DIY Passive Summing Mixer with a difference.

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Dj Subjective

New member
Hi there guys, ive been researching how to make Passive Summing mixers, and i have lots of info to build 1.

My question is maybe simple to answer, i have lots of schematics for allsorts of different configurations, but none of them seem to have different inputs in the same circuit eg 8 unbalanced ins, or 8 balanced ins.

I have just got a focusrite saffire pro 24, and it has 2 mic pres + 2 line ins, 6 balanced outs, and a stereo headphone out.

now im pretty shure i can make an 8 channel summing mixer out of this configuration, just dont have the expertise to solve my problem. I want to run the 6 balanced outs as 3 stereo mixes, and use the headphone out as a stereo channel to make up channel 7 and 8. im going to run it back through the 2 pre's for makeup gain.

So ill have 4 stereo groups getting summed, with the first 6 outs from my saffire running to balanced ins on the summing mixer, but what do i do with the 1/4inch headphone stereo out from the saffire ? as im shure headphone outs are not balanced.

Any ideas and schematics would be great thanks, plus this will be a passive box so no pots, switches or buttons, just copper rails, resistors, 8 ins & 2 outs stereo.
 
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DJ Subjective,

Pardon my ignorance, but what do you wish to achieve with this arrangement? If all the box is doing will be summing, why can't you just do that in software ie just using the console application that comes with the Focusrite? With my Audiofire 8, for example, I can create several routing scenarios and save them back into the device so it can be used separate from a computer.

You won't be adding colour with a handful of metal film resistors.

Obviously I am missing something?

Your best bet might be to convert the headphone output into impedance balanced - have a look at this Jensen white paper, and look at section 2.4. You can basically turn unbalanced into balanced with the addition of 1 resistor. Note: impedance balanced, not differential balanced.

Paul
 
PRHunt said:
Pardon my ignorance, but what do you wish to achieve with this arrangement? If all the box is doing will be summing, why can't you just do that in software ie just using the console application that comes with the Focusrite?
Click to expand...

Analog summing is in fashion with a subset of home and professional mixers these days. It's almost a religion. I've heard some really nice mixes that were summed in analog. I've heard really nice mixes that weren't. In one comparison of the same project summed both ways the difference was microscopic, like nudging the HF knob barely off the detent. I think it comes down to the person mixing more than the tool used.
 
I don't know if I'm ready to pray yet but I JUST got done building a 16 channels analog summing network. It's not even been connected up yet. It's passive with a stereo mic pre for makeup gain. I've never liked mixing ITB. It's not the mouse at all. I've just never made a mix ITB that sounded as good as what I could do witha console. Now I don't know if it's a function of analog or digital summing or what but I want to find out. I've always liked being able to use hardware and plugs side and with the summer, it'll be just plugins (except for the possibility of a compressor strapped accross the two buss). Not sure if I'll like it or not but I'll know in a few weeks.
 
bouldersoundguy said:
Analog summing is in fashion with a subset of home and professional mixers these days. It's almost a religion.
Click to expand...

Oh, I do understand what is behind passive summing. I was (am) still unclear about the OP's desired outcome. With no way to control levels or panning in the passive box, this will presumably all be performed in the DAW. But I do wonder if some of the magic comes from euphonic distortions from the make-up gain stage that would follow. The Focusrite won't really provide that.

I'd have thought that make-up gain would be better achieved using something like an amplifier with transformer input and output (something that distorts a bit, not high-end Jensens). An output level control after the output transformer would let the user drive the device hard, but provide digital interface-friendly levels for final capture.

I'm certainly not scoffing at the concept. More like thinking out loud.

OP, I suggest you use 0.1% resistors (or better) throughout. You don't need to buy 0.1%, just get a big bunch of 1% and match them. I also wonder if unused inputs on the summing box need to be shorted, since it seems to me that impedances of the whole network depend on all the resistors being in circuit?

Paul
 
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Got the unit done. Now to hook it up and road test for a few months,
 

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Is this a 4 stereo pair summing box? I am looking for a fun project just like this. Any rough schematics with resistor values you could share? It would be much appreciated!
 
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