Matching transistors for electret microphone?
I have some decent electret microphone capsules that I want to modify with phantom-power and balanced cable. This would accept higher sound pressure levels, SPL, and lower the level of hum and noice. A similar mod. could be done with all electret capsules. A circuit for my caplule, UEC-14, is available at:
staffwww.itn.liu.se/~nikro/diy/uec-14-at/UEC-14-AT.pdf
A circuit for other capsules at: http://www.epanorama.net/circuits/mi...g.html#phantom The transistor for my capsule is the BC560C. My supplyer haven't got that one, but recommend the BC860C from Philips or Samsung. They should be matched. The information about matching I've found have been different in relation to how you use the transistor. My circuit doesn't tell which data that should be matched. In a similar circuit it says "should be hand-picked for low noise and matched gain" Is that the same as HFe? My transistors will work at 36 Volt. The simple multimeter use just a few volt to measure HFe. Will that be close enough to match with?
Do you have other comments on this circuit? Are there better transistors for this purpuse? Someone called Chinn says: "My objection to this circuit is its total dependence on the CMRR of the balanced input as neither side of the microphone element is grounded; you must replace the cable with 2C shielded twisted pair" What does that mean? He argues for another solution at: http://www.uneeda-audio.com/pzm/pzm_kit2006.pdf
However, my version of the UEC-14 is different from the one in the circuit because I had to remove the bottom of the capsule to decouple the resistor of 2.2 kOhm. But that's easy.
|
