2. amps-preamps
  3. 1423772950

Woofer thump when preamp comes out of standby

Hello all, just recently purchased a Belles 350a power amplifier here on audiogon. Placed it in my system and now I have thumping woofers as soon as my preamp comes out of standby to ready mode. The preamp is a conrad Johnson PV-7. The Belles replaced a pair of Cary SLM -70 monoblock tube amps. I had never had this problem with them. Any ideas what might be causing this problem? Would a dedicated 20 amp line to the power amp solve the problem or is this more of a ground loop problem? Also wondering if the preamp is inverting and the amp isn't, could that be the culprit? Any help would be greatly appreciated.
Thank you!
skipper320
skipper320 OP
63 posts
 
I put my cary tube amps back in the system and the problem is there. I have tracked it down to the preamp. I tried changing tubes with some extras I had, no help. With the cover off and a power line glove on, I moved the line stage tubes just a bit and the hum changed with every tiny move. I swapped a SS preamp into the system and the problem isn't there so I am going with a problem with the preamp. I tried switching the speaker wires as indicated and that didn't work. I also tried as suggested to power up the source and allow the preamp to come out of standby before powering up the amp and the problem is still there. The only time the problem goes away is when intake the CJ PV-7 out of the system completely. I guess I'll send it back to CJ to have them troubleshoot it. Could it be just a bad tube socket? The preamp IS 30+ years old.
Thanks for all the help
jmcgrogan2's avatar
jmcgrogan2
6,346 posts
 
It sounds as if your PV-7 could be leaking DC to the output, possibly a faulty cap that needs to be replaced.
skipper320 OP
63 posts
 
Called Conrad Johnson. They said it sounds like a power supply problem. They want quite a bit of $$ to look at it. I'm wondering if it's even worth the cost as old as it is.
tom32
33 posts
 

I agree that preamp should be turned on before the final. If you do it that way, there is no thump, right? Some preamps output a wave when their power rises, then settle back to normal. There is such a thing as a no-thump circuit, but some preamps don't have it, and that is why people have the habit of not turning on the finals until the preamp settles. To make a no-thump input, the designer has add a circuit that disables the preamp output for a couple seconds until the channel settles in. It's one of those nice little touches to the design.

If the preamp output had a leaky output coupler cap you can measure the leakage with a DC meter. After the preamp is up, output should read zero volts if the output is OK. A quick meter will also show you the "thump" wave when you turn it on.

If you power the preamp first, then the final, and still get a thump, then you might have leaky coupler cap in preamp.
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