from oscillating when supplying a capacitive load (such as a coax cable).

With an input of zero volts, the output will also be at zero volts. As the input increases (or decreases in the case of the -10V control) the output will change by exactly the same value, but in the opposite direction. Wiring is not critical, the 1458 opamps specified are very cheap (but more than capable of doing the job), and they can be built very simply on Veroboard or similar. Supplies should be bypassed to common (ground) with 10uF electrolytic caps.

Firstly, I'd like to stress that the intended use of this circuit is only one of many possible applications. Apart from the obvious usage as a headphone amplifier, the circuit can be used for a range of applications where a wide bandwidth low power amplifier is needed. Some of the options include ...

• Reverb drive amplifier - ideal for low and medium impedance reverb tanks
• High current line driver - suitable for very long balanced lines
• Low power speaker amplifier - better performance than small integrated amps
• ... and of course, a headphone amp.

In short, the amp can be used anywhere that you need an opamp with more output current than normally available. Since most are rated for around ±20-50mA, general purpose opamps are not suitable for driving long cables or anywhere else that a relatively high output current is needed.

As a headphone amplifier, this design is very similar to others on the ESP site, but the main difference is that this one (and P70) has been built and fully tested. The design is fairly standard, and every variation was checked out before arriving at the final circuit. A photo of the prototype is shown below, and at only 64 x 38mm (2.5 x 1.5 inches) it is very small - naturally, the heatsink is not included in the dimensions.

The amplifier is capable of delivering around 1.5W into 8 ohm headphones, and 2.2W into 32 ohms - this is vastly more than will ever be needed in practice. The use of a 120 Ohm output resistor is recommended, as this is supposed to be the standard source impedance for headphones. Unfortunately, many users have found that their 'phones perform better when driven from a low impedance source.


The circuit is based on an opamp, with its output current boosted by a pair of transistors. Distortion is well below my measurement threshold at all levels below clipping into any impedance. Noise is virtually non-existent - even with a compression driver held to my ear, I could barely hear any, and I couldn't hear any with headphones.

WARNING
Headphones are rated in dB SPL at 1mW, and this amplifier (like many other similar headphone amps) is capable of producing extreme SPLs. The levels obtainable are sufficient to cause almost instantaneous permanent hearing damage! Never operate the amp at very high levels, and never switch the amplifier on with signal while wearing you headphones.

Always start with the volume control at minimum, and gradually increase the level until it is comfortable, but not too loud. Because of the very low distortion, it is easy to increase the level too far without noticing. Your ears are precious - safeguard them at all times.

Note the warning above - this is serious. Most headphones are capable of at least 94dB SPL at 1 mW, with some as high as 107dB SPL. Even 10mW is enough to create sound levels capable of causing hearing damage, so you must be very careful to avoid damaging levels.

Continuous dB SPL	Maximum Exposure Time
85	8 hours
88	4 hours
91	2 hours
94	1 hour
97	30 minutes
100	15 minutes
103	7.5 minutes
106	< 4 minutes
109	< 2minutes
112	~ 1 minute
115	~ 30 seconds

Table 1 - Maximum Exposure to SPL
Note that the exposure time is for any 24 hour period, and is halved for each 3dB SPL above 85dB. The above shows the accepted standards for recommended permissible exposure time for continuous time weighted average noise, according to NIOSH (National Institute for Occupational Safety and Health) and CDC (Centers for Disease Control). Although these standards are US based, they apply pretty much equally in most countries - hearing loss does not respect national boundaries.

Description

The amplifier itself is fairly conventional, and is very similar to another shown on this site (see Project 24). This amplifier does not include the active volume control, because in general it is far easier to get a good log pot (or simply 'fake' the pot's law as described in Project 01). Likewise, it does not include the cross-feed described in Project 109. If this is desired, it is very easy to implement on a small piece of tag board, or even 'sky hook' the few components off the bypass switch. Full details of how to do this will be included in the construction guide when PCBs are available.

The output transistors are biased using only resistors, rather than constant current sources. Extensive testing showed that using current sources made no discernible difference to performance, but increased the complexity and PCB size. Using separate caps for each biasing diode does make a difference though - and although it is relatively minor, the use of the two caps is justified IMHO.

The bias diodes should be 1N4148 or similar - power diodes are not recommended, as their forward voltage is too low. This may result in distortion around the crossover region, where one transistor turns off and the other on. As shown, crossover distortion is absolutely unmeasurable with the equipment I have available.


Above is the schematic of one channel. Resistors and caps use the suffix 'R' for the right channel. The second half of the dual opamp powers the right channel. Note that the volume control shown is optional, and is not on the PCB. If needed, it may be mounted in a convenient location and the output connected to the inputs of the board as shown. D1 and D2 (L and R) are 1N4148 or similar.

One of the reasons the amp is so quiet is that the entire board runs from a regulated supply, so hum (in particular) is eliminated. Although an unregulated supply can be used, this is not recommended. The supply should be separate from that used for your preamp, because of the relatively high current drawn by the amplifier (at least with low impedance 'phones). A P05 preamp supply can be used, and will ensure optimum performance.

The prototype amplifier has flat frequency response from 10Hz to over 100kHz. Distortion is below my measurement threshold with any level or load impedance, and output impedance is almost immeasurably low. Your headphones may be designed to operate from a 120Ω source impedance (many are), so this may be added if it improves sound quality. Adding any series resistance will reduce the available power, but it is already far greater than you can use. Without series resistance, the minimum power into various load impedances is given below (based on ±15V supplies).

Impedance	Power (Direct)	120 Ohm Feed
8 Ohms	1.5 W	35 mW
32 Ohms	2.2 W	99 mW
65 Ohms	1.1 W	136 mW
120 Ohms	595 mW	149 mW
300 Ohms	238 mW	121 mW
600 Ohms	119 mW	82 mW

Table 2 - Output Power Vs. Impedance

This is not especially comprehensive, but will cover the majority of headphones in common use. In all cases, the available power is more than needed ... not so you can damage your hearing, but to allow adequate headroom for transients.

