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In Reply to: RE: Tube amplifier hum posted by email@example.com on May 19, 2017 at 13:23:19
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.
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