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Hum problem

Been chasing a hum in my VAC Ren 30/30 for quite a while.  My technician has been all through it and does not find a problem.  Says he cannot hear it in his system, but it's quite apparent on my Horning speakers (94 dB maybe?).  As soon as the soft start relay starts to open, it begins to be audible and when it clicks open it's fully audible, maybe from 5-6' away, with the pre-amp fully attenuated.  Once you advance the attenuator past about 9 or 10 o'clock, it starts to get louder, but not before.  It's not a transformer mechanical hum; no sound at the amp but clearly audible through the mid range of the speakers.  Present w no other components turned on (or any/all turned on).  No change after swapping out power cords, lifting ground, swapping interconnects.  Changing the position of the ground switch on the amp has no impact.  Same w AC straight from the wall or w AC from a Dodd Audio Balanced Power System iso transformer.  Since this is a transformer/tube amp (not an OTL), I assume there can be no DC offset, and cannot really check that because I don't think I can operate it w/o a speaker load and the info I find on the web says it must be checked w/o a load.  

Any ideas before I ship this 85 lb beast back to VAC?

128x128swampwalker

Showing 4 responses by jea48

jea48
4,313 posts
 
My technician has been all through it and does not find a problem. Says he cannot hear it in his system, but it's quite apparent on my Horning speakers (94 dB maybe?).
See if you can hire the technician for an hour or so to come to your home and listen to the hum first hand.

Jim

jea48
4,313 posts
 
At this point all I can suggest in addition to what you are planning to do is to reiterate the suggestion of seeing if the hum is present with nothing connected to the amp’s inputs, if you haven’t already done so. Or better yet, with shorting plugs connected to its inputs.
Al,

I was thinking the same thing. That eliminates everything ahead of the amp. I would probably go so far as to unplugging all other associated equipment from the AC mains wall outlet.

What about the sensitivity of the speakers Michael is using. Not sure which model he has. One review for Horning speakers, I quickly read over, said the sensitivity was rated at 100db.
And is there any way the speaker cables could be causing the hum? Capacitance? Inductance? I have read where some speaker cable designs can cause problems for a power amp.

Jim
jea48
4,313 posts
 
@swampwalker,

By chance did you unplug all the other associated audio equipment from the wall AC power outlet? Audio equipment near the amp as well as the speakers?

By the way the reason you hear a loud buzz from the amp/speakers with the shorting plugs removed from the inputs is due to RFI/EMI that is allowed to enter the open input ports/jacks of the amp.

By chance do you have another pair of speaker cables you can try? If you still have the hum using different speaker cables that would rule out the cables, ime.

Edit:
Also are you using an aftermarket power cord on the amp? If so did you take the power cord with you when you took the amp to the repair shop to have it looked at?
Just for the heck of it install the factory power cord that came with the amp. Check for the hum.

Jim
jea48
4,313 posts
 
BTW, RFI was most likely from powered wireless connection to internet for my music server
Did you kill that thing when you were isolating things to find out what might be causing the hum?

Has this amp always had the hum issue? If not, can you think of anything you may have changed or added that maybe now is causing the hum?
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