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Have noise on mic preamp plus aux mixer
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Author:  Adafruit123 [ 01 Nov 2016, 04:00 ]
Post subject:  Have noise on mic preamp plus aux mixer

I'm a very new guy to audio hardware so please bear with me.Thanks advanced!
What I wanted to do was make a simple unit that takes a mic input, mixes it with audio playing from the headphone jack of an iPod/iPad/etc and spits it out to a self-amplified speaker.

Modified a circuit I found online to mix the mic with the left channel and have what's in the attached image. I'm just using basic electrolytic caps and 1% resistors.

The issue is that there's a high pitched noise coming out the speaker at all times, and seems to depend on the pot value from the mic preamp. The higher the mic volume, the worse the noise. Cranking the volume down to 0 causes the noise to reduce considerably, but not disappear.

I assume there's some noise being generated somewhere in the preamp stage and being forwarded on. It happens even with the mic disconnected and the closed circuit jack dropping the input to ground.

The OP Amps are RC4580 amplifier. I started out powering it from a 9V cell running through a MAX1044 to generate the -9V. Figuring power supply noise was a potential culprit, I pulled the MAX1044 out of the equation and used a second 9V cell to generate the -9V rail, with no improvement. In fact, the two new cells I used had slightly higher voltage than the original, so it was even worse to listen to

Sound quality isn't of much concern to me. If I could get something basically tolerable, it would be just fine, but the hiss makes it pretty difficult. Aside from the hiss, though, it does indeed amplify the mic and mix with the audio at appropriate levels.

Any pointers would be much appreciated!

NOTE: Missing feedback resistor on the preamp is 100k, same as the others.
Added 0.1uF ceramic bypass caps to + and - inputs on op amps and switched the MAX1044 to another 9V cell and it's reduced the noise a fair bit, but it's still there.

Author:  Peter W. [ 22 Nov 2016, 14:38 ]
Post subject:  Re: Have noise on mic preamp plus aux mixer

What happens when you short all the inputs?

Author:  laurie54 [ 26 Nov 2016, 20:52 ]
Post subject:  Re: Have noise on mic preamp plus aux mixer

Try using a 10K as the input cap from the mic rather than 1K. It will reduce mic gain a bit but may help the oscillation due to such high gain ratio in the amps. You could also change the 100K feedback Res to 50K for the same reason. Just to see if your problem is too much gain and osc because of it.

Author:  gofar99 [ 26 Nov 2016, 21:46 ]
Post subject:  Re: Have noise on mic preamp plus aux mixer

Hi, You don't show the power conenctions to the ICsso there might be the problem. There should be a 0.1uf poly or even ceramic capacitor from both the positive and negative power inputs on the ICs. The caps go to the ground. Without the capacitors nearly all IC op amps will generate noise. If you already have capacitors there then I would guess the issue is still power related, but elsewher in the circuit. Poor grounding or poor layout can also cause noise.

Good listening

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