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Here's what is, to me anyway, a weird scenario. I have a small tubed headphone amp that has a pretty noticeable hum no matter where and how I set it up, except for one position. The hum is independent of the volume, by the way, and is mostly relieved if I stand there and hold it. The tubed amp is connected to an outboard DAC.
If I have it sitting on another amplifier, a solid state amp called a Beta22, that is not on but is hooked up, plugged in and connected to my cd player, the tubed amp does not hum in the least. If I even pull one of the interconnects out of the solid state amp (at the cd player end), which is not on, the humming in the tubed amp starts up immediately.
To add to the mystery, if I put the tubed amp on a different amplifier that is hooked up to the cd player, just as the Beta 22 was, the hum is still there.
Any ideas what is going on?
Edits: 05/19/17Follow Ups:
I think it is a strong large metal object (like an even bigger transformer in the amp) that will affect the electrical field around the headamp.. So when on the amp the electrical field is 'caught' by the amp metalwork. (Really just like sucked in!)
The the stray field (HUM) is no longer affecting the tubes etc.
Perhaps a metal shield around the headamp transformer with a strap to the ground of the powercord could fix it.
A piece of soft copper cut to bend around the sides of the transformer facing the rest of the electronics?
Then attach a wire to ground the metal to the ground of the powercord.
Stray AC radiation
Also some ferrite around it might work. Or those VPI block made of all steel in a wood case..
I read some new ones are being made and sold now...
Actually any thick metal, dense. to work on the radiated magnetic field will do.
So the amp position may be the PERFECT location for your head amp!
I had a similar issue and a repair technician in Hong Kong told me to buy two cheap ($1 cheap) plug adapters.
So you buy a US to Asia/Euro Adapter - plug your amp into this adapter. Then plug that into a Asia/Euro to US adapter. Then plug it into the power bar/wall - this grounds the connection and he said in the vast majority of cases the hum will disappear. But do not use 3 prong to 2 prong plugs. So if your amplifier cord is a three prong plug make sure the last adapter is a three prong plug.
I have to say - it worked like a charm. Also - having your source component and your tube amp on the same power line bar or wall socket may also work. This latter suggestion I would try first as I am not sure about the safety of the first method - it works but is it strictly speaking safe? I don't see any Hong Kong apartments going up in flames and I don't see why it would be unsafe but I am not an engineer.
This is clearly a grounding issue. You don't mention if the headphone amp has a grounded power cord, but is seems as if it is not properly grounded. When sitting on the other equipment it is possible the other equipment is providing the ground; it could even be the CD player that is providing the ground through the SS amp upon which the headphone amp is sitting.
The tubes are not at fault, as I have tried others with the same hum, which btw, is through the phones, not in the amp itself.
The power supply cord is not grounded in that it only has a two pronged plug.
I am pretty sure that the other amp, on which the problem amp is sitting, is acting as a ground for the problem amp, as someone mentioned.
The tube amp is the ALO Pan Am, with the non-upgraded power supply.
Not being overly mechanically inclined, although I can use a soldering iron, is there anything I can do myself to alleviate the problem, or do I need to take it to a technician?
Thanks so much for your help.
It is possible you have a couple of things going on. Some background, electrical noise can be conducted or radiated. Meaning it can come through electrical connections or through the air. Some designs are more susceptible than others to one or the other or both. You should contact the manufacturer who may be aware of the problem and have a fix that you can implement.
When you tried the experiment of moving to another room, no interconnects. To really do this test, use shorting plugs on the inputs. Non loaded inputs can act as antennas. This is a common problem with higher gain mic and phono inputs, but still worth trying with shorting input plugs on your line level inputs.
In the days before polarized AC plugs, when you had a hum problem, the first thing to try was turning the AC plug around. You can't do that with today's polarized plugs, but sometimes home wall sockets can be incorrectly wired (hot and neutral swapped). Hardware stores sell test adapters that will tell you if a socket is incorrectly wired or you can check it with a voltmeter. You can also have resistance neutral to ground, which a voltmeter will also tell you. Plug the amp in and measure the other socket neutral to ground, AC volts should be very low, in the millivolts.
So what can you do? Redo your experiment with shorting input plugs and verify the rooms/circuits you are trying are correctly wired and low resistance neutral to ground. You should be able to determine if the problem is conducted or radiated. If your problem is radiated and as someone mentioned grounding the chassis doesn't help, this link will show you the thickness required of different materials to shield for 60/120Hz.
The graph you want to look at is mid way through on the right.
Did this amp do this suddenly or did it always hum like this?
What changed when you noticed the hum start?
two things to try. First move your head amp off the other amp. If the hum persists try running a ground wire from a case screw to the ground connection on your amp. By this you can at least tell if you have an internal grounding problem. If the head came with a two prong cord it should be double insulated, which in audio gear is unusual.
Sounds like a ground loop inside your tube headphone pre-amp. Maybe between grounding points of input ground and the grounding of the power supply.
You'll prolly need to play with the grounding scheme or add a star ground to the unit.
Sorry if I'm not under standing, but is it a "mechanical" hum you are hearing from the amp? Or do you mean the hum is heard through the headphones only?
"I know just enough to get into trouble. But not enough to get out of it."
It's a grounding issue or a grounding loop. I have to figure out what is grounding, or not grounding?
Unplug the whole thing and just setup the tube amp. See if there is any hum. post your findings.
Thanks... so I took the amp out of the room, plugged it in in another part of the house, no interconnects connections, and plugged in the phones.
Definitely a pronounced hum.
Ok, check for loose pin connections.
Gently wiggle each tube and see if the hum worsens or goes away.
BTW what unit and tubes are we talking about?
How old are the tubes and do you have a replacement set to test with?
When was the last time you cleaned the pins?
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