Construction

While it may be possible to build it using Veroboard or similar, there is a high risk that it will oscillate because of the very wide bandwidth of the amplifier. A capacitor may be added in parallel with R4 (L and R) to reduce the bandwidth if stability problems are encountered. Although I used an NE5532 opamp for the prototype, the circuit will also work with a TL072, but at reduced power. You may also substitute an OPA2134 or your favorite device, taking note of the following ...
The standard pinout for a dual opamp is shown on the left. If the opamps are installed backwards, they will almost certainly fail, so be careful.

The suggested NE5532 opamp was used for the prototype, and performance is exemplary. Devices such as the TL072 will be quite satisfactory for most work, but if you prefer to use ultra low noise or wide bandwidth devices, that choice is yours.

Construction is fairly critical. Because of the wide bandwidth of the NE5532 and many other audio grade opamps, the amplifier may oscillate (the prototype initially had an oscillation at almost 500kHz), so care is needed to ensure there is adequate separation between inputs and outputs. Even a small capacitive coupling between the two may be enough to cause problems.

As shown in the photo, this amplifier needs a heatsink. While it can operate without one at low power using high impedance headphones, you need to plan for all possibilities (after all, you may purchase low impedance 'phones sometime in the future). The heatsink does not need to be massive, and the one shown above is fine for normal listening levels. An aluminium bracket may be used to attach to the chassis - I recommend 3mm material. Note that the heatsink should always be earthed (grounded).

The output transistors must be insulated from the heatsink. Sil-Pads™ are quite suitable because of the relatively low dissipation, but greased mica or Kapton can be used if you prefer. If you use the suggested 3mm aluminium, you can drill and tap threads into the heatsink, removing the need for nuts.

Testing

Connect to a suitable power supply - remember that the supply earth (ground) must be connected! When powering up for the first time, use 56 ohm "safety" resistors in series with each supply to limit the current in case you have made a mistake in the wiring. These will reduce the supply voltage considerably because of the bias current of the output transistors.

If the voltage at the amplifier supply pins is greater than ±6V and the output voltage is close to zero, then the amplifier is probably working fine. If you have an oscilloscope, check for oscillation at the outputs ... at all volume control settings. Do this without connecting your headphones - if the amp oscillates, it may damage them.

Once you are sure that all is well, you may remove the safety resistors and permanently wire the amplifier into your chassis.

Crowbar circuits are so-called because their operation is the equivalent of dropping a crowbar (large steel digging implement) across the terminals. It is only ever used as a last resort, and can only be used where the attached circuit is properly fused or incorporates other protective measures.

A crowbar circuit is potentially destructive - if the circuitry only has a minor fault, it will be a major fault by the time a crowbar has done its job. It is not uncommon for the crowbar circuit to be destroyed as well - the purpose is to protect the device(s) attached to the circuit - in this case, a loudspeaker.

Description

There's really nothing to it. A resistor / capacitor circuit isolates the trigger circuit from normal AC signals. Should there be enough DC to activate the DIAC trigger, the cap is discharged into the gate of the TRIAC, which instantly turns on ... hard. A TRIAC has two basic states, on and off. The in-between state exists, but is so fast that it can be ignored for all intents and purposes.


The BR100 DIAC (or the equivalent DB3 from ST Microelectronics) is rated for a breakdown voltage of between 28 and 36V - these are not precision devices. Needless to say, using the circuit with supply voltages less than around 40V is not recommended, as you will have a false sense of security. The supply voltage must be higher than the breakdown voltage of the DIAC, or it cannot conduct. Zeners cannot be used as a substitute for lower voltages - a DIAC has a negative impedance characteristic, so when it conducts, it will dump almost the full charge in C1 into the gate of the TRIAC. This is essential to make sure the TRIAC is switched into conduction.

The TRIAC is a common type, and may be substituted if you know the specifications. It's rated at 12A, but the peak current (non-repetitive) is 95A, and it only needs to sustain that until the fuse (or an output transistor) blows. A heatsink is preferred, but there is a good chance that the TRIAC will blow up if it has to protect your speakers, so it may not matter too much. The 0.47 ohm resistor is simply to ensure that the short circuit isn't absolute. This will limit the current a little, and increases the chance that the TRIAC will survive (albeit marginally). Feel free to use a BT139 if it makes you feel better - these are rated at 16A continuous, and 140A non-repetitive peak current.

The peak short circuit current will typically be about 90A for a ±60V supply, allowing ~0.2 ohms for wiring resistance and the intrinsic internal resistance of the TRIAC, plus the equivalent series resistance of the filter capacitors. That's a seriously high current, and it will do an injury to anything that's part of the discharge path. Such high currents are not advised for filter caps either, but being non-repetitive they will almost certainly survive.

Construction & Use

Apart from the obvious requirement that you don't make any mistakes, construction is not critical. Wiring needs to be of a reasonable gauge, and should be tied down with cable ties or similar. C1 must be polyester. While a non-polarised electrolytic would seem to be acceptable, the circuit will operate if the capacitor should dry out over the years. This means it will lose capacitance, and at some point, the crowbar may operate on normal programme material. This would not be good, as it will blow up your amplifier!

Make sure that all connections are secure and well soldered. Remember that this is the last chance for your speakers, so it needs to be able to remain inactive for years and years - hopefully it will never happen. The circuit doesn't have to be mounted in the amplifier chassis - it can be installed in your speaker cabinet. Nothing gets hot unless it operates, at which point no-one really cares - it just has to save the speakers from destruction once to have been worthwhile.

Remember that the crowbar circuit absolutely must never be allowed to operate with any normal signal. A perfectly good amplifier that triggers the circuit because of a high-level bass signal (for example) will very likely be seriously damaged if the crowbar activates. To verify that no signal can trigger it, you may want to (temporarily) use a small lamp in place of R2, and drive the amp to maximum power with bass-heavy material.

A speaker does not need to be connected. If the lamp flashes, your amp would have been damaged. If this occurs, you may want to increase the value of C1. Note that bipolar electrolytics should never be used for C1, because they can dry out and lose capacitance as they age. This could cause the circuit to false-trigger.

