Newsgroup: it.hobby.hi-fi
Titolo: il suono degli amplificatori....

Conoscete il Richard Clarks Amp Challenge?
Costui mette in palio $10.000 per chiunque riesca a distinguere due
amplificatori di potenza (vale sia in ambito home che car)....ci sono
alcune restrizioni (in inglese):
There is no question that all amps are not the same. It is very easy
to measure large differences in the performance of amplifiers. This is
true in nearly every known specification, including power, noise,
distortion, etc. My experience has led me to believe that even though
these differences can be easily measured, hearing those differences
may not be so easy. Given the relatively small magnitude of
performance differences, there is a giant step between amplifier
performance and our ability to hear performance differences.
It is claimed by designers, manufacturers and especially salespersons
that differences in amplifiers are clearly audible. Reasons include
"obvious" advantages of one type of circuit topology over another. For
example, it is claimed that certain designs have a smoother midrange
response whereas other amplifiers exhibit tighter bass. Tube fanatics
claim that tube amplifiers have that "warm" sound we all need in our
systems.
Such descriptive terms are certainly subject to personal
interpretation. It is not my intention to determine if one particular
amplifier is better than another amplifier. Differences in the quality
of the discrete components and constructions are more appropriate for
settling the issue of "good - better - best." The sole purpose of my
amplifier challenge is to determine if the differences in amplifiers
are audible.
What differences are Audible?
I believe the perceived differences in amplifiers are all due to
various factors that can be explained with basic physics and
elementary psyco-acoustics. For instance, if two amplifiers are not
carefully matched in volume, and one amp is slightly louder than the
other, then it would be a simple matter to detect such a difference.
In such an example it is important to understand that it is not the
circuit topology, quality of the component, design excellence, or
superb marketing and packaging that caused the noticeable difference -
it was an error in the test setup! It is my present belief that as
long as a modern amplifier is operated within its linear range (below
overload), the differences between amps are inaudible to the human
ear.
Comparing Amps
The idea here is for a test subject to scientifically demonstrate
his/her ability to hear differences in amplifiers. It is our job to
carefully match the amps so that we are comparing "apples to apples"
instead of "oranges to frogs." This means that we sure wouldn't want
to compare one amplifier that had + 12 dB of high frequency boost
against another amplifier that was adjusted for + 12 dB of bass boost.
Such a test would be easy to pass - even on identical amplifiers with
consecutive serial numbers.
For our comparison test, we aren't concerned with which amplifier
sounds best to the test subject. We only require that the listener be
able to identify each amplifier when it is powering the speakers.
Since many folks seem to believe that amplifiers
have some kind of distinctive sonic character, this test should be
easy to pass. Right? After all, we're talking about comparing those
harsh sounding, high distortion, squeaky "widget As" to those warm
sounding, smooth, bass hog "widget Bs."
Now pay particular attention to the following sections. Since we're
looking for differences in amplifiers, and we already know that those
differences are probably going to be very, very small, it is important
that the parameters under our control be carefully adjusted so as to
be equal as possible. This means that we must be cognizant of
differences we might unknowingly introduce between amp A and amp B.
They must be adjusted as identical as possible. We already mentioned
the importance of volume. The same goes for the L and R balance. It
sure would be easy to choose an amplifier that exhibited left side
bias over a balanced amp. Right?
Well, in order to keep this amplifier comparison test fair, there are
a few other parameters that must be considered. I'll list them all in
the following section.
Amplifier Comparison Test Conditions
1. Amplifier gain controls - of both channels - are matched to within
+- .05 dB.
2. Speaker wires on both amps are properly wired with respect to
polarity. (+ and -)
3. That neither amp has signal phase inversion. If so correction will
be made in #2 above.
4. That neither amp is loaded beyond its rated impedance.
5. That all amplifiers with signal processors have those circuits
bypassed. This includes bass boost circuits, filters, etc. If
frequency tailoring circuits cannot be completely bypassed an
equalizer will be inserted in the signal path of one (only one and the
listener can decide which) of the amps to compensate for the
difference. Compensation will also be made for input and output
loading that affects frequency response. Since we are only listening
for differences in the sonic signature of circuit topology, the
addition of an EQ in one signal path only should make the test even
easier.
6. That neither amp exhibits excessive noise (including RFI).
7. That each amp can be properly driven by the test setup. Not
normally a problem but it is theoretically a problem.
8. That the L and R channels are not reversed in one amp.
9. That neither amp has excessive physical noise or other indicators
that can be observed by the listener.
10. That neither amp has DC OFFSET that causes audible pops when its
output is switched.
11. That the channel separation of all amps in the test is at least 30
dB from 20Hz to 20kHz.
In addition to these requirements the test will be conducted according
to the following rules.
Amplifier Test Comparison Rules
1. To make things easy we would prefer to use high quality home type
loudspeakers for the test. If our speakers are not acceptable, the
listener can provide any commercially available speaker system as long
as it uses dynamic drivers. The actual measured impedance cannot
exceed the rated load impedance of the amplifiers tested. If, however,
the tester would like to perform the test in a car, we will use a car,
however, it will have to be provided by the test subject. For
practicality we will have to limit the number of amplifier channels to
four or less.
