darTZeel NHB-10 model one User manual
darTZeel NHB-108 model one
Audiophile's technical manual
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TABLE OF CONTENTS
1. Introduction................................................................................................................................................... 5
2. Genesis of NHB-108 ..................................................................................................................................... 5
3. Criteria........................................................................................................................................................... 6
4. Structural aspects........................................................................................................................................... 6
4.1. The case modules............................................................................................................................ 7
4.2. Divide to conquer............................................................................................................................ 7
4.3. Eliminating vibrations..................................................................................................................... 7
4.4. Everything is transparent................................................................................................................. 8
4.5. TIO, Totally Identified Object......................................................................................................... 8
5. The audio electronics..................................................................................................................................... 9
5.1. What is distortion? .......................................................................................................................... 9
5.1.1. Keeping in harmony............................................................................................................ 9
5.1.2. Inter Modulation Distortion............................................................................................... 10
5.1.3. Temporal Distortion.......................................................................................................... 11
5.2. What application field? ................................................................................................................. 11
5.2.1. IGBTs................................................................................................................................ 12
5.2.2. FETs.................................................................................................................................. 12
5.2.3. Bipolars............................................................................................................................. 13
5.3. The circuit of darTZeel ................................................................................................................ 13
5.3.1. Criteria of choice............................................................................................................... 13
5.3.2. darTZeel schematics ........................................................................................................ 13
5.3.3. Component layout............................................................................................................. 14
5.4. Symmetrically balanced? .............................................................................................................. 14
5.4.1. From the microphone… .................................................................................................... 15
5.4.2. To the loudspeakers…....................................................................................................... 15
5.4.3. And into the air….............................................................................................................. 15
5.4.4. Via the darTZeel .............................................................................................................. 16
5.5. darT to Zeel 50Ω......................................................................................................................... 17
6. Onboard safety............................................................................................................................................. 18
6.1. Crowbar circuit.............................................................................................................................. 18
6.1.1. Crowbar activation............................................................................................................ 19
6.1.2. Crowbar cycle ................................................................................................................... 19
6.2. Supervision system........................................................................................................................ 20
6.2.1. Current sensing.................................................................................................................. 20
6.2.2. For its eyes only ................................................................................................................ 20
7. Power supplies............................................................................................................................................. 20
7.1. From mains to loudspeakers.......................................................................................................... 21
7.2. Voltage or current?........................................................................................................................ 21
7.3. Received idea................................................................................................................................ 22
8. The sound of darTZeel ............................................................................................................................... 23
9. Reliability.................................................................................................................................................... 23
9.1. Quantified longevity...................................................................................................................... 23
9.2. Long term availability................................................................................................................... 24
9.2.1. Spare parts......................................................................................................................... 24
9.2.2. Self-preservation instinct................................................................................................... 24
10. The next darTZeel ................................................................................................................................. 25
11. Keeping an open door............................................................................................................................. 26
12. Special adjustments ................................................................................................................................ 26
12.1. Output DC voltage drift................................................................................................................. 27
12.2. Monitoring circuit ......................................................................................................................... 28
12.3. Version B ...................................................................................................................................... 28
12.4. Future developments..................................................................................................................... 28
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darTZeel NHB-108 model one
Audiophile's technical manual
1. Introduction
Thank you for studying this manual,
demonstrating proof of your interest in
the darTZeel NHB-108 model one.
First of all, please do not think for a
minute that we consider audiophiles dif-
ferently from music lovers. We know
perfectly well that audiophiles love music
too, and we are very happy with this.
It is however true that music lovers are
generally less inclined to enjoy the tech-
nical and performance side of the music,
hence the existence of the Music lover's
manual.
In the light of what follows, you will bet-
ter understand why our technical choices
during the design of the darTZeel NHB-108
model one led to what one could call a
truly exceptional machine.
We trust that you will derive great pleas-
ure from reading this manual, and that
we succeed in communicating some of
the burning passion that has been boiling
inside us for more than 25 years.
Enjoy your reading!
2. Genesis of NHB-108
The NHB ("Never Heard Before") project
began in summer 1984.
Crazy about audio since childhood, we
gradually developed the idea of design-
ing a power amplifier.
Even though the NHB-108 model one is the
fruit of a single man, the moral, financial
and professional support from family and
friends over all these years has played a
huge part in this endeavor.
At the time it was only a dream, and we
are grateful to every gal and pal who has
been involved in it and contributed to its
achievement.
