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Last updated: 17 Aug 2012 |
1: SCIENCE AND SUBJECTIVISM.
Audio engineering is in a singular position. There can be
few branches of engineering science rent from top to bottom by
such a fundamental disagreement as the Subjectivist/rationalist dichotomy.
Subjectivism is still a significant issue in the hifi section of
the industry, but has made little headway in
professional audio, where intimate acquaintance with the
original sound, and the pressing need to earn a living with reliable and
affordable equipment, provide effective barriers against most
irrational flights of fashion. (Note that the opposite of
Subjectivist is not "Objectivist". I understand this term refers
to the followers- if any- of the philosophies of Ayn Rand)
Most technologies have universally accepted measures of performance car makers compete to improve MPH and MPG; computer manufacturers boast of MIPs (millions of instructions per second) and so on. Improvement in these parameters is universally accepted as progress. In the field of hifi, many people seem to have difficulty in deciding which direction forward is.
Working as a professional audio designer, I often encounter opinions which, while an integral part of the Subjectivist offshoot of hifi, are treated with ridicule by practitioners of other branches of electrical engineering. The would-be designer is not likely to be encouraged by being told that audio is not far removed from witchcraft, and that no-one truly knows what they are doing. I have been told by a Subjectivist that the operation of the human ear is so complex that its interaction with measurable parameters lies forever beyond human comprehension. I hope this is an extreme position for it was proffered as a flat statement rather a basis for discussion.
I have studied audio design from the viewpoints of electronic design, psychoacoustics, and my own humble efforts at musical creativity. I have found complete scepticism towards Subjectivism to be the only tenable position. Nonetheless, if hitherto unsuspected dimensions of audio quality are ever shown to exist, then I look forward keenly to exploiting them. No doubt that most of the esoteric opinions are held in complete sincerity.
2: THE SUBJECTIVIST POSITION. It may appear unique that a sizable part of a technical industry
has set off in a direction that is quite counter to the
facts; it might be felt that such a loss of direction in a
scientific subject would be unprecedented. This is not so.
Parallel events that suggest themselves include the
destruction of the study of genetics under Lysenko in the USSR.
[1] Another possibility is the study of parapsychology, now in
deep trouble because after some 100 years of investigation it has
not uncovered the ghost of a repeatable phenomenon. [2]
This sounds all too familiar. It could be argued that
parapsychology is a poor analogy because most people would accept
that there was nothing there to study in the first place, whereas
nobody would assert that objective measurements and subjective
sound quality have no correlation at all; one need only pick up
the telephone to remind oneself what a 4kHz bandwidth and 10% or
so THD sounds like.
A startlingly close parallel in the history of
science is the almost-forgotten affair of Blondlot and the N-
rays. [3] In 1903, Rene Blondlot, a respected French physicist,
claimed to have discovered a new form of radiation he called "N-
rays". This was shortly after the discovery of X-rays by
Roentgen, so rays were in the air, as it were, and so was a desire to keep up with the Germans.
The N-radiation was apparently mysteriously refracted by aluminium
prisms; but the crucial factor was that its presence could only
be shown by subjective assessment of the brightness of an
electric arc allegedly affected by N-rays. No objective
measurement appeared to be possible. To Blondlot, and at least
fourteen of his professional colleagues, the subtle changes in
brightness were real, and the French Academy published more than
a hundred papers on the subject.
Unfortunately N-rays were completely imaginary, a classic product
of the "experimenter-expectancy" effect. This was demonstrated
by American scientist Robert Wood, who quietly pocketed the
aluminium prism during a demonstration, without affecting
Bondlot's recital of the results. This was widely reported by
the famous reporter/explorer William Seabrook, and the N-ray industry collapsed
very quickly. It was a major embarrassment at the time, but is now
almost forgotten. For more on N-rays, see the new N-ray page
This demonstrates with brutal clarity that it is quite possible for
large numbers of sincere people to deceive themselves when
trying to perform subjective assessments of phenomena.
3: A BRIEF HISTORY OF SUBJECTIVISM.