This little project came about as a result of a design job for a client. One of the items needed was a mic preamp, and the project didn't warrant a design such as the P66 preamp, since it is intended for basic PA only. Since mic preamps are needed by people for all manner of projects, this little board may be just what's needed for interfacing a balanced microphone with PC sound cards or other gear. Unlike most of my boards, this one is double-sided. I normally avoid double-sided PCBs for projects because rework by those inexperienced in working with them will almost certainly damage the board beyond repair. I consider this not to be an issue with this preamp, because it is so simple. It is extremely difficult to make a mistake because of the simplicity.


As you can see, the board uses a PCB mounted XLR connector and pot, so is a complete mic preamp, ready to go. Feel free to ignore the terminals marked SW1 (centred between the two electrolytic supply caps), as they are specific to my client's needs and are not useful for most applications. The original use was to use them for a push-button switch that activated an audio switch via a PIC micro-controller. They are not shown on the schematic.

The DC, GND and output terminals may be hard wired to the board, you may use PCB pins or a 10-way IDC (Insulation Displacement Connector) and ribbon cable. Power can be anything between +/-9V and +/-18V with an NE5532 opamp. The mic input is electronically balanced, and noise is quite low if you use the suggested opamp. Gain range is from about 12dB to 37dB as shown. It can be increased by reducing the value of R6, but this should not be necessary. Because anti-log pots are not available, the gain control is not especially linear, but unfortunately in this respect there is almost no alternative and the same problem occurs with all mic preamps using a similar variable gain control system.


The circuit is quite conventional, and if 1% metal film resistors are used throughout it will have at least 40dB of common mode rejection with worst-case values. The input capacitors give a low frequency rolloff of -3dB at about 104Hz. If better low frequency response is required, these caps may be increased to 4.7uF or 10uF bipolar electrolytics. These will give response to well below 10Hz if you think you'll ever need to go that low.

The project PCB measures 77 x 24mm, and the mounting centers for the pot and XLR connector are spaced at 57mm. If preferred, a traditional chassis mounted female XLR can be used, and wired to the board with heavy tinned copper wire. The PCB pads for the connector are in the correct order for a female chassis mount socket mounted with the "Push" tab at the top.

This is yet another project born of necessity. It's a simple circuit, but does exactly what it's designed to do - dim LED lights or control the speed of 12V DC motors. The circuit uses PWM to regulate the effective or average current through the LED array, 12V incandescent lamp (such as a car headlight bulb) or DC motor. The only difference between the two modes of operation is the addition of a power diode for motor speed control, although a small diode should be used for dimmers too, in case long leads are used which will create an inductive back EMF when the MOSFET switches off.


The photo shows what a completed board looks like. Dimensions are 53 x 37mm, so it's possible to install it into quite small spaces. The parts used are readily available, and many substitutions are available for both the MOSFET and power diode (the latter is only needed for motor speed control). The opamps should not be substituted, because the ones used were chosen for low power and their ability to swing the output to the negative supply rail.

Note that if used as a motor speed controller, there is no feedback, so motor speed will change with load. For many applications where DC motors are used, constant speed regardless of load is not needed or desirable, but it is up to you to decide if this will suit your needs.

Description

First, a description of PWM is warranted. As the pot is rotated clockwise, the input voltage changes linearly with rotation. At first, the voltage is such that the comparator output is just narrow spikes, which turn the MOSFET on for a very short period. Average current is low, so connected LEDs will be quite dim, or a motor will run (relatively) slowly. As the input voltage coming from the pot increases, the MOSFET is on for longer and longer, so increasing power to the load.


Figure 1 shows how the PWM principle works. The red trace is the triangle wave reference voltage, and the green trace is the voltage from the pot. When the input voltage is greater than the reference voltage, the MOSFET turns on, and current flows in the load. Because the frequency is relatively high (about 600Hz), we don't see any flicker from the LEDs, but the tone is audible from a motor that's PWM controlled. The PWM signal is shown in blue. The average current through the load is determined by the ratio of on-time to off-time, and when both are equal, the average current is exactly half of that which would be drawn with DC.


The circuit is shown in Figure 2. U1 is the oscillator, and generates a triangular waveform. R4 and R5 simply set a half voltage reference, so the opamps can function around a 6V centre voltage. U2A is an amplifier, and its output is a 10V peak to peak triangle wave that is used by the comparator based on U2B. This circuit compares the voltage from the pot with the triangle wave. If the input voltage is at zero, the comparator's output remains low, and the MOSFET is off. This is the zero setting.

In reality, the reference triangle waveform is from a minimum of about 1.5V to a maximum of 9.5V, so there is a small section at each end of the pot's rotation where nothing happens. This is normal and practical, since we want a well defined off and maximum setting. Because of this range, for lighting applications, an industry standard 0-10V DC control signal can be used to set the light level. C-BUS (as well as many other home automation systems) can provide 0-10V modules that can control the dimmer.

While a 1N4004 diode is shown for D2, this is only suitable if the unit is used as a dimmer. For motor speed control, a high-current fast recovery diode is needed, such as a HFA15TB60PBF ultra-fast HEXFRED diode. There are many possibilities for the diode, so you can use whatever is readily available that has suitable ratings. The diode should be rated for at least half the full load current of the motor, and the HFA15TB60PBF suggested is good for 15A continuous, so is fine with motors drawing up to 30A.

Construction

While it's certainly possible to build the dimmer on veroboard or similar, it's rather fiddly to make and mistakes are easily made. Also, be aware that because of the current the circuit can handle, you will need to use thick wires to reinforce some of the thin tracks. This is even necessary for the PCB version. Naturally, I recommend the PCB, and this is available from ESP. The board is small - 53 x 37mm, and it carries everything, including the screw terminals. The PCB is double-sided with plated-through holes, and has solder masks on both sides.

The MOSFET will need a heatsink unless you are using the dimmer for light loads only. It is necessary to insulate the MOSFET from the heatsink in most cases, since the case of the transistor is the drain (PWM output). For use at high current and possible high temperatures, the heatsink may need to be larger than expected. Although the MOSFET should normally only dissipate about 2W or so at 10A, it will dissipate a lot more if it's allowed to get hot. Switching MOSFETs will cheerfully go into thermal runaway and self destruct if they have inadequate heatsinking. You may also use an IGBT (insulated gate bipolar transistor) - most should have the same pinouts, and they do not suffer from the same thermal runaway problem as MOSFETs.

