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help, please: hum on active buffer stage

I get some hum through a valve amp when I switch on using my passive preamp, which has an active buffer stage. The amp has been professionally checked and virtually no hum is caused by the amp. The buffer stage uses the AD846 and is a copy of the Audio Synthesis design, with a seperate hefty power supply in a seperate case far away.

Although the hum is not very loud it is disturbing when no music is being played- or during very soft passages.

The strange thing is that after the system has been on for a couple of hours or so the hum diminishes- still audible- but at a noticeably lower level. Why is this?

Can anyone suggest a way to eliminate it, or reduce the hum?

I have turned off the power on every component, one by one, and the hum is definitely being caused by or coming from the active buffer stage, and not an interaction between this and any other component in the system.
eguth

Showing 1 response by consttraveler

consttraveler
250 posts
 
Eguth:

The only tubes I have in my system are in the Phono Preamp, so my experience is limited.

That being said, I would think the reduction in hum is due to everything "settling in" and warming-up (heat sinks, circuit plates, wiring, etc) to equilibrium/balance. Just a guess, I could be wrong.

Have you tried the collars that fit snugly around the tubes? I have seen reports in threads here, that indicate they help reduce noise.

Dave
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