HENRY ENGINEERING
503 Key Vista Drive
Sierra Madre, CA 91024
Tel: 626.355.3656
Fax: 626.355.0077
DESCRIPTION
StereoSwitch is a three-input stereo audio switcher. It accepts up to three stereo balanced audio sources, selecting
one source that is routed to the stereo balanced output. It can be used as a line-level audio source selector, or "in
reverse" as an audio router, sending an audio source to one of three destinations. Audio switching is via sealed
relays; there is no active circuitry in the audio path. The unit can be controlled by any switch or contact closure,
TTL/CMOS logic, or DC voltage. Internal power-up programming permits the user to determine which input is
automatically selected when AC is applied. Tally outputs are provided to drive remote status indicators. Inputs that
are not selected can be automatically terminated.
INSTALLATION
All audio inputs and outputs are connected via the barrier strips.
Connect all audio inputs and outputs to the appropriate barrier strip terminals. For best performance, audio circuits
should be at professional line level, low impedance and balanced.
StereoSwitch can be controlled with any contact closure, or by applying a DC voltage (5 - 24 VDC) from an external
source, e.g., the TTL-level output of a computer interface or a logic circuit. Control inputs can be either momentary
or maintained; StereoSwitch always switches on the leading edge of the input. All control wiring is via the 15-pin D-
sub connector.
To control via contact closure (e.g., an external switch or relay), connect the contacts to the IN- and GROUND pins
of each input as follows: (Do not use to the IN+ pins.)
INPUT #1: Pins 2 & 3 INPUTS #2: Pins 5 & 6
INPUT #3: Pins 8 & 13
To control via an external DC voltage (5 - 24 VDC), it is necessary to remove the internal pull-up resistors for each
input. Refer to the component layout, and remove R3, R7, and R11.
Apply DC control voltages to the IN+ and IN- pins of each input (observe polarity):
INPUT #1: Pins 1 & 2 INPUT #2: Pins 4 & 5
INPUT #3: Pins 7 & 8
StereoSwitch provides a TALLY output to drive external LEDs or other status indicators. (30ma max.) The Tally
outputs are open collectors, which go LOW when the input is active. Connect status indicators between each
TALLY output and the +12V pins on the D connector: Use a 1K resistor in series with any LED status indicator.
TALLY #1: Pins 9 & (10 or 12) TALLY #2: Pins 11 & (10 or 12) TALLY #3: Pins 14 & (10 or 12)
USER OPTIONS
(Refer to the component layout.)
StereoSwitch can be programmed to automatically select one of the three inputs when AC is applied. The Power-
Up programming should be set to select the audio source used most often, or the source most likely to be required
when AC power is restored after a power interruption. To determine which input is selected, install the POWER-UP
jumper in either position 1, 2, or 3.
StereoSwitch is supplied with terminating resistors installed. Inputs that are not selected are terminated with 600
ohms. To defeat this feature, remove the terminating resistors:
INPUT #1: R17, R18 INPUT #2: R19, R20
INPUT #3: R21, R22
OPERATION
Operation is straightforward. Activating any control input will select the corresponding audio source. Switching
always occurs at the leading edge the the control input. Only one source can be selected at a time. Do not
"overlap" control inputs; doing so will lock-out all audio sources. The tally lights on the StereoSwitch front panel
indicate which input source is active. Remote Tally indicators will follow these lights.
If the INHIBIT input (PIN 15) is connected to any GROUND pin, ALL inputs will be OFF; no audio will be sent to the
OUTPUT terminals. When the ground connection is released, the input previously active will return ON. Note that
input selection is still active even if the INHIBIT function is enabled.
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