Lexicon RV-8 Service Manual page 83

Lexicon
The output of the DAC passes through a second order low-pass filter with its –3dB frequency at 187kHz.
The filter topology is a compromise between the flat pass-band Butterworth filter and the Bessel filter with
its superb transient response. The filter is flat out to 20kHz. It has an overall gain of 6.7dB when
measured at the test points. This means a 0 dBFS signal at the D/A converter will be 4 Vrms going into
the analog switches.
DG411 analog switches select either the output of the Rec/Zone DAC or the Rec analog input source
directly. The selected signal goes to the dual op amp U32 which buffers a 6dB voltage divider that feeds
the level control. The PGA2311 output level control is buffered by a 6dB gain driver. A direct path around
the level control provides a fixed output. Rec analog output has a maximum output level of 4 Vrms. The
signal passes through a muting relay on the way to the output jacks. The relays are controlled by the
ZONE_MUTE_RLY/ signal.
Zone Level Control (sheet 9)
U63 serves as a direct bypass level control for the rear channels. The input comes from the side channels
in the 5.1 direct bypass path.
The rest of the circuitry, U44, U33, U23 and U15 are similar to the Rec path (page 8). Zone analog output
has a maximum output level of 4 Vrms.
Main D/A Converters (sheets 10-13)
There are eight outputs for the Main Audio Path. The D/A circuitry is shown for two outputs on each
sheet. The circuitry is identical for all eight outputs.
The AK4395 is the same 24-bit D/A converter that operates Zone2. The AK4395 DACs are configured
through their serial ports (pins 8,10,11). MAIN_DAC_RST/ puts the pairs of DACs into reset.
The DACs are followed by a 2
filter with the –3 dB point at 127kHz and a pass-band flat to 20kHz. The filter amplifies the 1.7Vrms
differential signal to 7Vrms (+11.2 dB) and converts it to single-ended for the level controls. Note these
values assume a 0 dBFS digital input signal to the DAC.
ADG451 analog switches are used to select either the DAC output or analog input for the respective
output. These direct analog signal paths have been designed to support two modes:
•
2-channel analog direct or bypass mode. Any analog input can be routed directly to the L/R Front
outputs.
•
5.1-channel analog direct or bypass mode. When this mode is enabled, specific analog input signals
are routed to specific analog outputs according to the table below:
Analog Input
U
#3 or 6, left channel
#3 or 6, right channel
#4 or 7, left channel
#4 or 7, right channel
#5 or 8, left channel
#5 or 8, right channel
Two different pairs of control bits are used to select the DSP/DAC signals or analog input signals for the
Main outputs. MAIN_DACOUT_SEL/ selects the respective DAC outputs for all of the Main outputs (Front
order low-pass filter. The filter topology is a flat pass-band Butterworth
nd
Analog Output(s)
Left Front
Right Front
Center
Sub
Left Side and Left Rear
Right Side and Right Rear
6-7