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Balanced Line Output from MiniDSP 2x4HD (help!)

aasearles

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Having a problem with my first balanced set-up and could use some advice from the more experienced folks.

I have a Loxjie P20 headphone amp, which features balanced XLR 3-pin inputs and balanced 4-pin output. I'd like to feed the balanced inputs by fabbing a custom cable and driving with a MiniDSP 2x4HD that I'm told can be configured for balanced output.

Essentially, the MiniDSP's 4 RCA outputs would be configured this way (#1 and #3 are inverted):
#1 Out: Left channel (-) cold
#2 Out: Left channel (+) hot
#3 Out: Right channel (-) cold
#4 Out: Right channel (+) hot

MiniDSP development team says the configuration (and wiring below) should work.

My custom cable consists of 4 RCA plugs from the MiniDSP outs to 2 XLR 3-pin connectors, like this:
XLR Balanced Wiring Diagram.jpg

I've triple checked the wiring for both cables is as described above. I think this should be correct, but this is my first adventure in balanced cabling.

The problem is, when I switch the Loxjie P20 to balanced input mode, it's acting (in my opinion) as single-ended. Here are the symptoms:

For either L/R channel, if I pull either of the "cold" RCA plugs (#1 or #3) there this no audible change. It's as if they're not contributing to the input.

In the MiniDSP configuration, if I mute, non-invert, or change levels for the same plugs #1 or #3, there is no audible change.

Have verified the MiniDSP is outputting signal from both the #1 and #3 RCAs.

Conversely, if I mute, invert, change levels, or pull plugs for either if the "hot" RCA outputs (#2 or #4), I can hear the expected audible results.

If I pull all RCA plugs (leaving input XLRs connected), touching either of the hot RCA plugs creates hum. Touching the cold RCA plugs makes no sound.

It's as if the amp is operating in single-ended mode using the balanced inputs.

Trying to decide if the amp has a fault, or is my wiring the problem?

Is there a way to easily test the balanced inputs are working?

Thanks in advance for any help.
 
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