Matrix Audio HPA-3B - Manual (Page 7)

6
The definition of 4-pin XLR headphone interface：
The 1/4" headphone jack is a stereo interface, please do not connect the
mono plug into it, otherwise it will have the risk to damage your device.
HPA-3B can eliminate the power on/off impact noise with its own
protection circuit, if a failure caused the amplifier output a higher voltage
that the working LED indicator will be off, in this case, please disconnect
the headphone plug quickly from the HPA-3B to protect your headphone.
Settings & Usage