Sorry for the delay
Apologies for not coming back to my own thread...
The amp is working great. The mic that I am using is a Nady HM-10 non-noise cancelling headworn mic. Its not a Countryman but its not an el cheapo pc mic either.
When David Clark says their M7a mic is noise cancelling, they are at least mostly referring to the fact that the capsule is an open design allowing two sound paths to the front and back of the mic diaphram. Ambiant noise flows down both channels and cancels where voice only comes through the "front" passage of the mic assembly. The amp is located in the mic assembly and might have some filtering circuitry; I didn't want to go tearing one appart. Wouldn't know what the components were anyway. But you're right, it sounds like crap.
The aircraft that I fly in have noise gates built into the intercoms so all I had to do was to tweak down the pot until the background signal fell below the threshold during normal operations. BTW the cabin of a flying C172 is about 90-95dB at low altitude (drops as you go up) which is a little quieter than a live stage environment which is what this mic is intended for. If I really wanted to drop the noise I'd need a notch filter and its really not that bad.
I knew that I was going to have to place the mic very close to my mouth anyway so that's how I set it up. It could probably use a band pass filter but more for efficient transmission power usage rather than sound quality. There is no point in transmitting 5khz as the receiving station will filter it out anyway, just wasting power. Although, when I get on the radio, ATC knows who I am...
The only other problem I ran into is that on a TRS mic plug in the studio we use the tip as first signal and ring as second (stereo mics) with grounding on the sleeve but in aviation the mic signal is sent on the ring not the tip. Don't know why but it is.
As far as an open helicopter...? Geez, all I know is that helicopters typically use much lower Ohm mics than GA aircraft, same with military. It should be possible to build a splitter that sets the resistance and output for each type but basically you'd be wearing a little mixer on your belt.
Thanks to all,
Tom.