B Series Bass Amplifiers
D:\RST\MISC\B Series Ampifiers THD measurement.doc
Created by R&D3 On 08/12/00
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Traditionally, Class A and Class A-B amplifiers have dominated the amplifier marketplace. These purely analogue
devices have low power efficiency and most integrated circuit Class A-B amplifiers fall short of true high-fidelity audio
quality.
Class D amplifiers however, solve the efficiency problem by using switching pulse-width modulation technology.
However, this produces audio quality that is far inferior to class A or AB, so efficiency is gained at the expense of audio
fidelity. The ‘Holy Grail’ of amplification: High audio quality AND high power efficiency has long eluded the market.
Enter the B range of Bass Amplifiers delivering audiophile performance and very high efficiency.
If you have measured the output waveforms of B Series Bass Amplifiers using traditional
equipment and methods, you may have observed an unusual characteristic in the output
waveform when the amplifier is driven close its maximum output. This characteristic
appears to be a high frequency oscillation near the waveform peaks.
In reality, this apparent oscillation is an audio quality enhancement characteristic of the
B series amplifier. The apparent oscillations are well outside the audio bandwidth, and
are a unique feature of the digital power processor.
No Filtering on output
As the input signal, to any conventional amplifier is increased, at some point the output of the amplifier will distort/clip
i.e. ‘Square’ off. This is bad news because the for a split second the loudspeaker is not moving at all but still dissipating
potentially huge power levels, this is the main cause of (expensive) compression driver failure.
When the B Series Bass Amplifier goes into this distortion region, it exhibits high frequency break-up of the peaks as
shown above. This is actually the amplifier resetting the control processor overloads at a
frequency far above the audible
range.
This is not instability or oscillation, nor does it degrade the performance of the amplifier in any way. In addition
the B Series Bass Amplifiers also have the ability to run almost rail-to-rail, and thus will not lose negative feedback. In
other words, it will retain control of the loudspeaker at all times.
Measurement of THD%+N
Unless your distortion analyser is designed to reject out of band noise when measuring THD (And very few are), the THD
measurement actually reflects harmonic distortion plus any noise present up to the bandwidth of the analyser and so is
actually a measurement of THD+N. Since the B Series range of amplifiers have energy far outside the audible frequency
range (As do inferior class D amplifiers), a test designed for linear amplifiers (such as class A, AB) will not yield valid
results when applied to a B Series amplifier. A sharp cut-off low-pass filter must be inserted between the amplifiers
outputs and the test equipment’s inputs prior to making any distortion measurements. Some measurement equipment
(Such as Audio Precision2) have high quality low pass filters which can be used to limit the test bandwidth. Using these
filters (as shown below) you will achieve what ‘looks’ like a high quality linear amplifier. Even though there appears to
be ‘lots going on’ outside the audio bandwidth, the B amplifier is designed to be well within current and future EMC
guidelines, and as such is quieter than most linear amplifiers.
Output with 30Khz Low-pass filter
