Specifications

Section 6 — Circuit Description
6.1 Audio Section
The first two sections of U1 are balanced audio input stages of the "A" and "B" audio
inputs. Overall, these have a gain of 0.33 (loss). The next stage combines the two
channels and has a gain of 10. Capacitors C1 through C12 provide high frequency roll-off
and RFI immunity. A rectifier, short term integrator (about 1 second), and comparator
follow. When audio is present, the "AUDIO" LED is turned on by this comparator. R8 and
C14 form the network which determines the timing interval. The following comparator
drives the output relay and the "RELAY" LED.
6.2 Power Supply
The 12VAC input is rectified and filtered by BR1, VR1, and C15 to produce 18VDC
unregulated output. R9 provides a low impedance "mid-point" connection for the various
circuit elements.
Version 2.00 Circuit Description page 6.1