FSG 90 System
3 Installation
December 2014 FAV_D10024 27
3.6.2 Microphone Connection
Independent of each other, almost any non-amplified dynamic microphone, or amplified
Electret or amplified dynamic or carbon standard microphones can be connected to the
FSG 90 System, without internal jumper setting or soldering.
A amplified/carbon microphone is connected to pin 18, a dynamic microphone is
connected to pin 5 of the 25-pole D-SUB receptacle. Avoid any ground loop in
microphone wiring (dynamic non-amplified mike shield shall not be connected to any
other aircraft wiring, except directly at the assigned pin in the accessory connector).
Up to two IDENTICAL microphones for pilot and co-pilot can be operated in parallel
simultaneously. Even two low impedance (4 Ohm) dynamic microphones may be
connected in parallel as well. However, for microphones with less than 30 Ohms, it may
be necessary to use a suitable microphone transformer.
The associated microphone ground (shield) for dynamic microphones must be
consequently routed galvanically isolated, in order to avoid any ground loop.
This means, dynamic mike ground/shield must be separated from any other grounds for
DC input or DC output, AF input and AF output like speaker, phone, PTT and external
Audio).
In this context, due to the extremely wide mike input level adjustment range, it is
mandatory to understand that correctly matched FSG 90(X) mike input sensitivity (SET-
UP) will be good ONCE FOREVER for this configuration and to ensure stable operation
without audio feedback.
IMPORTANT!
NEVER use the 3-pole mike connector type PJ-069 when using non-
amplified dynamic microphones, because ground for mike and PTT is
identical. This would create a ground loop and cause unstable / distorted
modulation. This PJ-068 is allowed only for amplified/carbon mikes.
However, the PJ-068/PJ-55 connector system must be regularly cleaned;
contacts shall have sufficient spring-loaded force. This is especially
mandatory under vibration operating conditions.
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