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).

Balanced Microphone Amplifier Circuit

Balanced Microphone Amplifier Circuit Diagram

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 0.22 mA.
Author: T. Giesberts
Copyright: Elektor Electronics


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