On 20 Mar 2005 08:01:11 -0800, wrote:

Not strictly true - I can notice the hiss *more* when I turn up the
input trim. The original problem is that the desk is too hissy - when
used at *any* level :-)

It's used on stage for my band as it has better EQ (not to mention more
inputs !) than our previous PA head, and since we made the switch we've
noticed a distinct increase in hiss coming through the PA speakers -
which is coming from the mixer. I just wanted to see if I could reduce
the noise level as a lot of our numbers are quiet-ish and no doubt the
sound could be improved by some hiss reduction.

Put an empty plug with sleeve shorted to ground into the insert input.
Does the level or quality of the hiss change? If not, it is the
post-insert circuitry that is causing the hiss. There are quite a few
points along the signal path where you can effectively isolate the
preceding stuff to see where the source of the hiss is.

Whatever, if the hiss gets much greater as you turn the mic gain up,
that input stage needs addressing.

d

Pearce Consulting
http://www.pearce.uk.com