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Micro-Tech 600/1200/2400 Power Amplifiers

Reference Manual

5 Technical Information

5.1 Overview

Micro-Tech

amplifiers incorporate several technological

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 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.

Micro-Tech

amplifiers are protected from all common

hazards that plague high-power amplifiers including

shorted, open or mismatched loads; overloaded power

supplies; excessive temperature, chain-destruction

phenomenon, input overload and high-frequency blow-

ups. The unit protects loudspeakers from input

and out-

put DC, as well as turn-on and turn-off transients.

Real-time computer simulation is used to create an ana-

logue of the junction temperature of the output transis-

tors (hereafter referred to as the output devices).

Current is limited only when the device temperature

becomes excessive—and only by the minimum

amount required). This patented approach called Out-

put Device Emulation Protection (or

ODEP

) maximizes

the available output power and protects against over-

heating—the major cause of device failure.

Crown also invented the four-quadrant topology used

in the output stages of each

Micro-Tech

amplifier (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 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 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.

The two channels may be used together to double the

voltage (Bridge-Mono) or current (Parallel-Mono) pre-

sented to the load. This feature gives you flexibility 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 have been widely used for heat

sinks in power amplifiers due to their low cost and rea-

sonable performance. But measured on a watts per

pound or watts per volume basis, the extrusion technol-

ogy doesn’t perform nearly as well as the heat sink tech-

nology developed for

Micro-Tech

amplifiers.

The heat sinks in a

Micro-Tech

amplifier are fabricated

from custom convoluted fin stock that provides an ex-

tremely 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. Mak-

ing 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 utili-

zation of the available resources.

5.2 Circuit Theory

Each channel is powered by its own power transformer

T100 or T200. Both channels share a common low-volt-

age supply. The secondary output of T100 is full-wave

rectified by D109 and is filtered by a large computer

grade capacitor. A thermal switch embedded in the

transformer 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 provide a regulated ±15 volts.

5.2.1 Stereo Operation

For simplicity, the discussion of Stereo operation will re-

fer to one channel only. Mono operation will be dis-

cussed later. For specific circuit references, see the

block diagram in Figure 5.1.

The signal at the ¼-inch phone jack input passes di-

rectly to the balanced gain stage (U104-A and U104-B).

The balanced gain stage causes balanced to single-

ended conversion using a difference amplifier. From

there, gain can be controlled with a potentiometer. The

error amp (U104-C) amplifies the difference between

the output signal and the input signal from the gain pot,

and drives the voltage translator stage.

From the error amp, the voltage translator stage routes

the signal to the Last Voltage Amplifiers (LVAs) based

on signal polarity. The +LVA (Q105) and the –LVA (Q110),

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