For test flying purposes I use an external mic on my Panasonic dictaphone. If you get the sort of mic which attaches to a telephone (sticks on with a suction cup) this is just the job. It can be placed inside the ear-cup of a headset or flying helmet and is quite comfortable to use. However, it will pick up some extraneous noise. But it will always work, whatever the system impedence of the aircraft being flown.