Addition of Antisidetone Circuit to Vintage Telephones

Small, low profile
Easy to build
Cut spaces between tracks instead of etching
Straight cuts to make cuts easier
Cheap transformer, Mouser 42TL016-RC, specified to take only 100 volts for 1 minute, but it is okay as it only sees line voltage of 48 volts and not ringing voltage

The antisidetone circuit can be thought of as a transformer with three windings and a line impedance matching network. When a line signal is present, the antisidetone circuit works by applying the line signal to the transformer primaries connected in series and thus the current through the windings is in-phase producing an additive flux and induces a voltage in the secondary winding. When the signal is from the telephone transmitter, the signal is applied to the junction of the two primaries, and the windings are in parallel. Thus the current through the windings is out-of-phase and cancel so there is no flux and no voltage is induced in the secondary winding. The simplified diagrams below help illustrate the local transmitter signal cancellation and do not show the signal that is produced by the transmitter and sent to the line.

Anti-Sidetone Circuit Schematic diagram

Signal From Line Simplified Diagram

Signal From Transmitter Simplified Diagram

1910 Western Electric 20AL 3PPC hook switch (3-pole Progressive Contact)

1910 Phone With Anti-Sidetone Circuit Schematic diagram

Stem Board mounted in 1910 phone

1919 Western Electric 50AL Western Electric type 5H dial 3PPC hook switch Two 1P1T dial off-normal switches Normally-closed leaf connects receiver until dialing, at which time the receiver connection is opened. Normally-open leaf shorts the line during dialing.

1919 Phone With Anti-Sidetone Circuit Schematic diagram

Board with Connector mounted in 1919 phone

1922 Strowger PAX L427A Automatic Electric type 24 dial 1P2T hook switch 1P1T dial off-normal switch Dial 1P1T off-normal switch leaves are open until dialing.

1922 Strowger PAX L427A Phone With AE Type 24 Dial Anti-Sidetone Circuit Schematic diagram

Strowger Board mounted in 1922 phone

1928 Western Electric B1 base mount Automatic Electric type 51A dial with AK-26 AE switch stack or Western Electric type 2 dial 2P2T hook switch 1P2T dial off-normal switch Dial 1P2T off-normal switch center leaf connects receiver until dial center leaf switches during dialing to short the line, at which time the receiver connection is opened.

1928 B1 Phone With WE Type 2 Dial Anti-Sidetone Circuit Schematic diagram

Large Board mounted in 1928 phone

1928 Western Electric B1 base mount 1930 Western Electric D1 base mount Automatic Electric type 51A dial with AK-25 AE standard switch stack 2P2T hook switch 3PPC dial off-normal switch Dial 3PPC off-normal switch leaves are open until dialing. The first two leaves short the receiver before the third leaf connects to short line.

1928 B1 or 1930 D1 Phone With AE Type 51A Dial Anti-Sidetone Circuit Schematic diagram

Large Board mounted in 1928 phone

Small Board mounted in 1930 phone