In the '70s and ‘80s, here in Europe,
Japanese electronics were queens and
kings of the market. Very rare, and
above all virtually unknown, was the real
high-end audio gear coming from USA or
elsewhere. The finish was already very
good, but the sound quality did not come
up to scratch, simply because it was not
one of the original design goals! Only a
few companies were able to offer real
breakthrough products. Not all of them
are still in business, Alas!
The more the years passed, the more
our desire for designing a new machine
became tangible. In mid spring 1984, we
produced a first digital power amplifier in
a school lab, as a diploma project.
Paradoxically, the innovation here was
not the fact that the amp was digital, but
that no feedback whatsoever was used.
The output stage was totally open-loop.
The sonic result was so astounding that
we immediately started the NHB project,
although its code name did not come
until later.
The initial idea was to design a new very
powerful digital amplifier, totally open
loop if possible, with the purest sound
reproduction we could attain, without
any other consideration, especially the
price factor.
The main problem at the time was that
the technology did not yet offer the
monochip solution, so we had to build
the circuit entirely in hybrid technology,
using mainly discrete devices. The odd
thing is that even in this early 21st cen-
tury, we have still not surpassed the
speed of those old circuits.
Another problem then arose. Our circuits
were much more complex than a single
modern IC, and of course they were not
very reliable either. We especially en-
countered several performance consis-
tency problems from sample to sample,
and we gave up for a time. Some years
darTZeel NHB-108 model one Audiophile's technical manual Page 6 of 28
later, new ICs allowed further investiga-
tion, and we built several prototypes. We
quickly noted, however, that digital had
some limits when pushed hard.
We used a sample frequency of 500 kHz
at the time, corresponding to a theoreti-
cal bandwidth close to 250 kHz, far
higher than all competitors, even in the
analog domain. But – alas – even with
such a high sample frequency, the actual
measured bandwidth was strongly de-
pendant on the loudspeaker impedance.
Another problem, jitter, already well
known in labs at the time, was difficult to
cure, to say the least. The signal to noise
ratio was just above our minimum crite-
rion.
Since pure sound was the only goal, then
noise, if at acceptable level, could have
been tolerated by us. On the other hand,
interdependence between load imped-
ance and frequency response was not
our cup of tea. Not by a long chalk.
End of the digital trip.
It is interesting to note that several
manufacturers offer digital amplifiers,
with greater or lesser success. The theo-
retical limits remain the same, even if
better managed than in the '80s. Analog
fortunately does not have to cope with
such limitations.
After several months of deep thinking,
circuit designing, and circle squaring at-
tempts, the project was revived (in
1990) on brand new bases. The gear
would be fully analog, moderately pow-
erful, and as close as possible, philoso-
phically speaking, to the signal treat-
ment used in its digital ancestor.
Two new prototypes were built on the
basis of existing designs; even though
heavily modified to suit our tastes, they
produced unexciting results. Then we
asked ourselves what is the point of pro-
ducing nice looking gear if the sonics
bring nothing new under the sun?
In 1992, we decided to develop a revolu-
tionary design from scratch. This ulti-
mate quest (for ultimate sound) lasted
over 5 years, with more disappointment
than joy. But the fun was always here.
Countless schematics were studied, scru-
tinized and hooked together until we
obtained, in mid 1995, what we could
tained, in mid 1995, what we could call
the first theoretical design corresponding
to our aims.
From 1995 to 1999, on that first elemen-
tary circuit basis, dozens of different
versions were simulated, some of them
being built and listened to.
By 11 November 1999 the definitive cir-
cuit was built and assembled. This was
the very first darTZeel machine, the NHB-
108 model zero.
The results, sonically speaking, were
outstandingly better than expected. It
seems that this design was the right one.
So much so that most of our test listen-
ers said that this product must be put
into production, even on a very small
scale.
The darTZeel NHB-108 model one was born.
3. Criteria
Your darTZeel NHB-108 model one is not an
ordinary machine and this is no doubt
one of the reasons why you purchased it.
The entire concept is based on just three
criteria:
-Simplicity
-Purity
-Reliability
These words can often be deceptive.
Easy to pronounce, conjuring up ele-
gance and fascination, they nevertheless
constitute very tough technical chal-
lenges, since their respective meanings
are not at all easy to reconcile.
Nothing is eternal. As for the darTZeel
NHB-108 model one, not enough time has
yet elapsed for us to assert such a claim.
What we do assert is that everything has
been taken into account so that you can
benefit from your machine for a long
time. A very long time.
You will discover that more often than
not, simplicity means long lasting.
Sometimes, it also means purity,
whether of sound or of form.
