Stereo IN TRS kit shown connected to AMP HEAD headphone amp kit.The only connections needed then are 2 wires for power to make a complete stereo headphone amp.
Stereo TRS kit can also daisy chain AMP HEAD amps together so plugging into the frist AMP HEAD will feed audio to all channels but the chain can be broken into sections by plugging into any Stereo IN TRS in the chain.
TRS holes are 11.5mm spaced 25.4mm (1") apart.
Or
Stereo IN XLR kit shown connected to AMP HEAD headphone amp kit.The only connections needed then are 2 wires for power to make a complete stereo headphone amp.
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All measurements in millimeters