As noted above, there are many different MOSFETs (or IGBTs) and fast diodes that are usable. The IRF540 MOSFET is a good choice, and being rated 27A it has a generous safety margin. There are many others that are equally suitable - in fact any switching MOSFET rated at 10A or more, and with a maximum voltage of more than 20V is quite ok.

Testing

Connect to a suitable 12V power supply. When powering up for the first time, use a 100 ohm "safety" resisor in series with the positive supply to limit the current if you have made a mistake in the wiring. The total current drain is about 2.5mA with the pot fully off, rising to 12.5mA when fully on. Most of this current is in the LED, which is also fed from the PWM supply so you can see that everything is working without having to connect a load.

Make sure that the pot is fully anti-clockwise (minimum), and apply power. You should measure no more than 0.25V across the safety resistor, rising to 1.25V with the pot at maximum. If satisfactory, remove the safety resistor and install a load. High intensity LED strip lights can draw up to ~1.5A each, and this dimmer should be able to drive up to 10 of them, depending on the capabilities of the power supply and the size of the heatsink for the MOSFET.

As readers will know, there are already several power amplifier projects, two using IC power amps (aka power opamps). Both have been popular, and this project is not designed to replace either of them. However, it is significantly smaller than the others, so it makes building a multiple amp unit somewhat easier because the space demand is much lower. It's quite simple to include 4 amps (two boards) into a small space, but be aware that good heatsinking is essential if you expect to run these amps at significant power levels.

The TDA7293 IC uses a MOSFET power stage, where the others featured use bipolar transistors. The main benefit of the MOSFET stage is that it doesn't need such radical protection circuitry as a bipolar stage, so unpleasant protection circuit artefacts are eliminated. There are no apparent downsides to the TDA7293, although it was found that one batch required a much higher voltage on the Standby and Mute pins than specified, or the amps would not work. This is not a limitation, since both are tied to the positive supply rail and are therefore disabled.

This particular project has been planned for a long time, but for some reason I never got around to completing the board or the project description. This is now rectified, and it's ready to "rock and roll". The board is very small - only 77 x 31mm, so getting it into tight spaces is easy ... provided adequate heatsinking is available of course.

Description

The TDA7293 has a bewildering number of options, even allowing you to add a second power stage (in another IC) in parallel with the main one. This improves power into low impedance loads, but is a rather expensive way to get a relatively small power increase. It also features muting and standby functions, although I've elected not to use these.

The schematic is shown in Figure 1, and is based on the PCB version. All unnecessary functions have been disabled, so it functions as a perfectly normal power amplifier. While the board is designed to take two TDA7293 ICs, it can naturally be operated with only one, and the PCB is small enough so that this is not an inconvenience. A LED is included to indicate that power is available, and because of the low current this will typically be a high brightness type.

The IC has been shown in the same format that's shown in the data sheet, but has been cleaned up for publication here. Since there are two amps on the board, there are two of most of the things shown, other than the power supply bypass caps and LED "Power Good" indicator. These ICs are extremely reliable (as are most power amp ICs), and to reduce the PCB size as much as possible, fuse clips and fuses have not been included. Instead, there are fusible tracks on the board that will fail if there is a catastrophic fault. While this is not an extremely reliable fuse, the purpose is to prevent power transformer failure, not to protect the amplifiers or PCB.

I normally use a gain of 23 (27dB) for all amplifiers, and the TDA7293 is specified for a minimum gain of 26dB, below which it may oscillate. Although this is only a small margin, tests so far indicate that the amp is completely stable. If you wish, you may increase the gain to 28 (29dB) to give a bit more safety margin. To do this, just change the input and feedback resistors (R3A/B and R4A/B) from 22k to 27k.

The circuit is conventional, and is very simple because all additional internal functions are unused. The LED is optional, and if you don't think you'll need it, it may be omitted, along with series resistor R3. All connections can be made with plugs and sockets, or hard wired. In most cases, I expect that hard wiring will be the most common, as the connectors are a pain to wire, and add unnecessary cost as well as reduce reliability.

The TDA7293 specifications might lead you to believe that it can use supply voltages of up to ±50V. With zero input signal (and therefore no output) it might, but I don't recommend anything greater than ±35V if 4 ohm loads are expected, although ±42V will be fine if you can provide good heatsinking. In general, the lower supply voltage is more than acceptable for 99% of all applications, and higher voltages should not be used unless there is no choice. Naturally, if you can afford to lose a few ICs to experiments, then go for the 42V supplies (obtained from a 30+30V transformer).

Construction

Because of the pin spacings, these ICs are extremely awkward to use without a PCB. Consequently, I recommend that you use the ESP board because it makes building the amplifier very simple. The PCBs are double sided with plated-through holes, so are very unforgiving of mistakes unless you have a good solder sucker. The best way to remove parts from a double sided board is to cut the pins off the component, then remove each pin fragment individually. This is obviously not something you'd wish to do if a power amp IC were installed incorrectly, since it will be unusable afterwards.


The diagram above shows the pinouts for the TDA7293V (the "V" means vertical mounting). Soldering the ICs must be left until last. Mount the ICs on your heatsink temporarily, and slide the PCB over the pins. Make sure that all pins go through their holes, and that there is no strain on the ICs that may try to left the edge off the heatsink. When ICs and PCB are straight and aligned, carefully solder at least 4 pins on each IC to hold them in place. The remaining pins can then be soldered. Remember, if you mess up the alignment at this point in construction, it can be extremely difficult to fix, so take your time to ensure there are no mistakes.

This amplifier must not be connected to a preamp that does not have an output coupling capacitor. Even though there is a cap in the feedback circuit, it can still pass DC because there is no input cap on the PCB. I normally include an input cap, but the goal of this board was to allow it to fit into the smallest space possible, and the available board space is not enough to include another capacitor. A volume control (typically 10k log/ audio taper) may be connected in the input circuit if desired.

