DIY Passive Mixer Qs

Hey lads and lassies-
I'm looking to build myself a small passive submixer
for my synths.
The catch is: I want an aux out to send to an fx pedal, and
I want each channel (of which there should be 3) to have a post-
fader switch, not pot, allocating that signal additionally toward the aux out.
That is, with pots open, signal always passes to the master out,
but a switch determines whether each channel is also feeding the aux send.

My question is, after the 10k pot and a 1-2k resistor on each channel, and
furthermore splitting the signal, will I need to step outside the realm of passivity
and add some amplification in at any point?

Thank you all so much for your time and patience. I wish you the tastiest of cheeses.

First, welcome to the forum.

would be nice an scheme of what you are planning, I don't understand where the 1-2k resistors are, summing resistors should be bigger than the pot to reduce interaction between them. Think in something at least 47k for 10k pots. for channel one full up you would have 47k devider with (47+10)/2k so -8.5dB for three on top, and 47k/2 with all down, -9.5dB.

You probably would need a gain stage, but if your input of where are you going has some gain you aren't using it should work. 20dB should be enought for 3 channel mixer with usuable fader range, even less (10dB) if you don't mind to use faders full up most of the time to get the same level than no mixer. 20dB would be nice, 10dB boost for when you need little more than direct connection.

JS

A couple of questions:

Are the synth inputs stereo? In other words is it a three channel stereo mixer?
Is the AUX send therefore stereo?
Are the synth signals unbalanced?
What is the output of this mixer going to be fed into?

Cheers

Ian

Thank you, and thank you for the replies.
The schematics I had looked up for building, say an altoids tin
passive mixer suggested placing a 1k resistor after the 10k pots.
To clarify, this is a 3 channel mono mixer w/aux out, both outs going
to DI boxes then preamps (the aux out first going through an fx pedal).
All signals would be unbalanced.

Let's do numbers with 1K... right when I found my numberologist...

oh, came on!

here she is... so

pot one at half position, 2k5 plus 1k are 3k5 output, devider with 5k5 or 0k5:
-2.8dB
-5.5dB

Almost 3dB between them, so, when one band is at mid point, the other two from one extreme to the other the level of the first will change 3dB. The input impedance may be a problem too, drive two 1K resistors in parallel, to the other two channels tied to ground, all sends on gets you 500Ω series 250Ω are just 750Ω at worst case, hard to drive for most synth... I think it's too low, you could get it with 10k at least, I wouldn't go lower than 22k and 47k seems the way to go for me, but I may be loosing something. Don't be afraid of it, some API desk has 47k summing resistors, even with large driving capability from their DOA to drive all the sends it has, yours is other story, you don't have too much channels or send but neither the driving capability, I used 22k for my design because I do have the driving from an DOA but no so many sends or and channels doesn't count at this point in a VE summing. I'm just guessing you may have some trouble with 1k, you could try it and figure it out, maybe some distortion, maybe the interactions affects you, it depends, I guess it will if you move them too much, if you set it and stay there interaction isn't a problem.

JS

And we must not forget the AUX sends. From the description these would appear to be pre-fader via a switch. So when the AUX is on you have two pots in parallel across the input. I would be thinking in terms of 22K pots with 47K bus feed resistors. I would not worry too much about noise. The noise in a 47K resistor is about 106dBu. The bus sees three of these in parallel so the bus noise due to the 47Ks is only about -110dBu.

Cheers

Ian