Got a 706 which has a 500 or 600 ohm mic input impedance. I have a D104
which
best I can tell has a high output impedance.
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Don't know the D104 mic ( Shure ?) However many socalled High impedence
microphones are dynamic low imp. mics. with a small transformer fitted
inside the mic. housing. Examples : Mics supplied with Yaesu valve
transceivers from the 1960 -70s.
By disconnecting the transformer it is just a 500 - 600 Ohms microphone.

Frank GM0CSZ / KN6WH