Use this adapter to connect a headphone cable with a 2.5mm TRRS for Astell & Kern or a 3.5mm TRRS for Hifiman HM901 or RSA/ALO/Cypher Labs connector to a 4 pin Male XLR.
Converts below to a 4 pin male XLR:
Female RSA connector
3.5mm TRRS connector (can be used as an adapter between the HiFiMan HM-901 and a balanced amp with 4 pin XLR connector)
2.5mm connector (can be used as an adapter between the Astell & Kern AK240, AK120 II, AK100 II and a headphone amp with 4 pin XLR connector)