6 Principles of Operation; 6.1 Overview; 6.2 Circuit Theory- Page 15

Micro-Tech Series

 Power Amplifiers

page 15

Operation Manual

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.

6 Principles of Operation