lovesjsan:
Hi everybody,So what I've been trying to do is reading audio input from an electret mic, and I've cooked up the circuit below. The code is just reading voltage from A0 and send it back to my computer. Omitted from the circuit are 2 100nF "decoupling capacitor" bridging 5+ -> GND and +Vcc -> -Vcc which I don't know whether crucial or not (doesn't seem to affect the results).
One of the first mistakes in low-noise design is using large resistor values. 100k/10k feedback network is
too high, use something more like 220/2k2. All resistors are noise sources and the higher the resistance the
more voltage noise you get.
The TL082 is not a low-noise opamp(*), not a good coince for microphone amplication. If you want the best you could go to something awesome like the OPA1612, but a standard NE5532 will be cheap and much better than
the TL082.
And without band-limiting before sampling with the ADC you are going to see all the high frequency noise aliased down
into the audio band, so some sort of analog low pass filtering before the ADC will reduce noise substantially (even just a simple RC should make a big difference).
All of this pre-supposes you've already eliminated sources of EMI injecting noise directly into the circuit, which you
clearly originally had.
The problem is, without any sound and minimal gain with R3 at 10k Ohms, analogRead returns values ranging very quickly (only seems to be limited by the baud rate) between +-15 units around 511. For some reasons, with loud sounds the input still only varies just a bit more (mic input seems to be around 5-6 times the noise value). And pulling out C1 (grounding IN+ I think) doesn't affect the noise at all (still varies between 500 & 530 very quickly). Because of this changing R3 doesn't get rid of the noise.
(*) Not low voltage noise with a low impedance source, to be precise - its low current noise and
good with a high impedance source, and its by no means the worst, but there are many devices an
order of magnitude better for a 2k source impedance electret.