2 inputs 1 output, unity gain, summing mixer: design suggestions?

[mr_doyle]

Hello,

i need to mix outputs from 2 rack devices (both low z, line level, one is balanced, the other is not) together.
No controls (volume, pan, whatever), just: in 1, in 2, output.

Wondering if could i go passive (resistors) or if should use an active design.

Thanks in advance,
D.

[sean k]

I'd put a phase reversal on the balanced, or just decide which signal should be earthed, then sum them up. Bunch of 10k's would do It's line level so you ain't gonna miss much signal going through 20k. There may be an issue with ground loops but try it and if it works then don't worry about signal transformers on inputs.

[mr_doyle]

Thanks Sean,

so you think it's unnecessary going active?

D.

[sean k]

Well your going to have losses as the output impedance drives the resistors which then have an impedance which drives the next stage so you haven't got a buffer to do the work of re-lowering the impedance... so there will be looses but it ain't going to hurt, or cost much to try it out and listen for a difference.

They say an output inpedance should be looking at ten times that amount on the next input. So a 10k output would like to see 100k and 100k output would want to see 1M. So your resistors should be at least 47k and the input impedance of the next stage should be 1M. But the chances are your going out at 10k and looking at 100k so 10k-22k resistors should minimise your losses.

Friend of mine is using unbuffered piezos on a plate reverb then mixing them with lower value resistors with a mix pot between them, 47k I think, so it's all passive and it seems to work ok for him and thats being done in territory where its common knowledge YOU NEED buffers... but hes gotten away with it and is happy with the results.

You gotta remember that round here most of us are more than happy to throw in a few buffers because we etch our own boards and have little plastic bags fulls of jfets and transistors. See what happens. Be 

[Ripthorn]

A summing amplifier using an opamp is really quite simple, and you could use a blend pot as the two resistors prior to the input for summing.  Total parts count would be blend pot, opamp and maybe a couple resistors and a capacitor.

[Processaurus]

If you just use 2 summing resistors (i'd go lower, 4.7K, because if each of the outputs is low impedance it means each would be putting about a 10K load on the other (4.7K + 4.7K). You want to keep impedances fairly low to avoid noise) you can wire them into a Y cable.  That'd be a good solution if you have a little output volume to spare.  Should sound fine, as it is the same as an active summing amplifier, except you're counting on the next device to be the active part.