High-pitched hum in the headphones output when KeyStep is connected (except when I make skin contact)

#1

So I'm getting a high-pitched hum in just the headphone output when I plug in my KeyStep via USB.

  • I am using a choked USB cable from my Axoloti to my computer.
  • I am using a non-choked USB cable from my Axoloti to my KeyStep.
  • The noise goes away when I unplug my KeyStep.
  • The noise gets louder when I swap the cables.
  • The noise does not go away when I use a USB phone charger instead of my computer.
  • The noise gets quieter when I make skin contact with the metal backplate of my KeyStep.
  • The noise goes away when I apply pressure to the metal backplate (skin contact optional).
  • The noise goes away when I flip the KeyStep upside-down.
  • The noise sometimes goes away when I move my cables around (I can't find a pattern).

My KeyStep is modified to have an Axoloti (sans-midi/power) embedded inside it. I covered the metal parts up with a couple layers of scotch tape, so there shouldn't be any shorting or anything. The odd thing is, the noise is only present in the headphones out. I really hope this problem can be fixed, I'm using the line out to power a small phone speaker so the gain has to be as high as possible (not good for ears!).

#2

I would use a cable with the RF choke on both places. That is a clue to a badly shielded cable or connection. You hear of this all the time with USB connections to Beat Step Pros and other USB peripherals. Since it is a high frequency, it is not a ground loop (the other USB problem).

#3

If no other cable arrangement helps, try an (ADuM4160 based) USB isolator on one of the ports.

#4

It's high pitched, but it is a multiple of 60hz. I thought it'd be related.

#5

If the unit has a switching power supply (no big transformer) then that is a real possibility. Switching supplies are notorious for noise and would be filtered less on a device that doesn’t produce audio to worry about.