[sdiy] OT - DIY mixer

[harrybissell]

Hi Toby...

(a few cents worth...)

Toby Paddock wrote:

> I know, I know, just don't do it.
> But if I was thinking about doing it
> anyway, any suggestions?
>
> -8 or more stereo pairs

OK....

>
> -no panning or eq

OK as well... you have eliminated a lot of circuits here...

>
> -I think I have some dual pots

You will need them, or else you have to do a VCA per channel.
Do you need to 'fade' these during use, or just set and forget ?
Some of those new digital pots would be OK if you don't
move them...

>
> -line inputs (line/mic would be a bonus)

Line inputs have much lower gain (and can stand higher noise levels...)

I'd make a couple of dedicated mic input preamps (maybe NE5532 based)
with low impedance inputs...then patch those to line inputs as needed.

>
> -100k or higher input would be nice

Ouch.   If you need that high of impedance you will have to use
something like the
TL072...   The NE5532 is only a good choice at reasonably low input
impedances.

For your piezo inputs... 100K is appropriate (or higher)

> -line out
> -good, but not ultra-fi audio quality

yes yes...   (opamp based designs will do this)

> -easily expandable

This can be tricky. The summing amps have to be able to source / sink
enough
current to handle all channels. This gets worse as channels are added.

Another solution would be to sum currents from OTA stages.  I've done
this with
the CA3080 but noise performance is NOT optimum.

A solution I've used is a "pyramid" structure... where every four
channels sum into
a single channel... then four of them sum into two channels... then
those two into
one.  So its like a submix - stereo - mono configuration. Using insert
jacks everywhere makes it flexible.

> It would probably be used mostly with
> piezo contact mics. I just want a bunch
> of stereo channels and I can't afford
> a good mixer.

Make it cheap... because if it gets expensive you could afford a real
mixer.

IMHO mixers are sort of like BBD's  (no... not that they SUCK....  ;^)
in that
a lot of effort goes into them for not a lot of result.  A mixer is a
unit I'd buy, if
possible. Its just much easier...

H^) harry

>
>
> Thanks,
> Toby Paddock