Adjustable-volume input mixer?

[Doktor Jones]

I'm contemplating a multi-input audio mixer for my headphones; something I can hook up between my PC, Xbox 360, and portable audio devices (phone, etc). I am assuming drive-level input from all devices -- the 360 is the only exception, and if need be I can stick a small amp on it.

The key features I am looking for are individual input adjustment (so I can turn down one source or turn up another), as well as isolation between the inputs; the latter is the part I'm having trouble with. I thought optoisolators would provide a solution, but it seems that using them for audio is a journey fraught with peril (in terms of sound quality). Ideally I'd also like a master/output volume control that adjusts the volume of the mixed signal, but that's tertiary.

What would be the best way to go about this -- or is there just something on the market I can buy that would do the trick? Everything I saw online seemed to be professional-grade (1/4" TRS plugs or XLR, and all sorts of sliders and knobs I don't really need) or otherwise largely overpurposed for my needs -- and vastly overpriced.

 

[Nigel Goodwin]

It's a VERY, VERY simple and common application - you just need a single opamp to do it - I don't know what you mean by 'isolation' between inputs?, it's not something you have (or require) in an audio mixer. If you simply mean that the pots don't interact, that's what virtual earth mixers are for. The commercial mixers you mentioned don't have isolated inputs, or any requirement for them.

Have a look here:

http://www.all-electric.com/schematic/simp_mix.htm

One of a great many sites google finds, circuit 3 is what you need - although the second opamp isn't really required.

Incidentally, way back in the 70's I 'threw' a similar two channel mixer together for a disco that was 'dropped' on me - I built it from scratch, making up values in my head(it's that non-critical) in a tobacco tin powered by a PP3 battery - in about 20 minutes!. Needless to say it worked perfectly, there's really nothing to go wrong.