N_Cook wrote:
Meat Plow wrote in message
...
On Wed, 28 May 2008 18:31:00 +0100, N_Cook wrote:

Perhaps the sensible approach to this is extend the rubber feet by half
an
inch as this is a known cure. There is no obvious place to put a secoind
fan
at the air outlet.
So every XR 886 overheats?


Not to the extent of tripping any of the thermal switches but enough to
concern the owner into trying to improve airflow - admirable thinking for a
non-techie.
Just giving greater clearance to the airflow under an amp should not
normally affect the amp temperature though. I would suggest that it is due
to forcing the air to follow a tortuous path so a generic design flaw. I've
heated up the fan motor, etc and there seems nothing wrong with this fan as
well as no change of fan noise reported by the owner.


What say if an amplifier was designed in such a way that the thermal
protection was intended to operate at some given device temperature, say
the safe working temp of the output transistors, and the cooling
arrangments designed accordingly? Output transistors get hot, heatsink
gets hot, thermal devices trip, amplifier shuts down or whatever it`s
designed to do.

But, by increasing the airflow from the fan[1] or improving the cooling
to the heatsink in some other way, results in the heatsink remaining
cooler when the actual output devices themselves are still running at a
high temperature. Bearing in mind that the temp sensors may not be
directly monitoring the devices temp, but some other area on the
heatsink, that might lead to the thermal protection failing to operate.

By changing the design of the cooling, you could be blowing cool air
over the temp sensors and not the output devices. Result = a trip to
silicon heaven.

I have seen Peavey mixer amps shut down in hot weather when placed in
direct sunlight and driven hard, that`s the protection doing what it`s
designed to do tho. Generally speaking, if a decent quality amplifier is
overheating, it`s operator error, not a design fault. (IMO)

Ron(UK)


[1] This is hypothetical of course, I realise you havent changed the fan
or it`s cooling path in this instance, but I think it`s something to be
aware of. Maybe someone with experience in 'designing' power amplifiers
could comment?