Noise & Hum Rating

I did move the pre-amp to a different power supply, which helped, but the hum did not go away entirely. Unfortunately the cheap Monster power supply is not quite as clean sounding as PS Audio.

The hum itself is barely audible, and not at all when music is playing. I may simply be over-thinking the problem. It's certainly livable.

Would a dedicated line from the box in the basement improve this condition? It seems that if another circuit in the house is creating interference it would effect the dedicated line too, since all of the lines would originate from the same panel. Is there a way to install a sub-panel to keep EMI/RFI from traveling up the line?

I did switch to the 4 ohm taps, and they are quieter. Not silent, but definitely quieter. There also doesn't appear to be any difference in sound, which is good.

I was a bit disappointed in Cary's service for a number of reasons. I don't think they looked very hard at the amp, although they were quick to sell an overpriced capacitor upgrade, and to try to charge me $40 each for EH EL 34s that cost $13 from any number of sources. If I ever send it out in for service again, it won't be to them.

That's nonsense. My amp takes a dozen EL 34s. Cary tried to tell me that the tubes that were in it were bad. When I had them tested locally not only weren't they bad, they were actually very strong, and except for a little degradation in two of them, pretty well matched. Cary said they were all bad and needed to be replaced, and it simply wasn't true. They also said that the tubes were causing the hum, which also wasn't true.

The EH tubes they wanted to sell me are nothing special. Electro-Harmonix EL34s are an inexpensive tube. Tube Depot, Doug's Tubes, Amplified Parts - all of which were highly recommended by A-gon members - and a few others sell that same tube for $13 (or $27 a pair) matched and tested, and graded for early or late distortion - a pretty thorough QC process.

IMHO Cary was less than upfront about the state of my existing tubes, and trying to gouge me for replacements. Even if Cary does go through such a rigorous QC process themselves, that's not an excuse to triple the price of the tube, especially when the same service is available through some other reputable sources.

We actually did to tests. The first was on a B&K tube tester, set at a sensitivity of 53 - which was the spec for the machine for an EL34. All of the tubes tested out between 95 and 110, with two of them a little low at 85. Then we did a sine wave test for noise with an attached meter for output. Using a baseline control we got 21 watts in push-pull per pair of tubes - any combination of any of the dozen, even the two low ones (although I confess, I do not know the plate voltage). The sine wave - control at the bottom of the wave, and tested tube at the top - revealed no noise in any of the tubes.

The guy that did the test has been doing this for forty years - a really cool shop btw - and his opinion was that not only were the tubes still good, but they showed remarkably little wear at all.