BrianGT LM3875 Turn-Off Thump

[cantskienuf]

I built the BrianGT LM3875 premium NIGC kit and think it sounds great; however, I have a turn-off thump. I built the units as true stand-alone monoblocks with individual 22-0-22 Avel 160VA toroids per side. I used a 0.1uF, 450v Auricap in the Cz position and a 2.7 ohm 5 watt Kiwame in Rz. I added a 56K Riken shunt on the input and a .22R Mills on the output ala Decimal Dungeon. Other than that, it is stock. I have no hum or buzz, just a thump when power is turned off. It is not particularly obnoxious, but I'd like to get rid of it. Any ideas?

[Tazzy]

Power switched relays on the speaker outputs

[Peter Daniel]

Properly implemented kit will not cause turn off/on thump.
It must be all those extras Seriously talking though, check your grounding, this usually creates that kind of problems.

Power switched relays on the speaker outputs will help, but will not fix the problem.

[Sandy H.]

Does it happen if you leave the input (CD player etc) turned on? Does the same thing happen if the input is turned off first?

I had an amp which had both turn-on and turn-off thump unless the source was powered on first. I'm not sure what the real fix was, as I was fine with just turning the CDP on first and off last.

Sandy.

[Peter Daniel]

Also, what's your DC offset at the output?

[rnoble]

I would take your extras out of circuit and test. I have done the same setup recently, (minus the zobel and extras) with 2 225VA transformers and it's dead quiet on startup.

[sek]

Originally posted by Sandy H.
I was fine with just turning the CDP on first and off last.

Not to offend anyone.

But isn't this the ALWAYS recommended way of switching ANY chain of audio devices that don't ALL feature a turn-on/-off mute?

I even learned to switch on the computer screen on last and off first, back in school!



Sebastian.

[cantskienuf]

Also, what's your DC offset at the output?

Peter, I haven't tested DC offset lately. The last time I did they were both at approximately 40mV into an 8 ohm load. Brian thought that sounded OK, but I've seen from other posts that some are able to work the offset down much lower. Do you think that is the culprit? The thump has always been there, but I think I'll re-check the DC offset tonight and make sure it hasn't changed significantly.

and it's dead quiet on startup

rnoble, mine are quiet on startup also. It is the power-off thump that I want to work on.

Does it happen if you leave the input (CD player etc) turned on? Does the same thing happen if the input is turned off first?

Sandy, I always turn on the isolation transformer first, then the pre-amp, wait for warm-up, then power up the CDP, then power up the amps, then press play on the CDP, then bring up the attenuators. For turn-off I first turn down the attenuators, then press Stop on the CDP, then power off the amps, then power off the pre-amp, then power off the CDP. I don't like to hear what happens in the pre-amp (Foreplay) when the tubes warm up, or shut down.

[Peter Daniel]

40mV is fine, but maybe your source components pass DC as well so check with a source connected.

[sek]

Hi,

I've seen from other posts that some are able to work the offset down much lower.

40mV is not extremely much, but it depends on what the speakers make of it, though.

Do you have measurement equipment like an oscilloscope to verify what happens at turn off? I guess, not.

Another approach: have you tested the GCs with the input devices disconnected?

For further guessing, photos of the amp internals would be of great help...

Sebastian.

edit: Hehe, Peter was quicker, but pictures would still be great.