Rochey

mixing sources into single opamp headphone amp
« on: August 12, 2018, 10:11:40 PM »
i'm working on a small box that'll mix the line outs from 2 pc's into a headphone amplifier.
The mix is a straight 50/50 -- summing, essentially.

I could use a few opamps to do this - mix into one amplifier, then a pot, then feed an audio amp, but a good engineer is one that can do for $0.50 what others do in $5!

I'm wondering if I could use summing resistors are the input to an inverting opamp (with  vref on the positive input) and then use a single rotary pot as my feedback resistor (rf). As RF gets closer to zero, then my gain should drop to 0 as well.

What are the pitfalls I haven't thought of?

Cheers

Dafydd
Expat Audio Home: http://www.expataudio.com


PRR

Re: mixing sources into single opamp headphone amp
« Reply #1 on: August 12, 2018, 11:57:37 PM »
50 cents seems high. Just passively mix to the existing input. Mix-loss is half for each input; but if both inputs play together you *want* mix loss. If either/or, just turn-up whatever gain control you got.

JohnRoberts

Re: mixing sources into single opamp headphone amp
« Reply #2 on: August 13, 2018, 10:20:27 AM »
i'm working on a small box that'll mix the line outs from 2 pc's into a headphone amplifier.
The mix is a straight 50/50 -- summing, essentially.

I could use a few opamps to do this - mix into one amplifier, then a pot, then feed an audio amp, but a good engineer is one that can do for $0.50 what others do in $5!
Back when I was buying dual op amps for $0.13 I might be able to do that.
Quote
I'm wondering if I could use summing resistors are the input to an inverting opamp (with  vref on the positive input) and then use a single rotary pot as my feedback resistor (rf). As RF gets closer to zero, then my gain should drop to 0 as well.
Do you want a fixed 1:1 summation? You might be able to do that into the input of a typical headphone amp chip.

It will take more parts to keep input differential, but still possible with simple topology.
Quote
What are the pitfalls I haven't thought of?
obvious potential problem with using rheostat (variable resistor) across amplifier feedback is wiper bounce or pot failure can cause open feedback path (read very loud wiper noise). This can be somewhat mitigated if pot is in parallel with real resistors  that could limit  max gain but be better behaved.

JR
Quote
Cheers

Dafydd
John Roberts
http://circularscience.com
Tune it, or don't play it...


 

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