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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
swampwalker's avatar
swampwalker OP
5,138 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.  
almarg's avatar
almarg
9,670 posts
 
Hi Michael,

To clarify, on my 70/70 there is a slight mechanical buzz (which I suspect to be transformer-related) for a few seconds just **before** the B+ relay closes, about 20 seconds or so after the amp is turned on. (I assume the relay is closing, i.e., completing a circuit path, rather than opening when it presumably applies the high voltage B+ to the tubes at that time). Also, the relay closure itself is essentially instantaneous on my amp, at least since VAC worked the amp over a few years ago when I sent it in for an unrelated repair. Prior to that the relay would clatter for a couple of seconds when closing, which it should not do if in top condition.

Best regards,
-- Al

atmasphere's avatar
atmasphere
12,067 posts
 
Thanks- sounds like the relay is a secondary problem. The hum is nothing to do with the relay though. Its a process of the amplifier's operation. If you've already played with the tubes, then the filter caps are still on the table.
blindjim
3,092 posts
 

Thanks Ralph. Good to know.
slcaudioman
8 posts
 
I have  a few questions.

If your amp did not hum at the techs shop then it's not the amp.
1. Is your amp in a rack or free standing on the floor.
    ( if in a rack take it out of the rack  / unplug everything except the speakers )
2. You lifted ground, how did you do that . Switch on the amp or removing the ground leg off of the power cord. You need to lift the electrical ground of the power cable to the amp.Using a 3 to 2 adapter  does the trick put you have to pull off any metal loop that would touch the center screw that holds the wall plate on the wall outlet plug.
Take your power DODD transformer out,plug straight into wall jack.
3. Have you tried other speakers to make sure it's not your speakers crossovers.
If the ground is till there then it's the amp for sure.
 If you are clean then plug in the Preamp  with nothing in the input section,turn it on. ( do not put the frame of the preamp in contact with the amp frame ).
4. LED lights of any kind on the audio AC wall socket all the way to the circuit breakers ( notoriously buzzy and it goes down the ground leg)
Same applies to dimmers of any kind.
5. If you put a meter on your AC wall jack ,what is the reading between the + leg to ground ( should be 120V ac ) then - to ground ( that should be 0 ac volts ).
If you have dimmers on that AC line turn them low to midway and then check your ground voltage again.If it goes higher than previous test with them off you might have found your problem.Even completely off LED light circuits still burn power that is  leaked by the dimmers ( hence LED light glow when in the off position )

It makes no sense that the amp is quiet with another system and installed in your set up it starts buzzing.
Anyhow a lot of good thoughts in the upper threads but there is no mention on how your systems is installed in the room ( rack or no rack ) Components touching other components. I fight those issues daily with amp racks more so in permanent installation steel frames and rails.

Good luck.

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