0704-aa.ibw.doc page 6 of 23 25/05/98
TECHNICAL DESCRIPTION
The block diagram (ref.. 802827) shows that the AA-704 comprises:-
a) Two separate input stages, "Left" and "Right. Each input stage converts its balanced high impedance input to an
unbalanced low impedance output. The gain of these stages can be varied by 12 dB in two 6 dB steps by
changing the stage gain links.
b) A single optional 'plug-in' area. Should a plug-in option be installed Links 1 and 2 should be cut.
c) Two front panel adjustable gain controls. Links are provided after the gain controls to feed the Left or Right input
to both output stages for Mono operation, or the Left input to the Left output and the Right input to the Right
output for stereo.
d) Two separate output stages. Each converts its unbalanced input to 5 resistively isolated balanced outputs.
e) Power Supply:- See detailed description following.
Power supply:
Input power to the module may be fed by a number of means.
1. One or two 14 - 0 - 14 Vac supplies via connections on the 64 pin DIN rear connector of the module.
(Used when module is mounted in FR-700 chassis or FR-748A chassis fitted with one or two PT-701
PSU’s.)
2. Two ±16 Vdc supplies via connections on the 64 pin DIN rear connector of the module. (Used when
module is mounted in FR-748A chassis fitted with one or two PT-748A PSU’s.)
3. One 14 - 0 - 14 Vac supply and one ±16 Vdc supply via connections on the 64 pin DIN rear connector of
the module. (Used when module is mounted in a FR-748A chassis fitted with 1 x PT-701 PSU and 1 x
PT-748A PSU.)
4. A single 14 - 0 - 14 Vac supply connected to SK 2 on the rear assembly. (Used when module is mounted
in FR-722 chassis.)
The input power is isolated by fusible resistors F 1 to F 4. Should these fail they should be only replaced by a similar
type of the same value or protection of the common power supply will be compromised.
If an AC supply is provided, the isolated AC is full wave rectified by diodes D 1 to D4 and D5 to D 8 to provide a
raw DC voltage of approximately 20 V at filter capacitors C 15 and C 16. The actual voltage will vary depending on
the type of frame, loading of PSU by other modules and the local mains supply voltage.
If a DC supply is provided, the isolated DC passes through the diodes D 1 to D4 and D5 to D 8 which provide
protection against accidental reverse polarity connection of the DC supply. Where a DC supply is used the input
voltage must be at least ±15 Vdc under all conditions to ensure sufficient margin for the proper operation of the
following voltage regulators and losses in the input fusible resistors and diodes.
The raw DC power is fed to three terminal regulator IC’s which provide ±12 Vdc rails for the operating circuits.
The LED on the front panel is wired in series with a zener diode between +12 and -12 Volts. The zener is to ensure
that the LED extinguishes if either one of the regulators fails.
Wideband modification:
(For use with timecode, composite stereo and other extended frequency response signals.)
Capacitors C 11 & C 13 in the feedback loops of U 3A & U 4A respectively provide high frequency rolloff above 20
KHz. If a higher frequency response is required the value of C 11 & C 13 may be decreased from 120 pF to 22 pF.
This will provide a useful response to 100 KHz ±0.5 dB.
