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I just received a pair of used MA-2 MKII.3 amps and they both have 60Hz hum. Even with no input and pins 1 & 3 of the balanced inputs shorted, they both have this hum. It is worse with the input switch in the high resistance position. If I have to send these to Atma-Sphere it will be about $500 for shipping. Can bad tubes alone cause this? The speakers can’t be a problem. I have initially hooked up some Acoustic Research bookshelf speakers to test the amp. When I plug in my Golden Tube Audio SEP-1 preamp the problem gets a lot worse. The GTA isn’t a good match for these amps but at least it has balanced outputs. The amps make music but there is always the hum. If I can get these amps to work I will get an MP-1 preamp and some balanced sources and be in OTL hog heaven. I know these will be some really righteous amplifiers when they are working well. Any ideas?...
Follow Ups:
Thats sounds like a common grounding or filter cap problem..If they are using circuit boards they might have a solder connection thats needs resoldering..OTL amps are some of the best sounding I have ever heard but they do have problems with tube sockets and arcing..You may have a bad solder connection is all.My acoustat servo amps and my futtermans always used to have problems with resistors and tube sockets failing.If one of the grounds has a bad solder joint on one of the filtering sections,thats a good place to start.I dont think its major and those are fine amps.
As I understand it, he has the problem in both channels. Highly unlikely (though not impossible) that BOTH monoblocks sustained a bad solder joint which results in the exact same symptom in each.
Sounds like you have a ground loop. Try lifting the grounds on the amp's power cords and let the balanced interconnects ground the amps at the preamp. Should resolve the problem if it is indeed a ground loop. Are the amps and the preamp on the same electrical breaker?
I considered that possibility as well, but it seemed to me from memory of the insides of my (admittedly older production) MA240 amps that the AC ground is not used. That is, neither of the two AC cords that serve each monoblock is grounded to the chassis. (MA240s used the MA2 chassis' and transformers, with separate power cords for the input and output power supplies.) The monoblocks are grounded instead via the input jacks to the preamplifier. IOW, the AC ground is already "lifted". If these MA2s are wired the same way, this would make it hard to develop a ground loop, but maybe they are different. Tufwheel should tell his story to Ralph for a quick solution; I am sure this is a simple problem.
These amps are supposed to have two AC circuits per amp for four total, with two input transformers to a chassis. That is the way it is now. Truth be told you could get away with one breaker per amp. The preamp does indeed share one of the AC circuits. I will try what you are talking about and see what it does...
If you take "short" of, will humm go to worse? If not, you have not shorted them properly.
Tufwheel, call Atma-Sphere and talk with Ralph or Bill. They may have some suggestions for you to diagnose the problem. Ralph always supports the owners of his gear, whether first owner or third.You have indeed acquired a wonderful pair of amplifiers.
Ed
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