How to determine where hum is coming from

[WOPL Sniffer]

Well well well, the hum went away with no rca input connected, I use a schiit valhalla 2 as a preamp (for volume control) and an Audiocontrol C101. With C101 plugged in it hums! run just the Schiit. NO hum.

Thanks to everyone for helping diagnose this issue.

As I said, start with the easy shit. Tearing an amp apart is a PITA if it aint needed. That was a 5 minute fix.

 

[Gepetto]

I notice the Schitt Valhalla 2 has a 3 prong AC plug. Did you try floating the three prong plug on it? What other components do you have in the system? Is the Schitt the only one with a 3 prong AC plug? Usually if you have more than one safety ground, you end up with ground loops.

 

[MusicSteve]

I have issues with preamps picking up transformer hum from amps if they're stacked. So I keep them separated in the main system. The main system is dead silent.
Upstairs in my work room I keep them stacked due to space limitations and because the hum is very minor. Can't hear the hum when music is playing. Annoying, but really a non issue.

George
What do you have upstream. each component. Do you have a computer or tv hooked up?

 

[George S.]

The system with the hum is upstairs. A Lenovo desktop computer with a internal sound card, feeds stereo to a upgraded PL2000 S2 preamp that sits right on top of a PL400 S1 full WOPL, driving Pioneer CS-99 speakers. The preamp and amp are both wired 2 wire, using IEC sockets, with the line being wired to the fuses in both pieces, and then to the power switch in the preamp. Neutral goes directly to the transformer in both pieces. Nothing is attached to the ground lug on the IEC sockets, and the heavy 3 wire "indexed" cords cannot be reversed at the wall socket.
There is a low level hum in the system when the preamp sits directly on top the amp. The preamp has Budwig feet under it, so the only grounding between the two are the RCA cable shields. If I touch any part of the system, the hum does not change. If I unplug the preamp to amp RCA cables, the hum goes away. If I pickup the preamp, and move it away from the amp, the hum goes away. Pretty sure it's the amps magnetic field inducing hum in the preamp.
One thing I need to try is a chassis ground jumper between the two. The desktop has the only "wall" ground in the system. There's a more I can do trying to reduce the hum, just haven't got to it.

 

[George S.]

I do need to modify a old IEC cord so I can flip it at the wall, the use the scope to compare noise with plug orientation to ensure I have the transformer primaries wired for lowest noise. Another topic I need to read up on so I don't fry the scope. Haven't taken time to even learn how to safely look at line voltage. So much to learn and do and so little time available.

 

[MusicSteve]

The system with the hum is upstairs. A Lenovo desktop computer with a internal sound card, feeds stereo to a upgraded PL2000 S2 preamp that sits right on top of a PL400 S1 full WOPL, driving Pioneer CS-99 speakers. The preamp and amp are both wired 2 wire, using IEC sockets, with the line being wired to the fuses in both pieces, and then to the power switch in the preamp. Neutral goes directly to the transformer in both pieces. Nothing is attached to the ground lug on the IEC sockets, and the heavy 3 wire "indexed" cords cannot be reversed at the wall socket.
There is a low level hum in the system when the preamp sits directly on top the amp. The preamp has Budwig feet under it, so the only grounding between the two are the RCA cable shields. If I touch any part of the system, the hum does not change. If I unplug the preamp to amp RCA cables, the hum goes away. If I pickup the preamp, and move it away from the amp, the hum goes away. Pretty sure it's the amps magnetic field inducing hum in the preamp.
One thing I need to try is a chassis ground jumper between the two. The desktop has the only "wall" ground in the system. There's a more I can do trying to reduce the hum, just haven't got to it.

You can try this.
I like to put a Ground Wire from Frame for each Device ( I Use fine wire , Speaker wire works really good (lost of strands )
I always install a ground wire From my computer PS Frame (or sound card Frame) and the TV (not your setup) each go to a 5-10 lug Screw Ground Bar Mounted on wood I have two of then you could use wire nuts.
Now I do the same w the PL Outside Chassis Frame and Preamp outside frame
(and sometime a 120vac ground wire also)
Now I have a bunch of wires going to the Ground bar and not hooked up.
I play touch them together and every possible way and see if the buss gets better or worse. some never get hooked up
most of the time just the amp and preamp works by them self together.
Good luck, easy to try (also I have to redo this if I move stuff or add stuff LOL)

Other
I also found The 2 PL frame Grounds don't like each other (I have a wood rack)
I found that the Tv has to be Ground to the Computer and my Denon AVR as of today
than I play the wire game and get it to work w no buss

 

[VSAT88]

I use "fine wire" too.