Lexicon MPX G2, Руководство по обслуживанию

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MPX G2 Service Manual
6-2
In parallel mode, the DSP switches are set to loop and the mix switches are on. This treats the post-
dynamic gain DSP as if it were in a parallel effects loop.
Input
This section is found on page 7 of the main board schematic. There are two input jacks: one on the front
panel (front panel input board scheme) and one on the rear panel. They are wired so that plugging into the
front-panel jack overrides the rear-panel jack. The grounds of the input jacks are connected to the chassis
by small caps (C3 of main bd., C4 of front panel bd.) which are shorts at radio frequencies to permit any
EMI that may get radiated from the jack to be shunted to ground, while at the same time isolating the rear-
panel input ground plane/front panel ground from the chassis at audio frequencies to prevent ground loops.
The input is high impedance so that it will not load down the passive pickups used in most electric guitars.
This 1-Megohm input impedance will pick up quite a bit of noise when a guitar is not plugged in. For this
reason a switching jack is used to short the input to ground when both jacks are empty. C2 (main bd.)/C5
(front bd.) and FB1 (main bd.)/FB1 (front bd.) prevent RFI radiation from the box and give ESD protection.
D1 (main bd.)/D1 (front bd.) provides protection from input overvoltage, including ESD. R1 (main bd.)/R3
(front bd.) is current limiting to protect these diodes. C1 (main bd.)/C1 (front bd.) is a dc blocking cap, which
is necessary, when a piezo pick-up is connected to the input, otherwise bias currents from U1 (main bd.)/U1
(front bd.) would charge up the crystal and it would cease to work.
R2 (main bd.)/R1 (front bd.) gives the bias currents a path to ground and keeps U1 (main bd.)/U1 (front bd.)
operational. R3 (main bd.)/R2 (front bd.) is further current limiting for the opamp input if the input exceeds
the rails. U1 (main bd.)/U1 (front bd.) is wired as a noninverting buffer to keep its input impedance high. The
signal is then sent to the input-level pot, R4 (front panel pot board schematic). C1 blocks any dc that could
cause a current to flow through the pot and produce wiper noise. When R4 is set for no clipping, it has a -
2.2 dBu signal coming out of it. Because 0dBFS of the ADC is 2.2dBu single-ended, 2.2 dBu is the working
level throughout the circuit. The wiper of the pot is buffered by U1.
Next, the signal is split and routed to DSP bypass crossfade switch and to the soft-sat circuit (p. 7 of main
board scheme). U72 is a buffer that prevents soft-sat from loading down the input-level pot buffer. The soft-
sat circuit comprises voltage sources (Q33 and Q34) which turn on when the signal exceeds .88-V in the
positive direction or -0.88-V in the negative direction. This builds in a 4-dB margin before the ADC starts to
clip. R310, R311, R312, and R313 set these turn-on points. R308 forms a voltage divider with R309 to
determine the slope of soft-sat.
The control signals for the soft-sat circuit are created by the signal conditioning transistors found on page
15 of the main board schematic: Q25- Q27 enable and disable the soft-sat. When the soft-sat line is low,
Q26 is off and Q27 is on. With Q26 off, the base of Q33 (p. 7) is at 5 volts, which disables it. With Q27 on,
Q25 is off and the base of Q34 (p.7) is at -5 volts, which disables it. So when the soft-sat line is low, soft-sat
is disabled.
When the soft-sat line is high, Q26 is on and Q27 is off. With Q26 on, the base of Q33 is at .18 volts.
Whenever Q33’s emitter goes over .88 volts, it will turn on. With Q27 off, Q25 is on and the base of Q34 is
at -.18 volts. Whenever Q34’s emitter goes below -88 volts, it will turn on.
U73a and U73b form a differential driver to drive the inputs of the ADC, U76. R306 and R307 form a unity-
gain inverter out of U73b. C326 is compensation. R334 and R335 form the anti-alias filter with C346, as
well as provide latchup protection for the ADC. Because the ADC runs from a unipolar 5-V supply, it self-
biases its inputs to 2.2-Vdc. C327 and C330 block this DC voltage to eliminate a huge current flow due to
this bias being connected to the output of a ground referenced opamp. R337, R338 and C347-350 provide
filtering for the ADC supplies to prevent noise from entering the ADC on its supply lines. The ADC feeds
the pre-dynamic gain DSP.
The pre-dynamic gain DSP goes into the CS4390 DAC, U75. C342-345 and R333 filter its power supply.
U74b and its associated components make up the 3rd-order, differential, anti-imaging filter. The filter's