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Yamaha Power Amplifier
White Paper
August 2008
Table of Contents
1. About EEEngine ...............................................................................................................2
1.1.
Introduction...............................................................................................................................................................2
1.2.
Explanation of different amplifier topologies ...........................................................................................................2
2. Yamaha technology..........................................................................................................5
2.1.
Dual mono-amplifier structure..................................................................................................................................5
2.2.
Full resonance switching power supply ....................................................................................................................5
3. Behavior of the amplifier under heavy load condition ..................................................6
3.1.
Importance of stable 2 ohm load capability ..............................................................................................................6
3.2.
Comparison of amplifiers at lower impedance situations .........................................................................................7
3.3.
Explanation of results of listening test using music source.......................................................................................8
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1. About EEEngine
1.1. Introduction
Yamaha Power Amplifier Philosophy
Our approach to manufacturing power amplifiers is
simple; pure and natural amplification of the input signal.
Mixed audio is sent to the amplifier from the mixing
console, to be amplified before final “coloring” or
“flavoring” through the loudspeakers. The role of
amplifiers in a sound system should not be to add its
own color, but to be faithful to the input signal to give
you maximum control over the final sonic performance.
[ Fig.3 ] Output signal of Yamaha T5n amplifier; Natural and true to the input
Reliability is another important feature of Yamaha
amplifiers. All Yamaha products are also tested under
severe conditions and must comply with Yamaha’s strict
quality assurance standards. Read more about Yamaha’s
quality assurance testing and standards at:
http://www.yamahaproaudio.com/topics/leading_technol
ogy/quality_control/index.html
1.2. Explanation of different amplifier
topologies
There are many different amplifier topologies, or circuit
design principles that are used in professional power
amplifiers. The majority of the high power amplifiers
seen in the professional audio industry today can be
classified as derivatives of three major technologies;
Class H, Class D or a hybrid of Class AB and Class D
such as Yamaha’s EEEngine (Energy Efficient Engine).
[ Fig.1 ] The input signal, 70Hz burst sine wave.
Class AB
Class AB technology is the foundation of professional
amplification. Even to this day, Class AB amplifiers can
be found in many professional audio applications. This
topology, which had been the norm in the industry for
decades, offers a simple circuit configuration and superb
sound quality. Yamaha’s older amplifiers such as P2200
released in 1976 and PC2002M, released in 1982 were
Class AB amplifiers. Class AB topology, however, has a
drawback of always requiring its output stage to drive at
maximum voltage output, resulting in a great deal of
heat dissipation. This low efficiency is the reason why
Class AB amplifiers are comparatively limited in output
power considering their unit size and weight. When
[ Fig.2 ] Output signal of a typical competitor amplifier.
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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.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.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.6 ] Class D operational waveform.
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EEEngine
EEEngine combines the sound quality of Class AB
circuitry while maintaining the efficiency of Class D
circuitry. Combining positive aspects of both Class AB
and Class D may seem simple by concept, but it took
years of extensive engineering efforts to achieve this
technology on a mass production base.
[ Fig.7 ] EEEngine operational waveform
EEEngine overcomes problems conventional amplifier
topologies while providing advantages in all areas,
offering a dramatic leap in power amplifier design. It
realizes efficiency that matches Class D without
compromising the sound quality of a Class AB amplifier.
The patented EEEngine technology is scalable and can
be found on a wide range of Yamaha power amplifiers
from the value class P series to the flagship TXn series.
EEEngine vs competitor technology
There is one well respected amplifier manufacturer with
a proprietary amplifier topology which shares the same
concept of combining Class AB amplification and Class
D power supply operation. Both technologies track the
audio signal to always provide the minimum power
required for the final output stage. Two technologies are
different however, in its tracking operation methods.
EEEngine tracks the audio signal to always provide the
minimum power needed for the final output stage,
allowing for surprising improvement in efficiency. It
utilizes Class D operation to provide the power at the
final output stage of Class AB operation. Almost all of
the current energy is output as the audio signal, and just
a small fraction of the remaining energy is emitted as
heat dissipation through the heatsink.
Signals of higher frequencies require a higher slew rate*,
and are harder to track. Slew rate is a measure of the
ability of an amplifier to respond to very fast changes in
signal voltage. To compensate for the inability to keep
up with changes in signal voltage, this competitor
technology adds a delay to the input signal. This delay
gives the Class D power supply more time to respond to
sharp changes in voltage, but it must be noted that
manipulating the audio signal will inevitably have effect
on the final sonic quality.
With the final output stage operating as Class AB, the
output signal is of remarkably high sound quality. There
is none of the deterioration of frequency response and
damping factor or unwanted EMC, as conversion of the
audio signal to a PWM signal does not take place. Plus,
EEEngine is designed to operate perfectly while keeping
the power amplifier heat generation to a minimum,
regardless of the load requirements. All together
EEEngine offers Class AB sound quality with efficiency
that matches Class D. EEEngine circuitry was uprated
for TXn and Tn series amplifiers with a new high
efficiency electrical current buffer FET driver circuit to
withstand the power and 2 ohm loads that the amplifiers
will drive.
