On Jul 10, 2009, at 10:19 AM, Matt Syson wrote:
Hi
I would suspect that the left mix bus is shorted to ground or that
a channel is 'contributing' to it all the time which,without the DC
bus being engaged you won't hear in the monitors until you solo
something else.The FETs are used in a 'switching' mode rather than
in a linear amplifier fashion so I would not really be looking at
these until other things were checked.
I was at a loss to figure this out initially, but Matt's analysis
makes a ton of sense. To elaborate, if something severely reduced the
summing bus impedance, the noise gain of the summing amp would
increase drastically, causing lots and lots of hiss, yet the
transmission gain of any channel might not be affected.
I'd look for some sort of "soft" short, something that would
drastically reduce the summing bus impedance, but not completely
short out the summing bus. Maybe there's a whisker of stranded copper
that's sorta shorting the bus, or possibly some switching FET that is
somehow clobbering the bus, but not completely. Many switching FETs
have channel resistances of around 20-70 ohms when fully on, and this
would be about right - enough to make the summing amp run at a really
high noise gain, but not enough to clobber the signal currents sent
from the channels.
Best of luck tracking this down! One possible procedure would be to
pull all but one of the input strips and see what happens. If that
"fixed" it, cycle each strip in individually until you find the
culprit. (BTW, if it's not obvious, you need to power the console
down before you pull or add a module, then power it up to see what
the console does after you've reconfigured it ;-)
If the problem remains with most of the modules pulled, pull every
module that sums to the 2-mix bus (e.g. all inputs, effect returns,
etc.) and see what happens. If it's still noisy, then some sort of
connection in the frame is messed up and you can look there, and
avoid messing with the modules.
Regards,
Monte McGuire
[email protected]
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