I read in sci.electronics.design that wrote (in
. com) about 'Reducing
hiss by changing op-amps ?', on Sat, 19 Mar 2005:
I've just been playing around with the desk and have found that the
biggest source of noise does seem to mic preamps - If I sweep the input
trim pot next to the 20bD pad switch (sorry - can't work out where it is
on the schematic !), then I get a massive change in background hiss -
30dB's worth !

But are you not also changing the gain by 20 becidels? (;-) What you
should d, if possible, is to feed a 1 kHz signal at say 1 mV into the
mic input, set the gain controls (input gain trim at max. gain, 20 dB
pad OUT, channel gain full up, master gain giving attenuation) so that
you get 1 V output, reduce the input signal to zero BUT leave the signal
generator connected to the mic input, and measure the noise at the
output. Don't worry about using a true r.m.s. meter or a weighting
filter; the mixer itself limits the bandwidth and the unweighted S/N is
indicative of the level of noise performance.

Tell us what you measure.
--
Regards, John Woodgate, OOO - Own Opinions Only.
There are two sides to every question, except
'What is a Moebius strip?'
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk