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Flex6000 series microphone electrical options, including FHM-3
I'm working on some audio interface hardware to work with Flex radios (and others.) I'm seeing two very different signal levels coming out of two microphones that I expect to be very similar:
* FHM-3, stock hand-held mic that came with my Flex6400.
* A "PC Gaming Headset." Mic+ and Ground on the headset, wired directly to Mic+, and both Mic- and Ground on the Flex mic port.
They're both electret microphone elements, both require the "DC Bias" turned on. But the FHM-3 requires the "+20dB Boost" and still has the Mic Gain set to around 60% to sound right.
The HEADSET, with +20dB Boost enabled, I have to turn the Mic Gain all the way down to 0, and it's still compressing a fair bit. The output from the FHM-3 is A LOT lower than the output from the straight electret mic element in the headset.
Looking inside the FHM-3, there is a resistor divider (and some filter caps, but I don't think they're impacting this) that's cutting the DC bias voltage across the Electret element down to .7vDC, which is AWFULLY low. That'll lower the gain a lot. Between the lower DC bias actually across the mic element, and the voltage divider itself, that would explain why the FHM-3 has such a low output.
It's acting like the FHM-3 is trying to have the output of a dynamic mic, but still requires the DC bias voltage of an electret mic.
WHY is this? Does anyone know?
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