This unusual circuit from a Popular Electronics of the 90s uses LED indicators, operating as a bridge. The components used are common. It is based on the tuning of the signal by a PLL.
This low drift or offset circuit is from an 80's documentation. The configuration can be made with...
This circuit rectifies low voltage sinusoidal signals for measurement purposes, as the circuit is of low...
Indicated for vinyl media recovery, this circuit is from the 80's, having been found in an...