This adapter is designed for radios that do not have DC voltage for bias on their mic connector.
This also resolves the 'issues' with Kenwood's TS-590 noted by W1AEX on his website,. Good luck fitting all that in the mic connector.

Kenwood - 4 pin modular or round
TS-T599, TS-120/130/520/530/820/830 and now for the TS-590
Yaesu - 8 pin
FT-747, 757, 767, 736, 840, 890, 990, FT1000 or FT1000D
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