Lexicon MPX G2, Service Manual

Page: 82 / 165

MPX G2 Service Manual
6-4
FEEL GOOD
monitors the signal at the clip LEDs with a peak detector, U35. This signal is amplified, inverted
and offset in a positive direction by U33b. The amount of gain and offset are varied by U31a. This signal is
sent to the cathode of clip LED D39 and inverted by U33a and sent to the anode of clip LED D40. The
switches in U37 are used to defeat
FEEL GOOD
by connecting the LEDs to ground rather the output of the
FEEL GOOD
circuit.
The hi-cut filter (p. 10) comprises R164, C165, U31 and U32. It is a low-pass filter whose corner frequency
can be varied. Diode pairs D27 and D38 prevent digi-pot latchup. U14a (p.11) is the master volume control
for
DYNAMIC GAIN.
It is protected from latchup by D38 of the prior stage. U12, U13, U15, U16 and U17 form
the post-dynamic gain tone-control circuit. The bass control is shelving and has a +/-15 dB range. The
Treble circuit has a +/- 28 dB at 20 kHz. Diode pairs D23-D25 and D28 prevent digi-pot latchup.
Q14 and Q15 are JFET switches that crossfade. One fades from off to on, when the other fades from on to
off. Q16 is an inverting level-shifter which translates 0 Volts on its input to +5 Volts on its output and +5
Volts on the input to -15 Volts out. Q17 is also an inverting level-shifter. It tran5 Volts on its input to -
15 Volts on its output and -15 Volts on the input to +15 Volts out. These two transistors together have
translated 0 Volts on the net, GAIN_SWITCH/ to -15 Volts on Q17’s collector and +5 Volts on this net to
+15 Volts on Q17’s collector. R114 and C108 set the fade time for Q14. Q18 is an inverter, which drives
Q15 in the direction opposite to that Q14 is going. R115 and C109 set the fade time for Q15. When one of
the diodes (D30, D31) is reverse-biased, it turns on its JFET.
Send
The send select switch (p.12) is a 74HC4053. It is an IC containing three digitally controlled SPDT analog
switches. The output from
DYNAMIC GAIN
is buffered by U20a. This signal is split and sent to the send select
switch and to insert left (p.13). The output from
PRE-DISTORTION DSP
is buffered by U20b. This signal serves
as the gain block bypass (a.k.a. clean). It is split and sent to the select switch (p.12) and to insert right
(p.13). C99 and C100 eliminate offset from the prior stages. The send select switch chooses whether
DYNAMIC GAIN
or the gain block bypass (a.k.a. clean) is sent to the send output. Its output is buffered by
U19b and is fed to U14, the send-level digi-pot. This digi-pot is protected from latchup by D26. Offset from
the prior stage that could also cause a dc drop across the pot is eliminated by C95.
U19a boosts this signal by 15.8 dB. This allows us to get outputs as high as +18 dBu. C38 is dc blocking to
prevent offsets from reaching the bypass relay and causing a switching pop. It is 47uF to minimize low-
frequency loss when driving a low-input impedance. R52 provides a ground reference.
Q7 plays two roles: power-on/power-off muting and analog noise gate. R49 forms a voltage divider with Q7.
With Q7 on, this divider provides 36 dB of attenuation. R48, R50, and C36 maintain the cut-off state of the
FET during large negative excursions of the audio signal. D69 prevents this network from turning on the
FET during positive excursions of the audio signal. The gate/mute control signal from the Z80 is level
shifted and inverted by Q43. The Zout of R350, R51 and C417 sets the FET turn on ramp time constant
(ramp from audio out to audio muted). R451 and C417 set the FET turn off ramp time constant.
RY11 is the bypass relay that gives a buffered, low-noise path from the input of the box to send during
bypass. Q60 drives the relay’s coil. D66 protects Q60 from the coil’s flyback and R449/C415 filter the
supply to prevent relay noise from getting on the supply. C416 is dc blocking to prevent offsets from
reaching the bypass relay and causing a switching pop. It is 47uF to minimize low-frequency loss when
driving low-input impedance. R450 provides a ground reference.
C414 provides ac-coupling to the output. R447 provides a ground reference. R49 and 47 provide the 600-
ohm output impedance. D14 is ESD and overvoltage protection. R47 is current-limit protection for D14.
FB6 and C35 prevent RFI emission.