The smallest step-change in amplitude that can be detected is
about 0.3dB for a pure tone. In more realistic situations it is
0.5 to 1.0dB'". This is about a 10% change. [4]
The smallest detectable change in frequency of a tone is about
0.2% in the band 500Hz-2kHz. In percentage terms, this is the
parameter for which the ear is most sensitive. [5]
The least detectable amount of harmonic distortion is not an
easy figure to determine. Many variables are
involved, and in particular the continuously varying signal levels
mean the level of THD generated is also dynamically changing. With
mostly low-order harmonics present the
just-detectable amount is about 1%, though crossover-distortion can
be perceived at 0.3%, and probably lower. There is certainly no
evidence that an amplifier producing 0.001% THD sounds any
cleaner than one producing .005% [6]
THD measurements, taken with the usual notch-type analyser,
are of limited use in predicting the
subjective impairment produced by an imperfect audio path. With
music etc, intermodulation effects are demonstrably more
important than the harmonics themselves. However, THD tests do have the unique
advantage that inspection of the distortion residual on an oscilloscope gives
an experienced observer immediate insight into the
root cause of the non-linearity. Many other distortion tests
exist which, though yielding very little information to the
designer, exercise the whole audio bandwidth at once and
correlate well with properly-conducted tests for subjective
impairment by distortion. The Belcher intermodulation test (the
principle is shown in Fig 1.1) deserves more attention than it
has received. It may become popular now that DSP chips are
becoming cheaper and cheaper.
An objection often made to THD testing is that its
resolution does not allow verification that no non-linearities
exist at very low level; presumably some sort of micro-crossover distortion.
Hawksford, for example, has stated "Low-level threshold
phenomena... set bounds upon the ultimate transparency of an
audio system" [7] and several writers have claimed
that some metallic contacts consist of a net of so-called
'micro-diodes'. Actually, this sort of mischievous hypothesis can
be easily disposed of using enhanced THD techniques.
I evolved a method of measuring THD down to 0.01% at 200
microvolts rms, and applied it to large electrolytics, connectors
of varying provenance, and lengths of copper cable with and
without alleged magic properties. The method required the design
of an ultra-low noise (EIN= - 150 dBu for a 10 source resistance)
and very low THD. [8] The measurement method is shown in Fig 1.2;
using an attenuator with very low resistance values to reduce
the incoming signal keeps Johnson noise to a minimum. In no
case was any unusual distortion detected, and it would be nice
to think that this red herring at least has been laid to rest.
Interchannel crosstalk can obviously degrade stereo separation,
but the effect is not detectable until it is worse than 20dB,
which would be a very bad amplifier indeed.[9]
Phase and group delay have been an area of dispute for a long
time. As Stanley Lipshitz et al have pointed out, these effects
are obviously perceptible if they are gross enough; if an
amplifier was so heroically misconceived as to produce the top
half of the audio spectrum three hours after the bottom, there
would be little room for argument. More practically, concern
about phase has centred on loudspeakers and their
crossovers, as the only place where a phase-shift might exist without an accompanying
frequency-response change to make it obvious. Lipshitz appears
to have demonstrated [10] that a second-order all-pass filter (an
all-pass filter gives a frequency-dependant phase-shift without
level changes) is audible, whereas BBC findings, reported by
Harwood [11] indicate the opposite, and the truth of the matter
is still not clear. This controversy is of limited importance to
amplifier designers, as it would take truly spectacular incompetence
to produce a circuit that included an accidental all-pass filter.
Without all-pass filtering, the phase response of an amplifier is completely
defined by its frequency response, and vice-versa; in Control
Theory this is Bode's Second Law, [12] and it should be much more
widely known in the hi-fi world than it is. A properly designed
amplifier has its response roll-off points not too far outside
the audio band, and these will have accompanying phase-shifts;
there is no evidence that these are perceptible. [8]
The picture of the ear that emerges from psychoacoustics and
related fields is not that of a precision instrument. Its
ultimate sensitivity, directional capabilities and dynamic range
are far more impressive than its ability to measure small level
changes or detect correlated low-level signals like distortion
harmonics. This is unsurprising; from an evolutionary viewpoint
the functions of the ear are to warn of approaching danger
(sensitivity and direction-finding being paramount) and for
speech. In speech perception the identification of formants, (the
bands of harmonics from vocal-chord pulse excitation, selectively
emphasised by vocal-tract resonances) and vowel/consonant
discriminations, are infinitely more important than any hi-fi
parameter. Presumably the whole existence of music as a source
of pleasure is an accidental side-effect of our remarkable powers
of speech perception: how it acts as a direct route to the
emotions remains profoundly mysterious.
5: ARTICLES OF FAITH: THE TENETS OF SUBJECTIVISM.
"Sinewaves are steady-state signals that represent too easy a
test for amplifiers, compared with the complexities of music."
"Capacitors affect the signal passing through them in a way
invisible to distortion measurements."
"Passing an audio signal through cables, PCB tracks or switch
contacts causes a cumulative deterioration. Precious metal
contact surfaces reduce but do not eliminate the problem. This
too is undetectable by tests for non-linearity."
"Cables are directional, and pass audio better in one direction
than the other."
"The sound of valves is inherently superior to that of any kind
of semiconductor."
"Negative feedback is inherently a bad thing; the less it is
used, the better the amplifier sounds, without qualification."
"Tone-controls cause an audible deterioration even when set to
the flat position."
"The design of the power supply has subtle effects on the
sound, quite apart from ordinary dangers like ripple injection."
"Monobloc construction (i.e. two separate power amplifier
boxes) is always audibly superior, due to the reduction in
crosstalk."
"Microphony is an important factor in the sound of an
amplifier, so any attempt at vibration-damping is a good idea."
Let us for a moment assume that some or all of the above
hypotheses are true, and explore the implications. The effects
are not detectable by conventional measurement, but are assumed
to be audible. Firstly, it can presumably be taken as axiomatic
that for each audible defect some change occurs in the pattern
of pressure fluctuations reaching the ears, and therefore a
corresponding modification has occurred to the electrical signal
passing through the amplifier. Any other starting point supposes
that there is some other route conveying information apart from
the electrical signals, and we are faced with magic or
forces-unknown-to-Science. Mercifully no commentator has (so far)
suggested this. Hence there must be defects in the audio signals,
but they are not revealed by the usual test methods. How could
this situation exist? There seem two possible explanations for
this failure of detection: one is that the standard measurements
are relevant, but of insufficient resolution, and we should be
measuring frequency response, etc to thousandths of a dB. There
is no evidence whatsoever that such micro-deviations are audible
under any circumstances.
An alternative (and more popular) explanation is that
standard sinewave THD measurements miss the point by failing to
excite subtle distortion mechanisms that are triggered only by
music, the spoken word, or whatever. This assumes that these
music-only distortions are also left undisturbed by multi-tone
intermodulation tests, and even the complex pseudorandom signals
used in the Belcher distortion test. [16] The Belcher method
effectively tests the audio path at all frequencies at once, and
it is hard to conceive of a real defect that could escape it.
The most positive proof that Subjectivism is fallacious is
given by subtraction testing. This is the devastatingly simple
technique of subtracting before-and-after amplifier signals and
demonstrating that nothing audibly detectable remains.
It transpires that these alleged music-only mechanisms are not
even revealed by music, or indeed anything else, and it is clear that
the subtraction test has finally shown as non-existent these
elusive degradation mechanisms.
The subtraction technique was proposed by Baxandall in 1977.
[17] The principle is shown in Fig 1.3; careful adjustment of the
rolloff-balance network prevents minor bandwidth variations from
swamping the true distortion residual. In the intervening years
the Subjectivist camp has made no effective reply.
A simplified version of the test was introduced by Hafler.
[18] This method is less sensitive, but has the advantage that
there is less electronics in the signal path for anyone to argue
about. See Fig 1.4. A prominent Subjectivist reviewer, on trying
this experiment, was reduced to claiming that the passive
switchbox used to implement the Hafler test was causing so much
sonic degradation that all amplifier performance was swamped.
[19] I do not feel that this is a tenable position. So far all
experiments such as these have been ignored or brushed aside by
the Subjectivist camp; no attempt has been made to answer the
extremely serious objections that this demonstration raises.
In the twenty or so years that have elapsed since the
emergence of the Subjectivist Tendency, no hitherto unsuspected
parameters of audio quality have emerged.
6: THE LENGTH OF THE AUDIO CHAIN.
7: THE IMPLICATIONS.
A second consequence of placing Subjectivism above
measurements is that it places designers in a most unenviable
position. No degree of ingenuity or attention to technical detail
can ensure a good review, and the pressure to adopt fashionable
and expensive expedients (such as linear-crystal internal wiring)
is great, even if the designer is certain that they have no
audible effect for good or evil. Designers are faced with a
choice between swallowing the Subjectivist credo whole or keeping
very quiet and leaving the talking to the marketing department.
If objective measurements are disregarded, it is inevitable
that poor amplifiers will be produced, some so bad that their
defects are unquestionably audible. In recent reviews [20] it was
easy to find a £795 preamplifier (Counterpoint SA7) that boasted
a feeble 12dB disc overload margin, (another preamp costing £2040
struggled up to 15dB ( Burmester 838/846) and another, costing
£1550 that could only manage a 1kHz distortion performance of 1%;
a lack of linearity that would have caused consternation ten
years ago (Quicksilver). However, by paying £5700 one could inch
this down to 0.3% (Audio Research M100-2 monoblocs). This does
not mean it is impossible to buy an 'audiophile'amplifier that measures well; another example would be the
preamplifier/power amplifier combination that provides a very
respectable disc overload margin of 31 dB and 1 kHz rated-power
distortion below 0.003%; the total cost being £725 (Audiolab
8000C/8000P). I believe this to be a representative sample, and
we appear to be in the paradoxical situation that the most
expensive equipment provides the worst objective performance.
Whatever the rights and wrongs of subjective assessment, I think
that most people would agree that this is a strange state of
affairs. Finally, it is surely a morally ambiguous position to
persuade non-technical people that to get a really good sound
they have to buy £2000 preamps and so on, when both technical
orthodoxy and common sense indicate that this is quite
unnecessary.
8: THE REASONS WHY.
It has been universally recognised for many years in
experimental psychology, particularly in experiments about
perception, that people tend to perceive what they want to
perceive. This is often called the 'experimenter expectancy'
effect; it is more subtle and insidious than it sounds, and the
history of science is littered with the wrecked careers of those
who failed to guard against it. Such self-deception has most
often occurred in fields like biology, where although the raw
data may be numerical, there is no real mathematical theory to
check it gainst.
When the only 'results' are vague subjective
impressions, the danger is clearly much greater, no matter how
absolute the integrity of the experimenter. Thus in psychological
work great care is necessary in the use of impartial observers,
double-blind techniques, and rigorous statistical tests for
significance. The vast majority of Subjectivist writings wholly
ignore these precautions, with predictable results. In a few
cases properly controlled listening tests been done, and at the
time of writing all have resulted in different amplifiers
sounding indistinguishable. I believe the conclusion is
inescapable that experimenter expectancy has played a dominant
role in the growth of Subjectivism.
It is notable that in Subjectivist audio the 'correct'
answer is always the more expensive or inconvenient one.
Electronics is rarely as simple as that. A major improvement is
more likely to be linked with a new circuit topology or new type
of semiconductor, than with mindlessly specifying more expensive
components of the same type; cars do not go faster with platinum
pistons.
9: THE OUTLOOK.
10: SOME TECHNICAL ERRORS.
Discoveries like this can only be made because it is now
straightforward to make testbed amplifiers with ultra-low
distortion- lower than that which used to be thought possible.
The reduction of distortion to the inherent level that a circuit
configuration is capable of is a fundamental requirement for
serious design work in this field; in Class-B at least this gives
a defined and repeatable standard of performance that I have
named a "Blameless" amplifier, so-called because it avoids error
rather than claiming new virtues.
