both transists are low noise types. in the iginal circuit, i used bc650c which is an ultra low noise device. these transists are now hard to find but bc109c are a good replacement. the circuit is very device tolerant will set its quiescent point at roughly half the supply voltage at the emitter of the last transist.the electret condenser microphone (ecm) contains a very sensitive microphone element an internal fet preamp, a power supply in the range 2 to 10 volts dc is therefe necessary. suitable ecm's may be obtained from maplin electronics. the 1k resist limits the current to the mic. the output impedance is very low well suited to driving cables over distances up to 50 meters. screened cable therefe is not necessary.
