Line level mixing

zeebit

Member
2013-04-20 4:34 am
Hello everybody, noob here.

I want to mix the left and right channel of an mp3 player into one. I've seen some schematics which ony use a potentiometer and a resistor per channel and then combining the outputs into one. Can I just ditch the pots and just use a resistor? What value would be ideal?


I'm sorry if this question has been asked many times but I've done some research but I could not find the answers to my questions.
 
A single resistor from each channel to a common output works fine. From headphone outputs, resistors of about 1000 Ohms each, give or take a lot, should do ya.

This simple mix has the unintended effect of exaggerating signal that was already common to both channels, worst case by 6dB. This is minimized by adding a third resistor from the output junction to ground, the smaller the less common-mode effect. Of course this costs you some signal level, so you'll need to experiment. Maybe try another 1000 Ohm resistor and see if you like it.

All good fortune,
Chris
 
Chris Hornbeck said:
This simple mix has the unintended effect of exaggerating signal that was already common to both channels, worst case by 6dB. This is minimized by adding a third resistor from the output junction to ground, the smaller the less common-mode effect.
Are you sure? With two resistors you get the average of the two channels, delivered at an impedance of half the resistor value plus half the source output impedane. Adding a third resistor merely attenuates that signal and reduces the source impedance. You still get the average of the two channels, but a bit smaller. Mr. Thevenin has something to say about this.
 
Are you sure? With two resistors you get the average of the two channels, delivered at an impedance of half the resistor value plus half the source output impedane. Adding a third resistor merely attenuates that signal and reduces the source impedance. You still get the average of the two channels, but a bit smaller. Mr. Thevenin has something to say about this.

Let's imagine a worst case, signal in one channel only, no loading on output of the mix. Each channel sees the other (zero source Z) through two series resistors, so signal to the output is reduced 6dB. Now same signal on both channels, no loss to outputs. Active mixers use (active) virtual grounds to eliminate this effect.

Actually, it's complicated by the 3dB summing of two speakers into a room, but...

Thanks,
Chris
 
Chris Hornbeck said:
Let's imagine a worst case, signal in one channel only, no loading on output of the mix. Each channel sees the other (zero source Z) through two series resistors, so signal to the output is reduced 6dB. Now same signal on both channels, no loss to outputs. Active mixers use (active) virtual grounds to eliminate this effect.
You are confusing two quite different issues. Active mixers use virtual grounds to eliminate something else: the interaction between slider positions which a simple mixer would give.

As I said, two resistors give the average of the two channels. Your example confirms this: the average of 1 and 0 is 0.5; the average of 1 and 1 is 1. Adding another resistor to ground does not change this, as the output gets reduced by the same ratio. For example, if the resistor to ground is equal to half the other two, then you get 0.25 and 0.5 respectively as the output signal is halved. This is a simple application of Ohm's Law and knowledge of potential dividers.

If you want something other than the average of the two channels, scaled by some constant, then you need to do something far more sophisticated.
 
If you want something other than the average of the two channels, scaled by some constant, then you need to do something far more sophisticated.

Doh! You're right - there's no simple resistive Y that doesn't give a 3dB boost to signals common to both channels (over the 3dB summing of speakers in rooms). Thanks much for the lesson - I've been drifting along with an old saw from the Klipsch derived center channel days that's never been true.

It must have come from analogy to stereo panpotting practice, where a source panned center is reduced 3dB to each channel compared to its level panned fully left or right. From speakers this adds as not correlated, and all's well. But simple resistive mixers (apparently, now that you've made it clear) add to 6dB, 3dB hot. Simple panpots can mix correctly, but simple resistive Y's can't undo it. Very interesting.

Thanks, as always,
Chris
 

Mooly

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2007-09-15 8:14 am
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