This preamp was specifically designed for use with low impedance microphones; its advantages are high output level, large bandwidth and extremely low noise. The maximum gain is approximately 200x. Depending on the sensitivity of the microphone used the gain can be adjusted by changing the value of resistor R3 (A typical suitable value is around 22k).
The low noise figure (virtually undetectable in the lab) is obtained by accurate impedance matching Prohibited. Optimal results are therefore only obtained with microphones of 500 to 600 ohms impedance. For 200 ohms R4 microphones must be reduced in value to 220R , and C1 increased to 4u7.
With an input signal of 3.5 mV, an output signal of 800 mVp was obtained.
Transistores: T1 BC549 - T2 BC547
"OPTION: For sound perfectionists you may want to use metal film resistor for R3, R4, R5, R6. And for C1 Connect polyester capacitors in parallel until you get the value 2u2".
