Phone circuit (CB8709E)

With this circuit, found in a 80s Popular Electronics, it is possible to use two ordinary phones in a basic intercom.

Tunnel Diode and Transistor Oscillator (CB1340E )
The circuit shown is a documentation on 80's tunnel diodes. The frequency depends on the delay line...
Complete Wien Bridge Oscillator (CB22382E)
This circuit generates excellent sinusoidal signals and has the frequency adjusted by VR1 which is...
RIAA Preamplifier III (CB958E)
This circuit was adapted from an Italian publication that no longer exists. This is “Onda Quadra”...