Any one know if the 1/8 jack mic input on the 6400 is hi or low impedance I’m hooking a w2ihy box

Page 13 of the 6400/6600 Hardware Manual indicates 600 ohms nominal then indicates 200 to 10K ohms range.  I have used both a 600 ohm mics as well as an old HI=Z Astatic dynamic mic with no problems. 
------------------------------Here is a snip from the manual.-----------------------------


Microphone Impedance    600 Ohms Nominal, 200-10k Ohm

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The Manual is downloadable from the Flex web site.

GayLe  K0FLY

https://www.flexradio.com/downloads/flex-6400-and-flex-6600-hardware-reference-manual-pdf/


per the h/w manual fro 6400

7.20 MICROPHONE JACK

The MIC connector accepts a 1/8-inch (3.5mm) stereo (TRS) plug and provides a pseudo-balanced microphone input. Pseudo-balanced means that the mic (+) and mic (-) lines are balanced all the way to the input of the A/D converter, where the mic (-) line is DC grounded. The connector may be used with dynamic or electret microphone elements. A software enabled 3V bias voltage through a 2.1k Ohm source impedance may be applied to the Mic (+) line for electret microphones.

Note: The PTT signal is not accepted by this connector.

Paul K3SF

No, the mic runs directly into the radio. The mic input has plenty of sensitivity for almost any dynamic mic.

Your RE320 will work very well.  Just make sure to cut the low frequencies with the TX equalizer.

The transmit voice can be recorded on the Flex and then played back. When adjusting the TX equalizer, cut unwanted frequencies (mostly low frequencies) before trying to boost. What is not there can't be created with boost. After much trial and error, I ended with just a flat EQ curve and adjusting the TX bandwidth to cut below 250 Hz and top end cut above 3000hz or so. Excessive low transmit audio frequencies consume transmit power without adding much intelligibility.