Mic Preamp

© 2003-2011 by Harold Melton, KV5R. All Rights Reserved.

microphone audio preamp for all occasions

Text and schematic by Anthony Gargasz, KB8WOW

This nifty little audio preamp can be used for just about anything you can dream up, from microphones, QRP rigs, even guitars.

An emitter resistor value of 390 ohms works well in most cases.

All resistors should be 1/8 watt, 5% tolerance. Regular carbon composition types are OK. Use metal film types if you require ultra-low noise operation.

The emitter bypass capacitor can be eliminated for low source impedance devices such as dynamic microphones and guitar pickups. High impedance devices such as crystal and electric condenser mics will require the capacitor. Although a 10 mfd. electrolytic is a good starting value, you may use up to 100 mfd. to get maximum AC gain. Be judicious, however, too much gain in a single stage invites hum and oscillation instabilities. The capacitor should have a 35 VDC rating.

Want to experiment with the frequency response? For low impedance sources try reducing the value of the input capacitor (not past 0.01) for more treble. Increasing the output capacitor (up to 4.7 mfd.) will boost the bass. There is allot of gain interaction when you change the capacitor values, so be prepared for some trade-offs.

Construction is not critical, perf board works great. There is only one rule-of-thumb, keep all component leads that attach to the transistor as short as possible.


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