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