darTZeel Audio NHB-108 - Manual (Page 10)

darTZeel NHB-108 model one Audiophile's technical manual Page 10 of 28
Some amplifiers available on the market
claim extremely low THD figures, at the
very limit of instrumentation measure-
ment, assuming this result necessarily
leads to outstanding sound purity. We
take this with some caution, since it re-
calls for us the 70-80' Japanese philoso-
phy, by which "performance" implies
"quality". We have to be fair by acknowl-
edging that such designs can be liked
and even loved by audiophiles. The world
is big enough for multi musical tastes.
The idea is very nice indeed, and we do
admire such a philosophical approach,
even if it is not ours: the problem is that
those designs are highly sophisticated –
too much for our ears – involving lots of
added transistors, op amps, and higher
than reasonable NFB, leading finally to
altered sound structure.
These electronics can be considered as
very pure and detailed for the first listen-
ing hours or days, but once noticed, their
sonic signature becomes harder and
harder to bear.
So if THD does not seem to be a deter-
minant factor in the accuracy of the re-
produced sound, we have to look else-
where.
5.1.2. Inter Modulation Distortion
Now, let us listen to two flute players.
The flute is well known for its less rich
than usual sound - harmonically speak-
ing, of course! A kind of exception con-
firming the rule.
If each player produces a different, sus-
tained note, what will we hear? One in-
termediary note, two distinct notes, or
more than this?
In theory we should hear 2 distinctive
notes. In theory only? Well yes, because
in practice our hearing is not perfect.
Truly wonderful indeed, but less than
perfect.
Without entering into details, we can
nevertheless say that in fact we will not
only hear 2 distinctive sounds, but also
combinations of those primary notes.
We beg musicians and music lovers to
forgive us, but for a while we need to
replace notes by frequencies. Not very
musical, but much easier for the pur-
poses of our explanation.
If the first flute plays a 1,000 Hz tone,
and the second one a 2,500 Hz tone, we
will not only hear those two discrete
tones, but also the following combina-
tions:
2,500-1,000=1,500 Hz, also called the
beating frequency, and also the mirror-
ing part, say 2,500+1,000=3,500 Hz.
Fortunately, those combinations are of
much less amplitude than the discrete
notes themselves. But that’s not all! We
will also hear the harmonics of these
combinations! So, frequencies of 1,500,
3,000, 4,500 Hz, but also 3,500, 7,000
and 10,500 Hz will be perceived. Once
again, their level will be very low com-
pared to the 2 initial notes played.
It seems hardly believable, but it is
true… In practice, though, this is far less
embarrassing than you might think at
first glance. As said above, the relative
level of those "ghosts" is much lower
than the basic notes. The result will be
heard as if the flutes had some tremolo,
or vibrating behavior, a very easily per-
ceived phenomenon. And the combina-
tion’s harmonics will add some "warm-
ing" factor, or on the contrary some
"coldness" or "dryness" to the perceived
sound, depending on how they will com-
bine together.
So you can see that lMD is even less
easy to understand and quantify than
THD. And how can we appreciate IMD,
can we hear it, at what level does it be-
come a nuisance?
Well, the first thing we can admit is that
it is not musical instruments that gener-
ate inter modulation effects, but our own
hearing – brain included – that is the
cause. Some acousticians say than our
internal ear can "produce" inter modula-
tion artifacts at levels between 25 to
40%! Of course the brain then does
some necessary "correction".
However, this kind of "correction" cannot
be compared with an electronic chain,
say the microphone and analysis system.
So, we feel very uncomfortable about
advancing any IMD value from external
origin for which our ear is sensitive. In
practice it seems that IMD less than 2%