Stereo electret microphone preamplifier circuit

This is a simple stereo electret microphone preamp circuit. The design diagram below is for a mono design, but the PCB layout is designed for a stereo design and an electret microphone preamplifier. For best performance and better quality, use solid or film capacitors and metal film resistors (1% tolerance).

Schematic diagram, schematic diagram:

Stereo electret microphone preamplifier circuit

The 10k resistor on the far left provides plug-in power to the electret, forming part of the FET amplifier in the electret capsule. This can be anything from 2k to 10k, the higher the stereo separation the better (the other mic gets offset from the same track). Obviously higher values ​​will also reduce distortion, and the best bias supply circuits actually involve breaking the traces on the electret capsule to allow both drain and source resistors to be used, but I wouldn't go that far .

The 2.2uF capacitor on the far left blocks the bias voltage from the input. Together with the 27k resistor below, it forms a high-pass filter, but with a cutoff frequency essentially close to DC.

The input impedance is set by two 27k resistors and a 10k resistor. Because of the power cap, the +ve rail is also grounded as far as AC signals are concerned. So there are two 27k resistors connected in parallel to make 13.5k, and in parallel with 10k, the input impedance is about 6k. However, if you were to make it a proper dual supply, you wouldn't need the upper 27k resistor since the input no longer needs to be biased at the center rail.

The feedback loop has two resistors 27k and 1k5 from the inverting input to ground. When they are both in the circuit, the gain is slightly less than 2 ((28.5/33) + 1). The 27k resistor can be bypassed with a switch and then just 1k5 to set the gain to 23 ((33/1.5) + 1).

The 10uF capacitor in the lower half of the feedback loop reduces the DC gain to ~1. Value is not very important. If any DC input offset is amplified, it will create a larger output offset, pushing the output towards one of the rails and reducing headroom. (This may not matter when the expected input level is a gain of 23.)

An optional 2pF capacitor associated with the 33k resistor sets high frequency roll-off. Cutoff frequency is at 100 kilohertz. It has to go further than 20kHz to keep the phase shift of the audio frequencies small, and also because the output starts to drop off long before the cutoff. Op amps cannot maintain enough gain at these frequencies anyway, and their output has dropped, but the capacitor makes the circuit more stable, although it may work without it. The PCB trace alone may have 2pF of capacitance, and op amps tend to be well compensated these days so it's really not needed. In retrospect, I think this cutoff frequency should be much lower, like 30kHz-50kHz.

The 100ohm resistor section is used to limit the current to protect the op amp if the output is shorted, but the op amp has internal protection anyway. They mainly allow op amps to drive capacitive loads (long/cheap cables) without oscillation.

With an upper limit of 2.2uF on the output block DC, this value is not particularly important. It forms a high-pass filter with the 10k potentiometer, and the cutoff is actually DC.

If you think you might accidentally start connecting the battery incorrectly, you'd better put a diode in series with the battery clip or you'll smoke your IC. Put your IC in the socket as well, just in case you do want/need to replace it. There are several dual op-amps you can try, which are direct plug-in replacements.

PCB layout: