But the way human ears INTERPRET and PERCEIVE the sound experience in a
specific room with specific gear is different for each of us...
It is the reason why in the publicity of the marketing of electronical
equalizer company recommend it to make any consumers free to use it for
different kind of music, different room, different TASTES.... I met a reviewer that claimed he didn't like bass. Quite often people have preferences for flaws in equipment that isn't neutral. A great example is SETs which have a variety of flaws that interact nicely with human perceptual rules. Tubes exist OTOH because many solid state amps have brightness and harshness due to improper application of loop feedback. That's different from from saying that we can't measure what's going on. We can. But designers (particularly in high end audio) aren't always coming from an engineering background (or they probably would not be building SETs...) and to further muck things up some designers simply don't have the will (or are constrained by their employers) to make something that is actually neutral to the ear. |
- Listenings experiments is the ONLY way to tune and fine tune the quality we ask for and which qualities are IMPOSSIBLE to deduce only from any set of measurements nevermind how big it is and how precise...
Why ?
Because many dimensions are at play which no limited measuring tools in their range of application can take into account simultaneously when what is designed is designed FOR ANOTHER HUMAN EARS.... All humans use the same hearing perceptual rules. For example, to sense sound pressure all human’s ear use the higher ordered harmonics. All human’s ears have a masking principle and so on. Because there are a good number of measurements that never make it onto a spec sheet, IMO/IME the above quoted statement is false. If you understand the human hearing perceptual rules and design for them rather than a spec sheet you can easily design a circuit that will sound good the human ear. |
Test tones are used to show how equipment performs below audible levels
but how music performs at listening levels is the deciding criteria. In
that regard science fails miserably. Actually it doesn't. But science is rarely applied when doing measurements. If you want the measurements to show what you need to know in order to know how the equipment will sound, don't hold your breath because spec sheets are generally created as a sales tool. Now it is a simple fact that we can measure and correlate what we measure to what we hear and that is entirely due to studies of how the ear works, mostly done in the last 50 years. If you want to know what to look for, take a look at a post I made on another thread (sorry, its long): Distortion is in all forms of amplification. Of course we want it to be
as low as possible but the ear poses some real challenges.
It
uses the higher ordered harmonics (5th and above) to sense sound
pressure, and to do that it has to be keenly sensitive to them! The ear
is more sensitive to the higher ordered harmonics than almost anything
else. For this reason, a THD of 0.01% can be very audible if that’s
mostly higher orders.
The ear assigns tonality to all forms of
distortion! The higher orders get ’harsh and bright’. We’ve all been
hearing this in most solid state amps made in the last 70 years. Its why
tubes are still around!!
The lower orders (2nd, 3rd and 4th) are
nearly inaudible and mostly contribute to ’bloom’ and ’warmth’ using
audiophile terms. The ear has a masking principle where louder sounds
mask the presence of quieter sounds; if the lower orders are in
sufficient quantity, they will mask the presence of the higher orders.
When this happens, the amp will sound smooth and because the lower
orders are mostly inaudible it will appear to be relatively neutral. If
the amp has such a distortion signature this will be the case whether
tube or solid state. But for technical reasons (its very hard to build a
zero feedback solid state amp), until recently this has mostly been
describing tube amplifiers.
The problem has been up until
fairly recently that the the devices (whether tube or semiconductor)
didn’t exist to allow the amplifier design to have a sufficient amount
of feedback (on an engineering basis, the devices didn’t exist to allow
for sufficient gain bandwidth product). You have several hurdles to
cross; first when adding a lot of feedback you can’t exceed the phase
margin of the amp otherwise it becomes unstable and can oscillate. Phase
margin is an engineering way of saying that there’s a certain high
frequency above which the negative feedback applied is no longer
negative due to phase shift in the circuit.
The second problem is
you have to have an enormous amount of gain- and with gain you get
phase shift- because you really need north of 35dB of feedback in order
to allow the amplifier to clean up the distortion caused by the
application of feedback itself (which tends to be almost entirely higher
ordered harmonics, caused by the process of bifurcation occurring at
the point where the feedback is combined with the input signal). These
higher ordered harmonics are of course audible which is why feedback has
gotten a bad rap in high end audio over the last 40 years.
So
you have to blow off 35 db of gain with feedback and still have a good
25dB of gain left over- so this means that at a minimum any amplifier
that uses feedback properly will have a total gain (called ’loop gain’)
of at least 60dB! Most amps made have far less than that which is why
solid state has garnered a reputation for harsh and bright. Between 12
and 20dB is the area where feedback generates the most distortion: its
on a bell curve. Yes, it does suppress distortion but my point here is
that its makes some of its own too.
Because this is such a
tall order, most amps simply didn’t do it. To deal with this problem,
the industry (sweeping this under the carpet) only tests harmonic
distortion of amplifiers at 100 Hz. At this frequency almost any solid
state amp has enough feedback which is why they can play bass so well.
But if you measure the same amp at 1KHz or 10KHz you’ll find the
distortion is much higher- and of course that is why the amp sounds
bright and harsh (its not a frequency response error). This increase of
distortion with frequency is a sign that the amp lacks Gain Bandwidth
Product. GBP is to feedback what gas is to car. When you use it up by
increasing frequency, at some point there’s no more feedback. At any
rate distortion is increased!
Tubes avoid this for the most
part by having a greater amount of the lower ordered harmonics. So they
lack the harshness and brightness not because they are lower distortion
but **because the higher orders are masked**.
There are a number
of solutions. One way to get tubes to be much lower distortion is to
design the circuit to be fully differential and balanced from input to
output. In this way, even orders are cancelled not just at the output
but throughout the circuit. This results in a 3rd harmonic as the
primary distortion and since the 3rd is quite close to the fundamental
is treated by the ear the same as the 2nd. But it can easily be at a
level 1/10th that of an amp that does not employ this technique, and
succeeding harmonics will fall off at a faster rate according to a cubic
progression because distortion isn’t compounded from stage to stage.
For this reason such an amp is said to have a ’cubic non-linearity’ and
is considerably more neutral and transparent than amps that express the
2nd order as dominant (a ’quadratic non-linearity’), yet just as smooth.
This is true whether the circuit is tube or solid state.
Feedback
can be avoided altogether, thus avoiding the brightness that occurs
with its application. SETs are an example of this as well as our OTLs
(which are fully balanced and differential) and there are solid state
examples as well, such as the Ayre.
Another solution is to
simply have enough gain and bandwidth using newer semiconductors so
enough feedback can be applied so that the amp has consistent distortion
at 1KHz and 10KHz as it does at 100Hz, and won’t oscillate with +35dB
of feedback. This is a bit of a trick but it is doable and there are a
few solid state amps of traditional design that do this- the Benchmark
and Soulution come to mind.
Finally, class D amps can be
built that have so much feedback that their phase margin is grossly
exceeded and they go into oscillation as soon as they are turned on. The
oscillation is then used as the switching frequency. This type of class
D amp is known as ’self oscillating’ and can have very low distortion.
Because of non-linearities in the encoding scheme and also due to dead
time, lower ordered harmonics might be generated. If this is the case,
such an amp will sound every bit as smooth and transparent as the best
tube amps (due to masking) but with greater neutrality and transparency
due to vastly lower distortion overall (in case its not clear,
distortion masks detail). So the bottom line is the distortion signature is more important than how much distortion is actually present. That is what the spec sheets aren't showing and why there's often a disconnect between what you hear and what is measured. Its not that we *can't* measure it, its simple because most of the time we simple *don't* measure it. |
