Stereo Electret Mic Preamplifier

This a simple stereo electret microphone pre amplifier circuit. The below design diagram is the design for single channel, but the design of PCB layout is for stereo design og electret mic pre amp.? For maximum performance, better quality, use solid capacitors or film capacitors and metal film resistors (1% tolerance).

schematic diagram:

The leftmost 10k resistor supplies plug-in-power to the electret, forming part of the FET amplifier in the electret capsule. This could be anything from 2k to 10k, the higher the better the stereo separation (another mic derives bias from the same rail). Apparently higher values also lower distortion, and the best bias power circuits involve actually breaking a trace on the electret capsule to allow the use of both a drain & source resistor, but I”m not going that far.

The leftmost 2.2uF cap blocks the bias voltage from the input. In conjunction with the following 27k resistor it forms a high pass filter, but cutoff is essentially near DC.

The input impedance is set by the two 27k resistors and the 10k resistor. The +ve rail is also connected to ground as far as the AC signal is concerned because of the power supply cap. So there are two 27k resistors in parallel, making 13.5k, in parallel with the 10k, making about 6k or so for the input impedance. But if you’re making it proper dual supply, you don’t need the upper 27k resistor, as the input doesn’t have to be biased mid rail anymore.

The feedback loop has two resistors 27k & 1k5 from the inverting input to ground. When they are both in circuit, the gain is a bit under 2 ((28.5/33)+1). The 27k resistor can be bypassed with a switch, then only the 1k5 sets the gain, to 23 ((33/1.5)+1).

The 10uF cap in the bottom half of the feedback loop reduces DC gain to ~1. The value isn’t very important. If any DC input offset were amplified it would create a larger output offset, pushing the output toward one of the rails and reducing headroom. (At a gain of 23 with the expected input levels it probably doesn’t matter.)

The optional 2pF cap in relation to the 33k resistor sets the high frequency rolloff. The cutoff frequency is in the 100″s of kHz. It has to go further than 20kHz to keep the phase shift at audio frequencies small, and also because output starts falling long before cutoff. The op-amps cannot maintain enough gain at these frequencies anyway and their output will already be falling, but the cap makes the circuit more stable, though it will probably work without it. There will probably be 2pF of capacitance just from the PCB traces, and op-amps tend to be fairly well compensated these days so it”s really not needed. I think in retrospect this cutoff frequency should be much lower, say 30kHz-50kHz.

The 100ohm resistors are there partly to limit current to protect the op-amp if the output is shorted, but the op-amps have internal protection anyway. They mainly allow the op-amp to drive capacitive loads (long/cheap cables) without oscillation.

The 2.2uF cap on the output blocks DC and the value is not specially important. It forms a highpass filter with the 10k pot, the cutoff is virtually at DC.

If you think you might accidentally start connecting the battery the wrong way round, you’d better put a diode in series with the battery clip, or you’ll smoke your ic. Put your IC in a socket too just in case you do want/need to change it. You could try several dual op-amps against each other, they’re all direct plug in replacements.

PCB layout: