Found in American documentation from the 90's, this JFET stage has an input impedance of 500 M ohms. Bipolar transistors are general purpose types and the FET supports an equivalent.
This 1969 Philips documentation circuit makes it possible to control an engine through the sound...
We found this circuit in a 90's Radio Electronics. The circuit can still be easily assembled as all...
This circuit was found in the July 1992 Radio Electronics magazine. Sensitivity adjustment is made...