Overview; Circuit Theory; Stereo Operation - Crown STUDIO AMPLIFIER Owner's Manual

5 Technical Information

5.1 Overview

Studio Reference amplifiers incorporate several new
technological advancements including real-time com-
puter simulation of output transistor stress, low-stress
output stages, an advanced heat sink embodiment
and the Programmable Input Processor ( P.I.P. ) expan-
sion system.
Custom circuitry is incorporated to limit temperature
and current to safe levels making it highly reliable and
tolerant of faults. Unlike many lesser amplifiers, it can
operate at its voltage and current limits without self-de-
structing.
Studio Reference amplifiers are protected against all
common hazards that plague high-power amplifiers in-
cluding shorted, open or mismatched loads; over-
loaded power supplies, excessive temperature,
chain-destruction phenomena, input overload and
high-frequency blowups. The unit protects loudspeak-
ers from input and output DC, as well as turn-on and
turn-off transients.
Real-time computer simulation is used to create an
analogue of the junction temperature of the output tran-
sistors (hereafter referred to as the output devices).
Current is limited only when the device temperature
becomes excessive—and only by the minimum
amount necessary. This patented approach maximizes
the available output power and eliminates overheat-
ing—the major cause of device failure.
Crown also invented the four-quadrant topology used
in the output stages of each Studio Reference ampli-
fier (see Figure 5.1). This special circuitry is called
the grounded bridge . It makes full use of the power
supply by delivering peak-to-peak voltages to the load
that are twice the voltage seen by the output devices.
As its name suggests, the grounded bridge topology is
referenced to ground. Composite devices are con-
structed to function as gigantic NPN and PNP devices
to handle currents which exceed the limits of available
devices. Each output stage has two composite NPN
and two composite PNP devices.
The devices connected to the load are referred to as
"high-side NPN and PNP" and the devices connected
to ground are referred to as "low-side NPN and PNP."
Positive current is delivered to the load by increasing
conductance simultaneously in the high-side NPN and
low-side PNP stage, while synchronously decreasing
conductance of the high-side PNP and low-side NPN.
I I I I I II II II II II
The two channels may be used together to double the
voltage (Bridge-Mono) or the current (Parallel-Mono)
presented to the load. This feature gives you the flex-
ibility to maximize power available to the load.
A wide bandwidth, multiloop design is used for state-
of-the-art compensation. This produces ideal behavior
and results in ultra-low distortion values.
Aluminum extrusions are used widely for heat sinks in
power amplifiers due to their low cost and reasonable
performance. However, measured on a watts per
pound or watts per volume basis, the extrusion tech-
nology doesn't perform nearly as well as the heat sink
technology developed for Studio Reference amplifiers.
Our heat sinks are fabricated from custom convoluted
fin stock that provides an extremely high ratio of area to
volume, or area to weight. All power devices are
mounted directly to the heat sinks which are also elec-
trically at the Vcc potential. Electrifying the heat sinks
improves thermal performance by eliminating the insu-
lating interface underneath the power devices. The
chassis itself is even used as part of the thermal circuit
to maximize utilization of the available cooling re-
sources.

5.2 Circuit Theory

Power is provided by low-field toroidal power trans-
former T1. The secondaries of T1 are full-wave rectified
(by D1 through D4, D22 and D24) and filtered by large
computer grade capacitors. A thermal switch embed-
ded in the transformer protects it from overheating.
Monolithic regulators provide a regulated ±15 volts.

5.2.1 Stereo Operation

For simplicity, the discussion of Stereo operation will
refer to only one channel. Mono operation will be dis-
cussed later. Please refer to the block diagram in Fig-
ure 5.1 and the schematics included with this manual.
The input signal at the phone jack passes directly into
the balanced gain stage (U104-A). When a P.I.P. mod-
ule is used, the input signal first passes through the
P.I.P.'s circuitry and then to the balanced gain stage.
The balanced gain stage (U104-A) causes balanced to
single-ended conversion using a difference amplifier.
From there, gain can be controlled with the front panel
level controls and the input sensitivity switch. The error
amp (U104-C) amplifies the difference between the
Professional Studio Amplifiers