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 reconstructs 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 switching amplifiers to be as inefficient as they are (less than 50%
efficiency.) Class D amps approximately 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.
CLASS T TECHNOLOGY-- WHY IT'S BETTER
Class T is a combination of Adaptive Digital Signal Processing and Spread Spectrum
Switching. This design takes the efficiency of a Class D design 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 frequencies 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 out
put transistors and adaptively corrects
for variations 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 amp
lifier 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!
PAGE 6
D1200/D1201 INSTALLATION MANUAL & USER'S GUIDE ELAN HOME SYSTEMS
©ELAN Home Systems 2003 All rights reserved 07/03
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