Mackie M-2600, Руководство по обслуживанию

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that float along with the amplifier output line (D3 & D4 and associated parts on the 129 board).
/-15V supplies are shared throughout the product.
Referring now to the 170 schematic: The amplifier is muted for 3 seconds on power up as C29
charges via R56. When this voltage has charged above 11.6V, U3C goes low turning on Q7,
providing current for the differential pairs. The output of U3C, labeled “MUTE-VA”, when low turns
on the
V oltage A mplifier. Two seconds after MUTE-VA goes low, mute “MUTE-OP” goes low (Q27)
closing the O ut P ut relay passing signal along to the output terminals. (Wait a minute! Differential
Pairs? Read on…)
An immediately obvious departure from standard designs is the Differentials and Symmetrical
Voltage Amps . The reasoning behind this front end architecture is actually quite simple.
Transistor 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 transistors saturating, 2) Drive the LIMITER LDR.
Referring to the again to the 170 schematic: Q20 is a common base amplifier, it will turn on if
Q2’s collector rises more than three diode drops above the +100V supply, preventing Q2’s
collector from rising further. There are three diode drops between Q2’s collector and the base of
pre-driver transistor (Q26 on 129 board). Once the output from the collector of Q2 makes it to
the emitter of the output devices, there will be a total of six diode drops in series. Three “drops”
up and six down: As a result, the output transistor emitters will never see a voltage greater than
three diode drop below the +100V supply (even if the +100V supply fluctuates) and will never
saturate. This will consequently remove saturation of the output stage, as a source common
mode conduction. When the amplifier clips, Q20 and Q15 also send out “clipping pulses” at
their collectors that is coincident with baker clamp conduction. These clipping pulses are cross-
coupled to Q21 and Q22 and are stored in C47 and C51. These stored pulses allow for quick
attack and slow release required by a peak limiter. Output from Q21 and Q22 is coupled to the
LDR LED section via R110 and R152.
PROTECTION CIRCUITS
The M•2600 has several protection circuits in addition to main rail fuses on each channel.
Circuits mute the amp if the output devices are loaded beyond their safe operating area (SOA).
If there is DC on the output line, output relays open, disconnecting the load. The amplifier will
also mute if the power transformer or main heatsink get too hot, if the input AC line voltage is
low, or if either of the +/-15V supplies fails.
Turn-On Inrush Current Limiter
For the first second of operation there are two 15W power resistors (located on the 133
board) in series with the power transformer which eliminates what would otherwise be an
extremely high turn-on current. After 1 second these resistors are shunted with a relay. This relay is
turned on when the collector of Q26 (170 board) goes high. Note that either or both Q26’s on
the two 170 boards can turn on this relay.