"Phil Allison" wrote in message
...
Arfa Daily wrote:


While we're on slave amps,
anyone following this thread got the schematic for an Alto Mac 2.2 ? Very
odd fault. Both channels work just fine for a while, then all of a
sudden,
one channel starts to indicate 'clip' with the level control just a
gnat's
bollock above zero. There seems to be a dual opamp involved in the clip
detect circuit, but before going round in circles trying to see what's
happening and why, it would be handy to have a schematic to see how the
clip
detect circuit is designed and what its inputs are.


** The schem of the 2.3 should be very close:

http://www.mediafire.com/view/bmotmj..._manual+8.jpg#

The Alto MAC amps have basically copied early QSC models like the USA850
and 900. Changes include use of a toroidal transformer, flat pack output
transistors and SMD for all the small signal stuff.

The topology is very odd with output transistor collectors all tied to
chassis ground while the DC rails swing with output signal. Speaker drive
comes from the PSU centre points for each channel and the usual driver
transistors have simply been eliminated. The short circuit protection
scheme is weird too.

Advantages are a low parts count, cheap to make and will not pass DC
current into a speaker under fault conditions.

The clip LED is driven by U2-B (half an NE5532 supplying drive to the
output stage) via a diode bridge and also triggers the clipping limiter
(U1A) via R26 to ground.


.... Phil


Thanks for that, Phil. Much appreciated. Odd how it works for a short while.
Before having the schematic, I gave everything in the area a good dousing
with freezer when it failed, but nothing really that I would say was very
positive. Maybe - and it *was* a maybe - the 5532 was a bit sensitive, but
it was inconclusive. I did notice that when it was wrong, the waveform on
pin 7 I think it was, was much bigger than 'normal' and the output TRs on
that side got quite hot quite quickly.

I'll try and find time to get another look today or tomorrow with the
benefit of your schematic. Thanks again

Arfa