Lexicon MPX G2, Service Manual

Page: 79 / 165

Lexicon
6-1
Chapter 6 Theo r y of Operation
Analog
Overview
The MPX G2 is intended to be used in at least three ways: 1) guitar amp mode, 2) flat amp/direct-to-
console mode, and 3) mix mode. Its analog circuitry can be divided roughly into seven blocks: main input,
TONE, DYNAMIC GAIN
, insert send, insert return (a.k.a. aux input), main output and the power supply. The
main input and insert returns (a.k.a. aux input) are inputs. The send and the main output are outputs. The
TONE
block is the set of master tone controls for the unit. The
DYNAMIC GAIN
is a signal-processing block
consisting of analog EQ and distortion.
When the MPX G2 is used with a guitar amp, the guitar is plugged into the MPX G2 input and the MPX G2
send goes to the guitar amp input. This gives the user DSP before the guitar amp for wah-wah, phaser,
and other predistortion effects. It also allows the user to drive his amp with the
TONE
and/or
DYNAMIC GAIN
blocks. The guitar amp's preamp gives the guitarist his "tone." The guitar amp's send then goes into one of
the aux inputs for post distortion effects. Finally, the output of the MPX G2 goes to the guitar amp's returns
for amplification.
Use with a flat amp is simpler. The main outputs of the MPX G2 go to a flat amp, which feeds a full-range
speaker. The main outputs can also go directly into a recording console or a PA mixer. In these cases, the
MPX G2's send and aux input act like a normalized effects loop. There is a speaker simulator, which gives
a band-limited response like a guitar cabinet. This removes the high-end sizzle that would make the
DYNAMIC GAIN
sound cheesy and gives the low-end thump of the low-damping factor associated with tube
amps.
Use of mix mode is similar to use with a flat amp. The main difference is that the aux inputs can be used as
the mix inputs for an external sound source like a CD player, so that the guitarist can "Jam along with Jimi."
To accommodate these uses, the MPX G2 has plenty of signal-routing capability. If you look at the signal-
routing diagram, you can see the following switches:
DSP BYPASS, GTONE BYPASS, DYNAMIC GAIN BYPASS,
SEND SELECT, SUM, SPLIT, DSP LEFT/RIGHT, SPEAKER SIMULATOR BYPASS/SPEAKER SIMULATOR SPLIT,
and
MIX.
For the purist who wants a straight analog chain before his amp, the pre dynamic gain DSP can be
bypassed. The
SEND SELECT
switch bypasses the whole gain block including tone and
IN VOL
.
If a device with stereo outputs is plugged in to the aux inputs, the
SUM
allows the two channels to be mixed
together into a mono signal and sent to the left channel. Since the speaker simulator is mono and is part of
the left channel, the
SUM
allows both aux input signals to be sent to the speaker simulator. The
SPLIT
takes a
mono signal at the right aux input jack and splits it to feed both the right and left channels.
The
MIX
switches work in conjunction with the
DSP LEFT/RIGHT
switches. In mix mode, the
DSP LEFT/RIGHT
switches are set to normal so that
DYNAMIC GAIN
and
GAIN BLOCK BYPASS
go straight through to the post-
dynamic gain DSP, and the
MIX
switches are set to on to accommodate the external sound source.
In insert mode, the
DSP LEFT/RIGHT
switches are normalized for empty jacks and are set to Loop for an
inserted plug. Insert mode acts like a normalized effects loop and breaks the normalized connection when
a plug is inserted.
DYNAMIC GAIN
gets normalized to the left channel when the left aux input is empty, and
GAIN BLOCK BYPASS
is normalized to the right channel when the right aux input jack is empty. If the user
wants to bypass the outboard effect that is in the loop, he toggles the insert button and the
DSP LEFT/RIGHT
switch goes to normal.
DYNAMIC GAIN
is sent to the speaker simulator for use with a flat amp; the speaker
simulator can be bypassed for use with a guitar amp.