2. Amplifiers will be powered from the same power supply at a nominal
14 volts DC. (any voltage is OK as long as it is the same for both
amps)
3. The test can be conducted at any volume desired; however, the amps
will not be allowed to clip. In other words, listening volume can not
exceed the power capacity of the smallest amp of the pair being
tested. (power capacity will be defined as clipping or 2%THD 20Hz to
10kHz, whichever is less)
4. No test signals can be used - only commercially available music.
5. The listener can compare two amps at a time for as long as desired.
For practical reasons we would like to keep this at least no more than
a few hours. A test session will consist of 12 A/B sequences. Passing
the test will require a positive identification of each amp for all 12
sequences. Remember, guessing will get you about 6 out of 12. If the
differences are so great, and a subject can really hear the
difference, then he/she should be able to do so for all 12 sequences.
6. To win the $10,000.00, the listener must pass two complete sessions
of 12 comparisons. Passing the test means 24 correct responses.* The
amp of choice can be compared to the same or a different amp in each
session - challengers choice. We have many amplifiers in our demo
inventory such as, but not limited to, Alpine, Rockford, Kicker,
Phoenix Gold, Precision Power, MTX, Adcom, Kenwood, Pioneer, Sony,
etc. You can pick any of them or bring your own.
7. All amps must be brand name, standard production, linear voltage
amplifiers. This does not exclude high current amps. Amps can not be
modified and must meet factory specs. They must be "car audio
amplifiers designed to be powered from a car's electrical system."
8. Failure of an amp (this includes thermal shutdown) during the test
will require that the test be repeated after repair or replacement or
cooling of the amp. This means that the entire test session will have
to be repeated.
9. The amps will not be overloaded during the session from either a
voltage or current requirement.
10. To save time the listener will have to pass a quick 8 trial
session to qualify for the extended 2 session test for the money
prize. Any 2 amps can be used for this
test. Passing this qualifying test will require at least 6 out of 8
correct answers.
11. The amplifier power up and/or power down sequence will not be
acceptable for comparison. (The turn on/off noises of some amplifiers
would give it away.)
12. Although anyone is welcome to take the test, only subjects
employed in the car audio industry or Car Sound subscribers are
eligible for the $10,000.00 prize.
13. Cost to take the test is $100.00. $300.00 for people representing
companies. Payable in advance, scheduled appointments only. Done
correctly the test takes several hours and I don't have the time if
you aren't serious.
* Twelve correct responses in a row is certainly a lot of correct
listening but $10,000 is also a lot of money for a few hours of easy
listening. The way people describe the differences is that they are
like night and day. I would certainly not have any trouble choosing
between an apple and an orange 12 times in a row. When compared fairly
I believe the differences in amps are much too small to audibly detect
and certainly too small to pay large sums of extra money for. If I am
wrong someone should be able to carefully take this test and win my
money. Even if I am right, if enough people take the test eventually
someone will take my money due to random chance. This is the reason
for the large sample requirement. If you feel that you can easily pass
this test but 12 sequences will give you "listening fatigue" I am
willing to modify the requirements. Since the way it is being offered
is a challenge and only my money is at risk I am willing to let a
confident challenger "put his money where his ears are". If we are
willing to make this a bet instead of a challenge, I am willing to
drop 1 sequence for every thousand dollars put up by the challenger
against my money. This would mean:
____My___________ _ _Your________Trails Required to win__
$10,000 to $0 = 12 Tries
$9,000 to $1,000 = 11 Tries
$8,000 to $2,000 = 10 Tries
$7,000 to $3,000 = 9 Tries
$6,000 to $4,000 = 8 Tries
$5,000 to $5,000 = 7 Tries
$4,000 to $6,000 = 6 Tries
I will not do the test with less than 6 trails. It would be
statistically meaningless and reduce the challenge to mere gambling.
Fino ad ora nessuno č riuscito nell'impresa. C'č qualcuno che pensa si
possano vincere questi soldi?
per i piů abili ci sono addirittura $1.000.000.....
se qualcuno riuscisse a sentire gli enormi benefici effetti dello
smagnetizzatore di cd...
questo:
http://www.bedini.com/jsreview.htm
e questa č la sfida:
http://www.randi.org/jr/082704gluton.html#4
http://www.randi.org/jr/080504string.html#8
ciao

 alcune restrizioni (in inglese):
CUT
Bisogna ammettere che le condizioni non sembrano troppo restrittive.
Purtroppo la ''scommessa'' pone condizioni un po' diverse rispetto ad una 
normale indagine conoscitiva: per ovvii motivi RK chiede un 12/12, mentre 
statisticamente sarebbe gia' interessante un 10/12.
Cosa dire: sicuramente gli ampli quando sono lontani dalla zona di 
saturazione si somigliano molto piu' l'un l'altro che quando iniziano a 
distorcere.
Di solito, pero', su carico reale si riscontrano delle differenze nella 
risposta in frequenza fra l'uno e l'altro dovute alla diversa impedenza 
d'uscita. Mi sembra strano che nessuno avverta ''almeno'' queste ondulazioni 
della funzione di trasferimento.
Ciao!