Lexicon MPX G2, Service Manual

Page: 84 / 165

MPX G2 Service Manual
6-6
Q25- Q27 enable and disable the soft-sat. When the soft-sat line is low, Q26 is off and Q27 is on. With Q26
off, the bases of Q24 and Q21 are at 5 volts, which disables them. With Q27 on, Q25 is off and the bases of
Q23 and Q22 are at -5 volts, which disables them. So when the soft-sat line is low, soft-sat is disabled.
When the soft-sat line is high, Q26 is on and Q27 is off. With Q26 on, the bases of Q24 and Q21 are at .18
volts. Whenever the emitter of Q24 or Q21 goes over .88 volts, the transistor will turn on. With Q27 off, Q25
is on and the bases of Q23 and Q22 are at -.18 volts. Whenever the emitter of one goes below -.88 volts, it
will turn on.
U62 and U63 form differential input drivers for the ADC, U64. The anti-aliasing filters comprise R267, R268
and C280 for the left channel and R265, R266 and C279 for the right. R267, R268, R265 and R266 also
provide latchup protection by limiting current into the ADC. C270, C273, C275 and C278 are dc blockers
that prevent a dc current from flowing due to the difference in potential between the ADC's self-biased input
and the ground reference of the driving op amps. C281-C284, R269 and R270 filter the ADC's supply.
Output
The post-
DYNAMIC GAIN
DSP goes into the CS4390 DAC (p.16). C301- C304 and R289 filter its power
supply. The 0 dBFS output of the DAC is 8.2 dBu differential. U65a and U66a and their associated
components make up the anti-imaging filters. Their corner frequencies are set to 20 kHz.
U60a and U61a are the left and right
MIX
switches respectively. C260 and C265 eliminate offset from the
prior stages. U65b and U66b are the summing amps that sum the post-dynamic gain DSP signal with the
external sound source. The post-dynamic gain DSP signal is attenuated by 2 dB. For a mix ratio of 50/50,
the external sound source leg of the two channels has a 4 dB boost to bring it up to the 6.2dBu level of the
attenuated DSP signal. R241/R238 set this gain. R240/R235 are the bias-current paths.
This is followed by the output-level digi-pot, U3 (p. 17). C97 and C98 (p.16) block dc offset from the prior
stages. Diode pairs D8-D9 prevent digi-pot latchup. U2b and U4a form a balanced line driver with U2a and
U4b and have a gain of 15 dB to bring the signal up to 21 dBu single ended. When driving a balanced load,
the line driver puts out 27 dBu differentially. RC4556s were chosen for their ability to drive low impedances
(large loads). C16, C17, C22 and C23 are dc blocking caps. R8, R13, R24 and R29 provide a ground
reference for the outputs. R6 & R4, R11 & R9, R22 & R20, and R27 & R25 set the 600-ohm output
impedance. D2, D3, D6, and D7 protect the outputs from ESD and overvoltage originating from outside the
box. R4, R9, R20, and R25 are current limiting protection for these diodes. Q1, Q2, Q5, and Q6 are the
output mutes to eliminate turn-on and brownout pops. R5 & R7, R10 & R12, R21 & R23, and R26 & R28
linearize the FETs for lower distortion, and C6, C9, C12 and C15 provide the mute time constant. FB2-
FB5, C7, C8, C13 and C14 prevent RFI emission.
U5a and U5b are the stereo headphone amps. An RC4556 was chosen for its ability to drive low
impedances (large loads). C31 and C32 are dc blocking caps. R40 and R41 provide a ground reference for
the outputs. R39 and R42 set the 75-ohm output impedance. D10 and D11 are ESD and overvoltage
protection. R39 and R42 also provide current-limiting protection for these diodes.
Supplies
U71 and U70 provide the input +5 and -5-V supplies respectively. U30 and U29 provide the 5 and -
5-V supplies respectively. U10, U11, U68 and U69 form the digi-pot supplies. R89/90, R91/92, R296/297
and R298/299 set the output voltages of these regulators to +/-2.5 Volts.
There are also separate input and output supplies for the latch-up protection diodes. They comprise the
transistors Q3, Q4, Q28 and Q29. The diode resistor combination on the bases gives a 1.2-volt reference
for the positive supplies and a -1.2 volt reference for the negative supplies. The transistor itself adds an
additional .6 voltage drop, resulting in +/- 1.8-volt supplies. The positive supply sinks current and the
negative supply sources current.