Help with mixer: stereo to mono

[Pancra85]

I want to build a mixer using this schematic:
http://www.generalguitargadgets.com/diagrams/mixer_sc.gif

The only thing I want to change is that I want to make stereo inputs, so if a stereo device is plugged I can have both channels OUT mixed in mono.
would this schematic work so the top left input (the one marked with X4) is stereo? Are those 680 ohms resistors needed? Is there a better way??


Thanks!

[pinkjimiphoton]

i'm not sure, but i think you could run into phasing issues.

[Jazznoise]

That's a passive means of stereo-mono summing.  Phase issues could be a problem, being able to invert one channel relative to the other is often a useful addition (Mainly because douchebag mixing engineers thing flipping the phase on a channel makes things wide sounding, but forget that not every system is perfect stereo). A dual opamp with one acting as an inverting buffer and the other as a differential or summing amp is a good configuration, you just switch the inverting amp in and out of the path with an SPST.

Since you're already using a TL072, you'll be glad to know a TL074 will do all this. I'm sure the extra 2-3 mA won't bust the bank!

[fuzzlab808]

With these resistors (680 ohm) you are doing a passive mixer. Why don't you treat the stereo input as it was two different mono inputs connected to a stereo jack? You will need a stereo 10K potentiometer and that's all.
But if you want to do the passive premixing, dont use 680 ohms resistors (they are very low value and you will load in excess the stereo outputs) Use a minnimum of 10K.

[jsleep]

This is not really related to your question, but I thought I would chime in on this mixer.    What I have found with this mixer is that mixing 2 inputs (two channels) is okay, mixing 3 channels you lose fidelity, mixing 4 channels you lose a lot of fidelity.  It's basically summing whatever goes into the mixer section, so you have perceived, if not real volume loss for every input into the mixer.  I think, but I'm not 100% sure, that some active circuitry in each input would greatly improve this situation.  I've been meaning to upgrade this mixer for years, but it just hasn't gotten done yet.

JD

[PRR]

The resistors are needed. (You do not want to short the two outputs together.)

If the stereo is "good", it will sum to mono acceptably. As Chris says, there's bad stereo, which stands in two separate speakers but collapses in one. Until recently the need for AM radio play "enforced" sane stereo (altho sometimes a different mix was sent to radio). I suppose now that everybody has two earbugs, anything goes. You'll hear music, but the balance may be different.

All mixers are fundamentially passive. Passive mixing should not "lose fidelity". Wrapping an opamp inside to make-up the loss does not change that. What *can* happen in a too-simple passive mixer is that weak signals become weaker and dip too close to universal hiss. In some cases you should have boost before the mix network. However what you are showing looks like a Line mixer (not microphone or guitar) and that should be fine.

BUT: the connection between IC1A and IC1B seems wrong? I bet there needs to be a resistor between.

[aresaudio]

....dont use 680 ohms resistors (they are very low value and you will load in excess the stereo outputs) Use a minnimum of 10K.

Increase R9 for unity gain