Microphone Preamplifier LM833

Posted on May 25, 2012

This design was born of frustration with the mic preamp in my Sony R91. It clips at ridiculously low levels, basically it has 28mV of headroom to deal with an input that could be as high as 1800mV (depending on mic & volume), so this design attempts to cure that by : A: Providing a higher electret bias voltage, 9V through a 10k resistor to give the mic's fet more headroom. (The R91 provides 2.5V through a 6k8 resistor.) B: Having much greater preamp headroom, and bypassing the R91's mic preamp altogether, using a hi-fi or low noise op-amp to go straight into the line input.

The circuit needs to bring the signal from an electret microphone up to a level suitable for a line input. Based on measurement of the microphone output under a range of conditions, I have chosen two gain levels - 2 & 23. 23 is suitable for everyday general recording. 2 is suitable for live music such as a rock concert. There is a output pot to allow fine-tuning of the level if necessary, but it is intended that usually the wiper will be on the land. 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. The...

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