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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 2 responses by aball

aball
2,018 posts
 
Sounds to me like you either have a differential chassis voltage, or a voltage imbalance on the tube. If you have a digital multimeter, measure between the chassis grounds of both amp and preamp, AC and DC voltage, and see if you have anything. I bet you do. I have grounded all my chassis grounds together at my preamp and have since never had any more hum problems (and my stereo uses 18 tubes).

I take it you are saying this preamp is a DIY project. If the tube has an imbalance on the anodes, you need some experience before proceeding. The fact the sound changes with heat would indicate a bias imbalance probably caused by an asymmetrical/incorrect power layout for the tube. You can email me and I can tell you all about how to figure it out if you feel you are up the task.

Otherwise, have you tried other pairs of interconnects? Some of them don't have proper grounding and this too can cause hum in certain situations....

Arthur
aball
2,018 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.
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