Lexicon 200, Service Manual, Page: 38 / 90

MPX 200 Service Manual
6-2
Output Stage and Muting
The output of the CODEC circuitry re-enters this page as signals OUT_LEFT and OUT_RIGHT.
Provisions are made for an additional low pass filter of these signals by installing capacitors in locations
R45 and R33; additionally, the CODEC signal may be attenuated or “padded” by installing resistors in these
locations, but these have not been implemented in the design. R33 and R45 are left as uninstalled
locations.
C50 and C51 provide AC coupling of the OUT_RIGHT and OUT_LEFT signals. The AC side of these
capacitors are connected to two analog switches comprised of two sections of device U7. These switches
provide output muting during power up and power down conditions. When the MUTE/ signal at pins 9 and
10 of U7 is brought to a logic low, the output signals at pin 4 (Left channel) and pin 15 (Right channel) are
internally switched to pins 5 and 2 respectively. These pins in turn are tied to analog ground, thereby
providing a low impedance path to ground for the OUT_RIGHT and OUT_LEFT signals. Approximately
43dB of attenuation is achieved when the switch is in mute mode.
A dual op-amp (U6) and its associated circuitry comprise the output circuitry. Each stage is set to a fixed
gain of 10.42dB by way of R50/R51 (Right channel) and R52/R53 (Left channel). R48 and R49 set the input
impedance of the op-amp circuitry to 10K. C49 and C56 provide high frequency compensation for the op-
amps; this keeps the amplifiers from breaking into spurious oscillation. This dual op-amp is powered by +/-
10V unregulated supplies. In reality, because these supply voltages are unregulated, the actual measured
voltages are more akin to +12V and -15V. This was done to afford the widest possible headroom for the
output. C52 and C53 provide de-coupling for these supplies.
An output impedance of 75 ohms is developed by R59 and R79 for the right and left outputs respectively.
These provide output current limiting protection.
The ¼” unbalanced output jacks (J3 and J4) are configured so that when only the right output is used (J3),
the left and right signals are summed together to provide a monophonic output. The left jack J4) can
support stereo headphones if J3 is unused. The right channel is provided on the tip and the left channel on
the ring of a stereo phone jack. C63 and C73 provide RFI suppression at the output. Like the input jack pair,
these jacks AC couple analog ground to chassis ground via C61 and C71 and the respective ground lugs
on the jacks. This is again to prevent AC hum loops from forming at the outputs.
Although the output op-amp can drive high impedance (>100 ohms) headphones, for best results a
headphone amplifier should be used for low impedance phones.
Sheet 2
This sheet shows the CODEC device (U5), the input signal DC bias circuitry (R14 and R15), the single-
ended to differential input amplifiers (U3), and the differential to single ended output amplifiers (U4). Shown
also are the various clock and control signals used by the CODEC.
Single-ended to differential converter
Each section of dual op-amp U3 is a unity gain inverting amplifier. Gains are fixed at 0dB by R8 and R9 for
the right channel, and R23 and R24 for the left channel. The non-inverting inputs of each amplifier are tied
to analog ground, creating a virtual ground at the junctions of the input and feedback resistors for each
channel. Signals IN_RIGHT and IN_LEFT from the previous page are AC coupled into the these amplifiers
by C7 and C22. Each phase of the differential output signal pairs are impedance balanced via R10/R11 for
the right channel and R25/R26 for the left, and then AC coupled via C9/C10 and C24/C25 respectively.
Each phase of the differential signal pairs are then DC biased via the resistors R12/R13 and R27/R28. This
DC bias of 2.94V is provided by a resistive divider comprised of R14 and R15 while C12 and C13 de-couple
this bias voltage to remove ripple and noise. Because the CODEC samples the input at 256fs,
R10/R11/C41 and R25/R26/C42 form low pass filters. C9, C10, C24, and C25 simply provide AC coupling
into the CODEC.