I'm trying to fix a preamp (made by a colleague) that's noisy and somehow not amplifying enough. I should say right away that this project is pushing the edge of my electronics knowledge so I have a lot of questions. There's actually 2 copies on 2 boards, for one electret and one dynamic mics, but the design was the same except for the "plug-in power" for the electret (not phantom power, this expects about 5V, though I guess it's currently getting 12V). This is the current circuit (only R3 is different between the 2 boards).
From my research I found a few things maybe should be done differently. First, it seems it's mostly power line noise that I'm getting. (I tried getting cleaner power supplies but on a small budget that didn't work out.) Instead, I could add some filtering on the voltage divider, and on the electret bias circuit. I also don't like the idea of attenuating after amplifying so I'd move the potentiometer to control the gain instead, as such:
But these changes mean I have to pretty much redo the whole thing, and the product is in a "finished" state (in a box with connectors and everything), so I'd like to minimize the changes if possible.
1) (main question) I was wondering if "pre-filtering" the power supply would be equivalent/better/worse to having the filters in the preamp circuits?
I was thinking for example adding a 2nd order RC filter on a separate little board. It would reduce the voltage a bit, but I can use small resistors and then do one good filter instead of 3?
The other issue is the gain. Note that the goal is to get line level audio, so that voice from the mics is at comparable volume to other sounds coming from a computer. The outputs goes to speakers (this is part of an intercom/mixer). The gain is already at 2k and still pretty weak. I'm thinking maybe the electret mic isn't getting the appropriate power? I think it's supposed to get around 5V, but I read in another thread something I didn't quite understand about the electret inner workings and how the resistor value would change how much amplitude it would output.
2) Any tips on what value to use for the electret bias?
Note that the dynamic mic board (without R3) also sounds weak, but I'll have to verify, its R5 might have a different value.
Finally, I also read about resistor noise and how I'm better off not going too high (and use metal film) with such high gains. But I'm not sure which resistors would be most critical. In the second schematic, I'm guessing R8, R4... Not sure if the voltage divider also, R3, R9/11? Related to that, I'm unsure what values to use for the voltage divider.
3) Other than not wasting too much power (prefer large values), or resistor noise (prefer low values), are there other factors to consider?
Note that the dynamic mic has output impedance of 600Ω but the electret, 5kΩ.
4) Should I "tune" the circuits differently, or just go with the highest one and give them about 50kΩ input impedance (R8)?
Any other comments or tips on the circuit would be appreciated!
5) In the second circuit, should the output after Cout be left "floating" or add a R to neutral, and if so what R value?