Note that the metal tab of the TDA7293 is connected to the -Ve supply, so must be insulated from the heatsink. The more care you take with the mounting arrangement, the better. While you can use a screw through an insulating bush and a piece of mica to insulate the tab, a better alternative is to use a clamping bar of some kind. How you go about this depends a lot on your home workshop tools and abilities, but one arrangement I've found highly satisfactory is a suitable length of 6.25mm square solid steel bar. This is very strong, and allows good pressure on the mica (or Kapton) for maximum heat transfer. Naturally, heatsink compound is absolutely essential.

Do not be tempted to use silicone insulation washers unless you are using the amp at very low supply voltages (no more than ±25V). Its thermal transfer characteristics are not good enough to allow the amp to produce more than about 10 - 20W of music, and even that can be taxing for silicone washers. The amp will shut down if it overheats, but that curtails one's listening enjoyment until it cools down again.

Power Supply

A suitable power supply is shown below, and is completely unremarkable in all respects. The transformer may be a conventional (E-I) laminated type or a toroid. The latter has the advantage of lower leakage flux, so will tend to inject less noise into the chassis and wiring. Conventional transformers are usually perfectly alright though, provided you take care with the mounting location.

The bridge rectifier should be a 35A 400V type, as they are cheap, readily available and extremely rugged. Electrolytic capacitors should be rated at 50V. The cap connected across the transformer secondary (C4) should be rated at 275V AC (X Class), although a 630V DC cap will also work. This capacitor reduces "conducted emissions", namely the switching transients created by the diodes that are coupled through the transformer onto the mains supply. The power supply will work without this cap, and will most likely pass CE and C-Tick tests as well, but for the small added cost you have a bit of extra peace of mind as regards mains noise.


The supply shown includes a "loop breaker", which is intended to prevent earth/ ground loops to prevent hum when systems are interconnected. Please be aware that it may not be legal to install this circuit in some countries. The diodes must be high current types - preferably rated at no less than 3A (1N5401 or similar). The loop breaker works by allowing you to have the chassis earthed as required in most countries, but lets the internal electronics "float", isolated from the mains earth by the 10 ohm resistor. RF noise is bypassed by the 100nF cap, and if a primary to secondary fault develops in the transformer, the fault current will be bypassed to earth via the diodes. If the fault persists and the internal fuse (or main power circuit breaker) hasn't opened, one or both diodes will fail. Semiconductor devices fail short-circuit, so fault current is connected directly to safety earth.

Be very careful when first applying mains power to the supply. Check all wiring thoroughly, verify that all mains connections are protected from accidental contact. If available, use a Variac, otherwise use a standard 100W incandescent lamp in series with the mains. This will limit the current to a safe value if there is a major fault.

When the loop breaker is used, all input and output connectors must be insulated from the chassis, or the loop breaker is bypassed and will do nothing useful. The body of a level pot (if used) can be connected to chassis, because the pot internals are insulated from the body, mounting thread and shaft.

Note that the DC ground for the amplifiers must come from the physical centre tap between the two filter caps. This should be a very solid connection (heavy gauge wire or a copper plate), with the transformer centre tap connected to one side, and the amplifier earth connections from the other. DC must be taken from the capacitors - never from the bridge rectifier.

The order of the fuse and power switch is arbitrary - they can be in any order, and in many cases the order is determined by the physical wiring of the IEC connector if a fused type is used. With a fused IEC connector, the fuse is before the switch and it cannot be removed while the mains lead is inserted.

I have shown a 2A slow-blow fuse, but this depends on the size and type of transformer and your mains supply voltage. Some manufacturers give a recommended fuse rating, others don't. The fuse shown is suitable for a 150VA transformer at 230V AC, and is deliberately oversized to ensure that it will not be subject to nuisance blowing due to transformer inrush current. A 2A fuse will fail almost instantly if there is a major fault.

Make sure that the mains earth (ground) is securely connected to guarantee a low resistance connection that cannot loosen or come free under any circumstances. The accepted method varies from one country to the next, and the earth connection must be made to the standards that apply in your country.

WARNING: This power supply circuit requires experience with mains wiring. Do not attempt construction unless experienced, capable and suitably qualified if this is a requirement where you live. Death or serious injury may result from incorrect wiring.

Testing

Never attempt to operate the amplifier without the TDA7293 ICs attached to a heatsink!

Connect to a suitable power supply - remember that the supply earth (ground) must be connected! When powering up for the first time, use 100 ohm 5W "safety" resistors in series with each supply to limit the current if you have made a mistake in the wiring. If available, use a variable bench supply - you don't need much current to test operation, and around 500mA is more than enough. If using a current limited bench supply, the safety resistors can be omitted. Do not connect a speaker to the amplifier at this stage!

If using a normal power supply for the amp tests, apply power (±35V via the safety resistors) and verify that the current is no more than 60mA or so - about 6V across each 100 ohm resistor. No load current can vary, so don't panic if you measure a little more or less. Verify that the DC voltage at both outputs is less than 100mV. Using another 100 ohm resistor in series with a small speaker, or an oscilloscope, apply a sinewave signal at about 400Hz to the input and watch (or listen) for signal. The signal level needs to be adjusted to ensure the amp isn't clipping, and the waveform should be clean, with no evidence of parasitic oscillation or audible distortion.

If everything tests out as described, wire the amplifier directly to the power supply and finish off any internal wiring in the amp. Once complete, it's ready to use.

Voice manipulation device specially intended for props, 9V Battery operation

Although this kind of voice effect can be obtained by means of some audio computer programs, a few correspondents required a stand-alone device, featuring microphone input and line or loudspeaker outputs. This design fulfills these requirements by means of a variable gain microphone preamplifier built around IC1A, a variable steep Wien-bridge pass-band filter centered at about 1KHz provided by IC1B and an audio amplifier chip (IC2) driving the loudspeaker.

