- CROWN INTERNATIONAL, INC. LABORATORY AMPLIFIER INSTRUCTION MANUAL DC-300A

Section 4
CIRCUITRY
4.1 PRINCIPLES OF OPERATION
The DC-300A has two totally direct-coupled amplifier
circuits which employ a dual IC op amp and silicon
transistors in all stages. The CROWN-designed and
developed circuit represents a level of quality and per-
formance presently unequaled in the field of audio am-
plifier design.
As is implicit in the term “totally direct-coupled,” the DC-
300A has a perfectly flat frequency and phase response
extending to OHz or DC. Flat to DC response results in not
only low frequency amplification with absolutely no phase-
distortion, but also in perfect overloadcharacteristics. Non-
symmetrical waveforms (such as music) cause overload
thumping in all currently produced AC amplifiers. These
same amplifiers may, however, show no signs of thumping
when fed a symmetrical test waveform such as a sinusoid.
DC frequency response combined with ultra-low noise and
IM distortion results in the closest approach to a “straight
wire with gain.”
Another characteristic of a DC amplifier is the thump or pop
produced at turn-on and turn-off. For example, at turn-on
the input amplifier requires a finite period of time to reach
operating levels. During this time the output could be driven
to large DC offsets resulting in annoying thumps.
A supply voltage detector virtually eliminates this problem
in the DC-BOOA. The detector disconnects the regulated
supply voltages to the output stages during these turn-on
and turn-off periods, thus not allowing the DC offsets at the
output to occur.
The dual IC op amp is of a low noise type having a large gain
bandwidth. The result of using it for the input voltage
amplifier is that a maximum amount of feedback is applied
reducing distortion to record low values. This has been
confirmed by measurement with an elaborate test setup
employing CROWN-developed solid-state variable filters
and wave analyzing equipment. No other presently avail-
able harmonic distortion test apparatus is capable of
such low residuals. The 300A’s low distortion is achieved
by employing multiple feedback loops to allow a max-
imum of total feedback.
The lack of noise is evidenced by a typical 20Hz-20KHz ef-
fective input noise of 1.25~ volts which produces an
effective 8 ohm output of 80 micro-micro (pica) watts.
The output stage is a quasi-complementary format em-
ploying the CROWN class AB+8 technique which uses no
bias current in the output transistors. The result is
maximum efficiency with minimum crossover notch dis-
tortion and amplifier idling-heat. Thus there is no bias
current adjustment,
as the output circuit is not
temperature-tolerance critical. Temperature drifts of bias
are further controlled by bias servos which are mounted on
the heat sinks.
In the AB+8 output circuit, the driver transistors carry the
bias current, while the output transistors serve only as
boosters. The output transistors (12OOW dissipation/Ch)
sensewhen the driver transistors are delivering significant
current to the load and take over and deliver the large load
currents.
Protection against shorted and low impedance loads is
provided by the CROWN-developed SPACE (Signal Pro-
grammed Automatic Current Executor) control circuit. It
functions as an automatic current limiter at audio fre-
quencies and as a VI limiter at subaudio frequencies. The
threshold of current limiting is dependent on the history of
the signal, yet the no-signal threshold of current limiting is
high enough to allow full power tone bursting. The net
result is total protection with a maximum of headache-free
output power requiring neither an inventory of special fuses
or cumbersome load matching techniques.
The monolithic input amplifier stages result in extremely
low DC drift. The input terminal bias current is offset by a
unique temperature compensated source resulting in a
laboratory amplifier needing no user-accessible offset
controls.
The input amplifiers are powered by zener-regulated power
supplies. The bias regulators are also powered by zener-
regulated current sources with the result that line voltage
variations do not cause noise or distortion due to misbias-
ing.
The power supply is a continuous-duty type, capable of
1 KW loading. The power transformer, weighing 25 pounds,
is constructed of special grain-oriented steel. The main DC
supplies arefull-wavecapacitor inputtypewith heavy-duty,
chassis heat-sinked diodes. Computer grade electrolytics
furnish over 48 joules of energy storage. A higher voltage at
low current is derived from a half-wave voltage-doubler cir-
cuit. This voltage is used in the amplifier’s driver circuit.
The DC-300A represents nothing short of the highest
quality in both circuitry and components. It should provide a
lifetime of trouble-free service for the most discriminating
users.
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