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2.0 General arrangement
All the signal processing circuitry of the Circle5A
amplifier module is arranged on two PCB's - one
'passive' board, the C5AIPM holds the XLR, selec-
tor switch, volume control and phono socket. This
is connected to the C5APAM (filter/PSU/power
amp) board via a 6 way wire loom. 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 secondary of
the mains transformer (25-0-25 AC) is connected to
the bridge rectifiers and smoothing capacitors on
the board. These store considerable energy, and
are capable of creating a spark many minutes after
the C5A's mains power is removed.
The very low level of self noise generated in the cir-
cuitry means that, at idle, there is an extremely low
level of background noise generated in the circuitry.
If the C5A is powered up the front panel power-on
LED fitted to the tweeter face plate just above the
bass unit will glow green, indicating that the speak-
er is powered and awaiting a signal. If the power-on
LED is the illuminated, by definition, the PSU is
working, and the two LED's on the C5APAM will be
glowing, confirming that the +/- 12V rails are opera-
tional.
To minimise power-on and power off 'thumps'
appearing at the drive units, a circuit detects the
presence of mains 50/60Hz cycles, and should the
mains disappear (either because the user has
intentionally or unintentionally switched off the
speaker or because of mains supply failure) the
C5A will revert to muted mode a virtually instanta-
neously and the audio output circuitry will switch
off, and programme will not be heard.
The power-on LED is driven from the rectified regu-
lated supply rail, and is a positive indication that the
transfromer, rectifiers and smooting capacitors are
functioning correctly.
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 OUT-
PUT TERMINALS TO GROUND SINCE THIS MAY
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.
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