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A SET DC coupled amplifier can produce a turn on
and turn off "thump". I am thinking to put a shorting
relay on the SET output, i.e. across the secondary
of the output transformer. The relay would be on 30 seconds
after power up and immediately after AC power is removed.
I would prefer a shorting relay compared to a series disconnect
relay. Shorting relay is out of the circuit during normal
operation.
Does anybody have experience/warn against shorting/opening short
across SET amplifier output while it is on?
Follow Ups:
What happens if you turn off your source or pre-amp first, wait 5 - 10 seconds and then turn off your SE amp? Does it still thump on turnoff?
It has nothing to do with preamp.
You need a slow warmup for the HT . I was under the impression that an instant HT switchon could stress the valves in a DC amp as well as make that bump sound
Al
HT ramps up slowly but not slowly enough.
I will add more delay.
Hopefully if it is a DIY design , it should be simple to remedy . If it's a commercial item then it has flaws in the design or is faulty . What is the model and brand of amplifier ?
Al
It is easy to fix. The issue is not big and does
not cause any problems with my speakers.
Many commercial amps are worse. I want to
get rid of any turn on/off transients to use
drivers which are known to be sensitive to spikes.
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.
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.
--------------------------
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.
Usually it is a horrible idea to short an output transformer.
If I were you, I would focus on a B+ delay/voltage ramp.
Does your amp use a vacuum tube rectifier? I would not do a DC amp without one.
How much capacitance are you using?
~!
The Mind has No Firewall~ U.S. Army War College.
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