PS Engineering PMA7000H Installation Manual - Manual (Page 10)

PS Engineering
PMA7000H Series Audio Selector Panel and Intercom System
Installation and Operator’s Manual
200-780-0429 Page 2-3 Rev. 4, Dec 2012
2.4.1 Noise
Due to the variety and the high power of radio equipment often found in today's general aviation aircraft,
there is a potential for both radiated and conducted noise interference.
The PMA7000H power supply is specifically designed to reduce conducted electrical noise on the aircraft
power bus by at least 50dB. Although this is a large amount of attenuation, it may not eliminate all noise,
particularly if the amplitude of noise is very high. There must be at least 13.8 VDC present at the bottom
connector, pin 20, of the PMA7000H for the power supply to work in its designed regulation. Otherwise, it
cannot adequately attenuate power line noise. Shielding can reduce or prevent radiated noise (i.e., beacon,
electric gyros, switching power supplies, etc.) However, installation combinations can occur where interfer-
ence is possible. The PMA7000H was designed in a RFI hardened chassis and has internal Electromagnetic
Interference (EMI) filters on all inputs and outputs.
Ground loop noise occurs when there are two or more ground paths for the same signal (i.e., airframe and
ground return wire). Large power loads such as strobes, inverters, etc., can inject noise signals onto the air-
frame that are detected by the audio system. Follow the wiring diagram very carefully to help ensure a min-
imum of ground loop potential. Use only Mil Spec shielded wires (MIL-C-275000, or better). Under no
circumstances combine a microphone and headphone wiring into the same shielded bundle. Always use a 2-
or 3-conductor, shield wire as shown on the installation wiring diagram.
Radiated signals can be a factor when low level microphone signals are "bundled" with current carrying
power wires. Keep these cables physically separated. It is very important that you use insulated washers to
isolate the ground return path from the airframe to all headphone and microphone jacks.
Adding a high-performance audio control system, particularly in conjunction with high-performance active
noise canceling headsets, cannot improve on older avionics that were designed for cabin-speaker use. PS
Engineering makes no claim that the audio panel will provide a noise-free audio quality under all installa-
tion conditions, particularly with older avionics.
2.4.2
Power
The PMA7000H are compatible with both 14 and 28 Volt DC systems. A two (2) Amp circuit breaker is
required for 14 VDC installations, and a three (3) Amp breaker for 28 VDC aircraft. Power and ground
wires should be a twisted #18 AWG pair. Connect airframe power ground to J1 (bottom connector) Pin Z
only. No dropping resistors are required.
2.4.3
Audio Panel interface
The PMA7000H is designed to interface with standard aircraft avionics, and presents a 500  receiver im-
pedance. For best results, a twisted-shielded cable is recommended from the avionics audio source to the
audio panel, with the shield grounded at the audio panel end.
Some avionics do not provide a separate audio low, and may introduce additional electrical noise into the
system. For best results, connect the audio low from the audio panel to the radio ground, using one conduc-
tor of the twisted-shielded cable.
2.4.3.1 Fail Safe Audio
When the audio panel is off, or power is removed, the pilot’s right headphone will have the COM 1 audio,
and the Unswitched Audio #1 (J1-17). It is imperative that Pin 3, J1 (bottom connector) be connected to
pilot’s headphone tip contact.
2.4.3.2 Headphone Audio Output
The PMA7000H supports seven passengers, plus 2 crew. The passengers’ headphone outputs are provided
on two amplifier outputs in J2 (top connector); passengers 1, 3, 5 and 7 are connected to 12 (left) 11, (right)
WRT N. Passengers 2, 4 and 6 should be connected to 19 (left) 17, (right) also WRT N.