Mackie FR Series M-1200, Руководство по обслуживанию

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M•1200/M•1400 SERVICE MANUAL
CIRCUIT THEORY
8
CIRCUIT THEORY
Much of the circuitry in the M•1200/M•1400 is self explanatory from the schematic. This
section will explain the unique circuits and architecture. Samples in this section will refer
to Channel 1 (Left) for circuitry that is identical on both channels.
INPUT CIRCUITRY
Refer to schematic on page 23.
The signal path begins with the INPUT BOARD. Following Channel 1’s input, signal is fed
to a unity gain differential op-amp, U1A. The signal is next sent to U1B which serves as
both HPF and CD horn EQ. If not in SUBWOOFER mode, the summing amp (U2B) and
LPF’s (U3A, U3B) are bypassed. The signal is sent via J11 to the gain control on the
DISPLAY BOARD. After the gain control, the signal is buffered by U7A, returned to the
INPUT BOARD and routed to the MAIN BOARD via J16.
Channel 2’s input signal path is electrically identical to Channel 1’s in STEREO mode. In
PARALLEL MONO mode Channel 2’s input signal is ignored and Channel 1’s input is sent
to Channel 2’s gain control via SW5A. For BRIDGE MONO operation, Channel 1’s input
signal is inverted by U2A and routed to the MAIN BOARD via SW5C.
POWER AMPLIFIER CIRCUITRY
Refer to schematic on page 27.
The M•1200/M•1400 use a class AB triple darlington output with complementary output
devices. The main power supply is +/- 80V, there is also a +/- 90V supply for the front
end circuitry. Channels are muted for 4 seconds on power up as C53 charges via R203.
Muting is achieved by turning off Q73, removing current sources for the differential pairs.
(Wait a minute! PAIRS? Read on…)
An immediately obvious departure from standard designs is the Dual Differentials and
Symmetrical Voltage Amps
. The reasoning behind this front end architecture is
actually quite simple. Transistors transfer characteristics are not entirely linear, so even
the best conventional front end design will introduce some distortion. Most amplifiers
use negative feedback to reduce this problem (creating a few more in the process).
Mackie FR Series amplifiers take a different approach. By using two complementary
“mirror image” front end circuits any distortion caused by non-linear transistor curves is
effectively canceled out in the bias string, without feedback!
Another design feature unique to Mackie FR Series Amplifiers is the
Baker Clamp
. The
Baker Clamp
has two functions; 1) Prevent output/driver transistors saturating, 2) Drive
the LIMITER LDR. Referring to the left main amp schematic, Q27 is a common base
amplifier, it will turn on if Q31’s collector rises more than two diode drops above the
+80V supply, preventing it from rising further. There are three diode drops between
Q31’s collector and the base of driver transistor Q28 (D3, D2, Q29V
be
). Q29 will never
see a voltage greater than one diode drop below the +80V supply, even if the +80V
supply fluctuates. Q27 also drives the limiter LDR LED U2A. The negative side has the
same circuit mirrored.