Crown Audio Stereo Amplifier Class I User Manual

The Class-I Amplifier  
The Class-I Amplifier  
Class-I, also known as BCA (Balanced Current Ampli-  
fier) is Crown’s patented, cutting-edge technology that  
gets more power out of an amplifier with less waste than  
was ever before possible.  
Class-I technology offers several key advantages. It  
providesunprecedented efficiency,requiring lesspower  
from the AC supply than other designs and that can add  
up to significant cost savings over the life of the ampli-  
fier. Class-I handles reactive loudspeaker loads easily  
and gracefully, by reusing energy returned from the  
loudspeaker rather than dissipating it as heat or forcing  
the amp into premature current limiting. This character-  
istic means class-I amplifiers run better and longer—  
especially at lower impedances. It also makes them  
more reliable, since they are not constantly stressed to  
their limits or subjected to excessive heat. Best of all, as  
proud owners can attest, amplifiers with class-I technol-  
ogy sound great, with a powerful, accurate sound that  
stands out from the competition.  
Class-A Operation  
on and conduct all the time. Class-A amplifiers are  
generally considered to be the most accurate of all  
classes in low to moderate power ranges and are useful  
for applications such as preamp stages; however, they  
create tremendous amounts of heat due to their very low  
efficiency, making them impractical for high-power am-  
plification. Other amplifier classes have been devel-  
oped over time to overcome the class-A efficiency  
problem.  
Crown’s class-I “switching” technology is a completely  
new adaptation of switching (PWM) amplifier design.  
This paper provides a simplified overview of Class-I, but  
before we explore its inner-workings, we need to look at  
the foundation of all previous high-power amplifier de-  
signs in order to fully appreciate how class-I stands  
apart.  
CLASS-B AND TIME ALTERNATION  
Class-B was invented as a solution to the efficiency  
problem with class-A. The invention of class-B is signifi-  
cant in that with it came the concept of time alternation,  
which has been the foundation of virtually all power  
amplifier designs used for audio reproduction and in-  
dustrial power since about 1931 to the present. While  
many incremental improvements have been brought to  
market since that time, none have varied from the basic  
time-alternation paradigm.  
TRANSISTOR OPERATION  
To understand the different amplifier classes, it helps to  
understand a little about how transistors operate. Bias is  
a technical term referring to the static operating condi-  
tion of an electronic device, such as a transistor. In other  
words, bias defines how much conduction takes place  
inthetransistorwithnodynamic signalinput. Transistors  
may operate in three possible states: cutoff, saturation,  
or somewhere in between. The cutoff state is when there  
is not enough signal present to cause the device to  
conduct.Saturationreferstowhenthedevicehasreached  
maximum conductivity. Amplifiers referred to as “dissi-  
pative” control their output by operating in the region  
between cutoff and saturation.  
The basic class-B amplifier implements two devices in  
theoutputstageinapush-pull” arrangement, witheach  
amplifying half of the waveform, and the devices oper-  
ate in strict time alternation. When the signal goes  
positive, thepositivedeviceconductswhilethenegative  
CLASS-A  
Class-A amplifiers are the simplest in design, and can  
be the most distortion-free of all amplifier classes. In  
class-A, the output devices are biased on all the time  
with a current large enough to produce the largest  
output signal.  
Some class-A amplifiers may employ both a positive  
and negative device in a push-pull arrangement to  
increase output power, but both devices still are biased  
Class-B Operation  
 
The Class-I Amplifier  
amount. Class-I uses symmetrical interleaved PWM,  
meaning both the leading edges and trailing edges of  
the pulse are varied according to the amplitude of the  
signal, and the spacing between pulse centers remains  
constant. Both the positive and negative switch pulses  
remain aligned on-center, and the net output current is  
positive.  
tive energy returned from the loudspeaker to the ampli-  
fier is reabsorbed and output again to the loudspeaker  
with little loss. Non-switching, dissipative amplifiers are  
forced to dissipate all of the returned energy and more  
in the form of heat. This means class-I amplifiers are  
louder than other amplifiers with comparible power  
specs, yet they stay much cooler, and can be lighter  
since they don’t need nearly as much mass for heat  
dissipation.  
Class-I Amplifier  
Reactive Energy  
Class-I Switches, Positive Signal  
Dissipative Amplifier  
Reactive Energy  
Likewise, to produce a negative output signal, the  
output of the negative switching device is increased in  
duty while the positive switching device is decreased by  
the same amount. Again, both switch pulses remain  
aligned on-center, and the net output current is negative.  
Reactive Energy Returned to Amplifier  
From Loudspeaker  
With it’s rock-solid reliability, high quality audio repro-  
duction and unmatched efficiency, Crown’s patented  
class-I technology creates a totally new paradigm for  
amplifier design that represents the future of profes-  
sional amplifiers.  
Class-I Switches, Negative Signal  
The result of using interleaved PWM is that by operating  
theswitching devicesat250kHz, thesignaliseffectively  
modulated at500kHzsinceboththeleading and trailing  
edges of each pulse contribute to the output ripple  
current. This arrangement further increases efficiency,  
since switching losses are effectively halved by operat-  
ing the switching devices at 250 kHz, rather than at 500  
kHz as would be necessary with class-D designs to  
achieve the same effective sampling of the waveform.  
CrownAudio,Inc.  
P.O. Box1000  
Elkhart,IN46515-1000  
TEL:219-294-8200  
FAX:219-294-8FAX  
Class-I amplifiers also have all of the nearly ideal power  
converter attributes of class-D PWM amplifiers, in that  
reactive loudspeaker loads are easily driven. The reac-  
Trademark Notice:  
Crown® and BCA® are registered trademarks of Crown International.  
Printed in U.S.A.  
137234-1  
12/03  
 

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