I was delighted when I got a Cyrus Two amplifier and a PSX power supply to go with it. One of the finest combinations available in the 1980’s.
Sadly, when I set it up I became aware of an occasional mechanical buzz from the PSX transformer. It is located close to my sitting position, so I can’t ignore it. Something must be done.
Close visual inspection inside the PSX showed it to be in excellent physical condition. Only the filter capacitors showed any signs of their age.
The bulging and cracking was somewhat disconcerting. I replaced them as a mater of course.
Measurement showed them to have quite high VLoss of 5.9%, compared to less than 0.2 % for the new ones. Perhaps this high loss was drawing excesive current from the transformer and causing the buzz? Unlikely. Anyway, changing them didn’t make any difference, though I still think it was worthwhile general maintenance.
I’m out of ideas, so I disconnected the PSX and put it into storage for a while. Then I read a review of a commercial “DC Blocker” device. This piqued my interest. My symptoms were a mechanical buzzing when the amplifier was just idling. The buzz would come and go randomly. Just the thing that could be caused by a DC offset on the mains.
Now DC offset on mains and it’s effects are the subject of much conjecture, snake oil and lies, damn lies. As a student of Electrical Engineering I studied transformers and electricity distribution in depth. That was 40 years ago though! While you can’nae change the laws of physics, our understanding of such things does develop over time. So I hit Google. In amongs all the afore mentioned bullshit I found a magnificent article by Rob Elliot. Blocking Mains DC Offset From Transformers. I recommend you read it. Twice. “A varying DC offset on the mains is no longer uncommon”, he writes in 2008. This is 20 years after the PSX was created. Hum is worse in large transformers which have a low primary resistance. Yes, that’s the PSX.
Rob’s writings convinced me that my problem very likely was DC offset. A DC blocker is a simple circuit. However, the safety implications of working with mains voltage and power levels made me pause before lifting a soldering iron. I decided that, by the time I had sourced high quality components and suitable casework, I could spend a lot of time and money building something that would be a bit Heath Robinson. I bought an Audiolab DC Block for £100. It did the trick. At start up there is considerable hum for a few second as the rather large capacitors charge up. Then silence. Money well spent. The experimental results match the theory.
So I have “Removed the cause, but no the symptom”. As Frank’n’Furter said. The thing is that the DC offset hum was rather loud. More so than you would expect. This made me think maybe there was still some sort of mechanical problem. The PSX is extremely well engineered, so I didn’t expect to find cheap components or shoddy workmanship.
I found the culprit when I removed the Toroidal transformer. The rubber washer, which is supposed to provide mechanical isolation, had squashed flat over 30 odd years under the weight of the transformer. A replacement was quickly fabricated from a surplus mouse mat and installed under the original.
It’s connected up to the Cyrus Two now and sat next to me. No hum problems at all. I have, for science, occasionally removed the DC block unit. As expected the hum then occassionally returns at random times. Only to go away again as soon as the DC block is put back in circuit.
Best of all I am now enjoying the glorious thing that is a Cyrus Two amp with a PSX connected.