This amplifier, from Philips Databook is suitable for automotive applications. The input resistance is 60k ohms and the frequency range is 20 to 20 kHz.
This circuit of an Electronics Hobbyists Handbook uses a different configuration for a triac. The...
We found this circuit in the July 1988 Radio Electronics magazine. The frequencies are given by...
Found in an English documentation from the 90's, this circuit makes slow logical pulses audible,...