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Noise & Hum Rating

I am having a hum issue - steady state regardless of whether the pre-amp is connected, and no change with an increase in volume - and the amp has been back to Cary where it checked out fine. At this point I've assumed it's a ground loop.

The manual for my amp, a Cary V12R, states "Noise/Hum: > 84db below rated output." Output on the amp is 50 watts triode and 100 watts UL. 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.

Thanks!
grimace
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grimace OP
624 posts
 
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.
ether
64 posts
 
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.....
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