Lexicon MPX G2, Service Manual

Page: 81 / 165

Lexicon
6-3
corner frequency is 20 kHz. This filter takes a differential signal and sums it to make it single-ended.
Therefore its output is 8.2 dBu. R314 forms an attenuator with R315. This brings the signal level back
down to 2.2 dBu.
Q37 and Q36 are JFET switches that crossfade. One fades from off to on when the other fades from on to
off. Q39 is an inverting level-shifter which translates 0 Volts on the input to +5 Volts on its collector and +5
Volts on the input to -15 Volts on its collector. Q38 is also an inverting level-shifter. It tran5 Volts on
Q39’s collector to -15 Volts on its collector and -15 Volts from Q39 to +15 Volts on its collector. These two
transistors together have translated 0 Volts on the net, DSP_SWITCH, to -15 Volts on Q38’s collector and
+5 Volts on this net to +15 Volts on Q38’s collector.
R319 and C335 set the fade time for Q37. Q35 is an inverter, which drives Q36 in the direction opposite of
Q37. R318 and C334 set the fade time for Q36. When one of the diodes (D58, D59) is reverse-biased it
turns on its JFET.
Tone Block
The tone block is found on pages 8 and 9 of the schematic. This is the first circuit to use digi-pots. A digi-pot
is a digitally controlled pot. The digital control signals for the digi-pot are level shifted from 0/+5V range to
the -2.5/2.5-voltage range by U28 (p. 1).
The first digi-pot encountered is the InVol, U43. It allows you to attenuate the signal before the tone controls
so that they can be set to large amounts of boost without distortion. However, when they are set for cut, the
InVol is set to unity to preserve dynamic range. Diode pair D49 limits the signal level to prevent digi-pot
latchup.
The next digipots encountered are the tone controls. These are controlled by the pots on the front panel
labeled Low, Mid and High. These front panel pots are control voltages that are processed by the digital
section. The data generated from the digital section is level shifted and sent to U42-U44. Diode pairs D44,
D45, D47-D51 limit the signal levels to prevent digi-pot latchup. Offset from prior stage that could also
cause a dc drop across the pot is eliminated by C196/C205.
The low control is shelving, has a + 15 dB range. The mid control has +25 dB of range. However, the high
control is 30 dB of boost only. This is to prevent the user from dialing up bad guitar tones. It is a high-pass
shelf.
Q19 and Q20 are JFET switches that crossfade. One fades from off to on, when the other fades from on to
off. Q30 is an inverting level-shifter which translates 0 Volts on the input (GTONE_SWITCH/) to +5 Volts on
its collector and +5 Volts on the input to -15 Volts on its collector. Q31 is also an inverting level-shifter. It
tran5 Volts on Q30’s collector to -15 Volts on its collector and -15 Volts on Q30’s collector to +15
Volts out. These two transistors together have translated 0 Volts on the net, GTONE_SWITCH/ to -15 Volts
on Q31’s collector and +5 Volts on this net to +15 Volts on Q31’s collector.
R195 and C180 set the fade time for Q19. Q32 is an inverter, which drives Q20 in the direction opposite
from Q19. R196 and C181 set the fade time for Q20. When one of the diodes (D52, D53) is reverse-biased
it turns on it’s JFET.
Dynamic Gain
The input to the
DYNAMIC GAIN
(pp.10 &11) is buffered by U40. C185, R176 and U38 are the lo-cut (a.k.a.
Color) filter. This filter is a high-pass filter whose corner frequency is varied by the digi-pot, U41. Diode
pairs D42 and D46 prevent digi-pot latchup. U34b is the drive amp. Its gain is adjustable from 0 dB to 64
dB. R170 and C172 are compensation for U34b. D43 prevents digi-pot latch-up. D39 and D40 are the
clipping LEDs. R169 provides current limiting to protect the LEDs. The output impedance of this stage
changes depending on whether the LEDs are conducting or not. This would modulate the attenuator, which
follows. Therefore, U36a is required as a buffer.