User Manual

darTZeel NHB-108 model one Audiophile's technical manual Page 9 of 28
5. The audio electronics
Simplicity.
The whole darTZeel NHB-108 model one
electronic concept could come down to
this single word.
Simplicity does not mean simple, how-
ever.
Purity
Total cancellation or absence of har-
monic distortion does not mean "purity",
alas.
You certainly know that most musical
instruments are very rich, harmonically
speaking. Thanks to these harmonics, we
can distinguish between a saxophone
and a flute.
Did you know, though, that the level of
these natural harmonics can easily reach
20 to 40% of the fundamental note, de-
pending on the instrument played? But
do we say that an instrument distorts?
5.1. What is distortion?
In the audio world, any signal change is
called "distortion ". A lot of dif ferent kind s
of distortion exist, but we mostly speak
about "Total Harmonic Distortion", or
THD. THD is so well known that we more
than often forget that other ones do ex-
ist.
Negative Feed-Back, NBF, is a smart
electronic trick used for diminishing or
even eliminating all type of distortions,
and can be roughly described as follows:
At the amplifier input we put a signal
having an inverse deformation from the
one it naturally produces at its output.
So, the new output signal will be "puri-
fied" since the 2 inverse deformations
will cancel each other.
This is what the theory says.
Now let us see what actually happens in
practice. We will discuss only the best-
known distortions, given that their be-
havior is simpler to explain.
5.1.1. Keeping in harmony
Let us take an amplifier especially de-
signed to produce exactly 1% of THD.
Then let us feed it with a recorded musi-
cal instrument. We shall suppose that
this instrument is moderately rich har-
monically, say around 20%.
Now, let’s be a bit optimistic and sup-
pose that the speakers we will use are
truly perfect, without a single trace of
any distortion.
Well, now, let us try figure the THD we
will have at the output of the amplifier.
We could be tempted to say "21%, since
20 plus 1 makes 21, no?"
Okay… So then, which "golden ears"
could notice the THD difference between
20 and 21%? Who could swear that the
amplifier does add distortion?
In fact the distortion change is even less
than this. THD, like most other non-
correlated physical phenomena, does not
increase in a linear fashion. The "total"
THD is equal to the square root of the
sum of the squared individual THDs.
A little equation is even better than a
less-than-clear definition:
()() ()
22
2
2
1
....
n
HDHDHDTHD +++=
In our particular case, we only have 2
terms, so the equation becomes:
%025.20
100
1
100
20
100
22
=






+






=THD
And now? Who could distinguish between
a 20.000% harmonically rich musical
instrument, and another one producing
20.025%? Not us for sure.
And the very next note, how rich is it?
And the next one? Oh! The song has al-
ready finished? What sort of music was
it?...
Even though in reality THD is a bit more
complicated than this, we can still see
that a posteriori, it does not have the
importance that was attached to it for
decades – quite the contrary.
If at darTZeel we could completely elimi-
nate THD while maintaining our 3 main
criteria, then maybe we would do so.
Maybe. Just for the beauty of the ges-
ture.