Transformer Hum Source 'Unmasked'

Post from @atomicbob copied from another thread here. Original first post is below.

- @rhythmdevils

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For those of you experiencing hum with ZDT Jr try this.

Turn ZDT Jr power off. Disconnect power cord. Disconnect RCA cables.
Is hum still audible?
If so, while wearing headphones move JDT Jr around several cm / inches. Does the hum increase or decrease as moved around?
If true, there is a source of magnetic field nearby. ZDT Jr output transformers are sensitive to magnetic fields. Using ZDT Jr as a detector the source of magnetic field may be identified.
Now move ZDT Jr away from magnetic field source until hum is no longer heard. Or alternatively move the magnetic field source away from the ZDT Jr. until hum is no longer heard.

A common magnetic field source that many on this forum have is any Schiit DAC or Amp with wall wart transformers or the Cthulu. If ZDT Jr and transformer are located within approximately 17 cm hum will be heard. This distance varies with size / power of transformer involved.

Modi MB transformer wall wart at 7 cm


Modi MB transformer wall wart at 17 cm


Hum measured -64 dBu with Modi MB wall wart transformer at 7 cm


Hum measured -75 dBu with Modi MB wall wart transformer at 17 cm


Using Sennheiser HD800 with sensitivity of 98 dBSPL / mW and 300R impedance (ignoring frequency response for this calculation) the following SPL will be achieved:
-64 dBu produces 37 dB SPL
-75 dBu produces 26 dB SPL

37 dB SPL is audible in quieter ambient settings.
26 dB SPL is on the threshold of audiblity in my acoustic lab.

 

I'm posting this in the hopes that it can help anyone that is trying to find the source of hum in their system.

At present, I'm evaluating a Decware tube output stage for my Gungnir DAC, and running it with the cover removed. My setup starts with a Pi2AES to the Gungnir, balanced out's to a DIY balanced-to-SE converter (Jensen trannies)/passive preamp, to a Loki+, then to my DIY Decware SE84 clone using Transcendar heapdphone specific output trannies.

I have a bit of hum from the amp, but it is tolerable. A couple of days ago I rearranged my shelf system (Flexi rack) to put the DAC on the shelf below the amp instead of two shelves below. That evening after I shut everything down except of course the DAC, I could hear a pronounced hum mostly in the right channel. I thought that was quite odd since everything was powered down. I disconnected the inputs to the amp, but the hum was still there, so then I unplugged the amp from the power; still hum. Looked around the room to see if there were any ghostly figures creating havoc, but nothing. Then I grabbed the amp and slowly withdrew it from the shelf, and the hum slowly disappeared. Put the amp back and turned off the DAC, and the hum was gone. Apparently, the amp's output trannies which are mounted inverted inside the amp in the front, are picking up the magnetic fields from the two trannies in the DAC, and doing it through the shelf, which is made of 1.5" of mdf.

The amp btw was placed directly over the DAC in the shelf, with the pre and Loki to the right. The DAC's trannies are on the far left of the DAC. I switched the positions of the amp and pre/Loki, and now there if a barely perceptible hum (if the world shuts up for a moment).

 

More information on the interaction between power transformers and amp output transformers can be found here

 

Thanks. I kinda' figured that someone else here had posted about it. Makes me wonder if it's feasable to replace the DAC's trannies with toroids.

 

Your post draws attention to how transformers in the signal path are potentially susceptible to interference from power transformers located nearby. When hum is noticed on an amp, removing power from the amp and listening for hum persisting is a great diagnostic step. Physical arrangement matters, as you proved.

 

It seems that the Loki Mini+ is also producing hum, as Schiit states, because of the inductors inside. It's picking it up from the power tranny on my amp which it's next to. Maybe time to build an eight foot tall rack out of steel plate. At least the Jensen trannies on the back of my passive pre didn't join in the 'hum show'.