John Popelish wrote:

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.

If you want to experiment a bit, you might get a few LT1124CN8 dual
opamps to try as replacements for the 4558 types. They have make
about half the noise of the 4558.

I have no idea what the specs are for the KTA970BL PNP transistors at
the mic front end,

They'll be KEC's version of a 2SA970. Blue is the highest gain grade.


but I would probably also experiment with replacing
a pair of them with a pair of 2N5087. Keep in mind that the collector
and base leads are interchanged with this swap.

I wouldn't !

The 2SA970 is one of the quietest bipolar devices available for mic amps.


Graham