190-00917-01
GMA 240 Installation Manual
Rev. D
Page 2-5
detected.
MUTE ON COM TX: This input configures the audio panel to mute all selected radios (COM, NAV,
AUX) except the mic selected COM (COM selected for transmission) when a PTT key is activated for
COM transmit.
2.4 Noise
Because the audio panel is a point in the aircraft where signals from many pieces of equipment are brought
together, care must be taken to minimize effects from coupled interference and ground loops.
Coupled interference can sneak into audio system interconnecting cables when they are routed near large
AC electric fields, AC voltage sources, and pulse equipment (strobes, spark plugs, magnetos, EL displays,
CRTs, etc). Interference can also couple into audio system interconnecting cables by magnetic induction
when they are routed near large AC current-carrying conductors or switched DC equipment (heaters,
solenoids, fans, autopilot servos, etc).
Ground loops are created when there is more than one path in which return currents can flow, or when
signal returns share the same path as large currents from other equipment. These large currents create
differences in ground potential between the various equipment operating in the aircraft. These differences
in potential can produce an additive effect at an audio panel signal input.
The audio panel may "see" the desired input signal plus an unwanted component injected by ground
differentials, a common cause of alternator-related noise. This is the main reason why all audio jacks
should be isolated from ground. Terminating audio shields just at one end eliminates another potential
ground loop injection point.
Single-point grounding cannot be overstressed for the various avionics producing and processing audio
signals. Single-point, in this context, means that the various pieces of equipment share a single common
ground connection back to the airframe. Good aircraft electrical/charging system ground bonding is also
important.
The wiring diagrams and accompanying notes in this manual should be followed closely to minimize noise
effects.
