6.1 Overview
Micro-Tech amplifiers incorporate several tech- nological advancements including real-time computer simulation of output transistor stress, low-stress output stages and an advanced heat sink embodiment.
Custom circuitry is incorporated to limit tem- perature 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-destruct- ing.
Micro-Tech amplifiers are protected from all common hazards that plague high-power amplifiers including shorted, open or mis- matched loads; overloaded power supplies; excessive temperature, chain-destruction phe- nomenon, input overload and high-frequency blowups. The unit protects loudspeakers from input and output DC, as well as turn-on and turn-off transients.
Real-time computer simulation is used to cre- ate an analogue of the junction temperature of the output transistors (hereafter referred to as the output devices). Current is limited only when the device temperature becomes exces- sive—and only by the minimum amount required). This patented approach called Out- put Device Emulation Protection (or ODEP) maximizes the available output power and pro- tects against overheating—the major cause of device failure.
Crown also invented the four-quadrant topol- ogy used in the output stages of each Micro- Tech amplifier (see Figure 6.1). This special circuitry is called the Grounded Bridge. It makes full use of the power supply by deliver- ing 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 constructed as gigantic NPN and PNP devices to handle currents which exceed the limits of available devices. Each output stage has two composite NPN devices 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 deliv- ered to the load by increasing conductance simultaneously in the high-side NPN and low- side PNP stage, while decreasing conductance of the high-side PNP and low-side NPN.
The two channels may be used together to dou- ble the voltage (Bridge-Mono) or current (Par- allel-Mono) presented to the load. This feature gives you flexibility to maximize power avail- able 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 distor- tion values.
Aluminum extrusions have been widely used for heat sinks in power amplifiers due to their low cost and reasonable performance. But mea- sured on a watts per pound or watts per volume basis, the extrusion technology doesn’t perform nearly as well as the heat sink technology developed for Micro-Tech amplifiers.
The heat sinks in a Micro-Tech amplifier 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 massive heat spreaders that are electrically at the Vcc potential. Making the heat spreaders electrically alive improves ther- mal performance by eliminating the insulating interface underneath each power device. The chassis itself is also used as part of the thermal circuit to maximize utilization of the available resources.
6.2 Circuit Theory
Each channel is powered by its own power transformer T100 or T200. Both channels share a common low-voltage supply. The secondary output of T100 is full-wave rectified by D109 and is filtered by a large computer grade capac- itor. A thermal switch embedded in the trans- former protects it from overheating.
The low-voltage fanformer is rectified by diodes D1, D2, D3 and D4 to generate an unregulated 24 volts. Monolithic regulators U1 and U2 pro- vide a regulated ±15 volts.
