I found tis circuit in na old documentation from 1958. The circuit uses germanium transistor availble that time and hard to obtain today. The circuit is indicated for phono applications using variable reluctance transducer.
This basic configuration can be found in a lot of articles on this site. In this case, the relay...
This circuit with the 741 operational amplifier was found in the March 1995 issue of Electronics Now...
This sound relay or Vox was found in 70s RCA documentation. SCRs can be common types today and...