darTZeel Audio NHB-108 B version - Manual (Page 23)

darTZeel NHB-108 model one
User Manual, version B 1.0 Page 23 of 35
ally is. It is one of our favourite subjects for discus-
sion, 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 chronometer
precision".
Jitter is nowadays well known, and well explains
why temporal errors, even when small, lead to am-
plitude distortion. Effectively, a "0" or a "1" not
arriving on time will be translated into output am-
plitude which will not be proportional to the input
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 distortion will alter the precious and deli-
cate 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, everything
has been done to preserve the temporal integrity of
the music.
There are two principal means for reducing, 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 trans-
fer 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 de-
scribed above. The only problem is that they con-
tradict each other. High bandwidth generally re-
quires 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 band-
width 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? Everything is okay
up to the Megahertz range (1,000,000 Hertz), say
fifty times the audio range!
T5.2. 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 harmonics are not directly
responsible for the sonic signature of an amplifier.
The apparent simplicity of our audio circuit has
been effectively made possible through our choice
of semiconductors technology.
If the THD and IMD produced by the amplifier are
kept low enough, say lower than the audibility
threshold, then no further correction will be needed,
and the audio circuit can remain simple.
At the start of this new century, 3 main technolo-
gies coexist in the construction of transistors, these
being in chronological order:
- Bipolar, in the early 1950s.
- Field effect, in 1962.
- IGBT, a mix of the previous two, in the ‘eight-
ies.
At darTZeel, we strongly believe in new technolo-
gies. On the other hand, we readily admit that noth-
ing can replace experience.
So, "our" winner is…
Bipolar technology!
T5.2.1. IGBTs
For your own information, we are ready to tell you
an old secret. What does the word "transistor"
mean?
It comes quite simply from the contraction of trans-
fer and resistor.
IGBT stands for "
I nsulate G ate B ipolar T ransis-
tor". It behaves like a bipolar transistor at its output,
while being driven like a FET at its input.
IGBTs are mostly used in power applications, like
inverters, switching power supplies, heart defibril-
lators, and… for some audio power amplifiers.
We do not have any preconceived notion about the
use, and more specifically, the sound of IGBTs.
Results can vary with the samples used.
Their linearity is close to FETs, which is, alas, not
enough for us.
IGBT technology is by far the youngest, and we
lack time to fairly judge them as to their long-term
sound quality, reliability and availability.