simple passive summing device

Tweakers' Asylum

Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ.

I record live / on location directly to 2 track utilizing a stereo pair of mics. But when I need to add 1 spot mic, my "system" falls apart as I then need a mixer. I've investigated nearly everything available commercially without luck and I'm back to looking at a DIY solution.
All of the negatives associated with passive summing seem to be related to the size / scale of the mixer. As I'm considering a 3x2 mixer (line level only), I'm hoping to avoid any serious impacts due to passive summing. To make things even more basic, 2 of the 3 inputs would have no controls at all; the inputs for the stereo mic pair would go directly to the left and right busses. Only the third input, intended for the spot mic, would have a level fader. (to vary the ratio between the spot mic and the stereo pair). After the fader, the signal from the spot mic would be split and added equally to the left and right output busses; no panning required.
To clarify, Input #1 would go directly to the Stereo-Left bus, Input #2 would go directly to the Stereo-Right bus, and Input #3 would hit a fader and then be split evenly to both summing busses. Each bus would consist of a simple 2x1 passive summing network and there would not be any output faders.
My questions:
1. As the circuitry is so simple, I'd like to keep the signals balanced all the way from input through to output (unlike most mixers that use transformers to change the signal to unbalanced inside the mixer). Although I would not be using transformers on the input and outputs to unbalance and then balance the signals, would it still make sense to transformer-couple the I/O?
2. Any suggestions for where I might find info on selecting appropriate values for the passive components (summing resistors, and the one fader)?
3. Has anybody out there built a simple passive summing device and have any insights that they would like to share?
Thanks.
steve
lex125@pacbell.net

digital? I'm still not sure about transistors....

Lexington 125 recording studio (Open in New Window)

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Topic - simple passive summing device - hollywood_steve 17:30:47 01/17/03 (0)

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