Noise & Hum Rating

Best way to go is separate dedicated lines for digital and analog equipment.

Especially if a home computer is on the same branch circuit as the audio equipment.... They put filthy hash back out on the AC line.

Dedicated lines will decouple the power supplies from one another.

Digital equipment non linear switching power supplies put a lot of hash back out on the AC line in the form of odd harmonics an AC transients.

Take two dedicated branch circuits that are ,say, 50ft long each. Because of the impedance of the wire the switching power supply transient spikes will dissipate very fast in the branch circuit conductor on its way back to the electrical panel. Even a lot of the odd harmonics will end up out at the utility power transformer and be dissipated in the secondary winding in the form of heat. Not all....

If both digital and analog are fed from the same duplex receptacle from the same branch circuit the two power supplies are basically coupled together.

Good power cords on digital equipment that shield RFI/EMI can help from the hash coupling to the analog equipment.

A Sub panel can be fine provided the branch circuits fed from them are not too short.

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

Al,

Thanks for the kind words.

I Like your idea about trying the 4 ohm tap for the speakers.

Jim

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.

and to try to charge me $40 each for EH EL 34s that cost $13 from any number of sources.

Like ARC tested tubes,..... there are tested good tubes and then there are very good closely matched tested tubes.

Out of all the tubes Cary buys from the distributor I would bet they probably throw at least half of them away or sell them to another tube vendor because the tubes did not meet their testing standards.

You pay for what you get when buying new production tubes....
Quality control in tube factories ain't what it used to be in the 1950s and 1960s. Even the 1970s and 1980s....