This circuit was found in the January 1999 issue of Electronics Now magazine. The circuit uses an integrated circuit from the period and has earphone listening.
This circuit from an old Motorola manual uses an integrated circuit that is no longer easily...
This step was found in the 1960 Mullard transistor reference manual. The transistor can be a BC558...
This circuit is from the Philips Databook of 1990. The circuit has an input resistance of 60k ohms...