Electronic – Voltage Divider with low noise

EDIT: This all is unimportant. The questioner found the error by himself. See the comment. The 47nF capacitor in series with the 2200 ohm resistor kills all bass.

It's not white noise if you filter it. I bet your system oscillates and you cant stand the high pitched whistle. Why do I think so: I can't see any big capacitor at the 741's supply voltage input terminals. Connect one 50uF 10V to 25V between the same points where the wires from the battery come to the opamp.

The noise, if it is white, can sound quite high pitched and fizzy if you have a little speaker that has no bass and gives enormous boosts to some frequencies. Actually the white noise should have absolutely no pitch ( distinquishable frequency).

If you need a less treble content sound, attenuate some high frequencies (and kill the whiteness at the same time) Connect a little capacitor parallel with the 1MOhm resistor Start tests with 150 pF (that 1MOhm which is connected over the opamp).

There are a number of problems with this circuit:

1. The input to the reference is not filtered.

   

   R5 and R4 make half the supply voltage, but also transfer half of whatever noise is on the supply. There should be cap to ground across R4. I'd start with around 2 µF.

2. There is no bias supply for the microphone.

   

   You said this was a electret, so there should be a pullup to 5 V. Usually these should be a few kΩ. Check the datasheet of your mic.

   You should also filter the pullup voltage to keep power supply noise from being fed directly into the mic. Perhaps 1 kΩ from 5 V followed by 20 µF to ground, then 2 kΩ to the mic. Again, see the mic datasheet.

3. There are no bypass caps anywhere!

   

   The above shows the power to only one opamp, but all of them have the same problem. For even better noise rejection, especially for the first stage amp shown above, put a ferrite chip inductor in series followed by 10 µF or so ceramic to ground. That reduces high frequencies, like from radio station pickups, that the active circuitry in the opamp can't handle well.

4. Low gain. The first stage amplifies by a bit over 6. That by itself is OK, although I'd like to see a bit more from the first stage if the opamp can handle it. You didn't provide a link to the opamp datasheet, so I didn't look it up. In any case, there are certainly opamps available that can run from 5 V and provide a closed loop gain of 30 or so at 20 kHz with plenty of gain headroom for the feedback to do its job.

   A microphone amp usually needs a gain of around 1000 at full volume to get to line level signals. Two gain stages at front with a gain of 30 each is usually about right, with a volume control between the two of them.

   After the signal has been amplified, any noise added is a much lower fraction of the whole.

5. Decouple the Vref inputs to each stage. U5 isn't perfect. It will have some noise. It's output impedance also isn't 0, so there will be some cross talk between stages. Even just 1 kΩ in series followed by 20 µF to ground at each point of use would help.

6. Do you really need all that filtering? This is way excessive for any normal audio. Eliminating filter stages will reduce noise. Everything adds noise, so don't put more stuff in the signal path than you really need.