Technical Information; Principles Of Operation - Crown DC-300A II Reference Manual

DC-300A II Power Amplifier

5 Technical Information

5.1 Principles of Operation

The DC-300A II has input amplifiers that are powered
by zener-regulated power supplies. The bias regula-
tors are also powered by zener-regulated current
sources with the result that line voltage variations do
not cause noise or distortion due to misbiasing.
The power supply is designed for continuous duty and
supports 1 kilowatt loading. The power transformer
weighs 25 pounds (11.3 kg) and is made of a very
special steel. The main DC supplies are full-wave sup-
plies with heavy duty, heat-sinked diodes. Computer
grade electrolytic capacitors furnish over 48 joules of
energy storage. A higher voltage at low current is de-
rived from a half-wave voltage-doubling circuit. This is
used in the amplifier's driver circuit.
The amplifier has two direct-coupled amplifier circuits
employing dual IC opamps and silicon transistors in all
stages. Because of its direct coupling, the amplifier
has a perfectly flat frequency and phase response ex-
tending from DC (0 Hz) to 20 kHz. Flat-to-DC response
not only results in low-frequency amplification with ab-
solutely no phase distortion, but it also offers excellent
overload characteristics. DC frequency response is
combined with ultra-low noise and intermodulation dis-
tortion to deliver the classic rendering of a "straight
wire with gain." In fact, before the advent of Crown's
Reference series amplifiers, the DC-300A II enjoyed a
reputation of being the most accurate amplifier avail-
able.
The two low-noise IC opamps have a large gain band-
width. Using them as the input voltage amplifiers re-
sults in a maximum amount of feedback which reduces
distortion to exceptionally low levels.
The output stage is a quasi-complementary format
employing Crown's AB+B class circuitry which uses no
bias current in the output transistors. The benefit is
maximum efficiency, minimum crossover notch distor-
tion and minimum thermal dissipation at idle. The out-
put circuit does not have a critical temperature
tolerance, so there is no bias current adjustment.
In the AB+B output circuit, the driver transistors carry
the bias current while the output transistors serve only
as boosters. The output transistors sense when the
driver transistors are delivering significant current to
the load, then they take over and deliver the large cur-
rent levels.
The IOC (Input/Output Comparator) circuitry works
with the error correcting signal of the main opamp. If a
small non-linearity exists in the amplifier, an error signal
appears at the output of the main opamp via the unit's
feedback loop. This produces an abnormally high
value, exceeding the threshold of the IOC and illumi-
nating the LED. Because transient overload can hap-
pen very quickly, a pulse stretching circuit is added so
you can see the LED flash.
Protection against shorted and low impedance loads
is provided by Crown's JTS (Junction Temperature
Simulation) control circuit. It functions as an automatic
current limiter at audible frequencies and as a V-I lim-
iter at subaudible frequencies. The threshold of cur-
rent limiting is dependent on the history of the signal,
yet the threshold of the limiting with no signal is high
enough to allow full power tone bursting. The result is
maximum protection with trouble-free output power
without an inventory of special fuses or cumbersome
load-matching techniques.
The DC-300A II gives you the highest quality in both
circuitry and components. It should provide you with a
lifetime of trouble-free service.
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