Schematic Reference

Balanced Microphone Amplifier

We published a design for a stereo microphone preamplifier with balanced inputs and a phantom power supply. The heart of this circuit was a special Analog Devices IC, the SSM2017. Unfortunately, this IC has been discontinued. In its place, the company recommends using the pin-compatible AMP02 from its current product line. However, and again unfortunately, the specifications of this opamp make it considerably less suitable for use as a microphone amplifier. By contrast, Texas Instruments (in their Burr Brown product line) offer an integrated instrumentation amplifier (type 1NA217) that has better specifications for this purpose.

Incidentally, this IC is also recommended as a replacement for the SSM2017. It features internal current feedback, which ensures low distortion (THD + noise is 0.004 % at a gain of 100), low input-stage noise (1.3 nV/√Hz) and wide bandwidth (800 kHz at a gain of 100). The supply voltage range is ±4.5 V to ±18 V. The maximum current consumption of the 1NA217 is ±12 mA. The gain is determined by only one resistance, which is the resistance between pins 1 and 8 of the IC. The circuit shown here is a standard application circuit for this instrumentation amplifier. R1 and R2 provide a separate phantom supply for the microphone connected to the amplifier (this is primarily used with professional equipment).

This supply can be enabled or disabled using S1. C1 and C2 prevent the phantom voltage from appearing at the inputs of the amplifier. If a phantom supply is not used, R1 and R2 can be omitted, and it is then better to use MKT types for C1 and C2. Diodes D1–D4 are included to protect the inputs of the 1NA217 against high input voltages (such as may occur when the phantom supply is switched on). R4 and R5 hold the bias voltage of the input stage at ground potential. The gain is made variable by including potentiometer P1 in series with R6. A special reverse log-taper audio potentiometer is recommended for P1 to allow the volume adjustment to follow a linear dB scale.

The input bias currents (12 µA maximum!) produce an offset voltage across the input resistors (R4 and R5). Depending on the gain, this can lead to a rather large offset voltage at the output (several volts). If you want to avoid using a decoupling capacitor at the output, an active offset compensation circuit provides a solution. In this circuit, a FET-input opamp with a low input offset (an OPA137) is used for this purpose. It acts as an integrator that provides reverse feedback to pin 5, so the DC output level is always held to 0 V. This opamp is not in the audio signal path, so it does not affect signal quality. Naturally, other types of low-offset opamps could also be used for this purpose. The current consumption of the circuit is primarily determined by the quiescent current of IC1, since the OPA137 consumes only 0.22mA.