This common circuit using operational amplifiers was found in the 1993 issue of Radio Electronics Special Projects. The circuit is sandwiched between the pickup and the amplifier input.
In this transmitter for telephone conversations, found in a 1985 publication, the L1 coil has 4...
The circuit shown is only for analog telephone lines. I found it in Popular Electronics (1991). It...
This circuit is intended to manipulate a transmitter (CW) from coded audio signals recorded on a...