Audio Mixer Design Questions

[Paul Schreiber]

1) Use at least 100K input resistors.
2) On each input jack, connect a 100 to 220pf cap to ground. Goodbye radio.
3) The 074's can't drive headphone. Look at the National Semi "Boomer" parts, or the Philips TDA7052.
4) On the ouput stage to DAT, use a series 1K resistor.

Paul Schreiber
Synthesis Technology
www.why.net/users/paults

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From: 	john miller[SMTP:moron5000 at juno.com]
Sent: 	Tuesday, March 11, 1997 9:29 AM
To: 	bode at veda.is
Cc: 	synth-diy at horus.sara.nl
Subject: 	Audio Mixer Design Questions

I am putting a small audio mixer in my synth and was trying to decide the
best way to build it. I would like it to be 4 to 8 channels each with its
own op-amp buffer (inverting) with a single dual op-amp out (inverting
again) to drive headphones or patch into a DAT machine. I was also
thinking about doing it the SERGE way and make it a matrix mixer but was
unsure exactly how they did theirs. Anyone have schemos?:) I have had good luck with the output stage driving headphones directly but was a little
unsure of proper input/output impedance.

Here's what I have done so far:
 I have a 1/4" and banana jack going into a 1K Resistor to - input of a
TL082. + input has a 1k to ground. Feedback loop is a 50k pot.(I would like to be able to distort it!). The output of this to the second op-amp stage which is the same as the first. Oh, I'm powering it of a 15v +/- supply. 

So here are my questions:

I plan on using commercial synths and home built  VCO's (3340's) e.t.c.
on the inputs. Should I use a pot for input impedance? What is the best
all around impedance to use. (no guitars!)

Should I use a 3-10 pf cap in the feedback loop? Would this cut down the
HF response? Is this why I'm getting "Radio Honduras" through some
channels? Megusta mi Synthesizador!

What would be the best output impedance to the DAT?

Does a 1uf or similar value cap on the input cut down on HF response? I
have seen all sorts of different values, types e.t.c. on commercial
mixers and am very confused.

What are the problems with the way I have done it. Getting Shortwave
Radio is sure nice!

How on earth do you make a panning circuit? I have tried with no luck. Can it be done with a single pot?

Sorry for so many questions but I have been trying all sorts of ways of
doing this and can't figure out which way is best! Any tips suggestions would be appreciated.
..and I sold my Mackie to my brother because I though it would be easy to
build a mixer!

Muchas Gracias Amigos !
John