help, please: hum on active buffer stage

Aball

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.
Eguth

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!

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