I measure my distortion with a FFT analyser and a super low distortion sound source directly coupled to the capsule ...
Is this a B&K 4231 Calibrator?
bv
I dont want to give away all my little secrets.... The name and make of the device shall remain my little secret but its not a regular calibrator if that is what your asking. 
Just my opinion, but I don't like this secrecy. I've said this before, and I'll say it again, there is lots of room for both vendors and diy types here. Hiding this information does no good. People in the know (competitors) will figure out how to do this anyway. You (Chris) are offering a great product at a great price. And you're constantly innovating, with new products that we (think we) need. That is what sells your product. IMO this is what puts you above others (like Soundpros, Core sound, and sonicstudios, for example).
Richard
Just as a comment you can DIY a small Hi-Q piston chamber and get some very low distortion at very high SPL (without ear damage
. Exciting it at resonance gives excellent extra rejection of harmonics. I used a 2.5" Yamaha driver scavenged from a computer speaker and a 3-4" piece of heavy mailing tube. It measured <.1% at 140dB. I also used a medical grade MEMS pressure transducer as a measurement device. It makes a VERY insensitive and fairly low frequency device (15 microV/Pa, <3kHz) but it is calibrated to the equivalent of .1dB, has response to absolute DC, and using it way below its rated range it is inherently very linear. I was able to easily measure several mics all the way to hard clipping. Obviously this is only for omni's.
The second way to measure THD is to use two tones and two speakers and derive the mic's transfer function from the IMD spectrum which can only come from the mic (pretty elegant). This is documented in a couple of AES papers.
As for commercial interests polluting DIY, Chris is definately one of the good guys with the white hats.
Topic: Battery/PiP powered jfet buffer for TSB-120a --> Korg MR-1 (Read 16085 times)