[Vitalstatistix]

Just received my Schiit Valhalla 2 tube amp. It's a great amp and I love it. I'm listening with Senn HD650 (on high gain) and my source is NAD c542.

When the music gets quiet, I could hear a hum/buzz. It's only noticeable when the house is quiet eg listening at night. The hum has some interesting characteristics:

(1) the hum can only be heard through the headphone.
(2) the hum can only be heard 5 seconds after the amp is switched on, and not initially (could be a safety feature of the amp to protect headphone I think)
(3) it gets louder as the volume is increased.
(4) the hum is there even if it's not connected to any RCA cable.
(5) the hum is noticeably reduced when rca cables are connected.
(6) the hum is present regardless of the wall outlet, or the use of a noise suppressing surge protector board.
(7) it's still there when hooked up to a portable CD player which uses a 2-prong power supply (whereas the amp uses a 3-prong).

What's your impression? Ground loop, mechanical, transformer or tubes? I'm not so sure it's ground loop as the hum gets louder with volume, whereas the typical ground loop hum remains constant (I think).

Any help is greatly appreciated! I have about a week to decide if I want to return the amp but I'd just like to make sure where the source of the hum is first.

 

[NA Blur]

From Schiit's site:
 

Amp Problems

Hum and Noise
1. Got hum? It could be a ground loop. Try a GFCI to isolate the ground.
2. Got hiss and hum? High-gain, high-powered tube amps aren’t the best choice for IEMs and highly sensitive headphones. Using a Lyr with 115db/mw efficient headphones is overkill—and yes, you’ll get hiss and hum.
3. Tube amps are going to be more variable in terms of noise than solid-state. Some tubes will be very quiet. Some won’t. That’s why we say, “If you’re looking for certainty, go solid-state.”
 
I think it is the tube and design of the amp since you are using the HD650 which has a somewhat high impedance.

 

[tomb]

The Valhalla is an OTL amp and match well with the high-impedance HD650's.
 
I suspect the tubes.  If it were transformers, I don't think it would change with volume level.  Plus, you remark that it's not audible until some time after turn-on.  That could mean if the hum is in the tubes, you wouldn't hear it until the tube heaters have warmed up and the tubes are starting to produce a signal.

 

[Vitalstatistix]

Awesome thanks very much tomb!