We found this circuit in a 1980s American documentation. The operational amplifiers of the integrated circuit used can be common types in an experimental setup.
This circuit came out in an English magazine from 1977, but it can be easily assembled today, as...
This Baxandall tone control was obtained in an Argentine publication from the 70s. The circuit uses...
A touch of the X1 sensor causes the motor to be activated for a time that can be adjusted in P1....