Author Topic: Simple audio mixer - splitting mono, avoiding noise  (Read 710 times)

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Offline fsky

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Simple audio mixer - splitting mono, avoiding noise
« on: February 03, 2016, 08:08:52 am »

I'm trying to design basic audio mixer. Right now, as shown in the schematic, it should provide a mono input,
with optional microphone (electret type) bias and optional +20dB gain switch (+highpass switch, but I didn't figured it out yet).

My main question is - how to mix mono op-amp buffered output onto both left and right channels, with adjustable balance?
My current approach shown in the schematic, with potentiometer, doesn't work in simulation - both channels are at the same level
on the left/right channels output.

My second question is about microphone input ac-coupling. Previously I've put capacitor after the potentiometer, but then I remembered
that DC current from bias circuit flowing trough pot could create a noise problem. So I've placed the cap before it. Is it OK?
Will that change anything in terms of bandwidth?

Third question is about input noise with disconnected input. I've tested the circuit of U8A/U10A opamp alone (responsible for right/left channel
amplification). With input disconnected, amplitude of noise depends on potentiometer setting - which is counter-intuitive for me, as no DC current should pass trough it. I checked that noise on oscilloscope, and there was a lot of very low frequency noise. Any lecture on that?

Question on the side - can I import PSpice schematics to the Eagle :) ?

I've made some research on the forum before posting this, but nothing was particularly helpful. Anyway, if anyone with similar problem
will search here, I'm putting links to topics found:

In case anybody asked, my supply is symmetrical 5V.
Please point out my mistakes and provide any advice.
Looking forward to your replies, have a good day!

I've noticed my mistake - by accident I've joined output channels into one node, that's why it wasn't working.
Uploaded corrected schematic. Still I don't know if my solution is a good one, so please comment :)
« Last Edit: February 03, 2016, 08:34:44 am by fsky »

Online mikerj

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Re: Simple audio mixer - splitting mono, avoiding noise
« Reply #1 on: February 03, 2016, 08:21:06 am »
Try something like this.  I leave the resistor value calculations to you :)


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