Crown BCA Supplementary Manual

Reinventing the Power Amplifier - BCA
THE BALANCED CURRENT AMPLIFIER - A NEW
PARADIGM FOR THE 21ST CENTURY
In 1931 Loy Barton, a research worker employed by
David Sarnoff , unearthed the paradigm that has domi-
nated all electronic power amplifiers used for audio re-
production and industrial power to this present day. Many
incremental additions have embellished Loy's original
invention since its inception. The embellishments have
many names, ultra-linear, Williamson, full complemen-
tary, quasi-complementary, quasi-linear, class-G, class-
H, grounded-bridge, class-D, etc.
One common thread in all of the above is the use of
push-pull circuitry. Loy did not invent push-pull circuitry.
Class-A push-pull amplifiers were around before 1931
and were used whenever larger output powers were
needed than could be derived from a single device
vacuum tube output stage. Loy was the first to describe
class-B push-pull amplifiers which he developed to
power both the large audio modulators of AM broadcast
stations and the output stages of home radios. Both
applications had a common need, the need to produce
more high-quality power output with less electricity and
natural resources.
In 1931 Loy Barton published "High Audio Output from
Relatively Small Tubes" in the Institute of Radio Engi-
neers proceedings. The very theme of the article is in
harmony with the goals of every designer who has ever
wrestled with the power amplifier problem of wanting
unlimited output power from a small box of affordable
cost. There are some things which do not change.
One thing has changed since 1931 and that is the vari-
ety of electronic devices which are available to imple-
ment circuits. The original electronic power devices were
vacuum tubes which were characterized by large out-
put impedance's and high saturation resistance's. They
made poor power switches and were most useful when
used with transformers to match their output impedance's
to lower impedance loads such as loudspeakers. Today's
solid-state devices such as power MOSFETs offer char-
acteristics which are most appropriate to make high-
speed switches, not linear output stages as practiced
by Loy.
Loy's genius was to operate the two tubes of his class-B
output stage in strict time alternation. To produce one
polarity of output current he would turn on one tube; to
produce the other polarity of output current he would
turn on the other tube. Previously with class-A designs,
both tubes were always turned on and even at no signal
were dissipating large amounts of quiescent power. By
] Balanced Current Amplifier
careful selection of the class-B bias point, he was able
to produce essentially undistorted output without hav-
ing a massive quiescent power loss. This greatly in-
creased the power output that could be obtained from a
pair of tubes and reduced the wastage of electricity.
While many variations on this basic theme have been
developed since 1931, Loy's class-B paradigm has sur-
vived unchallenged. Operation of the push-pull power
devices in time alternation has been part of all high per-
formance designs for the last 66 years. Even when the
devices became class-D PWM (Pulse Width Modulation)
switches, they were operated using the class-B para-
digm, first one on and then the other, in strict time alter-
nation.
While switching and PWM methods are the methods of
choice to all modern power electronics engineers, PWM
amplifiers have remained relatively useless for precision
power amplification. Ironically the class-B paradigm lies
at the heart of the problem.
To produce a class-D PWM amplifier with low amounts
of distortion near zero output current, it has been neces-
sary to operate the time alternating power switches with
very precise sequencing of the two switches. If the
switches have any dead time (no switch on) between
their activation large amounts of distortion will form. If
they overlap, the circuitry would self-destruct with large
amounts of shoot-through current. The circuitry has been
designed around the paradigm and is therefore not tol-
erant of any violation of time alternation.
So pervasive has been the paradigm that it has gone
unchallenged until now. While Loy's class-B paradigm
has served us all well for most of a century, its days are
numbered.
With a marked bifurcation in design concept, the para-
digm for the next century uses simultaneous activation
of its push-pull switches and has been appropriately
dubbed a "Balanced Current Amplifier". This is the very
antithesis of the time alternation paradigm.
In the Crown BCA design, when there is no intended
output signal, the power switches are being turned on
and off simultaneously with a 50% duty cycle. The result
is the formation of two balanced and canceling high-
frequency output currents with no output at the no-sig-
nal condition.
To produce an output signal the output of one of the
switches is increased in duty while the remaining switch
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