User Manual

3
driven with typical program material with occasional
clipping (1/8 power), Class AB topology typically
achieves around 20% efficiency*, meaning that 80% of
power drawn is lost as heat. Various methods have been
developed to overcome this drawback which led to the
development of Class H and Class D topologies.
* Efficiency rate within this document refers to overall
efficiency of the power amplifier including its mains power
supply. Efficiency is calculated at 1/8 of rated output power,
which is a reference of typical program material with
occasional clipping.
[ Fig.4 ] Class AB operation waveform.
Class H
Class H uses a method that switches the power supply
voltage level according to the input signal. This can
vastly improve output stage heat dissipation by
providing low voltage when the signal level is low.
However, as the signal level increases, the system
functions in the same way as a Class AB system, and
efficiency is lost. Class H loses efficiency when fed
music signals with a wide dynamic range. A system that
uses a multi-step voltage switching method may easily
come to mind to overcome this problem, but this would
create many complications such as increased switch loss,
making it impractical as a solution. Class H amplifiers
typically have efficiency of around 30%. Yamaha’s
P5002 amplifier released in 1982 was an early adopter
of Class H topology.
[ Fig.5 ] Class H operational waveform.
Class D
Often misunderstood as an abbreviation for “digital”,
Class D utilizes PWM, or Pulse Width Modulation. First,
a PWM signal is created from the input audio signal.
The power supply voltage is then switched according to
the pulse width, creating a high power PWM signal to
drive the loudspeaker. The elements used for the
switching operation require only a minimum of voltage,
allowing vast improvements in efficiency compared to
previous amplifier topologies. Class D amplifiers
typically have efficiency of around 60%. However, to
convert the audio signal to a rectangular wave PWM
signal, a high power consuming low-pass filter must be
used at the output stage to eliminate pulse, or the
original audio signal cannot be recovered. The audio
signal’s frequency response, distortion, and damping
factor are affected by the low-pass filter. High power
PWM signals also have the side effect of emitting
harmonic electromagnetic (EMC) waves within the
radio frequency range of up to a few megahertz. Class D
amplifiers may be convenient on the efficiency side, but
often face difficulties in achieving optimal sonic quality
and many manufacturers are attempting to work their
way around this problem.
[ Fig.6 ] Class D operational waveform.