Lexicon RV-8 Service Manual page 82

RV-8 Service Manual
SCHEMATIC WALKTHROUGH
Analog Audio Inputs (sheets 1,2 & 3)
Sheets 1 and 2 are identical. The Left input jacks and associated circuitry are on sheet 1 while sheet 2
includes the Right input jacks and circuitry. Each input pair is buffered by a dual TL072 op amp. Each
buffer connects to three DG408 8x1 CMOS switches. There are separate switches for the Main and
Zone2/3 analog source selection with independent switches for left and right channels, for a total of six
DG408s.
At the bottom right-hand corner of each sheet is a quad-switch. These switches are used when routing a
5.1-analog source. One routes the Center and Subwoofer signals from Inputs 4 or 7 while the other
buffers the Surround L/R signals from Input 5 or 8.
Sheet 3 hold buffering and switches for tuner and phono, and buffering for phono75, and mic1/2. Three
sets of switches are used to select inputs for each zone separately. These switches work in parallel with
the muxes on sheets 1 and 2 to provide a 1-of-10 analog input mux function for each zone.
Main Inputs, C/Sub and Side Inputs (sheets 4,5,6 & 7)
The outputs of the Main source selectors feed the Main Input Level control on sheet 4. A dual op amp on
sheet 4 buffers a 6dB voltage divider that feeds the level control. The output of the level control is used
for the direct analog path to the Front L/R outputs and feeds the A/D converter.
Two microphone inputs are provided from the microphone preamplifiers. A DG411 analog switch can
select Mic inputs 1 and 2 or the phono75 to be passed to the Main Input level control and A/D converter.
These switches work in parallel with the muxes on sheets 1 and 2 to provide a 1-of-12 analog input
function. When the Mic input is selected, the Analog inputs are disabled by bringing MAIN_ANLG_EN low
on sheets 1 and 2.
The Main Input level control is the PGA2311, which has a range from +31.5 to –95.5 dB in 0.5 dB steps.
The PGA2311 operates on ±5 volt rails and cannot handle signal levels greater than 7.5 Vpp. Two dual
op amps provide the left and right differential audio signals to the A/D converter. The op amp circuits bias
the signals at 2.5 V and attenuate it by 7dB. This means a 2 Vrms signal at the output of the level control
will be equivalent to 0 dBFS after the A/D conversion.
The PCM1804 stereo A/D converter incorporates a multi-bit delta-sigma architecture. It outputs 24-bits at
a 96kHz sample rate under normal operation. The serial audio data from the A/D converter goes directly
to the Main board. Control signals are used for reset (MAIN_ADC_RST/) and to place the converter in
88.2k/96k or 192k sample rate mode (MAIN_ADC_96K_EN and MAIN_ADC_192K_EN). The main board
provides three clocks: MAIN_ADC_MCKI/, which is 256xFS for 44.1k and 48k sample rates, 128xFS for
88.2k and 96k sample rates and 64xFS for 192k; MAIN_FS64/, which is 64xFS; and MAIN_FS/ which is
1xFS (where FS = sample rate).
Sheets 5, 6 and 7 cover the C/Sub, Side and Rec A/D channels. They are similar to the main zone except
for various analog source differences and separate clock sources and data outputs.
Rec & Zone D/A converter (sheet 8)
The AK4395 24-bit delta-sigma stereo D/A converter operates up to 192kHz. The DAC is configured
through its serial control port (pins 8,10,11) with a separate Reset pin.
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