Modulated light receiver (CB21790E)

We found this circuit in a Mullard manual from 1960. The transistor is a phototransistor of the time. The gain is 3 dB.

Electronic Crossover (CB20648E)
This circuit separates the bass from the treble in an audio signal. The circuit is from a Hands On...
Car Sound System (CB17339E)
This circuit was obtained in a 1986 documentation consisting of an active frequency divider for...
Voice Scrambler (CB22462E)
We found this circuit in a Popular Electronics from 1992. It consists of a voice scrambler used in...