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Over the holidays I built a pair of Williamson monoblocks for an old friend in Florida. They arrived fine and he likes them a lot. Circuitwise, the amps are dead quiet. But the two Hammond 274BX power transformers emit mechanical hum. I didn't notice this when I tested them, but then my listening room is large and I really wasn't listening for it. His room is small and he has very good hearing. I'm trying to trace the possible causes. It's the first time I've used Hammonds in a build. It's also the first time I've taken the B+ from the 5 volt center tap (because it was there?) instead of directly from the rectifier socket. The circuit pulls about 140mA and the HV secondaries are rated for 200mA. Filament draw is 3.5A from a 6A tap. They're mounted on a Delrin top plate so it's not interaction with a metal chassis. Here's my list so far:

1) Taking the B+ from the 5V center tap is not something the Hammonds like.

2) He has some DC offset on his AC. I know toroids don't like this but can it cause EI trannies to hum?

3) The Hammonds suck and just hum--but both of them?

Any other ideas before he sends them back? I'm hoping a local DIYer from Audiokarma or some other forum could try swapping the B+ wire, but I haven't found anyone yet. Anyone here in the area? He's in Ormond Beach.

I've always used vintage trannies and never experienced this problem.

I have an iso-tranny on my low-level equipment that buzzes periodically. I added this circuit a while back and have never had the problem since

Taking the B+ from a tap on the 5-V winding would not cause hum. If the hum is mainly low frequency rather than a buzz, the transformer is probably running at too high a flux level--indicating a somewhat cheap design. If buzz, I would look for dirty power line, perhaps caused by SCRs in light dimmers etc.

Thanks, David. That narrows it down. If the iPhone recording he sent me is any indication, it's definitely a "buzz". The house is full of ceiling fans, dimmers, etc. I'm thinking there's some DC on the line or some other type of interference. He says he never heard it with his previous amp but the transformers were enclosed in heavy aluminum cases so it's hard to tell if it was a problem. I think I might have him try a DC blocker or some other type of line filter. We could also slip a steel case over the tranny, that might help.

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Walt

Or some series impedance with the caps like a choke. That will help smooth out the current pulses.

I seem to remember that some Hammond models had problems with buzzing/humming noises and also got really hot. I think at one point Hammond started adding higher quality (more expensive) models that did not have these problems. An internet search might determine if your models are the crappy ones?

Oh, these are the cheaper ones. Some people have trouble with them, others don't. I didn't notice it here. The fact that BOTH transformers hum makes me suspicious. He's going to take them to a dealership next week, where he knows the owner, and see if they hum there. If they do, I'll have to replace them, but I have some good candidates here. If they don't he's going to try a DC blocker.

I was trying to see if the bottom-tier Hammonds would work. They sound fine, but if they hum that's a deal-breaker.