Arfa Daily wrote:
"jakdedert" wrote in message
.. .
Arfa Daily wrote:
wrote in message
...
Speaker connected (switch on or off for them), right channel power amp
still very hot. The sound is good on both speakers but I cant leave it
on for long, I am afraid to fry the power amp ($20.00)
On Mon, 06 Mar 2006 19:37:03 +0100, Damir wrote:
On Mon, 06 Mar 2006 13:12:54 -0500, wrote:
Thanks Charles,
I checked the voltages(DC) on all pins on both hybrid power amps and
they are the same. I swapped the input to the hybrid power amps (left
and right channels) and I still have the same power amp getting very
hot. Not sure what to check next.
If the sound is good, but the hybrid is frying, then I would suggest that
it is hooting, maybe as high as RF. You need to get a 'scope on the
output to check for this. There may be a Zobel network ( R & C in
series ) on the output that was damaged by the previously faulty hybrid.
Alternatively, there may be a defective decoupling cap on one of the
bypass pins of the hybrid, or an open or high ESR supply decoupling cap,
and this is allowing it to hoot, which may be what caused the demise of
the original hybrid, and would ultimately do the same to the replacement,
if left.
Arfa
'Hooting', you say. I suppose by that--and your description of the
solution--you refer to high-frequency oscillation.
Hooting...that's a new one on me; but I like it.
jak
Oh, ok. Perhaps it's a funny British thing !! It's a term that was taught to
me when I was an apprentice nearly 40 years ago, by my apprentice mentor,
and I've always used it ...
Your assumption of its meaning Jak, is basically correct. It is a parasitic
oscillation of a stage which may be, but is often not, detrimental to the
basic operation of that stage. The OP's amp is a classic example, if that is
indeed what the problem was - pins 3,4 and 5 of that hybrid are all up the
front end, and typically have caps connected to them. The stage basically
carried out its job of amplifying ' low freq ' music perfectly well, but was
also outputting an additional signal either at an ultrasonic, or low HF
frequency, probably at close to full power, depending on how good a
bandwidth the output stage has - hence it gets very hot. I don't know if you
remember the old Texas Instruments SN76003 / 013 / 023 series of power amp
ICs. They had a sort
of " fan " shaped hetsink bonded to them. Well those used to suffer from it
badly, if the supply decoupling wasn't perfect.
I've also had genuine oscillators in mixers that have suffered from
parasitic oscillations. Their primary function has been basically fine,
except that every other radio in the vicinity is going bananas, due to the
high amplitude 'hoot' that is also occuring.
I used a Studiomaster desk that just sounded like utter sh*t. I
couldn't get a decent mix out of it to save my life, although the rest
of the PA system was really excellent. I couldn't put my finger on it,
but something was just not 'right'. I subbed in a cheap Sunn console,
and the whole system came to life!
Turned out the Studiodisaster had parasitics (up into AM radio
frequencies) in almost every stage.
Sorry if I've caused any confusion to anyone. I'll stick to ' contemporary '
terminology in the future ... !!
Hooting is fine. I like it, but it must be a British thing...like 'valve'.
jak
Arfa