Class 'T' Explained - Elan D1650 Installation Manual And User's Manual

ELAN HOME SYSTEMS
'Class T' Digital Technology Explained
The ultimate objective of any audio amplifier design
is to make a high fidelity amp with high efficiency
and high reliability. There are several basic audio
power amp topologies that have been developed to
attain these objectives: Class-A, Class-B, Class-
AB, Class-H, Class-G and Class-D are the most
common. The D1650 utilizes proprietary Class-T
topology from Tripath™. Class T combines the
best attributes of several of these designs and mini-
mizes deficiencies in each design, as well.
Class-A, Class-B and Class-AB amps have been
around for over fifty years. Basically, these classifi-
cations designate the amount of time that the amp's
output devices conduct during one full cycle of a
periodic signal. Class-A amps are in a state of con-
duction 100% of the time. Class-B amps have a
complimentary pair of outputs, which are biased so
that each output is conducting only 50% of the time.
Class-AB amps also have complimentary output
pairs but they are biased so that each output is con-
ducting slightly more than 50% of the time: this low-
ers crossover distortion. The vast majority of audio
amps in use today are Class-AB. A well-designed
Class-AB amplifier has good linearity (high fidelity)
and poor efficiency (less than 50%). Class-H and
Class-G are both voltage-supply varying techniques
which are usually applied to Class-AB type, linear
amplifiers. These techniques give marginal
improvement in efficiency at the cost of a more
complex and less reliable power supply.
Class-D amplifiers use output devices which switch
on and off at a fixed frequency. This frequency is
usually more than ten times higher than the highest
frequency to be amplified. A passive filter recon-
structs the wave form passing through the amplifier
and removes switching artifacts that distort sound.
Class D amplifiers use output devices that are either
ON or OFF; never in a state of mid-conduction. This
mid-conduction state is what causes linear switch-
ing amplifiers to be as inefficient as they are (less
than 50% efficiency). Class D amplifiers are approx-
imately 85% efficient: a 35% increase!
As mentioned, each of these amplifier designs has
drawbacks. Class D amps have tendencies toward
high distortion rates. Crossover distortion, ground
bounce, and high frequency artifacts create most of
the distortion in these designs. Imperfectly
matched transistors lead to inexact ON/OFF timing
results and crossover distortion issues. Ground
Bounce caused by high-current switching of the
output transistors manifests itself as noise on the
audio output. In some Class D amplifiers, this high-
frequency noise is not completely filtered out,
resulting in high frequency distortion.
Page 8
D1650/D1651
Advantages of Class T Design
Class T is a combination of Adaptive Digital Signal
Processing and Spread-Spectrum Switching. This
design takes the efficiency of a Class D amp and
combines it with the fidelity of a Class AB amplifier
by dramatically improving signal integrity.
Class T offers the following improvements over
Class D:
1. Class D has a fixed switching input. Class T
has an adaptive switching frequency which is
dependent upon both input signal frequency
and magnitude. Switching artifacts are
removed in this way, reducing distortion. The
switching signal is constantly being optimized
to match the input signal in order to yield the
highest possible fidelity.
2. Class D amplifiers have nominal switching fre-
quencies between 200kHz and 300kHz which
creates artifacts in the 20 to 50kHz audio band.
This can be heard as audible noise. Class T
amplifiers have nominal switching frequencies
between 600kHz and 700kHz; artifacts from this
frequency are not audible.
3. Class T design constantly monitors the output
transistors and adaptively corrects for varia
tions between and within these transistors. The
Class T design also monitors and corrects for
ground bounce that the transistors produce
when switching large currents.
4. Typical power efficiency with a Class T amplifier
is 85% (unreachable by class A-B amps).
Typical THD + Noise is less than 0.04%
(unreachable by Class D amps.)
Truly the best of both worlds!
© ELAN Home Systems, Inc. 2004 • All rights reserved. 7/04
INSTALLATION MANUAL