It has proved possible to take the standard Class-B power
amplifier configuration, and by minor modifications, reduce the
distortion to below the noise floor at low frequencies. This
represents approximately 0.0005 to 0.0008% THD, depending on the
exact design of the circuitry, and the actual distortion can be
shown to be substantially below this if spectrum-analysis
techniques are used to separate the harmonics from the noise.
11: ABSOLUTE PHASE.
It is unclear how this applies to instruments less physical
than a kickdrum. For the drum the situation is simple- you kick
it, the diaphragm moves outwards and the start of the transient
must be a wave of compression in the air. (followed almost at
once by a wave of rarefaction) But what about an electric guitar?
A similar line of reasoning- plucking the string moves it in a
given direction, which gives such-and-such a signal polarity,
which leads to whatever movement of the cone in the guitar amp
speaker cabinet- breaks down at every point in the chain. There
is no way to know how the pickups are wound, and indeed the
guitar will almost certainly have a switch for reversing the
phase of one of them. I also suggest that the preservation of
absolute phase is not the prime concern of those who design and
build guitar amplifiers.
The situation is even less clear if more than one instrument
is concerned, which is of course almost all the time. It is very
difficult to see how two electric guitars played together could
have a "correct" phase in which to listen to them.
Recent work on the audibility of absolute phase [21], [22]
shows it is sometimes detectable. A single tone flipped back and
forth in phase, providing it has a spiky asymmetrical waveform
and an associated harsh sound, will show a change in perceived
timbre and, according to some experimenters, a perceived change
in pitch. A monaural presentation has to be used to yield a clear
effect. A complex sound, however, such as that produced by a
musical ensemble, does not in general show a detectable
difference.
Proposed standards for the maintenance of absolute phase
have just begun to appear, [23] and the implication for amplifier
designers is clear; whether absolute phase really matters or not,
it is simple to maintain phase in a power amplifier (compare a
complex mixing console, where correct phase is vital, and there
are hundreds of input and outputs, all of which must be in phase
in every possible configuration of every control) and so it
should be done. In fact, it probably already has been done, even
if the designer hasn't given absolute phase a thought, because
almost all amplifiers use series negative feedback, and this must
be non-inverting. Care is however required if there are stages
such as balanced line input amplifiers before the power amplifier
itself. REFERENCES.
A short definition of the Subjectivist position on power
amplifiers might read as follows:
I believe this is a reasonable statement of the situation.
Meanwhile the overwhelming majority of the public buy
conventional hifi systems, ignoring the expensive and esoteric
high-end sector where the debate is fiercest.
The early history of sound reproduction is notable for the
number of times that observers reported that an acoustic
gramophone gave results indistinguishable from reality. Such
such statements throw light on how powerfully
mind-set affects subjective impressions. When interest in sound
reproduction grew in the post-war period, technical
standards such as DIN 45-500 were set, though they were soon
criticized as too permissive. By the late 1960s it was almost universally
accepted that the hi-fi requirements would be met by:
"THD less than 0.1%, with no significant crossover distortion,
frequency response 20-20kHz, and as little noise as possible,
please". The early 1970s expanded this to include slew-rates
and properly behaved overload protection, but the approach was
always scientific and it was perfectly normal to read amplifier reviews in
which measurements were dissected but no mention made of
listening tests.
Following the growth of subjectivism through the pages of
one of the leading Subjectivist magazines (HiFi News), the first
intimation of things to come was the commencement of Paul
Messenger's column "Subjective Sounds" in September 1976. He said
"The assessment will be (almost) purely subjective,
which has both strengths and weaknesses, as the inclusion of
laboratory data would involve too much time and space, and
although the ear may be the most fallible, it is also the most
sensitive evaluation instrument". Subjectivism as an expedient
rather than a policy. Significantly, none of the early instalments
contained any references to amplifier sound.
In March 1977, an article by Jean Hiraga was published attacking
high levels of negative feedback and praising the sound of an amplifier with 2%
THD. In the same issue, Paul Messenger stated that a Radford
valve amplifier sounded better than a transistor one, and by the
end of the year the amplifier-sound bandwagon was rolling. Hiraga
returned in August 1977 with a highly contentious set of claims
about audible speaker cables, and after that no hypothesis was
too unlikely to receive attention.
