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.
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
