Mackie M-2600, Руководство по обслуживанию

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10
Circuit Theory
Much of the circuitry in the M•2600 we hope is self explanatory from the schematics. This
section will explain the unique circuits and architecture. Examples in this section will refer to
Channel 1 for circuitry that is identical on both channels.
INPUT CIRCUITRY
The signal path begins with the INPUT BOARD (055-131-00). 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 a
12dB/Oct. highpass filter. If not in SUBWOOFER mode, and if running in STEREO mode, the
summing amp (U3B) and 24dB/Oct. Likwitz-Riley Crossover (U3A, U2, U4) are bypassed. The signal
is sent via J11 to the gain control on the DISPLAY BOARD (055-132-00). After the gain control, the
signal is buffered by U1A, returned to the INPUT BOARD and routed to the Channel-1 amplifier
board via J6.
Channel 2’s input signal path is electrically identical to Channel 1’s in STEREO mode.
In PARALLEL MONO mode, Channel-1’s and Channel 2’s input signal is summed via U3B and is
sent to both channel’s front panel level controls via SW3A and SW3B.
For BRIDGE MONO operation, these summed inputs are sent to the channel-1 front panel
level control which in turn feeds the channel-1 amplifier and also the inverter U10A. The output of
U10A is routed to the channel-2 amplifier via SW3C. Also in BRIDGE mode, SW3D connects both
channel-1’s and channel-2’s protection circuits together. The result being that when one
amplifier detects a short circuit condition it will mute and immediately tell channel-2 to mute.
Muting lasts around 5 seconds and the amplifier un-mutes until the next shorted condition is
detected.
The limiters (U6, U5, U7 and their associated components) are always in the signal path. When
the limiter is switched off (via SW4) the LDR (Light Dependant Resistor) is in parallel with the
source resistor (R39) and will not reduce signal amplitude. When the limiter is switched on, the
LDR (U5B) is switched as a shunting element to ground. The Baker clamp in the power amplifier
detects clipping and the LDR shunts away driving voltage until clipping almost disappears. This
type of limiter is know as a feedback style (As opposed to feedforward style) of limiter.
U6 is also used for the amplifier input slew clamps, comprised of local +/-5V supplies (D7, D8
and associated) and clamping diodes (D13 and D14). These diodes clamp the voltage feeding
the power amp to around 10V pk/pk or about 6dB above the amplifier’s clipping point. This
ultimately limits the risetime of the signal presented to the power amplifier, controlling common-
mode conduction in the power amplifier output stage.
Output from each crossover can be routed to the rear panel THRU jacks via SW2 and SW6. In
the THRU position the THRU jack is hardwired to the input jacks. In LOW and HIGH output modes,
the lowpass and highpass outputs of the crossover respectively are fed to the THRU jack. In these
modes the output is impedance balanced via R28, R29 and R23.
POWER AMPLIFIER CIRCUITRY
The M•2600 use a class AB triple darlington output stage with complementary output
devices. The output stage and the bias trackers are on the channel boards (129 and 130 boards)
while the voltage amplifier and amplifier protection circuits are housed on two identical v-amp
/ protection (055-170-00) boards. Each channel has local main +/- 100V power supplies, and a +/
- 115V stacked supply for the front end circuitry. Each amplifier also has fl/-16V supplies