In evaluating the Subjectivist position, it is essential to
consider the known abilities of the human ear. Contrary to the
impression given by some commentators, who call constantly for
more psychoacoustical research, an enormous amount of hard scientific
information already exists on this subject, and some of it may
be briefly summarized thus:
All of the alleged effects listed below have received
considerable affirmation in the audio press, to the point where
some are treated as facts. The reality is that none of them has
in the last fifteen years proved susceptible to objective
confirmation. This sad record is perhaps equalled only by
students of parapsychology. I hope that the brief statements
below are considered fair by their proponents. If not I have no
doubt I shall soon hear about it:
This is presumably meant to imply that sinewaves are in some
way particularly easy for an amplifier to deal with, the
implication being that anyone using a THD analyser must be
hopelessly naive. Since sines and cosines have an unending series
of non-zero differentials, "steady" hardly comes into it. I know
of no evidence that sinewaves of randomly varying amplitude (for
example) would provide a more searching test of amplifier
competence.
I believe this outlook is the result of anthropomorphic
thinking about amplifiers; treating them as though they think
about what they amplify. Twenty sinewaves of different
frequencies may be conceptually complex to us, and the output of
a symphony orchestra much more so, but to an amplifier both
composite signals resolve to a single instantaneous voltage that
must be increased in amplitude and presented at low impedance.
The rate of change of this voltage has a maximum set by the frequency
response and amplitude capability of the channel and is not generally
greater for more complex signals; you do not get hgher slew rate with bigger orchestras.
You must remember that an amplifier has no perspective on the signal arriving at its
input, but literally takes it as it comes.
Several writers have advocated passing pulse signals through two
different sorts of capacitor, and subtracting the result,
claiming that the non-zero residue proves that capacitors can
introduce audible errors. In fact such tests expose only
well-known capacitor shortcomings such as dielectric absorption
and series resistance, and perhaps the vulnerability of the
dielectric film in electrolytics to reverse-biasing. No-one has
yet shown how these imperfections could cause capacitor audibility
in properly designed equipment.
Concern over cables is widespread, but it can be said with
confidence that there is as yet not a shred of evidence to
support it. Any piece of wire passes a sinewave with unmeasurable
distortion, and so simple notions of inter-crystal rectification
or "micro-diodes" can be discounted, quite apart from the fact
that such behaviour is absolutely ruled out by established
materials science. No plausible means of detecting, let alone
measuring, cable degradation has ever been proposed.
The most significant parameter of a loudspeaker cable is
probably its lumped inductance. This can cause minor variations
in frequency response at the very top of the audio band, given
a demanding load impedance. These deviations are unlikely to
exceed 0.1 dB for reasonable cable constructions. (eg inductance
less than 4 uH) The resistance of a typical cable (perhaps 0.1 Ohm)
causes response variations across the band, following the speaker
impedance curve, but these are usually even smaller at around
0.05 dB. This is not audible.
Corrosion is often blamed for subtle signal degradation at
switch and connector contacts. By far the most
common form of contact degradation is the formation of an
insulating sulphide layer on silver contacts, derived from
hydrogen sulphide air pollution; the problem seems to have become worse in recent years. This typically cuts the signal
altogether, except when signal peaks temporarily punch through
the sulphide layer. The effect is gross and completely inapplicable
to theories of subtle degradation. Gold-plating is the only
certain cure. It costs money. A switch with gold-flashed contacts can cost five times as much as the silver version.
Audio signals are AC. Cables cannot be directional any more
than 2 + 2 can equal 5. Anyone prepared to believe this nonsense
won't be capable of designing amplifiers, so there seems no point
in further comment.
The "valve sound" is one phenomenon that may have a real
existence; it has been known for a long time that listeners
sometimes prefer to have a certain amount of second-harmonic
distortion added in, [13] and most valve amplifiers provide just
that, due to grave difficulties in providing good linearity with
modest feedback factors. While this may well sound nice, hi-fi
is supposedly about accuracy, and if the sound is to be thus
modified it should be controllable from the front panel by a
'niceness' knob.
