LT0371 Issue 1.10
7 February 2018
Page 1 of 6
PA0688
Microphone Pre-Amplifier Module
Installation and Operating Instructions
Description
The PA0688
Microphone Pre-amplifier board is intended to provide emergency and non-emergency
microphone pre-
amplification to “line” level with or without PTT function.
Overview
The PA0688 Microphone Pre-amplifier Module is designed for use with Vigilant EWIS systems; T-Gen2,
T-GEN 50 and QE90. It contains a microphone pre-amplifier with automatic level control, and two relays
which are operated by the PTT switch on the microphone. Refer to Figure 1.
The first relay switches the line level output between (a) line level input terminals (SPEECH IN) and (b) the
output of the pre-amplifier, and can be used to allow several PA0688 pre-amps to be connected in series to
the same panel input. The second relay is a two-pole changeover relay which can, for example, be used to
disconnect speakers in close proximity to the microphone when the microphone is in use to prevent audio
feedback. It can also be used for various other functions, e.g. to provide a switch input to a QE90 to indicate
that Paging is required.
Audio transformers should be used in conjunction with the PA0688:
for isolation, e.g. where the module is to feed multiple T-GEN units
for balancing, in order to reduce noise pickup in situations where long lines are to be used to feed
unbalanced inputs.
Refer to Figures 2, 6 and 7 for details.
SPEECH OUT
SPEECH IN
Relay Coils
+24V
0V
PTT Switch
PTT Out
MIC IN
SPARE
SPEAKER OUT
SPEAKER IN
Figure 1 - PA0688 Pre-amp Block Diagram
Connection and Operation
QE90
The PA0688 microphone pre-amplifier can be connected to the QE90 in many ways. One example is to
provide local paging to several QE90 amplifiers, using the QE90 amplifier local inputs. Refer to Figure 2.
In Figure 2 there are two pre-amps connected in a daisy chain so that either may be used. In the event of
PTT being pressed on both at the same time, the one closest to the QE90 will have priority. The connection
to the QE90 amplifier local inputs has been isolated by a transformer as these inputs are unbalanced and the
use of the transformer will provide a reduction in hum and noise. The SPKR IN+ and SPARE+ terminals on
the PA0688 have been used to obtain clean contacts which close when PTT is operated, and these have
been wired to a GP or FIP input on the QE90 that has been programmed by the manufacturer to disable
background music to the relevant amplifiers when the local paging is in use.