Chapter 6 Theory Of Operation; Sheet 1; Input Stage; Output Stage - Lexicon MPX 110 Service Manual

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Chapter 6 Theory of Operation

SCHEMATIC WALK-THROUGH

Sheet 1:

This sheet shows the analog input section (U21-23), and the analog output section (U17 & U18) and their
associated circuitry.

INPUT STAGE

Separate unbalanced 1/4" phone jacks (J8 and J9) are provided for left and right input signals. A single
input source will be routed to both left and right input stages if only the right input (J9) is used. J8 and J9
also provide chassis ground through an integrated ground lug from the PCB to the cover.
Capacitors (C81 and C85) are found at the inputs to prevent unwanted high frequency interference from
entering or leaving MPX 110 through the input cables.
DC blocking is incorporated by capacitors (C80 and C84) in line with the signal path.
The input impedance of the MPX 110 is set by R86 and R99, 1M Ohm per channel and 500K Ohm when
sum-to-mono by plugging in the Right I/P only. This allows the MPX 110 to be used with a wide variety of
input sources including electric guitars. However, this high impedance makes the input susceptible to noise
pickup. For this reason the left and right inputs short to ground when empty.
D16 and D17 protect U21 and U23 from being destroyed by potentials > |5V|. R87 and R100 limit the
current through D16 and D17 during conduction to protect them. This insures signals at the input of the U21
and U23 are never greater than 0.7V above the op-amp rails.
The non-inverting inputs of one half U21 and U23 are used as the signal input buffer to maintain a high
input impedance, provide unity gain at low frequencies, and pre-emphasis.
R88, R89, C89 and R101, R102, C101 are feedback networks around U21 and U23 respectively. They form
a high pass shelf for pre-emphasis. It is a 10-dB shelf starting at 3 kHz and ending at 9 kHz. This is
15/50uS curve with matching de-emphasis on the output. The pre/de-emphasis improves the SNR of the
unit. Capacitors C91, C103 provide high frequency compensation for this input buffer stage.
C94 and C106 remove offset so potentiometer wiper noise in R92 is eliminated. R92 is a ganged dual pot
for input level control.
The second half of U21 and U23 is a gain stage, which provides approximately 25db of gain. R90, R91 and
R103, R104 set the gain of the respective channels. C95 and C107 are used for op-amp compensation at
high frequencies.
IN_LEFT and IN_RIGHT are passed on to Sheet 2 for further signal conditioning.

OUTPUT STAGE

OUT_LEFT and OUT_RIGHT come in from Sheet 2. Two analog switches (U17) provide output muting
during power up and power down conditions. MUTE/, pins 9 and 10 of U17, control these switches and are
low on power reset. When MUTE/ is low, pins 4 and 5 of U17 are connected for the right output and pins
15 and 2 of U17 are connected for the left output, creating a low impedance signal path to ground. These
switches are place in parallel with the output level potentiometer before the output stage. Approximately
43db of attenuation is achieved when the switch is on (MUTE/ is low).
A dual op amp (U18) and its associated circuitry (R54, R55, C60 and R56, R57, C61) make up the final
output stage. This stage provides 10.5db of gain and the complimentary de-emphasis curve to compensate
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