The use of valves leads to some intractable problems of
linearity, reliability and the need for intimidatingly expensive
(and once more, non-linear) iron-cored transformers. The current
fashion is for exposed valves, and it is not at all clear to me
that a fragile glass bottle, containing a red-hot anode with
hundreds of volts DC on it, is wholly satisfactory for domestic
safety.
A recent development in Subjectivism is enthusiasm for
single-ended directly-heated triodes, usually in extremely
expensive monoblock systems. Such an amplifier generates large
amounts of second-harmonic distortion, due to the asymmetry of
single-ended operation, and requires a very large output
transformer as its primary carries the full DC anode current, and
core saturation must be avoided. Power outputs are inevitably
very limited at 10 Watts or less. In a recent review, the Cary
CAD-300SEI triode amplifier yielded 3% THD at 9 Watts, at a cost
of $3400 [14]
Negative feedback is not inherently a bad thing; it is an
absolutely indispensable principle of electronic design, and if
used properly has the remarkable ability to make just about every
parameter better. It is usually global feedback that the critic
has in mind. Local negative feedback is grudgingly regarded as
acceptable, probably because making a circuit with no feedback
of any kind is near-impossible. It is often said that high levels
of NFB enforce a low slew-rate. This is quite untrue; and this
thorny issue is dealt with in detail in Section 2.5.1. For more
on slew-rate see also [15]
This is usually blamed on "phase-shift". At the time of
writing, tone controls on a preamp badly damage its chances of
street (or rather sitting-room) credibility, for no good reason.
Tone-controls set to 'flat' cannot possibly contribute any extra
phase-shift and must be inaudible. My view is that they are
absolutely indispensable for correcting room acoustics,
loudspeaker shortcomings, or tonal balance of the source
material, and that a lot of people are suffering sub-optimal
sound as a result of this fashion. It is now commonplace for
audio critics to suggest that frequency-response inadequacies
should be corrected by changing loudspeakers. This is an
extraordinarily expensive way of avoiding tone-controls.
All good amplifier stages ignore imperfections in their
power supplies, op-amps in particular excelling at power-supply
rejection-ratio. More nonsense has been written on the subject
of subtle PSU failings than on most audio topics; recommendations
of hard-wiring the mains or using gold-plated 13A plugs would
seem to hold no residual shred of rationality, in view of the
usual processes of rectification and regulation that the raw AC
undergoes. And where do you stop? At the local sub-station?
Should we gold-plate the pylons?
There is no need to go to the expense of monobloc power
amplifiers in order to keep crosstalk under control, even when
making it substantially better than the - 20dB that is actually
necessary. The techniques are conventional; the last stereo power
amplifier I designed managed an easy - 90dB at 10kHz without
anything other than the usual precautions. In this area dedicated
followers of fashion pay dearly for the privilege, as the cost
of the mechanical parts will be nearly doubled.
Microphony is essentially something that happens in
sensitive valve preamplifiers. If it happens in solid-state power
amplifiers the level is so far below the noise it is effectively
non-existent.
Experiments on this sort of thing are rare (if not unheard
of) and so I offer the only scrap of evidence I have. Take a
microphone preamp operating at a gain of +70 dB, and tap the
input capacitors (assumed electrolytic) sharply with a
screwdriver; the preamp output will be dull thump, at low level.
The physical impact on the electrolytics (the only components
that show this effect) is hugely greater than that of any
acoustic vibration; and I think the effect in power amps, if any,
must be so vanishingly small that it could never be found under
the inherent circuit noise.
An apparently insurmountable objection to the existence of
non-measurable amplifier quirks is that recorded sound of almost
any pedigree has passed through a complex mixing console at least
once; prominent parts like vocals or lead guitar will almost
certainly have passed through at least twice, once for recording
and once at mix-down. More significantly, it must have passed
through the potential quality-bottleneck of an analogue tape
machine or more likely the A-D converters of digital equipment.
