This crystal or galena radio uses a germanium diode. C1 is an old radio variable and L1 has 100 turns of 26 or 28 wire in a 1 inch PVC pipe. The antenna must be a long wire.
Found in a 1960 Sylvania manual, this circuit is intended to measure audio signals frequency....
This circuit is indicated for measurement bridges as a null detector, thanks to the logarithmic gain...
National Semiconductor suggests the circuit in the figure that uses two operational amplifiers of type...