Competing Paradigms in Amplifier and Loudspeaker Design, Test and Measurement.
Article By Ralph Karsten Of Atma-Sphere
Music Systems, Inc.
For the last thirty years I have
been working with low feedback and zero feedback amplifiers. Initially I was
convinced that audio power amplifiers had to include certain elements (such as
negative feedback) to control distortion. After some exposure to Robert Fulton
of FMI (Fulton Musical Industries) in the late seventies, a larger picture began
to emerge. Robert Fulton was emphatic that the quality of the topology came
first, then quality of components, and if everything was right, the distortion
would already be low. Negative feedback could be reduced or eliminated.
Back in the days when
feedback was being experimented with, it was common knowledge that it was a
compromise. In the succeeding decades it became accepted as a fact of audio
design. More recently the negative feedback idea has been challenged by elements
of the high-end audio community.
My own explorations
resulted in an amplifier that was designed to reduce distortion in every way
possible without feedback. In this way it was possible to examine the effects of
feedback, since the amplifier was very functional without it. During the
process, I also learned about common engineering practices that tend to hold
back development in fields such as audio. In their recent book "Control
Design And Simulation," Jack Golten and Andy Verwer discuss this phenomena
in chapter two, with regard to applying mathematical models to the real world:
"...mathematical models invariably involve simplification. Assumptions
concerning operation are made, small effects are neglected and idealized
relationships are assumed."
It is the mark of a
good engineer to know when and which things should be assumed, neglected or
idealized, and we see this in audio all the time. The problem here is human
nature. We tend to stay within the limits thus set by the existing paradigms and
to resist changes that threaten one's viewpoint of the world. When someone else
creates challenges to the paradigms, it is normal also to try to protect one's
world view by preventing the new idea from gaining ground.
As alluded earlier,
negative feedback has been found to be an inexact solution to amplifier
distortion. This is due to propagation delays (the very small but measurable amount of time it takes for a signal to move from the input of an amplifier to
the output) which are a normal phenomenon of amplifiers. In order for negative
feedback to work according to the math, it must be applied to counter the input
signal in real time. Propagation delay in the amplifier circuit prevents this;
the negative feedback will always be lagging the original input signal. This lag
results in ringing effects and enhancement of odd-ordered harmonics that the
human ear/brain system is particularly sensitive to: in General Electric's tests
conducted in the 1960s, amounts of only hundredths of a percent were found to be
audible and irritating to the human ear. In other words, a disparity exists
between the mathematical proof for negative feedback and its actual application,
and is example of the engineering phenomena to which Golten and Verwer refer.
Despite this, negative feedback is commonly accepted in the audio world, causing
the reigning design, test and measurement paradigm to have a built-in weakness.
The Voltage Paradigm
is the reigning design, test and measurement paradigm for amplifiers and
loudspeakers, and is seen in many audio magazines. It assumes that the voltage
response of an amplifier is the only aspect of the amplifier that matters. If a
speaker is being tested, the test signals will be voltages (the power of the
test signal is not considered). The ideal Voltage Paradigm amplifier produces
zero THD, has wide bandwidth and is "Load Impervious."
Being Load Impervious
is not exactly what it sounds like! It is called load impervious as the
amplifier will make the same voltage regardless of load; it has a "constant voltage" characteristic. It works like this: if a 'constant
voltage amplifier' were to produce 100 watts into 8 Ohms, it could produce 200
watts into 4 Ohms, possibly 400 watts into 2 ohms (and 50 watts into 16 Ohms).
In all of these cases the output voltage is about 28.28 Volts RMS. We are very
familiar with these characteristics, typical of solid state amplifiers. Under
this model, to be 'load impervious', the amplifier will make different amounts
of power depending upon the load impedance.
under this paradigm are said to be 'voltage driven', as they expect the
amplifier driving them will produce constant voltage despite the speaker's
variable load impedance.
amplifiers inherently employ a fair amount of negative feedback. However as
General Electric has proven, negative feedback is out of sync with the the rules
of human hearing, due to added odd-ordered harmonic generation. We cannot change
our ears, but we can do something.
