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27
SIGNAL PATH CIRCUITRY
NOTE:
Left and right program channel circuits are identical, so only
the left channel will be described here. A skilled technician should find
it within his capability to extrapolate for the right channel.
INPUT STAGES
Line Input
Balancing
IC44B is an “active-balancing” stage for the left channel program line
input, affording rejection of unwanted common-mode signals. When
jumper JMP5 connects R268
into
the network (H), the Line Input
accepts “High” program levels between 0dBu and +15dBu. With R268
jumpered
out
(L), “Low” program levels from –15dBu to 0dBu are
accommodated. Input range setup is described on Page 8.
“Bass Enhance”
IC50B performs the dual function of a variable-gain stage and the
exclusive “Bass Enhance” equalizer.
With jumper JMP6
closed
(F), low frequency response of the stage is
“Flat” to at least 20Hz. With JMP6
open
, the stage becomes a high-Q
high-pass filter with a low-end boost. With this jumpering the
response of the stage rises below 500Hz, peaking with about 4dB of
boost at 80Hz, then falling rapidly. (See Figure 4 on Page 10.) The
effect of this special equalization is to accentuate frequencies which are
perceived
as “bottom-end,” while attenuating sub-audible program
components which can compromise overall modulation efficiency.
The feedback path of IC50B includes the front-panel
INPUT GAIN
control, R242. This affords the 15dB of continuous level adjustment
within each jumpered input gain range.
The “design center” sinewave signal level at the output of IC50B (the
level which will maintain AGC gain at 0dB) is –5dBu.
AGC STAGE
From the output of IC50B, the program signal takes one of two paths.
One path bypasses the audio processing section of the
DAVID-II
entirely, with analog switch IC40A re-inserting the program signal
downstream of processing and pre-emphasis. The program normally
encounters IC46C, the first analog-switch PWM attenuator.
IC47B and IC48B comprise a 4-pole low-pass “reconstruction” filter
which removes the 152kHz PWM switching component and restores
the program signal to its original (audio) spectrum. The small amount
of high frequency “shelving” boost provided by C123 and R282 is
important for maintaining compressor/limiter flatness downstream.
This boost is canceled by complimentary equalization in a later stage.
