wrote:



"So, when using a variac, a condition must exist where
the needed reverse bias is missing but the V-FETs have
enough DC supply available to fry themselves if held there."


I understand that, but there should be a way to design around it,
prevent it from happening. **** poor IMO. Asking for trouble.

** Only if you use a variac with one.


How many times the power went out and came back dim, like half voltage.
And varied.

** I have seen that happen only once in my life, about 20 years ago.

Of course you need to have the amp switched on and not bother to turn it off when the power outage happens. Prolonged brown can damage stuff, like the compressor motors in fridges - and they are always left on 24/7.


****, there are so many ways to prevent that failure mode
it is ridiculous. One stupid power relay that removes the
rail voltage form the outputs would do it, and then they
wouldn't need one at the speaker output. Very low cost.

** The amp is already bristling with add on circuits that constantly tweak the bias, protect the V-FETs from current overload and stop DC appearing at the speaker terminals. All not shown in the simplified schem I linked.

FYI: the DC bias across R737 is anywhere from 20 to 50 volts, depending on the particular V-FETs actually fitted, the ambient temp and the DC supply voltage.

****ing assholes.


** Crikey, it don't take much to wind "jurb" up.



.... Phil