PS Engineering
PMA7000M-S Series Audio Selector Panel and Intercom System
Installation and Operator’s Manual
200-070-0006
Page 2-3
Rev. 6, Nov 98
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 PMA7000M-S 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 at-
tenuation, 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
PMA7000M-S for the power supply to work in its designed regulation. Otherwise, it can-
not adequately attenuate power line noise. Shielding can reduce or prevent radiated noise
(i.e., beacon, electric gyros, switching power supplies, etc.) However, installation combi-
nations can occur where interference is possible. The PMA7000M-S 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 cyclic loads such as strobes, inverters, etc.,
can inject noise signals onto the airframe that are detected by the audio system. Follow the
wiring diagram very carefully to help ensure a minimum of ground loop potential. Use
only Mil Spec shielded wires (MIL-C-275000, or better).
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.
2.4.2 Existing KMA-24 Installation
If the installation replaces a KMA-24 (series -01, -02 or -03), the existing 44 pin connec-
tor can be used for the bottom connector of the PMA7000M-S tray as is, providing it is
properly installed and wired. No other changes are required except for external marker
lights (see Section 3.7.2 for details). The "key" in the existing connector must be located
between pins 7 and 8. This connector will be used in the bottom connector position. (See
Appendix for complete wiring harness details.)
The existing ground bus may be reused for radio shield connections, if it was constructed
so it can be relocated to the PMA7000M-S tray.
2.4.3 Existing PMA6000 installations
In 28-Volt aircraft, the dropping resistor may be removed, however, the 2 Amp breaker
should be changed to 3 Amp. If both old unit is stereo (PMA6000S or PMA6000MS), no
rewiring is necessary unless additional features are added.