Noise on mic preamp + aux mixer

[clpalmer]

Very new to audio hardware so please bear with me 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.

Found a simple circuit online that mixes several mic's with several channels, with a pre-amp for each mic and a final amp to offset the mixing losses.

I modified that a bit, because I didn't want to mix the left and right channels together, just mix the mic with the left channel, so I now have what's shown 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 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. 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

I don't have any personal knowledge of why the author of the original schematic chose the resistor and cap values so I'm not sure where to start to figure out where the noise is coming from and how to minimize it.

It's just for my daughter to play karaoke with, so 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!

https://cdn.pbrd.co/images/3PWWl2I.png

An externally hosted image should be here but it was not working when we last tested it.

 

[nigelwright7557]

Have you used any decoupling ? The op amps might oscillate without it.

On your inputs I would use jack sockets with switches to short out unused inputs.
Noise could be getting in there.

 

[clpalmer]

I expect the jacks to always be connected, but I do have closed circuit jacks so it'll short when they aren't.

Please elaborate on decoupling. Something like a .1uF cap on V+ and V- to GND for each op amp?

 

[nigelwright7557]

I expect the jacks to always be connected, but I do have closed circuit jacks so it'll short when they aren't.

Please elaborate on decoupling. Something like a .1uF cap on V+ and V- to GND for each op amp?

Yes that's a good start close to each op-amp.

 

[clpalmer]

Added the caps and that, combined with switching back to 2 9V cells instead of the inverting charge pump seems to bring the hiss down to a barely tolerable level. It still hisses, but it's probably low enough that when the music is played and she's singing it won't be too noticeable.

Any other quick fix ideas?

 

[clpalmer]

Also, noticed that when I turn the pot down all the way, it still hisses (though quieter). Beginning to believe it's just general system noise (ie. crappy mess of wires out to the jacks, pot, etc).