Yamaha’s EEEngine takes a different approach to
compensate for Class D power supply’s limitation in
keeping up with sharp changes in voltage by adding an
auxiliary “high speed buffer” power supply. This high
speed power supply circuit is activated only when Class
D power supply alone is not able to keep up with the
speed. This “high speed buffer” mechanism allows
EEEngine to respond to quick voltage changes without
manipulating the audio signal and degrading sound
quality. The elimination of unwanted and excessive
components to the audio line is a reflection of Yamaha’s
philosophy of delivering natural output signal that is
faithful to the input signal.
* Slew rate affects the ability of an amplifier to accurately
render complex waveforms at high power levels. A higher
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slew rate is however, preferable only to a point. A higher slew
rate will give the amplifier a wider bandwidth, and when in
excess it will ultimately result in amplification of signals even
in the radio frequency range. This will waste energy, create
distortion and also put undesirable stress on the speaker unit.
channel amplifiers incorporating a symmetrical dual
mono-amplifier design, with each mono amplifier
having its own power supply. Dual mono-amp structure
plays an important role in achieving separation between
the two channels. Having a dedicated power supply on
each mono-amplifier minimizes interference between
the channels, preventing powerful bass notes on one
channel from taking power away from the other channel,
for example. The two power supplies operate in opposite
phases, synchronizing to cancel noise and lowering
electromagnetic interference.
[ Fig.8 ] Circuit of a competitor amplifier. To allow Class D power supply more
time to respond to quick changes in voltage requirements, all audio goes
through a delay. Effect on sonic quality cannot be avoided with this
manipulation of the audio signal.
[ Fig.10 ] Dual mono-amp structure. Each channel has a dedicated power
supply.
The amplifiers are also carefully designed to suppress
internal vibration within the amplifier that could have a
negative impact on sound quality. The top surface of the
heatsink is reinforced to reduce vibration to the power
transistors that are located on top of it. The heatsink
itself is fastened to the chassis side panels at numerous
strategic points with special insulators that are designed
to absorb vibration and chassis resonance that interfere
with optimum reproduction.
[ Fig.9 ] EEEngine’s “High speed buffer” is activated only when Class D power
supply is not able to keep up with sharp increases in sound. This circuitry
allows EEEngine to maintain a preferable slew rate without manipulating and
degrading the audio signal.
2.2. Full resonance switching power supply
The power supply plays a crucial role in the quality of
any amplifier. Full resonance switching power supplies
found on TXn, Tn and PC1N series amplifiers processes
two types of switching; Zero Voltage Switching and
Zero Current Switching. Full resonance power supplies
provide voltage and current waveforms with natural
curves, significantly reducing harmonic components
from switching noise. Typical switch mode power
2. Yamaha technology
2.1. Dual mono-amplifier structure
Yamaha power amplifier technology – mechanical
design
The TXn, Tn and PC9501N series amplifiers are 2
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supplies employ what is typically called “hard
switching,” which induces more noise into the DC
output and gives square waveforms rich in high
frequency harmonics, requiring an additional filter to
remove them. “Soft switching” as seen in full resonance
switching on the other hand, produces natural
waveforms that are desirable for music playback.
3. Behavior of the amplifier under heavy
load condition
3.1. Importance of stable 2 ohm load capability
Tn and TXn series were developed with the concept of
stable operation under 2 ohm load. We do not
necessarily suggest power amplifiers to be configured
for a 2 ohm load setup. However, we recognize that
stability under extreme low impedance is very important
for professional use power amplifiers. For example, in
the use of dual subwoofers, woofer units with nominal
impedance of about 6 to 8 ohms are typically connected
in parallel, giving the amplifier a load of 3 to 4 ohms.
Line array speakers are also often connected in parallel,
requiring stability at lower impedance. The actual
impedance curve of a speaker unit is complex and its
load varies greatly depending on frequency. A
loudspeaker’s lowest actual impedance is usually lower
than its nominal impedance. Because of this impedance
curve, an operator may unknowingly put extreme stress
to the amplifier with a source that repeatedly hits the
frequencies most demanding (lower impedance) for the
loudspeakers. Because an amplifier is put under
extremely demanding conditions at times, it is important
that there is enough headroom to keep the amplifier
from clipping.
[ Fig.11 ] Current and voltage of a typical competitor power supply. Visibly
much higher noise content can be observed (circled in red). Voltage
waveform shown in yellow, and current waveform shown in blue.
