]
Balanced Current Amplifier
is decreased by the same amount. Both pulses remain
centered on each other or balanced in time. The result
is that the difference ripple current has a minimum fre-
quency which is twice the operating frequency of the
individual switches.
The frequency doubling character of the output is re-
markable and further allows advancement towards Loy
Barton’s goal of more from less. The switching losses
are effectively halved by this property as it is only nec-
essary to switch at 250KHz to make a 500KHz amplifier!
The result is that the operating frequency is taken to its
theoretical maximum of N (the number of switches) x fs
(the switching frequency). This is a full factor of two faster
than any known previous design.
The modulation process makes two decisions per switch-
ing cycle for each switch, as both the turn-on time and
the turn-off time are independently controlled by the
modulator. A 250KHz Crown BCA design thus has one
million switch decisions made each second. This is what
is required for full bandwidth audio operation. Previous
to the BCA the conventional wisdom correctly held that
any full-bandwidth audio amplifier would need operate
at 500KHz. Low quality or limited bandwidth PWM de-
signs have operated at lesser frequencies.
The result of the new paradigm is a convection cooled
2.5KW amplifier which mounts in two rack spaces. This
is approximately an order of magnitude larger amplifier
than could have been built previously in the same space
without any cooling fan. With no fan there is no need for
filter maintenance, no fan noise and no contamination of
the unit resulting from normal use.
The Crown K2 amplifier has all of the nearly ideal power
converter attributes of class-D PWM amplifiers in that
reactive loads such as loudspeakers are easily
driven. The reactive energy returned from the
load to the amplifier is reabsorbed and
reoutput with little loss. Non-switching
amplifiers are forced to dissipate all
of the returned energy plus much
more (the latter ratio is a function of
the topology used) and is typically
three fold or more.
Difficult loads are driven with grace
and ease. Cur rent overload is
smooth and sonically identical to
voltage overload. Thermal overload
is rendered a thing of the past as it
is difficult to produce large amounts
of heat. Conventional amplifiers
tackling the same difficult loads be-
come overloaded within minutes
and become sonically dysfunctional with either large
amounts of distortion or shutting off entirely. The result is
that a BCA output Watt is operationally larger than that
of previous amplifier designs.
Real-world high power operation of most large amplifi-
ers reveals that rated bench Watts and distortion ratings
often bear little if any relationship to what can be sus-
tained under normal field conditions by the typical user
using loudspeakers and music.
In critical studio environments the K2 is sonically flaw-
less and will outperform the best large studio amplifiers
in that it does not have the one sonic flaw that any unit
with a low-speed fan has fan noise. The K2’s over 100dB
of electrical signal to noise (A-weighted) is not rendered
superfluous by fan noise.
With a low-frequency damping factor of over 10,000 and
low distortion (<0.1%THD), the K2 is ready to give your
music the quality of presentation that it deserves.
One final footnote: One (the larger) of Loy Barton’s 1931
design examples was a 2.5KW amplifier. Ironically there
are some things that do not change, no matter which
century’s paradigms are in force.
PUSH-PULL
The basic concept of push-pull amplification is quite old
(1920’s) and can be described as an amplifier in which
there are two similar signal branch circuits operating in
phase opposition and whose outputs are combined in a
difference (summing) circuit to produce an increased
power output.
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