109A Loudspeaker

This loudspeaker with integrated amplifier was part of the Western Electric 4A (COM KEY 416) communication system.  Per the Bell System Practices (BSP) Supplement 518-450-105, Issue 1, June 1975:

"The loudspeaker set (Figures 9, 10, and 11) includes speaker, amplifier, and two volume controls in a wood housing designed for wall mounting.  They are intended for typical indoor business offices and not outside or high noise level environments.  Customer-accessible individual volume controls are provided for background music level (right control) and page level (left control)."

The 109A uses an IC op-amp labeled "502A" which drives a push-pull output stage (66G NPN and 51A PNP transistors).  Although the BSP does not indicate the supply voltage for the 109A, it does show that the 4A system runs on 24 VDC.  The 109A has a 75uF capacitor rated at 30 VDC across the input power leads, consistent with 24 VDC operation.  The amplifier works well with a power supply of 12 VDC or more.  The 51E, like the 51A is a PNP transistor.

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