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2.0 General arrangement
All the signal processing circuitry of the
Circle5A amplifier module is contained on one
double-sided plated-through PCB. There are no
user-adjustable parts.
Although mains voltages do not appear on the
PCB, but care should be taken when examining
and working on the module, since the sec-
ondary of the mains transformer (25-0-25 AC)
is connected to the bridge rectifiers and
smoothing capacitors on the board.
A mute relay prevents power on and power off
'thumps' appearing at the drive units, and gives
a silent switch on and off. The correct operation
of the mute relay can be detected by carefully
listening to the module as the speaker is pow-
ered-up: after approx. 3 seconds a small
mechanical 'click' should be heard in the vacini-
ty of the heat sink (from the outside of the
speaker), or from the relay, when the module
has been removed.
The very low level of self noise generated in
the circuitry means that, at idle, the best indica-
tion that the speaker is powered and awaiting a
signal is the illuminated power on LED, fixed in
the tweeter face plate.
The power-on LED is driven from the rectified
unregulated transformer secondary, and is a
positive indication that the transfromer, recti-
fiers and smooting capacitors are functioning
correctly. (It does not, directly, confirm that the
12v regulated supply is functioning).
Provision is allowed for optional physical isola-
tion of the audio grounds by use of an input
transformer. Fitting the transformer requires
removal of the PCB.
Two presets - one for bass drive level, the
other tweeter drive level, and both with a range
of approx. ±2dB are pre-set at the factory and
should not be adjusted.
The bass unit is directly driven from a pair of IC
power amolifiers in bridge push-pull mode. The
tweeter is driven from one one IC.
NOTE CAREFULLY:
DO NOT CONNECT EITHER OF THE BASS
OUTPUT TERMINALS TO GROUND SINCE
THIS WILL INSTANTANEOUSLY DESTROY
ONE OR BOTH BASS POWER IC'S. THIS
WILL REQUIRE THE REMOVAL AND
REPLACEMENT OF ONE OR MORE POWER
IC'S, A DIFFICULT AND EXPENSIVE TASK.
USE ONLY BATTERY OPERATED OR
BALANCED AND FLOATING INPUT
ISOLATION TO MAINS OPERATED
EQUIPMENT.
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