Noise & Hum Rating

Unplug the other equipment from your AC outlets, one at a time, while still disconnected from the Cary.

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?

Does the rating suggest that there will always be some hum and that this is normal? Does anyone know how this rating works at a practical level? Cary didn't have much of an explanation except to say that yes, some hum is normal.

Your speakers appear to be rated at 89db/1W/1m. Neglecting room effects, and assuming you are using triode mode, it can be calculated that 84 db below 50 watts into the two speakers would result in an spl at a 10 foot listening distance of 15 db. Since the 84 db spec represents hum and noise combined, and since the spec hopefully provides at least a little bit of margin, the 15 db figure should be at least several db less, with respect to hum. I would expect that spl's of around 10 db or so would be completely inaudible in a typical room environment, particularly at deep bass frequencies where our hearing mechanisms lose sensitivity at low volumes.

So my suspicion is that the amp is not up to snuff, even though Cary checked it out, although your statement about the improvement that resulted from changing the preamp's power source suggests that external factors are contributing to the problem as well. Cary's statement that "some hum is normal" is simplistic at best, because it doesn't take speaker sensitivity into account.

In saying this, I'm assuming that the hum is at 60 Hz and/or 120 Hz, as opposed to a higher frequency buzz, and that there are no nearby sources of low frequency emi that might be coupling into the amp through the air. To reach a firm conclusion, also, you should verify that the hum still exists when shorting plugs are placed on the amp's inputs.

Another thought is that if you are presently using the amp's 8 ohm taps you could reduce the hum level a little (close to 3 db) by going to the 4 ohm taps. I couldn't readily find an impedance curve for the S8e, but I found some indications that its 8 ohm nominal impedance probably drops down to 4 or 5 ohms at a lot of frequencies. That MIGHT mean that sonics on the 4 ohm taps would be at least as good, or possibly even better, than on the 8 ohm taps.

Excellent comments by Jim, as usual. I would add, though, that I don't think dedicated lines will help a 60 Hz or 120 Hz hum problem very much. They would be helpful mainly with respect to higher frequency hash.

Regards,
-- Al

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.

I think some of it depends on your level of tolerance.
I also tried Cary but it was the 120s MKII and was getting irritating by the hum coming from the transformes and some low hum from speakers when music wasn't playing.
All this wasn't even really audible from my listening spot but as I was near the equipment I could surely hear it and it drove me nuts.....