In its long path from here to ear the audio passes through at
least a hundred op-amps, dozens of connectors and several hundred
metres of ordinary screened cable. If mystical degradations can
occur, it defies reason to insist that those introduced by the
last 1% of the path are the critical ones.
This confused state of amplifier criticism has negative
consequences. Firstly, if equipment is reviewed with results that
appear arbitrary, and which are in particular incapable of
replication or confirmation, this can be grossly unfair to
manufacturers who lose out in the lottery. Since subjective
assessments cannot be replicated, the commercial success of a
given make can depend entirely on the vagaries of fashion. While
this is fine in the realm of clothing or soft furnishings, the
hi-fi business is still claiming accuracy of reproduction as its
raison d'etre, and therefore you would expect the technical
element to be dominant.
Some tentative conclusions are possible as to why hifi
engineering has reached the pass that it has. I believe one basic
reason is the difficulty of defining the quality of an audio
experience; you can't draw a diagram to communicate what
something sounded like. In the same way, acoustical memory is
more evanescent than visual memory. It is far easier to visualize
what a London bus looks like than to recall the details of a
musical performance. Similarly, it is difficult to 'look more
closely'; turning up the volume is more like turning up the
brightness of a TV picture; once an optimal level is reached,
any further increase becomes annoying, then painful.
It might be difficult to produce a rigorous statistical
analysis, but it is my view that the reported subjective quality
of a piece of equipment correlates far more with the price than
with anything else. There is perhaps here an echo of the
Protestant Work Ethic; you must suffer now to enjoy yourself
later. Another reason for the relatively effortless rise of
subjectivism is the 'me-too' effect; many people are reluctant
to admit that they cannot detect acoustic subtleties as nobody
wants to be labelled as insensitive, outmoded, or just plain
deaf. It is also virtually impossible to absolutely disprove any
claims, as the claimant can always retreat a fraction and say
that there was something special about the combination of
hardware in use during the disputed tests, or complain that the
phenomena are too delicate for brutal logic to be used on them.
In any case, most competent engineers with a taste for
rationality probably have better things to do than dispute every
comtroversial report.
Under these conditions, vague claims tend, by a kind of
intellectual inflation, to gradually become regarded as facts.
Manufacturers have some incentive to support the Subjectivist
camp as they can claim that only they understand a particular
non-measurable effect, but this is no guarantee that the dice may
not fall badly in a subjective review.
It seems unlikely that subjectivism will disappear for some
time, given the momentum that it has gained, the entrenched
positions that some people have taken up, and the sadly
uncritical way in which people accept an unsupported assertion
as the truth simply because it is asserted with frequency and
conviction. In an ideal world every such statement would be
greeted by loud demands for evidence. However, the history of the
world sometimes leads one to suppose pessimistically that people
will believe anything. By analogy, one might suppose that
subjectivism would persist for the same reason that
parapsychology has; there will always be people who will believe
what they want to believe despite the hardest of evidence.
Misinformation also arises in the purely technical domain;
I have also found that some of the most enduring and widely held
technical beliefs to be unfounded. For example, if you take a
Class-B amplifier and increase its quiescent current so that it
runs in Class-A at low levels, ie in Class AB, most people will
tell you that the distortion will be reduced as you have moved
nearer to the full Class-A condition. This is untrue. A correctly
configured amplifier gives more distortion in Class-AB, not less,
because of the abrupt gain changes inherent in switching from A
to B every cycle.
Concern for absolute phase has for a long time hovered
ambiguously between real audio concerns like noise and
distortion, and the Subjective realm where solid copper is
allegedly audible. Absolute phase means the preservation of
signal phase all the way from microphone to loudspeaker, so that
a drum impact that sends an initial wave of positive pressure
towards the live audience is reproduced as a similar positive
pressure wave from the loudspeaker. Since it is known that the
neural impulses from the ear retain the periodicity of the
waveform at low frequencies, and distinguish between compression
and rarefaction, there is a prima facie case for the audibility
of absolute phase.
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