Maestro C & FHM-2 - The definitive word

Bill Wuttke

I recently received my new Flex Maestro C and wanted to try it out. I connected the FHM-2 microphone that I received with my 6400M years ago to the supplied ‘Y’ cable. The 6400M connected via WiFi keyed, but there was no transmit audio.
So, I initiated a help desk ticket with Flex. The help desk was zero help. So then I pored over messages on the Flex Community board relating to this issue. The most definitive answers were such as, “Flex does not support the FHM-2 with the Maestro C”, or “The FHM-3 wiring is different from the FHM-2.” Why can such a simple thing as connecting a microphone be so much of a problem?
Then, I commenced to find out what the problem is. First I looked through the radio manuals and found that in every radio that I looked at, the mic input was a “pseudo-differential” input with the + on tip, - on ring, and ground on sleeve. The - input is connected to the radio ground at the physical input to the mic audio amplifier section. OK, that is simple enough. Then I looked at the Maestro C mic input. The (+) mic input is on the TRRS jack sleeve and mic (-) (ground) is on the ring2. Therefore I surmised, if the FHM-2 mic + is connected to the TRRS sleeve and the FHM-2 mic - is connected to the TRRS ring2, it should work properly. I looked like the supplied ‘Y’ cable should do the trick. But, it doesn’t. Why? So I broke out my old Radio Shack multimeter to do some continuity checks. Lo and Behold! The ‘Y’ cable mic jack has the tip and ring connected together. This effectively shorts the mic input. I thought this must be a manufacturing error, but I did some research on Amazon, and found that all the commercial TRRS to TRS ‘Y’ cables were wired the same. Flex is using a commercial ‘Y’ cable. If the tip and ring were not connected, all would be fine.
To solve the problem, I constructed an adapter connecting the mic plug tip to the Y mic jack tip and the mic plug ring and sleeve to the Y mic jack sleeve. Works like a champ!
But, the question remains, why does the FHM-3 work with the ‘Y’ cable while the FHM-2 does not? I can only surmise that the engineers determined that the “pseudo-differential” input did not make any difference and used only the mic tip and sleeve for the connection.