Amplifier Hum - Only Amp Connected

To clarify, hum begins as soon as the soft-start period ends.

Is there any mechanical low buzz/hum emanating from the amp itself, or is it quiet and hum coming purely from the speakers?

It's possible you may have a DC offset issue, or your amp may have an intrinsic transformer hum.  This may be curable by tightening down transformer(s) or other kinds of damping, or there may not be anything you can do (as I found out to my cost a while ago). In my experience, be prepared for frustration.

I am going through almost the same thing right now.
One question:
Are the speaker cables anywhere near the power cords?
If so, try moving them as far away as you can.
B

Is there any mechanical low buzz/hum emanating from the amp itself, or is it quiet and hum coming purely from the speakers?

Hum is coming only from the speakers and only from the mids as far as I can tell.

What do you recommend for a DC offset issue?

Are the speaker cables anywhere near the power cords?
If so, try moving them as far away as you can.

I don't have a ton of room, since everything is fairly tight. I will try to move things to a different room at least to experiment.

DC offset is a b***ch.
As I said, I am dealing with a similar issue.
First, I would get an electrician to go over the circuit. If possible make a clean run to your system. Replace outlets with hospital grade outlets-cheap upgrade.
And, at the very least, make sure the wires are properly installed. Sometimes the ground and neutral get mixed up, then all hell breaks loose. 

Bob

Does the hum sound like this?  
120Hz hum.   

https://www.youtube.com/watch?v=pC4Uzt0qm2E  

If yes it could be failing electrolytic caps in the power supply. 

How old is the amp?

Manufacturer and model?

DC offset should not sound like a hum. You need AC to get sound from speakers. In extreme cases DC offset might push the cone in or out some but the cones will stop there and you won’t hear anything.
On the plus side DC offset is easy to troubleshoot.   Put any multimeter set on DC across the speaker terminals.   If you see more than 100-200 mV you may have a problem (it should be zero).

I had a hum problem and it turned out to be the wall wort from my Ikea lamp over my turntable.

The hum sounds like the 120Hz hum in the video in @jea48 's link, albeit perhaps with less sizzle.  Also, I listened again this morning and the hum is slightly louder in the right channel than the left channel.

The amp is supposedly 5 years old.

What Brand is it?  Best JohnnyR

VAC.

It is normal for an amplifier to pick up a little bit of buzz with no input or with the preamp turned off. Does it have the buzz with the preamp turned on?

If no you're off the hook. If yes, this type of buzz can be caused by a ground loop. For testing purposes only, place a ground cheater adapter on the power cord of the amp. You can get these at almost any hardware store. Run the system with the preamp ON. Did that fix it? If yes, you have a ground loop and some exploration will be needed to sort out a solution. If no, other exploration will be needed.

Try these and report back- we'll see what the next step (if any) is needed.

There absolutely is a hum when the preamp is turned on.  The hum does not change in any way when the preamp is off or on, including when the interconnects between the amp and preamp are removed or connected.

I will try the cheater plug this evening.  Does it make sense that a ground loop would be worse in one channel?

If the buzz is 120Hz, given the amp's age, the power supply filter caps are probably not suspect. Based on what I've read, a 120Hz buzz is typically caused by something faulty in the rectification stage depending on whether the signal is half wave or full wave rectified. If the VAC amp is tube rectified, that might be an area for investigation. That might also explain the uneven amplitude of the buzz. I'm sure others with more hands-on experience can correct me if I'm mistaken.    

Been through this one myself. I agree with cheater plug to start off or break ground pin off a cheap stock power cord. Have you swapped power tubes left to right since you said it's louder in one speaker, or maybe a rectifier tube. Electrolytic cap, transformer in power supply would cause hum but that would be last stop after trying all the free solutions listed.

I will try the cheater plug this evening. Does it make sense that a ground loop would be worse in one channel?

That hasn't been my experience but that does not mean it couldn't happen.

Cheater plug did not make a difference.  Hum was still there.

I swapped the power tubes and it is possible the worse side 

jdpawnbroker OP6 posts

12-13-2019
5:33pm

Cheater plug did not make a difference. Hum was still there.

Did you put a piece of tape on the bottom of the cheater plug ground tab, or over the trim screw that hold on the electrical outlet cover plate, so the ground tab does not make electrical contact with the metal screw?
The trim screw is electrically connected to the equipment ground.

I did not use tape but the ground tab was not in contact with the screw.

This sounds to me like a power supply problem such as a failing filter capacitor in the power supply. Such a part can indeed cause more hum in one channel than the other but the problem would be in both. Sometimes you might be able to get it to stop humming for a little bit by simply turning the amp off and on again, but only sometimes. My recommendation is to seek service on the amplifier.

Thanks for your responses and advice.  Seller asked me to ship it back to them so they can take a look.

Follow up:  I sent the amplifier back to the seller, which sent the following:

"I was able to get the amp on the bench today and test everything out. Everything is working great. The hum that you were hearing is remedied with a slight tweak of the bias to get the pairs of tubes biased correctly. The bias light system can get you very close but very slight adjustment of the bias cleans up all of the remaining noise and gets everything perfect."

The "bias light system" mentioned above is the VAC amp's bias system whereby a light comes off or on while you turn a screw to determine whether bias is correct.  Does it make sense that a slightly incorrect bias could create a hum audible from 8 feet away?

FYI, I bias my current amp all the time with a multimeter and without use of a light system and have had no such issues.

If there are separate bias controls for each power tube then this explanation makes plenty of sense. If the currents are not balanced, the sawtooth waveform in the power supply can be heard in the speaker. When properly balanced (both bias controls set correctly) then the sawtooth (buzz) waveform is cancelled.