[sdiy] DIY Mixer - please help!

[Byron G. Jacquot]

>I am currently collecting ideas as well as resources. It seems as
>though it's not so easy to find the schematics for most mixers that -
>at least I consider - are interesting. The first step is to come up
>with a good summing bus design. I think solid state is good for a
>start, but since it's all DIY, it'll be possible to swap it out for a
>tube stage if there's a need for that.

The best reference is he Steve Dove stuff in the Handbook for Sound Engineers...about 100 pages on console design.

Of similar note is the article by John Roberts (there are PDFs around the web).  Maybe the old Bel Losmandy article in JAES...or see if Opamp Labs will still send you a free catalog full of design notes.

There's some decent summing theory on Fred Forssell's website, too.

>I understand this list might not be the best forum to find out about
>this stuff - but I searched about for mixer/console DIY with people
>who actually *knew their stuff* when it comes to analogue music
>hardware and I simply couldn't find much interesting. If anyone has
>any recommendations, please do tell me. I trust the people here so I
>ask you guys first :)

A while back, every mixer shipped with complete documentation...a service manual with schematics.  See what you an track down.  The Yamaha PM1000 and Soundcraft 600 are both downloadable as PDF from their respective manufacturers...and both use very different approaches to implementation

>Each channel is one VCA that has CV inputs, a single input, and a
>single output to a single bus. Of course a stereo channel is that
>multiplied by 2. With a CV approach it's going to be very easy to
>control two channels with just the fader of the left channel, but
>that's a trivial comment.
>The bus sums and gives a single high-headroom output.

I don't see a problem with the basic description - the faders are replaced by VCAs, and we have VCA designs all over the web.  The summing is standard...maybe followed by more VCAs as master faders.

>From a modular synth perspective, it might break down into a bank of VCAs as a group of modules, plus a summing module or two.

One thing to think about is "panning law."  Panning & crossfading usually follow a log curve, so that things move smoothly between the speakers.  Also consider using an inverting summer - you can add channels, and they compensate by adding gain.  Noninverting summers work best with a fixed number of channels, as more channels means more loss in the summing network (Forssell hs a paper that details this nicely).

The thing you're going to be looking at is replicating the same couple of circuits a bunch of times...and thanks to prototype-quantity PCB production, and VCAs on a chip, it's not insurmountable.

Byron Jacquot