Amplifier Design 729BCA is zero. This condition corresponds to the times
when Sp and Sn are simultaneously both on or both off.
Position one of the three-position switch corresponds to
those intervals of time when Sp is on and Sn is simulta-
neously off. This position generates a positive audio
output to the load. Finally, position three of the
three-position switch corresponds to those intervals of
time when Sn is on and Sp is simultaneously off. This
final position occurs when a negative audio output is
being generated.
The type of pulse width modulation employed with
the BCA is called natural double-sided interleaved with
n=2. The n designator indicates the fundamental ripple
frequency of the BCA output when generating audio
signals relative to the fundamental frequency of the
triangle waveform. The advantage of this type of modu-
lation in addition to the fundamental ripple frequency
being twice that of the triangle waveform is that there
are no harmonically related distortion products to the
audio frequency being processed and there are no odd
integer multiple bands related to the fundamental
frequency of the triangle waveform. As a consequence,
only a relatively simple output filter is required for
handling normal loudspeaker loads. Additionally, when
operating a two-channel amplifier in the full-bridge
mode, if the modulator in the second channel is in
quadrature with that of the first, n becomes 4 rather than
2 so that the lowest ripple components appear at 1 MHz
rather than at 500 kHz. Crown terms such amplifiers as
being opposed current interleaved amplifiers )OCIA).
This constitutes a new class of amplifier or class I. This
type of operation is incorporated in Crown’s I-Tech
series of switching power amplifiers.
The required modulator circuitry is actually quite
simple as is illustrated in Fig. 20-41.
In reference to Fig. 20-41, A 1 is the error signal
amplifier while A 2 is a simple inverter amplifier. C 1 and
C 2 are high-speed comparators.The innovations described in this section have led to
the production of switching amplifiers having power
ratings up to 8000 watts while producing only 0.35% of
total harmonic distortion at rated power. The size and
weight figures are equally as impressive being only 2
rack units and 29 pounds.20.3.7 Signal Processing in Power AmplifiersMost high-power audio amplifiers for professional
applications currently feature a wide variety of signal
processing functions. This was not always the case.
Originally power amplifiers were just what the name
implied with the possible exception of a selectable high-
pass filter at the input for the protection of high-
frequency compression drivers when such loudspeaker
elements constituted the only load on the amplifier.
During this period, full-range loudspeaker systems
employed passive dividing networks and high-powered
systems featured compressors contained in dedicated
units preceding the power amplifier. The modest
consoles of this period usually provided only high and
low shelving filters. Modern changes began to occur
first with the introduction of -octave real-time
analyzers and dedicated -octave equalization units.
These changes were further accelerated by the advent of
TEF and similar computer-based analysis systems.
Electronic crossover networks, signal alignment, and bi-
or triamplification shortly came into vogue in order to
correct system problems discovered by the new sophis-
ticated analysis systems. Two-channel power amplifiers
with the ability to operate the two channels indepen-
dently or in the bridged mode became the de facto stan-
dard. The electronic crossovers and signal delays of this
period were dedicated separate units and initially wereFigure 20-40. Equivalent circuit of BCA for all three modes
of operation.Effective voltage L/21
23- VCC
VCCIoutLoadFigure 20-41. Basic OCIA pulse width modulator.Rf
R 1RRC 1A 2 C 2A 1- Verror
+VerrorTo Sn controlTo Sp control
Audio input
Triangle
generatorFeedback from amplifier output++(^1) » 3
(^1) » 3