Have made a number of changes to the original circuit. For one the original 5 volt supply permitted only very little output power even when overdriven, so this circuit uses a 12 volt supply. Linearity was pretty poor in the original and has been improved with a 180ohm emitter resistor in the 2nd voltage amplification stage. This reduces overall sensitivity and an additional gain stage could be added at the input if millivolt sensitivity is needed.
Original circuit from: www.techlib.com
