Stereo Electret Mic Preamplifier Circuit
When choosing an op-amp, it’s important to find out the open loop gain at 20kHz. The circuit has roughly 27dB of gain, and at least 47dB at 20kHz (i.e. 20dB higher) is desirable. This is because op-amp circuits are predicated on an ideally infinite open loop gain, while in reality 10 times (20dB) higher than the closed loop gain at highest frequency of interest is acceptable. Otherwise the amplifier becomes progressively non-linear. The TL072 at roughly 45dB just makes it. The LM833 is good at ?55dB and the NE5532 sparkles at ?60+dB.
If you’re going for ultra small size, you can build it single supply rail, but an LM833 won’t run like that. A TL072 will, with reduced output swing, it’ll swing about 1.8VRMS off a single ended 9V supply, which is adequate for the purpose. Otherwise two 9V batteries will do the trick.
Fairly high quality components have been used – 1% metal film resistors, solid aluminium caps, though the output pot is a bit miserable.
