There is no signal going to the amp and it's time to shutdown until
another day of listening arrives. Flip the power switch to off like I
always do EXCEPT this time after about five seconds or so there is this
brief sound out of the speakers. Sound lasts about two seconds and
sounds like a sine wave sweep down in Hz and amplitude.

BBBBBEEEEEoooooooppppp p p

How bad is this?

"Neil Peters"
There is no signal going to the amp and it's time to shutdown until
another day of listening arrives. Flip the power switch to off like I
always do EXCEPT this time after about five seconds or so there is this
brief sound out of the speakers. Sound lasts about two seconds and sounds
like a sine wave sweep down in Hz and amplitude.

BBBBBEEEEEoooooooppppp p p

How bad is this?



** Crown DC300As did that as a matter of course.

According to Crown , it was not possible to fix it.



............... Phil

sounds like a power supply cap discharging......

Stu

"audiodir"
sounds like a power supply cap discharging......



** Nah - it sounds like a party balloon discharging !!!


LMOA ......




............ Phil

Neil Peters wrote:

There is no signal going to the amp and it's time to shutdown until
another day of listening arrives. Flip the power switch to off like I
always do EXCEPT this time after about five seconds or so there is this
brief sound out of the speakers. Sound lasts about two seconds and
sounds like a sine wave sweep down in Hz and amplitude.

BBBBBEEEEEoooooooppppp p p

How bad is this?

You might have a supersonic oscillation happening when at full power
and it goes sonic when the B+ is dying just after you turn it off.
Supersonic oscillations can cause distortion of the audible
audio, and be hell on tweeters. So I would dig into the amp
to find any such oscillations and fix it. Maybe grid stoppers
might help. Use a scope if you have one.

Interesting.

Frankenhouse (check my website) does that, or at least used to. A short
couple cycles of say, 5-10Hz as it discharges, about a second after throwing
the switch and maybe half second duration. Not quite as dramatic as yours,
which apparently is tuned based on power supply voltage. An excess of
capacitance somewhere perhaps? (Charging slower, thus the dropping
frequency, as decaying supply voltage reduces supplied current.)

Apparently it comes and goes... hasn't done it the last couple of
shutoffs...

In this case I suspect the soft power supply (a 5Y3) and the low capacitance
in it. Going over the circuit might not hurt either, probably should adjust
some coupling caps. You'd be suprised how often a global NFB amp has good
infrasonic gain! A VLF sweep can be very helpful...

Tim

--
"California is the breakfast state: fruits, nuts and flakes."
Website: http://webpages.charter.net/dawill/tmoranwms

"Neil Peters" wrote in message
...
There is no signal going to the amp and it's time to shutdown until
another day of listening arrives. Flip the power switch to off like I
always do EXCEPT this time after about five seconds or so there is this
brief sound out of the speakers. Sound lasts about two seconds and
sounds like a sine wave sweep down in Hz and amplitude.

BBBBBEEEEEoooooooppppp p p

How bad is this?

Neil Peters wrote:

There is no signal going to the amp and it's time to shutdown until
another day of listening arrives. Flip the power switch to off like I
always do EXCEPT this time after about five seconds or so there is this
brief sound out of the speakers. Sound lasts about two seconds and
sounds like a sine wave sweep down in Hz and amplitude.

BBBBBEEEEEoooooooppppp p p

How bad is this?

Your amplifer is oscillating and unstable during the discharge
period for the PS caps where the circuit can become an oscillator.

Some amps let out a screach or whoop when turned on.

Unconditional stability means that the amp never oscillates
under **any** condition.

Its not uncommon to find amps with NFB which suffer noisy turn on
or turn off behaviour.

I recently built an SE amp which whooped during turn on, hust as the
current
just began to flow in the output tubes during heat up.
The only thing that would stop it was to have a delayed B+ turn on, and
a very long time constant for the screen supply.

SE amps may be more prone than PP types due to the balanced nature of PP
amps likely
to produce cancelling currents preventing turn on or turn off noises.

Oscillations such as this can be difficult to diagnose.

The problem can affect all types of feedback amplifiers including SS.

SS amps often are fitted with relays between the output and speakers
which immediately disconnect the speakers at turn off,
also only connect the speakers to the output when the protection circuit
gives the all clear.

One has to worry that oscillation as you say does not fry some amp part
while its happening.

Therefore it should be stopped or prevented, and never ignored.

Patrick Turner.