Parts:
P1______________10K Log. Potentiometer
R1,R10__________10K 1/4W Resistors
R2_______________1K 1/4W Resistor
R3______________50K 1/2W Trimmer Cermet or Carbon
R4,R6,R7,R14___100K 1/4W Resistors
R5______________47K 1/4W Resistor
R8______________68K 1/4W Resistor
R9_______________2K2 1/2W Trimmer Cermet or Carbon
R11_____________33K 1/4W Resistor
R12_____________18K 1/4W Resistor
R13_____________15K 1/4W Resistor
C1,C2,C3,C8,C9_100nF 63V Polyester Capacitors
C4______________10µF 25V Electrolytic Capacitor
C5_____________220nF 63V Polyester Capacitor (Optional, see Notes)
C6_______________4n7 63V Polyester Capacitor
C7______________10nF 63V Polyester Capacitor
C10____________220µF 25V Electrolytic Capacitor
IC1___________LM358 Low Power Dual Op-amp
IC2_________TDA7052 Audio power amplifier IC
MIC1__________Miniature electret microphone
SPKR______________8 Ohm Small Loudspeaker
SW1____________DPDT Toggle or Slide Switch
SW2,SW3________SPST Toggle or Slide Switches
J1____________6.3mm or 3mm Mono Jack socket
B1_______________9V PP3 Battery (See Notes) Clip for PP3 Battery

Notes:

• The pass-band filter can be bypassed by means of SW1A and B: in this case, a non-manipulated microphone signal will be directly available at the line or loudspeaker outputs after some amplification through IC1A.
• R3 sets the gain of the microphone preamp. Besides setting the microphone gain, this control can be of some utility in adding some amount of distortion to the signal, thus allowing a more realistic imitation of a telephone call voice.
• R9 is the steep control of the pass-band filter. It should be used with care, in order to avoid excessive ringing when filter steepness is approaching maximum value.
• P1 is the volume control and SW2 will switch off amplifier and loudspeaker if desired.
• C5 is optional: it will produce a further band reduction. Some people think the resulting effect is more realistic if this capacitor is added.
• If the use of an external, moving-coil microphone is required, R1 must be omitted, thus fitting a suitable input jack.
• This circuit was intended to be powered by a 9V PP3 battery, but any dc power supply in the 6 - 12V range can be used successfully.

There will be many times where it is desirable to use the P05 supply module from a higher voltage source. For example, if you want to add balanced inputs to a power amplifier, then you need a +/-15V supply, but the amp's supply voltage will be much too high for the regulator ICs.
This project is about as simple as they come, and is very cheap to build. It is designed for exactly this purpose - to reduce the amplifier supply voltage to a safe value for regulator ICs.

Description:

The circuit is shown in Fig. 1 and it is very simple indeed. You will need to make a few simple calculations to determine the resistor value, but this is explained below.

The circuit shown uses the 24V zener diodes (D1 and D2) to regulate the output voltage to a little under 24V. This is a perfectly safe input voltage for standard 3-terminal regulators, and using this circuit will provide even better regulation and supply noise rejection then normal. Using MJE3055 and 2955 transistors will allow for supply voltages up to 70V quite safely, but they will need to be mounted on a heatsink (with insulating washers).

The purpose of R5 is to isolate the main power amplifier ground from the supply, to prevent hum loops. The 10Ω resistor shown will be fine for the vast majority of applications, but may need to be changed. This is up to you to experiment with if necessary. I suggest that R5 should be 1W. R2 and R4 may be 1/4W or 1/2W resistors, and 1W zeners are recommended.

The only calculation is to determine the value for R1 and R3. First, measure the power amp supply voltage (V1). The resistor value is calculated to provide a maximum zener current of 20mA, and this will ensure sufficient base current for the pass transistors for up to 100mA or so output current at ±15V.

V2 = V1 - 24 (Where V1 is amplifier supply voltage, and a 24V zener is used)
R1 = R3 = V2 / 20 (R1 and R3 values are in kΩ)
P = V2² / R1 (P is power dissipation of R1 and R3 in mW)

Let's assume a supply voltage of ±56V for an example calculation ...

V2 = 56 - 24 = 32V
R1 = R3 = 32 / 20 = 1.6k (use 1.5k)
P = 32² / 1.5 = 680mW = 0.68W (use 1W)

The dissipation in Q1 and Q2 may also be calculated, but you need to know the current drawn by the external circuits. For example, if the external circuitry draws 50mA, the transistor power dissipation is ...

Pt = V2 * Iext = 32 * 50 = 1600mW = 1.6W (it will need a small heatsink)

That's it - it could hardly be simpler.

Construction:

Construction is non critical, and the resistors, zener and power transistors can be mounted on a tiny piece of Veroboard or similar. There are no stability issues, and you only need to make sure that the transistors have an adequate heatsink. Mounting to the chassis will normally be quite sufficient - even a steel chassis will keep the temperature well within limits. Remember that the transistor cases must be electrically isolated from the chassis, and Sil-Pads will be fine due to the low dissipation.

A suggestion for assembly is shown in Figure 2 (note that the 10Ω resistor from the main supply has not been shown). This construction method will be quite acceptable for most applications. The earth (GND) terminal point should ideally be isolated from the heatsink to prevent earth loops.
Testing:

Connect to a suitable power supply - remember that the supply earth (ground) must be connected! When powering up for the first time, use 100 ohm to 560 ohm "safety" resistors in series with each supply to limit the current if you have made a mistake in the wiring. There is very little that can go wrong (other than wiring mistakes), so any fault you may find is easily rectified.

A power supply suitable for use with the 60W amplifier presented in the predeeding project is perfectly simple, and no great skill is required to build (or design) one. There are a few things one should be careful with, such as the routing of high current leads, but these are easily accomplished. The first thing to choose is a suitable transformer. I suggest toroidal transformers rather than the traditional "EI" laminated types because they radiate less magnetic flux and are flatter, allowing them to be installed in slimmer cases.

They do have some problems, such as higher inrush current at switch on, which means that slow blow fuses must be used. For the 60W amplifier, a nominal (full load) supply of +/- 35V is required, so a 25-0-25 secondary is ideal - however, see Updates, below. The circuit for the supply is shown below, and uses separate rectifiers, capacitors and fuses for each channel. Only the transformer is shared, so channel interactions are minimised. A single ±35V supply (i.e. using only a single bridge and set of filter capacitors) will work just as well in the majority of cases.

