darTZeel Audio NHB-108 - Manual (Page 11)

darTZeel NHB-108 model one Audiophile's technical manual Page 11 of 28
(3 to 5% for THD) is considered as not
disturbing, even not audible in a musical
context, judging by the Single Ended
Triode aficionados or by all the psycho
acoustical experiments conducted over
the past few decades.
By way of conclusion, it appears that
IMD, while perhaps more disturbing than
THD, remains practically harmless, pro-
vided the levels are not outrageously
high.
Okay. Now bring on the next suspect…
5.1.3. Temporal Distortion
In the ‘seventies, the famous Matti Otala
highlighted a new kind of distortion, not
so easy to measure, but clearly audible,
called Transient Intermodulation Distor-
tion, TID (also called TIM, for Transient
Inter Modulation).
TID occurs when the negative feedback
(NFB) loop is in a state of overflow,
something that arises more often than
you might think since the NFB correction
always applies after the phenomenon to
be corrected appears. During these very
short instants, the amplifier can produce
more than 100% THD and/or IMD.
At darTZeel, we prefer to call it Temporal
Distortion, or TD, because this is obvi-
ously what it actually is. It is one of our
favorite subjects for discussion, since it
is here that we can find the key to the
problem.
Since the CD came on the scene, lots of
water has flowed under bridges. In the
‘nineties, a big bug reared its ugly head
on the "perfect sound for ever" road. Its
name is "Jitter", and it is, more scientifi-
cally speaking, an "uncertainty of chro-
nometer precision".
Jitter is nowadays well known, and well
explains why temporal errors, even when
small, lead to amplitude distortion. Effec-
tively, a "0" or a "1" not arriving on time
will be translated into output amplitude
which will not be proportional to the in-
put amplitude of the signal. We all know
how jitter "sounds", when not cured.
Thanks to – or because of – this famous
jitter, a truly digital calamity, we can
better understand that temporal distor-
tion will alter the precious and delicate
analog, musical signal. In the analog
world, though, TD is more subtle and
more difficult to treat and cure.
In the darTZeel NHB-108 model one, every-
thing has been done to preserve the
temporal integrity of the music.
There are two principal means for reduc-
ing, or even eliminating, TD.
The first is to use several small but local
NFB loops, instead of a bigger and
slower, global NFB loop. This approach
greatly improves signal transfer speed
and propagation delay time.
The second is based on the principle that
temporal error is equivalent to phase
shift. So if one can enlarge the frequency
response by a factor of ten (say 200
kHz), phase shift will be also greatly re-
duced.
Ideally, the best would be to apply both
means described above. The only prob-
lem is that they contradict each other.
High bandwidth generally requires higher
global NFB, while low NFB leads to
poorer frequency response.
Here is where the darTZeel NHB-108 model
one comes in, the first very low NFB and
high bandwidth power amplifier. No
global NFB is used, and both 1
st and 3 rd
stages are even open loop! So what
about the frequency response? Every-
thing is okay up to the Megahertz range
(1,000,000 Hertz), say fifty times the
audio range!
5.2. What application field?
The electronic schematics of the darTZeel
NHB-108 model one's audio circuit, as we
will soon see, is astonishingly simple.
"It's because the NHB-108 has high THD
and IMD values", would say our well-
meaning detractors.
If you have read us from the beginning,
you are now perfectly aware that har-
monics are not directly responsible for
the sonic signature of an amplifier.
The apparent simplicity of our audio cir-
cuit has been effectively made possible
through our choice of semiconductors
technology.