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.

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.

The first thing I would do would be to separate the PS from the buffer. If the hum is on both channels, the PS is the more likely culprit.

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....


Thanks, I found your suggestions helpful…

Things are getting complicated.

My ‘preamp’ consists of three stages: i) a MC transformer (no hum from this of any appreciable amount, and it stays the same); ii) a Quad 44 preamp, converted to a ‘phono stage only’ powered by a beefy separate power supply in separate case with umbilical cord; iii) the passive preamp, which has the Audio Synthesis active stage inside its case; this passive preamp active stage is also powered by its own beefy separate power supply in a separate case far away, connected by its own umbilical cord.

So far as I know there is no hum coming from or caused by the valve amp, a modified Radford STA iii. The Radford has an exceptionally low hum level; the serviceman who last worked on it told me that the hum was so low that he could not measure it with his sophisticated equipment.

When you say ‘connect all preamp grounds together’, I am not certain what you mean , or how to do this. I should emphasize that no part of my preamp is run on valves; its all transistorised.

The Radford amp is used for mid range only in a tri amped system. The tweeter and bass have their own dedicated amps, transistor ones.
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.
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!

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

DC= 0.00

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

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.