We found this circuit in the July 1969 Radio Electronics magazine. The circuit uses a coupler made with a lamp and an LDR. The power of the lamp determines the types of diodes.
We found this circuit in a 1998 documentation. The circuit can also be used as a minutery...
The TAA131 used in this circuit is not very easy to find today, but an equivalent low power audio...
These three circuits have different treble responses. We found them in the February 1970 issue of...