When an amplifier clips, its output signal is distorted
and a rectangular waveform is observed. A rectangular
wave contains very high frequency and this causes voice
coils of the loudspeakers to burn out. Clipping of the
output signal, which may potentially destroy speaker
units in the system, must be prevented in a professional
audio system. An amplifier’s ability to maintain stable
operation at lower impedance is essential as an amplifier
is more likely to clip under lower impedance.
[ Fig.12 ] Yamaha’s Full-resonance switching power supply. Smooth, natural
waveforms with minimum switching noise. Voltage waveform shown in yellow,
and current waveform shown in blue.
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3.2. Comparison of amplifiers at lower
impedance situations
Below are oscilloscope measurements to visualize
differences in behaviors of some of the better known
power amplifiers available today. The test signal is sine
wave (200 cycles of 500 Hz = 0.4 sec) followed by 1.2
seconds of interval (no signal). This frequency can be
found in many typical program materials, and an interval
was set because continuous playback of sine waves is
not realistic in actual sound reinforcement applications.
[ Fig.13 ] A typical impedance curve of a bass reflex woofer. The nominal
impedance is 4 ohms but the lowest impedance is below 4 ohms.
1.2sec interval
A+
A-
0.4sec
Time
500Hz x 200 cycle
sine wave
Oscilloscope screen
B+
B-
Signal Generator
0.4sec
This is a comparison of various power amplifiers in the
market, all of which are rated from 2500W to 3000W at
2 ohms. Voltage gain and input levels have been
carefully measured and adjusted for a fair comparison.
A+
AB+
B-
2 ohm
Dummy load
Digital
Oscilloscope
2 ohm
Dummy load
[ Fig.14 ] Set up overview
[ Fig.15 ] The input signal. The same waveform with a higher amplitude is
desired for the output signal of the amplifier.
[ Fig.16 ] Output of Yamaha’s T5n amplifier (2500W @ 2ohms). Output signal
is very true to the input signal.
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[ Fig.17 ] Output signal of an amplifier, “Competitor A”. (2500W @ 2ohms)
[ Fig.19 ] “Competitor C” - Rated at 3300W into two ohms, this amplifier
muted quickly after its limiter kicked in. (3300W@ 2 ohms)
The oscilloscope measurement on Fig. 17 shows strong
compression-like behavior on its output signal. The
output signal shows no resemblance of the input sine
wave. This behavior was not seen when the amplifier
was driven only on one channel, but quickly became
unstable when driven on both channels. We believe this
to be result of overstressing of the one power supply that
supplies power for both channels.
Fig.19 shows the output signal of amplifier model,
“Competitor C”. Though this amplifier is rated at
3300W into two ohms and specified to have the highest
power of all the amplifiers in this comparison, the
oscilloscope trace shows contradicting results. Its limiter
kicked in, drastically dropping output voltage. Though
not apparent on one still image of the measurement, it
took a few seconds for the output voltage to recover,
only to have the limiter activated once again shortly
afterwards. This behavior repeated for the entire
duration of this test.
These results show that different amplifiers behave
differently under low impedance operation. The results
of the comparison also prove that actual performance of
an amplifier cannot always be predicted from its catalog
specifications. Because there are no industry standards
for amplifier specifications, paper comparison of figures
such as output power is not very practical.
[ Fig.18 ] An oscilloscope measurement of amplifier model “Competitor B”.
(2900w @ 2 ohms)
3.3. Explanation of results of listening test
Fig.18 shows that Competitor B, rated 400 watts higher
than the T5n at two ohms, seems to start out well but
quickly loses power and its output voltage drops. This
behavior was observed when the amplifier was driven
on both channels.
using music source
We conducted the above experiment using a music
source. To replicate a more realistic setup, we replaced
the dummy load on one channel with four loudspeakers
connected in parallel. To reduce interference between
the four loudspeakers and also to reduce stress on our
ears, we verified our results from one reference
loudspeaker; the remaining three loudspeakers were
placed in a remote location.
The results of this listening test were basically
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reproductions of the oscilloscope measurements. The
kick drums on “Competitor A” were heavily distorted,
extremely harsh on the ears and harmful to the
loudspeakers as well. Its playback level fluctuated after
the kick drum, as was observed in the oscilloscope
measurement.
The amplifier muted for a few seconds before returning.
This limiter may protect the amplifier from damage, but
this behavior is unacceptable in a live situation.
Yamaha’s T5n showed positive results in this test. The
T5n showed minimal limiting and had the most
headroom among the competitors. The amplifier’s
output did show slight distortion when levels were high,
but the playback remained musical and had the best
performance in this comparison.
Competitor B’s output was considerably distorted when
louder notes were repeated. The amplifier’s limiter
kicked in on “Competitor C” after the kick drum beat.
[ Fig.20 ] Listening test set up
P.O.BOX 1, Hamamatsu, Japan
http://www.yamahaproaudio.com
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