Designing a Mic Preamp/Mixer - Help w/ First Attempt?

Archived from groups: rec.audio.tech (More info?)

I hope this is not off topic here -

I am designing a mic premamp/mixer for our band's practice sessions. We
have one main singer and at least one or two backup singers. I built a
simple circuit that has 4 balanced inputs mixing to one output with about 52
dB maximum gain per channel. Channel One has an effects loop. There is no
phantom power as we are using dynamic mics for now. While it worked OK in
practice, we had trouble matching levels to the rest of the system. I am
wondering what other features common preamp/mixers have and how to best
implement them - like perhaps clipping indicators, VU meters, an FX bus
(rather than just a simple loop on channel one only), built in compression,
eq, etc. Also, I want to operate as a +4dB device but Im not sure what
that means when you dont even know where "zero" is (without a meter) Is
this correct? I am assuming that +4dB means 4dB over .775 volts into a 600
ohm load, which would be about 1.228 volts. From what I have read, its good
to shoot for a 20dB headroom which I am guessing means being able to swing
over 12 volts into a 600 ohm load.
Also, is it more common to have the fader for a channel affect the send
level for the effects or would the fader come after the send and return?
If anyone knows any good sources of technical info on such matter, I would
appreciate hearing from you. Are there any good web sites that outline the
process of desiging custom live/pro audio equipment? Most of my previous
design experience is in the areas of home audio and guitar amplification, so
some aspects of this project I have not enountered before now.

thanks in advance,


David Forsyth
foda01@ncx.com