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In Reply to: RE: Shorting (muting) SET amplifier output posted by TonyB on June 30, 2015 at 14:42:51
There is no problems shorting the secondary BUT:
1/ it does not help at turn off , a relay won't be fast enough, it
has to be active before the amp is turned off.
2/ i think your problem is another, it's the input signal that has
a "hump" when power is removed. Test this by : short the input of the
amp, turn on and off. If there is no "bump" this theory is right.
Follow Ups:
I have played with turn on B+ delay. It fixes turn on
thump but will not fix one on turn off. Electronics
(or 2 switches)could be used to turn off power to B+
first. However, that will not fix a potential problem
of power black out. I am thinking about electronics
monitoring incoming AC power and shutting power off
to B+ if a single AC half wave is missing. That would
mean only 8ms delay for B+ turn off.
The amp has tube rectifier and only 2 x 50uF B+ caps.
"I have played with turn on B+ delay. It fixes turn on
thump but will not fix one on turn off."
If the turn-on noise is resolved, you only need to add a relay to short the output when AC power is switched off.
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Buy Chinese. Bury freedom.
The only problem with turn off shorting relay is that
it has to act instantly. The biggest problem is in the case
of power black out when AC power to both tube heaters and B+
goes off at the same time. B+ always has some caps which
provide B+ for a while. AC heated driver and output tube have
no caps to keep them on after AC power is gone. They have
to rely only on their thermal inertia. This thermal inertia
may be shorter than the B+ time constant due to B+
capacitors. That is where the output shorting relay could help
but I am not sure whether output transformer core magnetizing
could happen.
"The only problem with turn off shorting relay is that
it has to act instantly."
In no way should that be necessary. Is this a thump resulting from circuits discharging, or is it a spike induced by the power switch?
--------------------------
Buy Chinese. Bury freedom.
I would think it is due to how the driver and output
tube high voltages go down to zero and how heaters
cool down. The instant when the power switch turns on/off
with respect to AC mains sine wave could also influence turn on/off
behaviour.
"I would think it is due to how the driver and output
tube high voltages go down to zero and how heaters
cool down."
There's nothing instantaneous about any of that. Why do you think the solution requires an instantaneous mute? Maybe you could describe the symptom in more detail.
--------------------------
Buy Chinese. Bury freedom.
I will measure how long it takes for the turn off thump
to come after AC is turned off.
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