Crown Audio MA-36X12, MA-24X6 Transformer Thermal Protection, Circuit Breakers, page

Macro-Tech MA-24X6 & 36X12 Power Amplifiers

4 Advanced Features

and Options

4.1.4 Transformer Thermal

Protection

All Macro-Tech amplifiers have transformer thermal protection. It protects the power sup- plies from damage under the rare conditions of transformer temperatures rising too high. A thermal switch embedded in each transformer removes power to the channel if there is exces- sive heat. The switch automatically resets when the transformer cools to a safe temperature.

It is very unlikely that you will ever see a Macro-Tech amplifier activate transformer ther- mal protection as long as it is operated within rated conditions (see Section 7, Specifica- tions). One reason is that ODEP keeps the amplifier working under very severe conditions. Even so, higher than rated output levels, exces- sively low impedance loads and unreasonably high input signals can generate more heat in the transformer than in the output devices. These conditions can overheat the transformer and activate its protection system.

Macro-Tech amplifiers are designed to keep working under conditions where other amplifi- ers would fail. But even when its limits are exceeded, it will still protect itself—and your investment—from damage.

4.1.5 Circuit Breakers

The power supplies are protected by circuit breakers. With rated loads and output levels, the circuit breakers should only shut down the amplifier in the rare instance of a catastrophic failure. Other protection systems like ODEP keep the amplifier operational under most other severe conditions. The circuit breakers can also shut down the amplifier in cases where extremely low-impedance loads and high out- put levels result in current draw that exceeds their rating. Again, this should only be possible when operating outside rated conditions, like

when the amplifier is used to drive a 1-ohm load in Stereo mode, or when a signal over- loads the input and is clipped severely.

Macro-Tech amplifiers do not trip their break- ers unless something is wrong. If a breaker trips, try to identify and correct the problem before resetting the breakers with the back panel Reset switches. If the problem persists, refer the unit to a qualified technician.

4.2 Circuit Designs

4.2.1 Grounded Bridge

Grounded Bridge is the name of Crown's unique four-quadrant amplifier topology. The Grounded Bridge topology takes full advantage of the power supplies delivering peak-to-peak voltages to the load that are twice the voltage seen by the output devices and twice the volt- age generated by the power supplies.

The power supply bridge rectifier is not ground referenced, and the transformer secondary is not center-tapped. This allows the power sup- ply to deliver +VCC and -VCC from the same bridge rectifier and filter as a total difference in potential regardless of their voltages with respect to ground.

Composite output devices are arranged to func- tion as gigantic NPN and PNP devices. Each output stage has two composite NPN and two composite PNP devices. The devices con- nected 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 decreasing conductance of the high-side PNP and low-side NPN in synchrony.

Crown's Grounded Bridge design delivers large voltage swings without stressing output tran- sistors. The results are higher efficiency, lower distortion and superior reliablility.

4.2.2Variable Impedance (VZ) (Macro-Tech 36X12 only)

VZ is the name of Crown's patented articulated power supply technology. This innovative tech- nology permits us to pack large amounts of power into a compact package while achieving ultra-low distortion and without generating excessive heat.

An amplifier power supply must be large enough to handle both the maximum voltage and maximum current necesary for the ampli- fier to drive its rated power into a specified load. In order to meet this requirement, most conventional supplies are heavy, large, and produce lots of heat. In contrast, the VZ supply gets more current AND voltage out of a smaller, lighter, and more efficient package by dynami- cally adapting to both signal and load require- ments in real-time. This provides the best power match to the widest range of loads.

The VZ supply is divided into two segments. When the output stage requires high-voltage, the segments are arranged in series to deliver twice the voltage of a single segment. When the output stage requires high-current, the seg- ments are arranged in parallel to deliver twice the current of a single segment. Sensing cir- cuitry "watches" the voltage of the signal to determine when to switch VZ modes. The switching circuitry is designed to prevent audi- ble switching distortion to yield the highest possible dynamic transfer function—you hear only the music and not the amplifier.

With VZ, you get not only maximum power and safety, but you also get the best power match- ing to your load.

Operation Manual page 15