So, when I switch off my DIY preamp, I get a *thump* in my speakers. I think the preamp output actually trips the input protection circuits in my VTL amp, as it momentarily mutes the music before it comes back and dies out slowly as the preamp tubes cool down.
So what actually causes the thump? Is it the drop in B+ voltage? Is it a change in current? Is it the AC heaters cutting out?
I'd like to see what I can do to my PSU to mitigate the thump...I currently have a CLCLC PSU.
(I know I could add a mute switch on the outputs or some sort of relay, but I would much rather address the source than treat the symptoms).
Thanks!
Jayme
So what actually causes the thump? Is it the drop in B+ voltage? Is it a change in current? Is it the AC heaters cutting out?
I'd like to see what I can do to my PSU to mitigate the thump...I currently have a CLCLC PSU.
(I know I could add a mute switch on the outputs or some sort of relay, but I would much rather address the source than treat the symptoms).
Thanks!
Jayme
I do follow the proper order for switchon.
I am more concerned that the POP is stressing the tubes in my preamp, hence trying to figure out the origin.
If it is just from the output cap discharge, then I am comfortable that the preamp tubes aren't seeing any huge voltage or current spikes, and I'll just add a switch to short output to ground.
If it could be something else, especially in the PSU, then I am still VERY interested in the source...
Thanks.
I am more concerned that the POP is stressing the tubes in my preamp, hence trying to figure out the origin.
If it is just from the output cap discharge, then I am comfortable that the preamp tubes aren't seeing any huge voltage or current spikes, and I'll just add a switch to short output to ground.
If it could be something else, especially in the PSU, then I am still VERY interested in the source...
Thanks.
burnedfingers said:
Nup....worstcase happens when opposite partner decides at random to pull any plug out of the wall socket...to run the hair dryer.
-->I always run my main amps from the preamp AC supply (i.e = main amp is powered from socket back of preamp chassis per quad II which can only be switched on when preamp is on and fit DIL relays to input of main amp. These are run from heater supply via small rect/cap which provides the fastest dropout. One of loads of ideas.
In reality every piece of amp equipment should be fitted with an anti thump to avoid the picture and plaster coming off the wall.
richj
This is an excellant question, it really isn't being answered here. What if you sell a diy preamp and your customer makes a mistake turning things off? This thump is stresssing the speakers the most, so what do the pros do to solve this problem. I've tried a K&k delay/mute board it mutes the output fantastic on power up. But, I to still have that pop if the preamp is turned off first. How do we resolve this? It does mutes when shuting off just not fast enough to kill that pop. What can change this? And yes it's probably the output caps discharging in the tube preamp.
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You can add an "instant mute upon power off" feature to the muting circuit. For example, if it uses a 555 timer,
connect a diode's anode to the timing capacitor, and the diode's cathode to a positive power supply node that
collapses quickly upon turn off. If you have a DC filament supply, use that.
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This is an excellant question, it really isn't being answered here. What if you sell a diy preamp and your customer makes a mistake turning things off? This thump is syresssing the speakers the most so what do the pros do to solve this problem. I've tried a delay board it mutes fantastic on power up but, I to still have that pop if preamp is turned off first. How do we resolve this? It mutes when shut off just not fast enough to kill that pop. What can change this?
Is there a capacitor across the primary of the preamp mains transformer? If not, put an X-rated cap of about 10nF there. That may cure the problem.
Preamp power supply rails usually collapse quite slowly so unlikely to be the cause unless there is something unusual about your preamp.
Preamp power supply rails usually collapse quite slowly so unlikely to be the cause unless there is something unusual about your preamp.
Unlikely, unless the output coupling cap is particularly large and the PSU is particularly weak. If this was the case, you are already likely to be seeing lots of woofer flapping from subsonics getting through.JBLman said:
This is unlikely to help. A ground leak resistor helps in the situation where you switch something on, then connect it to the next item in the system.DigitalJunkie said:
What is pop or thump? Could be a one shot motor-boating, which implied bass insatiability. You maybe able to test bass insatiability by playing a very bassy track to see if sub cone moves in and out. If so you maybe having too short filtering cap timing constant, causing the voltage falling too fast and thus oscillation instability (remember the moment you turn off the tubes are still warm and amplifying), the easy way is to add some more capacitors to PSU of pre-amp to see what happen. Please also refer to https://forum.audiogon.com/discussions/amplifier-capacitance
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The trick is to pull the plug and see if the noise goes away. If so, then it's the switch contacts.
It strikes me that the best approach in figuring this out would be to figure out what causes the thump rather than trying different "solutions" until the thump goes away.
Have you looked at the output voltage with an oscilloscope when the amp is turned off? That could provide some clues as to what's going on.
Tom
Have you looked at the output voltage with an oscilloscope when the amp is turned off? That could provide some clues as to what's going on.
Tom
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