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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
aball
2,016 posts
 
Ok, sounds like a complicated system. I think the problem has to do with grounding. Do you have a multimeter? If so, take one lead and put it on an exposed metal part of the valve amp chassis. Put the other lead on the passive's chassis and see if there is any voltage. Then keep the first lead on the amp and move the second one to the pre's power supply chassis and see if there is voltage there (check both AC and DC for each). We'll go from there.
eguth OP
10 posts
 
Aball: Great. Will do. I'll get back to you in due course. I have managed to isolate the cause to the Radford valve amp. I did this by removing the mains plug from the wall socket to the passive preamp active buffer stage. There was virtually no change in the hum. This with both sources and all other amps and the active crossover plugged in but no sources turned on. The hum is still coming from the mid range speaker, which is driven only by the Radford valve amp. Many thanks for you assistance so far. I am hopeful that with your help I can get to the bottom of this problem. I am dismayed that I have been told by the engineer that reworked my Radford that he could not measure any hum with his sophisticated test equipment!
eguth OP
10 posts
 
Aball

I set the digital multimeter to the 20V range. The following results were obtained:

RADFORD valve to PASSIVE PREAMP AC = 0.00
DC= 0.00

RADFORD to QUAD preamp PS AC = 0.00-0.01
DC= 0.00


RADFORD to PASSIVE BUFFER PS AC =0.00-0.01
DC = 0.00

The only voltage found is from valve amp chassis to preamp PS, and from valve amp to active buffer PS. The 0.01 voltage was short- lived- an estimated ½ second (approx). Most of the time it stayed at 0.00, but sometimes it fluctuated for half a second to 0.01 before returning to 0.00. What can we try next? The hum reduces noticeably after the system has been on for 2 hours, and reduces further after 3 hours. However, it is still audible from the listener seat after 3+hours, and particularly noticeable on voice through my tuner. I, of course, rebiassed the output valves after swopping them from one side of the amp to the other.

Regards,

Eguth
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