I read in sci.electronics.design that wrote (in
.com) about 'Reducing
hiss by changing op-amps ?', on Wed, 23 Mar 2005:
Hmmm...... what value resistors are in your potential divider ?

75K in series with both the live and the shield and 560 Ohms across
them. These figures (and configuration) were arrived at with the help
of the folks over in rec.audio.pro.

You are attenuating the mixer output by nearly 50 dB and then amplifying
up again. This is not good. I suppose you are feeding into the
low-impedance balanced input of the Carlsbro. You would do better to
feed into the high impedance unbalanced input, provided you
interconnecting cable is not more than about 2 m long (to minimise
interference pickup, not anything to do with frequency response. You
need a different attenuator pad: 22 kohms in series with the live,
nothing in series with the shield and 1 kohm across them.

Using a mic input is a really bad idea. Performance is limited by the
noise figure of the " PA head's " own mic amps.
It is easy to show using theory that this will *always* be worse than
just using the PA head's mic amps on their own ! No matter how good
the mixer is.

I know, I know... :-( We're not gigging at the moment (band reshuffle
!) and have spent a bit on other gear recently, but a separate PA amp
is next on my shopping list. I can finally run in stereo then ! (I
just want to hear my stereo chorus pedal through the PA :-) )

I really don't think the desk is to blame.

Perhaps it's not. I will do some more testing on Friday. I'm only
really basing my accusations of the desk on the fact that when we plug
into the PA head directly (mics and guitars), there's almost no hiss.
Bit of mains hum (diabolical electrics in the place we practice in !),
but no hiss. When we bring the mixer into play and plug into that,
there's a noticeable increase in hiss from the PA speakers when we're
not playing. I'm also presuming that this hiss will be 'polluting' the
sound when we are, - I can't hear it then, but I know it's there! We
have nice guitars and good mic's and I want it as clean as I can get it
:-)


Doesn't the PA head have a line level - ish input ( like a stereo
input for example ) ? Or an insert point. That would produce far
better results.

Yes, but none that I can route via the internal reverb of the PA head.
I need to use two mic inputs on the PA head, one with a bit of reverb
for the vocals & rhythm guitar, and the other one dry (I have a better
quality reverb in an effects pedal) for my guitar. I can't see any way
of routing a line level input on the PA head through it's internal
reverb - that would solve everything if I could !

http://homepage.ntlworld.com/anengin...7%20Manual.pdf
(though sadly it's not a schematic).

The Carlsbro doesn't have a proper line-level input, only an 'effects
return'.

Incidentally, the mixing desk has it's own 'reverb'. Possibly the
worst I have ever heard though ! It's a simple delay, more an echo
than a reverb (unless it's faulty and not supposed to sound like that
!). Once again, it's stuff I plan to sort when funds are available - a
decent outboard reverb unit - eventually.

Is it electronic or electromechanical? If the latter, it will certainly
have been damaged. Sometimes they can be repaired.


--
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