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Old 21st September 2007, 10:05 PM   #1
jayme is offline jayme  United States
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Default turn off thump

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).

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Old 21st September 2007, 10:34 PM   #2
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Is there an output coupling cap?
Maybe the thump is from the cap discharging?
Maybe try a resistor to ground,to help bleed off any DC.
Just a thought.
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Old 22nd September 2007, 01:35 AM   #3
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Turn off the amplifier first followed by the line stage and this will eliminate the thump factor.

The amplifier is always turned off first and allowed to bleed down the power supply before any line stage/preamp is turned off.
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Old 22nd September 2007, 03:57 AM   #4
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Yes, and the opposite also applies: Switch on the preamp before the main amp.
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Old 22nd September 2007, 07:13 PM   #5
jayme is offline jayme  United States
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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...

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Old 22nd September 2007, 11:57 PM   #6
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What do you have for an amplifier?

Are you turning off the amplifier first when powering off the system?

No need to try to re invent the wheel here. Proper turn off procedure will eliminate the noise.
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Old 23rd September 2007, 07:40 PM   #7
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Originally posted by burnedfingers
Turn off the amplifier first followed by the line stage and this will eliminate the thump factor.

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.

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