SPA-4S Rev B Operating.indd
This instruction sheet describes the operation of the Sigtronics
SPA-4S intercom system. For information on SPA-4S installation
The SPA-4S series of aircraft intercoms incorporate voice activated (VOX) intercom with “transmit through the aircraft radio
capability” using your push-to-transmit (PTT) switches. In addition they allow for stereo music input.
Three controls are provided on the SPA-4S units:
ON/OFF Switch - Turns the intercom “on” or “off”.
INTERCOM VOL - Volume Control - Controls the intercom volume.
(Except in OFF mode when the intercom is switched off )
INTERCOM SQ - Squelch Control - Sets the intercom turn-on
threshold for voice activated intercom (VOX) mode.
CAUTION - As is standard practice with all aircraft avionics equipment, be sure that the aircraft radio master switch is turned off
when you start up the aircraft engine.
SQUELCH SETTING - To use the SPA-4S intercoms voice activated (VOX) feature, the Squelch Control will have to be set. This
can be performed by the following procedure:
1. To set the Squelch controls it is helpful to have some background noise present and music sources off. With aircraft
electrical power on, set the SPA-4S ON/OFF switch to the “ON”
position. Turn the SPA-4S unit volume (VOL) control to the 10
o’clock position.
2. Turn the Squelch control all the way counter-clockwise. Now,
without speaking, rotate the Squelch control clockwise just
until you hear the background noise in your headset. Next,
rotate the Squelch counter-clockwise a small amount and
wait (approximately 1 second) until the background noise
disappears. Finally, make small adjustments until your voice
triggers the unit at comfortable speaking levels. This procedure is necessary because the squelch is a “Fast-on, Slow-off”
Once set, the intercom stays silent until someone speaks in their
headset microphone (mic). Then it instantly turns on and relays
the conversation. After about a second of no conversation the
intercom goes silent again.
In most aircraft the squelch controls will not need to be set again
unless you change the number or type of headsets used with the
system. In some high noise aircraft it is better to set the squelch
during climb or cruise.
SPA-4SN PUSH-TO-INTERCOM - Install a separate momentary
switch at each headset position. To use the push-to-intercom
feature, turn the squelch control on the intercom fully counterclockwise. Then to activate, press the push-to-intercom switch(s).
See the separate Installation Instructions Figure 8 for switch
wiring instructions.
VOLUME SETTING - The Volume control should be set to the
minimum level for best performance. Intercom Volume levels set
too high allow excessive background noise to enter the headset.
This promotes whispering into the headset microphone. Excessive background noise and whispering combine to degrade what
is called the “signal to noise ratio”. This decreases intelligibility
and interferes with proper Squelch (VOX) operation.
Specialists in “SOUND” Management
Sigtronics Corp. 178 East Arrow Highway, San Dimas, CA 91773 (909) 305-9399
Made in the U.S.A.
Music is controlled at the music source itself. To adjust the
music volume level set the volume control on the music source.
Similarly, to turn on or off the music use the appropriate control
on the music source. When the music source is “on” the SPA-4S
system automatically mutes and restores the music to the headsets at the appropriate time. The SPA-4S default mode of operation is to partially mute during intercom conversations and full
mute during any aircraft radio communications
The Sigtronics SPA-4S systems are designed for use with general
aviation Stereo headsets with high impedance speakers (300 to
600 ohms). Headsets with low impedance (less than 100 ohms)
speakers should not be used with SPA-4S systems without
modification. Contact Sigtronics for details. In general, headsets
with speakers of high and low impedance and/or unmatched
audio efficiencies should not be used together without modifications.
MUSIC MUTE LEVEL DURING ICS - During normal intercom
operation there are two possible muting levels, full or partial.
Partial mute is the factory default. Alternately, “full mute” mode is
user selectable prior to intercom installation. Note: during radio
transmission and reception, the music will always full mute. This
is for communication safety reasons.
Since the intercom is not needed during solo flights, it may be
turned to the “OFF” position. The pilot will still hear the aircraft
radio(s)), since this circuit is always active, and may transmit to
ATC via his headset and push-to-transmit switch. If the aircraft
radio does not provide sidetone, then switch intercom mode to
the “ON” mode. To hear music, the intercom will also have to be
in the “ON” mode.
If power to the SPA-4S unit fails, the pilot can still transmit and
receive on the aircraft radio(s) (assuming the radio(s) are still
functioning). If a problem is ever suspected, switch the intercom
“OFF”. In this mode you will still be able to transmit and receive
on the aircraft radio(s) from the pilot’s position. If the radio does
not provide sidetone, then the pilot will not hear his voice in this
“FAIL-SAFE” or “OFF” mode.
Sigtronics stereo headsets are specifically designed for the aircraft high noise environment and give excellent noise attenuation. They also provide full frequency response stereo for
maximum enjoyment. They are compatible with aircraft mic circuits and can be used as general aviation headsets in aircraft that
are not equipped with stereo headphone jacks. This is because
they include a switch to change from “Stereo” to “Monaural”. No
adapters required.
NOTE: General aviation headset (monaural) phone plugs should
not be plugged into SPA-4S stereo phone jacks. A monaural plug
in a stereo jack shorts out one of the audio channels. This will not
damage the SPA-4S system in any way but will cause reduced
performance. General aviation headsets may be used only if one
of the following three changes are made:
1. Monaural to stereo adapters are used on the headset headphone plugs. (Only monaural music will be heard.)
2. The general aviation headsets are re-wired for stereo reception.
3. Install the Sigtronics SPA-4S system for monaural operation.
See note 6 on page 2 in the “WIRING INSTRUCTIONS” section.
Most automotive stereo units operate from 12V-14V sources. If
you want to use a 12 volt music source in a 28 volt aircraft do
not connect them directly to power. Regulators or converters are
available to permit operation from 24V-28V sources.
Some AM-FM music receivers are capable of causing interference
with aircraft COM and NAV receivers. The aircraft panel should be
placarded accordingly. Most player only units (cassette or CD) do
not cause interference with aircraft receivers.
Line level stereo music outputs can also be used into the SPA-4S
system. Some line level outputs however, are a fixed level and are
not adjusted by the devices volume control. If this is the case you
will not be able to change the volume of the music you hear in
the headsets. Headphone level or Speaker level outputs would
be better in this case.
Sigtronics Corporation
178 East Arrow Highway
San Dimas, CA 91773
Phone: (909) 305-9399
E-mail: [email protected]
Web Site:
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