Lexicon MPX G2, Service Manual

Page: 83 / 165

Lexicon
6-5
Aux Inputs (including Speaker Simulator)
AUX INPUTS
The aux-input circuit is found on page 13 of the main board schematic. The aux input jacks, J8 and J9,
each have a switch that senses when a plug has been inserted and sends out a logical high on the
corresponding insert status line. C39, C44, FB7, and FB8 prevent RFI radiation from the box and protect it
from ESD. D15 and D16 provide protection from input overvoltage, including ESD. R53 and R58 are
current limiting to protect these diodes. C40 and C45 are dc blocking caps, which protect the circuit from
any external dc.
R54 and R59 give the op amps’ bias currents a place to go and their value set the 50K Ohms input
impedance of the aux input jacks. R55 and R60 give additional protection to the input buffers by limiting
current if the input signal exceeds the supply rails. R61 is a ganged pot giving us a 28-dB attenuation
range. This accommodates input signals from -10 dBu to +18 dBu. R62 and R63 set the maximum
attenuation. U7a and b buffer the output of R61.
The
SUM
switch allows the two-aux inputs to be mixed together into a mono signal and sent to the left
channel. This circuit is comprised of switches U22a and U27a as well as the buffer U23 and the summer
U26. In addition the summer boosts the summed signals by 12.2 dB to bring it up to the +2.2 dBu level.
R155 sets the gain for the sum signal. C119 and C134 eliminate offset from the prior stages. R128/129 and
R152 are bias-current paths for the opamps. The right channel is boosted 12.2dB as well. U21b supplies
this boost.
U26a and U21b can clip, if the Aux Input Level pot is improperly set. To aid the user in proper setting of this
control, we have clip detectors. Each clip detector comprises an envelope follower and comparator. The
envelope follower is a rectifier, U25a/D32 or U25b/D33 combined with a low pass filter, R140/C125 or
R146/C127. This extracts the envelope of the waveform, which is sent to the comparator, U24. Whenever
one of the envelopes is greater than the reference set by R142/R143 and R133/R134, its comparator pulls
CLIP_RET/ low.
The
SPLIT
takes a mono signal at the right aux input jack and splits it to feed both the right and left channels.
It comprises switches U22c and U27b, the buffer U23 and the inverter U26. U21a and U26b feed the post-
DSP input switches, U22b and U27c, which switch between the send signals (insert_L and insert_R) and
the aux input signals from R61. C118, C112, C113 and C139 eliminate offset from the prior stages. R126,
R127, R158 and R159 are bias-current paths.
SPEAKER SIMULATOR
Because the left channel has the
DYNAMIC GAIN
signal, it has a speaker simulator (p. 14). The speaker
simulator is formed by three blocks: 1) a low-pass shelf (R221, R222, and C255). The shelf switch, U58b
enables and disables it. It is cascaded with 2) a low-pass, state- variable filter (U57, U54, U56, U51, U50,
and associated components). The peak switch, U52c switches this filter’s Q from no peaking to peaking.
U54 and U51 switch its corner frequency of the state-variable filter between four positions. It is cascaded
with 3)a high-pass filter (U49a and associated components).
The speaker simulator is bypassed using the speaker sim switch, U52b. C241 and C239 eliminate offset
from the prior stages. R211 and R212 are bias-current paths. Also the output can be routed to the right
channel by way of the speaker split switch, U52a and U60c. C238, C261 and C262 eliminate offset from the
prior stages. R209, R236 and R237 are bias-current paths.
POST DISTORTION ADC
Beyond this point the two channels are identical (p. 15). The soft-sat circuit comprises voltage sources
(Q21-Q24) which turn on when the signal exceeds .88V in the positive direction or -0.88V in the negative
direction. R245-R248 and R251-R254 set these turn-on points. R256 and R250 form voltage dividers with
R255 and R249 to determine the slope of soft-sat.