Lexicon 200, Service Manual

Page: 37 / 90

Lexicon
6-1
Chapter 6 Theo r y of Operation
Main Board
Schematic Walkthrough
Sheet 1
This sheet shows the analog input circuitry (U1-U2), the analog output (U6), and the output mute circuitry
(U7).
Input Stage
Separate unbalanced ¼” phone jacks (J1-J2) provide input for the left and right signals. A single input
source will be routed to both left and right input stages if the right channel (J1) is the only one used. These
jacks provide AC coupled chassis ground connections through integrated ground lugs. This AC coupling is
provided via C26 and C45. These components effectively AC couple the analog ground plane to the
chassis by way of the enclosure. This scheme is used to minimize AC hum loops from forming at the inputs.
Capacitors C26 and C47 provide protection from high frequency interference from entering or exiting the
MPX200 via the input cables.
DC Blocking is incorporated by capacitors C29 and C48 in series with the input signal paths.
The input impedance for each channel is set by R7 and R22 (Right and Left channels, respectively). Each
channel has an input impedance of 1M when used in Stereo mode, and a combined input impedance of
500K
Ω
when used in Mono mode (Right channel only). This was incorporated as a concession to guitarists.
Due to this relatively high input impedance, the unit is susceptible to noise pickup from radiating sources,
particularly if the input cables are routed near the power supply of the unit, so care must be taken when
routing cables into the unit. J1 and J2 short the input paths to ground when no input cables are plugged in.
This prevents noise voltages from being generated by this high impedance.
D1 and D2 provide clamping protection of the input signal paths into U1 and U2, while R6 and R19 provide
input current limiting to U1 and U2. The diodes ensure that the non-inverting inputs of the op-amps never
see voltages more than 0.7V above or below the positive and negative supply rails.
One half of U1 and U2 are used as non-inverting amplifier stages. These first stage components provide a
DC gain of 10.55dB. The input and feedback circuitry comprised of R5, R4, and C6 for the right channel
and R21, R20, and C21 for the left channel form high pass filters shelving at 3kHz and 9kHz. This is the
standard 15/50uS pre-emphasis curve. C5 and C20 provide closed loop frequency compensation for the
input buffer stages.
C2 and C17 remove DC offset from the buffer output stages so that wiper noise from the Level
potentiometer R1 is eliminated. R1 is a dual ganged pot. The mechanical frame of R1 ties into the analog
ground plane on the PCB via R16 and makes contact with the front panel. R16 is currently not installed on
the PCB, therefore direct connection of analog ground and earth ground is not implemented.
The wipers of R1 connect directly to the non-inverting inputs of the second half of devices U1 and U2.
These are simple DC gain stages. The gain of each stage is 21.97dB set by R2/R3 for the right channel and
R17/R18 for the left channel. Feedback capacitors C1 and C16 provide frequency compensation.
The outputs of these stages provide input to the CODEC circuitry on the next page. These are signals
IN_LEFT and IN_RIGHT.