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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?

swampwalker
atmasphere's avatar
atmasphere
7,068 posts
 
The problem is not the feedback switch. The hum changes because with increased feedback, some of the hum is wiped out.

The hum is independent of tubes, so right now its sounding like you have filter capacitors in the power supply that are failing. It could also be a bad ground somewhere that relies on a mechanical connection, but at any rate it sounds like you're going to have to send it out for repairs.
blindjim
2,837 posts
 

@Atmosphere > “…. capacitors in the power supply that are failing. It could also be a bad ground somewhere that relies on a mechanical connection, but at any rate it sounds like you're going to have to send it out for repairs.”

Blindjim > As I alluded to these two items earlier(caps & grounds), and you have been more specific as to which caps, wouldn’t those devices which have physical movement associated with their functionality be more suspect for abnormal operation of the amp? Eg., relays, power sw, etc., generally speaking?

Just wondering out loud. Thanks.

@swampwalker
I reckon its just a matter of time and well, you know, now. My deepest condolences and best wishes.
I dig a thorough systematic eval to ensure the or those, physical components which have failed, and or are failing to be found out and replaced is in order.


jea48
2,944 posts
 
I thought it might be a bad filter cap in the power supply as well. The thing that kept me looking elsewhere was swampwater’s statement,
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?).
I would think a bad filter cap in the power supply of the amp would hum in the tech’s shop as well.
I also would think the tech would have checked the DC outputs of all DC power supplies in the amp with a scope. I believe swampwalker said he has used the tech in the past over the years without any issues. People do make mistakes though.

swampwalker,

Just grabbing at straws,.... if you have a multimeter check the AC voltage at the wall receptacle outlet with the VAC amp powered on. Maybe the AC voltage is higher than it was in the tech’s shop. That would make the power supply DC high voltage higher than it was at the tech’s shop. I would think that might be causing the problem in your home that didn’t show up in the tech’s shop. 124Vac and higher with a load connected is considered high. Definitely 126Vac or above with a load connected.

Just curious, did you call the tech and talk to him? What did he say when you told him the amp still has the hum problem? Did you ask him if he makes house calls?

Jim
atmasphere's avatar
atmasphere
7,068 posts
 
As I alluded to these two items earlier(caps & grounds), and you have been more specific as to which caps, wouldn’t those devices which have physical movement associated with their functionality be more suspect for abnormal operation of the amp? Eg., relays, power sw, etc., generally speaking?
Not really, and not on a hum problem like this. Initially- the unit is better, gets worse as it warms up. That's not a mechanical issue.

@swampwalker here's a test for you- run the amp with shorted inputs and wait for it to hum. Then turn the unit off for 5 seconds and then back on again. Did the hum die instantaneously with the switch?

If yes that's a power supply problem.

Did the hum not come back as loud when you turned it back on?

This is a bit of a trick- failing filter caps will sometimes 'heal' for a bit with an off/on surge, but the hum will come back. So if the answer is 'yes' its just about certain its a filter cap, but if it was no, filter caps are still at the top of the list regardless.
swampwalker's avatar
swampwalker OP
5,132 posts
 
@atmasphere- Thank you for taking the time to think about this and to reply.  It appears that I was a little unclear in describing the problem.  The amp is dead silent (inputs shorted, no other items plugged in to nearby receptacles) until the soft-start relay starts to open.  For a few seconds I get a buzz which I attribute to mechanical noise as the relay opens up.  Almarg confirms that his Ren 70/70 does the same.  When the relay fully opens, the buzz goes away and then there is a hum from the speakers that is constant in volume and does not change over time (or at least not significantly).  When the amp is inserted into the system, the hum remains the same volume from full attenuation, until reaching around 9 or 10 o'clock. 

I tried shutting the amp down and then immediately restarting as you suggested.  The soft-start circuit kicks in, so we are back to square one.  Quiet, a buzz (only as the relay is opening), and then the hum.  
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