The 5A slow-blow fuse shown is suitable for a 300VA transformer, if a 120VA transformer is used, this should be reduced to 2.5A (or 3A if 2.5A proves too hard to get). If you are even a little bit concerned about the fuse rating, contact the transformer manufacturer for the recommended value for the transformer you will use. The correct fuse is critical to ensure safety from electrical failure, which could result in the equipment becoming unsafe or causing a fire.

The capacitance used is not critical, but is somewhat dependent upon one's budget. I suggest 10,000uF capacitors, but they are rather expensive so at a pinch 4,700uF caps should be fine - especially in the arrangement shown. When unloaded (or with only light load), the voltage will normally be somewhat higher than 35 Volts. This is Ok, and should not cause distress to any amp. The voltage will fall as more current is drawn, and may drop below 35V if a small transformer (or one with unusually poor regulation) is used.

Two parts of this circuit are critical:

• Mains wiring must be cabled using approved 240V rated insulated cable, and all terminations must be insulated to prevent accidental contact. The mains earth must be securely fastened to the chassis, after scraping away any paint or other coating which might prevent reliable contact.
• The centre-tap of the transformer and the ground points of each capacitor must be connected to the main signal earth point via heavy duty copper wire, or (preferably) a copper bus-bar. Large currents flow in this part of the circuit, containing nasty current waveforms which are quite happy to invade your amplifier. The supply voltages must be taken from the capacitors (not the bridge rectifiers) to prevent unwanted hum and noise.

When wiring the bridge rectifiers to the transformer, connect exactly as shown to ensure that ripple voltages (and currents) are in phase for each amp. If not, mysterious hum signals may be injected into the amp's signal path from bypass capacitors and the like. This is somewhat unlikely unless huge caps are used on the amp board(s) - not recommended, by the way - but why take the risk?

Bridge rectifiers should be the big bolt-down 35A types (or something similar) to ensure lowest possible losses (these will not require an additional heatsink - the chassis will normally be quite sufficient). The transformer primary voltage will obviously be determined by the supply voltage in your area (i.e. 120, 220 or 240) and be suited to the local supply frequency. Note that all 50Hz transformers will work just fine at 60Hz, but some 60Hz devices will overheat if used at 50Hz.

The transformer should be rated at a minimum of 120VA (Volt-Amps) for home use, but a 300VA transformer is recommended due to its superior regulation. Going beyond 300VA will serve no useful purpose, other than to dim the lights as it is turned on. Where it is possible, the signal and power ground should be the same (this prevents the possibility of an electric shock hazard should the transformer develop a short circuit between primary and secondary. Where this will give rise to ground loops and hum in other equipment, use the method shown.

The resistor R1 (a 5W wirewound resistor is suggested) isolates the low-voltage high-current ground loop circuit, and the diodes D1 & D2 provide a protective circuit in the event of a major problem. These diodes need only be low voltage, but a current rating of 5A or greater is required. The 100nF capacitor (C1) acts as a short circuit to radio frequency signals, effectively grounding them. This should be a device with very good high frequency response, and a 'monolithic' ceramic is recommended.

Updates:

The transformer secondary voltage will probably need to be higher than described above. I tested some stock and custom transformers I have, and found that unless the transformer has extraordinarly good regulation, a nominal 28-0-28 secondary will be needed, more with an average (i.e. poor) regulation unit. Also be careful when you test, since a relatively small (10%) variation in the mains voltage makes a big difference to measured output power - the secondary voltage also falls by 10%, so 60W becomes 48W if the mains is 10% low.

You also need to remember that the output voltage of transformers is typically quoted at full power with a resistive load. This means two things:

1. The no load voltage will be higher than expected
2. The loaded voltage will be lower than expected

The first point is true because there is no loading, so the output voltage must rise. The second is more complex, but happens because the conventional rectifier circuit uses a capacitor input filter (the rectifier feeds directly into the capacitor(s)). Since the diodes only conduct at the peak of the waveform, the current is much higher, so the transformer and supply line impedance will cause the peak voltage to fall, and the DC voltage cannot exceed the peak output voltage (less two diode forward voltage drops).

High Quality Moving Magnet Pick-up module
Two-stage Series/Shunt feedback RIAA equalization

Any electronics amateur still in possess of a collection of vinyl recordings and aiming at a high quality reproduction should build this preamp and add it to the Modular Preamplifier chain. This circuit features a very high input overload capability, very low distortion and accurate reproduction of the RIAA equalization curve, thanks to a two-stage op-amp circuitry in which the RIAA equalization network was split in two halves: an input stage (IC1A) wired in a series feedback configuration, implementing the bass-boost part of the RIAA equalization curve and a second stage, implementing the treble-cut part of the curve by means of a second op-amp (IC2A) wired in the shunt feedback configuration.

Parts:

R1_____________270R 1/4W Resistor
R2_____________100K 1/4W Resistor
R3_______________2K2 1/4W Resistor
R4______________39K 1/4W Resistor
R5_______________3K9 1/4W Resistor
R6_____________390K 1/4W Resistor
R7______________33K 1/4W Resistor
R8______________75K 1/4W Resistor (or two 150K resistors wired in parallel)
R9_____________560R 1/4W Resistor
C1_____________220pF 63V Polystyrene or Ceramic Capacitor
C2_______________1µF 63V Polyester Capacitor
C3______________47µF 25V Electrolytic Capacitor
C4______________10nF 63V Polyester Capacitor 5% tolerance or better
C5_______________1nF 63V Polyester Capacitor 5% tolerance or better
C6,C9__________100nF 63V Polyester Capacitors
C7,C10__________22µF 25V Electrolytic Capacitors
C8,C11________2200µF 25V Electrolytic Capacitors
IC1___________LM833 or NE5532 Low noise Dual Op-amp
IC2___________TL072 Dual BIFET Op-Amp
IC3___________78L15 15V 100mA Positive Regulator IC
IC4___________79L15 15V 100mA Negative Regulator IC
D1,D2________1N4002 200V 1A Diodes
J1,J2___________RCA audio input sockets
J3______________Mini DC Power Socket

This module comprises also an independent dual rail power supply identical to that described in the Modular Preamplifier Control Center. As with the other modules of this series, each electronic board can be fitted into a standard enclosure: Hammond extruded aluminum cases are well suited to host the boards of this preamp. In particular, the cases sized 16 x 10.3 x 5.3 cm or 22 x 10.3 x 5.3 cm have a very good look when stacked. See below an example of the possible arrangement of the rear panel of this module.