The Power Paradigm
assumes that amplifiers produce power and speakers are power-driven. Current
produced by a power amplifier is not ignored and is considered in the
amplifier's power response. Under this model, the ideal amplifier will make the
same power into all loads, 4, 8 and 16 Ohms. The typical amplifier in this case
is a tube amplifier. which usually makes its power into these loads via taps of
its output transformer. There are a small number of transistor amplifiers that
are designed with this behavior in mind also. Ideally amplifiers under this
paradigm have little or no feedback. The Voltage Paradigm refers to such
amplifiers as 'current source' amplifiers but the term is not accurate- in
reality they are what they are called- power amplifiers.
Zero feedback power
amplifiers have seen a resurgence in the last two decades, based mostly on their
sonic character. Voltage Paradigm adherents will state that that character is
based on distortion, but the truth of the matter is that what is really at the
heart of it is the lack of distortions that humans find
objectionable. In other words this approach is based on the reality of real
world human hearing, rather than a thought model.
In addition to a
constant power characteristic, the ideal Power Paradigm amplifier will be low in
objectionable distortions, while otherwise having similar qualities to
Voltage Paradigm amplifiers if possible: wide bandwidth being an example.
operate under Power Paradigm rules are speakers that expect constant power,
regardless of their impedance. Examples include nearly all horns, ESLs, magnetic
planers, a good number of bass reflex and acoustic suspension designs. Horns,
ESLs and magnetic planers do not get their impedance curve from system resonance
and so benefit from a constant power characteristic and indeed, many of these
speaker technologies are well-known to sound right with Power Paradigm amplifier
Designers make a
choice regarding which Paradigm they design for. Many speaker designers make the
choice based on creating a speaker that is 'tube friendly' or by default of
owning a commonly accepted transistor amplifier. In other words this choice is
not always conscious, despite there being very concrete rules that govern each
paradigm. In some cases this is not important, but in the case of acoustic
suspension and bass reflex designs, the impedance curve is often derived from a
combination of resonant elements in the enclosure and crossover so proper
performance may only be obtained in these cases by application of an
understanding of these rules.
Objectivist/Subjectivist debate has been raging in audiophile circles for nearly
three decades. Objectivists operate exclusively in the Voltage Paradigm while
Subjectivists tend to operate in the Power Paradigm.
In the world of speakers, efficiency of
the speaker has been an issue that the Voltage camp has had to address, as the
older Power Paradigm specification of 1 watt/1 meter was a 'chink in the armor.'
The new Voltage Paradigm specification, Sensitivity, illustrates the
point: 2.83V/ 1 meter is the spec, resulting in a certain sound pressure level,
expressed in dB, just like the Efficiency spec. 2.83 Volts into an 8 Ohm load is
1 watt. 2.83 Volts into 4 Ohms is 2 watts. Thus, a speaker can have a sensitivity
rating that looks the same as the efficiency rating, but the speaker can
be several decibels less efficient if the impedance is lower. This is an easy
way to cover up how much power it really takes to drive a speaker, and also
creates an expression that moves the efficiency issue into the Voltage Paradigm
nomenclature. It would also seem to create a 'buyer be ware' situation: you have
to know how to interpret the numbers to get to the truth of the matter.
Transistor amplifiers are almost entirely in the
Voltage Paradigm camp whereas most tube amplifiers are in the Power Paradigm.
This is the main distinction that separates Voltage Paradigm from Power Paradigm
amplifier designers but the use of negative feedback is obviously another.
amplifiers measured under the Voltage Paradigm will not tell you anything about
the way that amplifier sounds. It is very easy to tell how an amplifier will
sound using measurements based on the Power Paradigm as the measurements are
made with regards to understanding what is important to the human ear.
Any audiophile will
agree that the most valuable thing they have with respect to their audio system
is their own hearing. In fact human hearing defines the reality of audio. As
these words are written, the high-end audio industry has been experiencing a
shrinking market for over ten years. It is no surprise- in order for the market
to expand, the industry has to touch, move and inspire the marketplace with the
possibility of real music. In order to do this, the industry will have to accept
the importance of the rules of human hearing in the quest for improved
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