Low level mains hum with tube amp

Hey guys,

Recently acquired a transformed coupled headphone amp which was custom built to my specifications. Its been fantastic so far and synergises well with most of my headphones. One thing I recently discovered though is if I any hyper sensitive headphones off it like my 25ohm 108db/mw Denon D7000 or IEMS I can hear a tiny bit of 50hz mains hum when no music is playing. Now whats interesting is this when I increase the volume to 100% the level of the hum doesn’t increase at all leading me to believe its the tube heaters themselves rather than the transformer noise. It is not audible with all my other less efficient headphones. Curious to hear what your opinions are and whether or not this is normal? or what you think the cause is? (Its not a ground loop btw this happens when nothing is connected to it).

It’s hard to say.

Many transformer coupled amps couple the +VE line directly to the output through the transformer.
So literally any noise on it ends up on the output, and since that isn’t running through the “amp”, it doesn’t change volume when you increase volume.
BUT It can also be RF interference.

I have an A&S amp that does this, went back to A&S and they said it wasn’t the amp, mine is the same with no input connected, so it’s not the source, or a ground loop.
I have other valve amps that don’t do this.

I have not been able to find any sort of mains filter that resolves the issue on the A&S, I’ve tried at least HumX, floating Ground, Isolation Transformer and a Furman Power Conditioner, about the only thing I have left to try is Power Regeneration, but that would cost pretty much what the amp did, and I have no way to know IF it will fix the issue.

On my amp it’s only noticeable with the High Impedance output and high sensitivity headphones, so I just use the Low Impedance output.

Good to see it isnt just me. I’m going to try and roll in another set of substitute tubes (from 6ew7 to 6de7) to see if there is any change. To me it really isn’t that big of a deal as I said it only occours with my really really sensitive headphones and tbh I didn’t buy this amp to run with the Denons anyway I prefer the sound of those on my solid state/hybrid amps. I was just curious if people have experienced similar with their tube amps. I heard hum is a big issue on the Darkvoice aswell.

I did some further research turns out that its most likely heater cathode leakage… there are 2 possible solutions for this easiest one being to roll in new tubes but this could potentially make it worse… so we will see.

While it could be the case I know that’s not the issue with mine, it’s had 5 or 6 sets of EL84’s and probably the same number of 12AU7’s and they don’t affect the hum.