A preamp with a.g.c. and a power amp with switched
filtering for communications and surveillance

With a preamplfier designed around discrete components this project should overcome the problem of specialist i.c.s disappearing from the hobbyist market that has dogged previous projects published over the years.

Using widely available transistors, the design featured here is sensitive enough to permit the direct connection of dynamic microphones, effective in compressing the dynamic range, and has low noise and distortion levels.

Also included is a single i.c. (TBA820M) audio amplifier with switched filtering. Teamed with the Preamplifier, the two circuits will considerably improve the performance of simple communications receivers and, when used with electret microphones, will ensure good performance for surveillance purposes.