Total beginner question please - XLR ground loop

[pedalhead]

Hi all. I'm afraid I'm a total electronics dummy, but nevertheless I'm trying to troubleshoot a ground loop I'm getting on the XLR inputs of my headphone amp, but could use some assistance please 🙂. (btw I've elminiated the DAC and the cables, power etc. as the source of the issue).

I only hear the hum when using the amp's XLR inputs, the RCA inputs are fine. The amp simply appears to have the XLR inputs wired into the RCAs, as follows:

XLR chassis ground connection ok.
XLR socket pin 1 --> RCA socket signal ground, but also connected to XLR pin 3 (negative) via a resistor (Dale 1210J)
XLR socket pin 2 --> RCA socket positive

I've read up a bit and it seems that pin 1 should actually be connected to chassis ground (although this seems debatable)?

Any thoughts please on whether this is inherently a ground loop-prone configuration and if so what I could do to fix it? Thanks!

 

[DF96]

My understanding is that XLR pin 1 should go to chassis.
Pin 2 to RCA signal.
Pin 3 to RCA outer/'ground'/return. This may need a resistor, if the source is merely bi-phase and not truly balanced.

 

[pedalhead]

Thanks for the reply. The source is a fully balanced DAC (DiDiT DAC212XLR). I'm guessing the resistors bridging pin 1 & 3 are to attenuate the XLR signal so it's similar to the RCA volume (which it is).

I'm wondering if the fact that XLR pin 1 is connected to RCA signal ground could be causing a ground loop on the XLR inputs...?

 

[rayma]

I'm wondering if the fact that XLR pin 1 is connected to RCA signal ground could
be causing a ground loop on the XLR inputs...?

It could, disconnect the wire to pin #1 and see.