Notes:

• The circuit diagram shows the Left channel only and the power supply
• Some parts are in common to both channels and must not be doubled. These parts are: IC3, IC4, C6, C7, C8, C9, C10, C11, D1, D2 and J3.
• IC1 and IC2 are dual Op-Amps, therefore the second half of these devices will be used for the Right channel
• This module requires an external 15 - 18V ac (50mA minimum) Power Supply Adaptor.

Technical data:

Sensitivity @ 1KHz: 4.3mV RMS input for 200mV RMS output
Max. input voltage @ 100Hz: 53mV RMS
Max. input voltage @ 1KHz: 212mV RMS
Max. input voltage @ 10KHz: 477mV RMS
Frequency response @ 200mV RMS output: flat from 30Hz to 23KHz; -0.5dB @ 20Hz
Total harmonic distortion @ 1KHz and up to 8.8V RMS output: 0.0028%
Total harmonic distortion @10KHz and up to 4.4V RMS output: 0.008%

Prevents the complete discharge of the battery when the door is left open accidentally

I recently forgot to close the door of my car after parking in the garage and I found the battery completely exhausted after the week-end, when I tried to start the engine on Monday morning. This inconvenience prompted me to design a simple circuit, capable of switching-off automatically after a few minutes the inside courtesy lamp, the real culprit for the damage.

Parts:

R1______________10M 1/4W Resistor
R2______________10K 1/4W Resistor
C1______________47µF 25V Electrolytic Capacitor
IC1____________7555 or TS555CN CMos Timer IC
D1___________1N4148 75V 150mA Diode
Q1____________BD681 100V 4A NPN Darlington Transistor
LP1___________Existing Lamp Bulb, usually 12V 5W
SW1____________SPST Existing Door-Switch
SW2____________SPST Existing Bypass Switch

Circuit operation:

When the door is opened, SW1 closes, the circuit is powered and the lamp is on. C1 starts charging slowly through R1 and when a voltage of 2/3 the supply is reached at pins #2 and #6 of IC1, the internal comparator changes the state of the flip-flop, the voltage at pin #3 falls to zero and the lamp will switch-off. The lamp will remain in the off state as the door is closed and will illuminate only when the door will be opened again. The final result is a three-terminal device in which two terminals are used to connect the circuit in series to the lamp and the existing door-switch. The third terminal is connected to the 12V positive supply.

Notes:

• With the values specified for R1 and C1, the lamp will stay on for about 9 minutes and 30 seconds.
• The time delay can be changed by varying R1 and/or C1 values.
• The circuit can be bypassed by the usually existing switch that allows the interior lamp to illuminate continuously, even when the door is closed: this connection is shown in dotted lines.
• Current drawing when the circuit is off: 150µA.

This circuit is a relay driver that is based on a PIC16F84A microcontroller. The board includes four relays so this lets us to control four distinct electrical devices. The controlled device may be a heater, a lamp, a computer or a motor. To use this board in the industrial area, the supply part is designed more attentively. To minimize the effects of the ac line noises, a 1:1 line filter transformer is used.

The components are listed below.
1 x PIC16F84A Microcontroller
1 x 220V/12V 3.6VA (or 3.2VA) PCB Type Transformer (EI 38/13.6)
1 x Line Filter (2x10mH 1:1 Transformer)
4 x 12V Relay (SPDT Type)
4 x BC141 NPN Transistor
5 x 2 Terminal PCB Terminal Block
4 x 1N4007 Diode
1 x 250V Varistor (20mm Diameter)
1 x PCB Fuse Holder
1 x 400mA Fuse
2 x 100nF/630V Unpolarized Capacitor
1 x 220uF/25V Electrolytic Capacitor
1 x 47uF/16V Electrolytic Capacitor
1 x 10uF/16V Electrolytic Capacitor
2 x 330nF/63V Unpolarized Capacitor
1 x 100nF/63V Unpolarized Capacitor
1 x 4MHz Crystal Oscillator
2 x 22pF Capacitor
1 x 18 Pin 2 Way IC Socket
4 x 820 Ohm 1/4W Resistor
1 x 1K 1/4W Resistor
1 x 4.7K 1/4W Resistor
1 x 7805 Voltage Regulator (TO220)
1 x 7812 Voltage Regulator (TO220)
1 x 1A Bridge Diode
The transformer is a 220V to 12V, 50Hz and 3.6VA PCB type transformer. The model seen in the photo is HRDiemen E3814056. Since it is encapsulated, the transformer is isolated from the external effects. A 250V 400mA glass fuse is used to protect the circuit from damage due to excessive current. A high power device which is connected to the same line may form unwanted high amplitude signals while turning on and off. To bypass this signal effects, a variable resistor (varistor) which has a 20mm diameter is paralelly connected to the input.
Another protective component on the AC line is the line filter. It minimizes the noise of the line too. The connection type determines the common or differential mode filtering. The last components in the filtering part are the unpolarized 100nF 630V capacitors. When the frequency increases, the capacitive reactance (Xc) of the capacitor decreases so it has a important role in reducing the high frequency noise effects. To increase the performance, one is connected to the input and the other one is connected to the output of the filtering part.

After the filtering part, a 1A bridge diode is connected to make a full wave rectification. A 2200 uF capacitor then stabilizes the rectified signal. The PIC controller schematic is given in the project file. It contains PIC16F84A microcontroller, NPN transistors, and SPDT type relays. When a relay is energised, it draws about 40mA. As it is seen on the schematic, the relays are connected to the RB0-RB3 pins of the PIC via BC141 transistors. When the transistor gets cut off, a reverse EMF may occur and the transistor may be defected.

To overcome this unwanted situation, 1N4007 diodes are connected between the supply and the transistor collectors. There are a few number of resistors in the circuit. They are all radially mounted. Example C and HEX code files are included in the project file. It energizes the next relay after every five seconds.