Table of Contents
Advertisement
Technical Information
6-7
5) PRE-LIMITER CROSSOVERS
Card#Al
The crossovers are realized as follows:
Band 1 (150Hz low-pass):
Second-order 420Hz low-pass filter cascaded with
second-order 150Hz low-pass filter.
Band 2 (420Hz band-pass):
Second-order 420Hz low-pass filter cascaded with
third-order 420Hz high-pass filter.
Band 3 (700Hz band-pass):
Second-order low-pass filter cascaded with third-order
high-pass filter.
Band 4 (1.6kHz band-pass):
Two cascaded second-order stagger-tuned band-pass filters.
Band 5 (3.7kHz band-pass):
Two cascaded second-order stagger-tuned band-pass filters.
Band 6 (6.2kHz high-pass):
Second-order 6.2kHz high-pass filter.
Bands 3 through 6 are fed from a first-order all-pass filter which provides phase correction
for the crossover summation.
Component-level description:
All filters are built with second-order sections of
the type found in any modem text on active filter
design (see, for example, Wong and Ott, op. cit.').
Some filters are of the " multiple feedback " type,
and are basically negative-feedback filters. Oth
ers are of the " Sallen and Key " type, and are
positive-feedback filters. The two types are muted
as necessary to achieve desired inverting (neg
ative feedback) or non-inverting (positive feed
back) responses.
6) VOLTAGE-CONTROLLED AMPLIFIERS (VCAs)
Card #A3
The voltage-controlled gain block used throughout the XT2 is a proprietary Class-A VCA
which operates as a two-quadrant analog divider with gain inversely proportional to a cunent
injected into the gain-control port. A specially-graded Orban IC contains two matched, non
linear gain-control blocks with differential inputs and current outputs. The first of these is
employed in the feedback loop of an opamp to perform the gain control function. The
inputs of the first and second gain-control blocks are connected in parallel, and the output
of the second block is a distortion-corrected
current
which is transformed into the desired
gain-controlled voltage by an opamp current-
Component-level description:
This is a general description of the VCA circuitry
used throughout the XT2. Only band 1 will be
specifically described.
The operation of the VCAs is based on precisely
matched pairs of gain control blocks with differ
ential voltage inputs and current-source outputs.
The gain of each block is controlled by a control
current.
If used alone, one such gain-control block would
introduce considerable distortion. Therefore, the
first of the two matched blocks (IC13a) is used as
the feedback element in an operational amplifier,
to-voltage converter.
ICla. The second of the matched blocks (IC13b)
is then driven by the predistorted output of ICla.
The output of ICla is first attenuated by R4,
R5 and then applied to the input of the feed
back element IC13a. The output of ICla is
pre-distorted as necessary to force the current
output of IC13a to precisely and linearly can
cel the audio input into the " virtual ground "
summing junction of ICla. This same pre
distorted voltage is also connected to the in
put of IC13b. Thus, the output of IC13b is an
undistorted current, which is converted to a
voltage in current-to-voltage converter IClb,
R9, RIO, Cl, C2. The output of IClb is the
output of the VCA.
Advertisement
Chapters
Table of Contents
