Here is a really good 7-Input Mixer Matrix design. This design is from a 1970 "Radio Electronics" Article on "Building
a Modular 6-Channel Stereo Mixer Preamp". The circuits were all published in the RCA Solid-State Hobby Circuits Manual,
HM-91" This book sold in 1969 for $1.95 and is "out of Print" now. But the mixer circuit is here for the inquisitive at
heart.
An NTE-123 general replacement transistor can be used if you have a desire to build one of these.
The most interesting quality about this circuit is that the mixer outputs can be "stacked" to effectively give you 7-channels, 17-channels, or even 21-channels of mixing. This is because the output "buffer" transistor circuit is sufficiently isolated to allow for output combining with minimal interaction. The actual limit before significant drop below "unity gain" is three. Still, I would recommend a simple resistor combiner network without gain control to combine more than two mixer boards - you cannot be too careful.
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