Welcome to darTZeel.
darTZeel NHB-108 model one Audiophile's technical manual Page 7 of 28
4. Structural aspects
It would be all too easy to compare the
darTZeel NHB-108 model one and wrist-
watches, both being made in Switzer-
land. There are no springs, cogwheels, or
hands in a NHB-108 model one. Here, eve-
rything is a tad… bigger.
4.1. The case modules
The case of your machine is exclusively
machined from AW-5754-ALMG3 alloy alu-
minum, offering a remarkably even surface and
hardness.
Every item is CNC machined from the
billet in order to ensure a very high den-
sity and outstanding rigidity, thus con-
siderably lowering all unwanted reso-
nances.
The elements are finished in the three
hard anodized darTZeel colors, namely
electric blue, blazing red, and gleaming
yellow. This unique finish gives to the
darTZeel NHB-108 model one its inimitable
appearance.
The three main elementary modules are
the mother plate, the transformer plat-
forms and the heat sinks. They are
tightly assembled together with non-
magnetic, stainless steel screws.
Each basic module is first carefully hand
assembled, and individually tested. Only
then is it dated and signed before being
fitted.
If you dismantle your darTZeel NHB-108
model one, you will see that modules are
not assembled in a hurry. Some of them
are installed with several weeks elapsing
between construction and assembling.
In line with this non-stress approach, we
take all the time necessary to achieve
the best job we can. If our tests show
that a component is out of specification –
this can sometimes occur after its as-
sembling – the entire module is disman-
tled, checked, and reassembled before
being tested a second time.
The darTZeel NHB-108 model one housing
features a 25mm false bottom, sand-
wiched between the 6mm thick mother
plate module and the 4mm thick bottom
plate. All cables and wires are routed in
this sandwich, minimizing the influence
of electromagnetic fields.
Furthermore, such an arrangement is of
extreme rigidity ensuring exceptionally
good mechanical coupling between the
mother plate and heat sinks modules.
4.2. Divide to conquer
The inside of the darTZeel NHB-108 model
one is divided into four distinct volumes.
The left and right channels are insulated
from each other, as are their respective
power supplies. This unprecedented con-
struction allows extremely low crosstalk
behavior across the whole audio fre-
quency range. You are either true dual
mono, or you are not. The darTZeel NHB-
108 model one definitely is.
The two front compartments are devoted
to the power supplies, and are them-
selves divided into a two-storey struc-
ture. The Crowbar elements and the soft
start circuitry are located on the lower
level, while the suspended toroidal trans-
formers are fixed on the 6mm thick, up-
per decks.
We insist on the fact that in the final as-
sembly process, all parts are first care-
fully aligned before being tightly screwed
together. This additional precaution adds
still more rigidity, and the entire case
then behaves as a single block. As a free
bonus, the case is thus virtually dust
proof.
4.3. Eliminating vibrations
The fastening of the power supply trans-
formers was carefully thought out as re-
gards noise reduction and trouble-free
life. A specially developed suspension
ensures that even in case of failure, the
transformers will not fall off.
So, in 40 or 50 years, when the rubber
absorbers have become – perhaps –
worn out, they will withstand their last
trip to the factory for their replacement.
The same is true for the silentblocs used
for suspending the lighter audio and
monitoring circuits.
These various suspensions, tuned on
different resonance frequencies, are
coupled quite tightly to the chassis, ab-
sorbing especially the medium and high
darTZeel NHB-108 model one Audiophile's technical manual Page 8 of 28
frequencies, most important in terms of
sound reproduction accuracy.
And while on this subject, have you ever
heard an amplifier singing? This can oc-
cur when the transistors of the final
stages and/or the power transformers
are not properly decoupled.
They start to vibrate at the excitation
signal frequency – in this case the music
– this vibration interfering with the
electrical origin of the signal. You can
hear it if the amp is hooked to a dummy
load, since in normal use, loudspeakers
fortunately mask this disconcerting
internal “singing”.
No such mechanical singing on the
darTZeel NHB-108 model one! The electric
signal is transmitted stage to stage, from
input to output, free from any mechani-
cal disturbance, internal or external.
The darTZeel NHB-108 model one stands on
3 pads incorporating rubber inserts.
Oddly enough, very few manufacturers
rely on 3 legs instead of four. We can
recall that the Lunar Explorer Module
(LEM) also had three legs, but that is
another story after all.
Weighing around 30kg, the darTZeel (not
the LEM) needs to have a good stable
base. Those three pads are evenly lo-
cated around the horizontal center of
gravity of the amplifier, ensuring excel-
lent stability on virtually any surface.
The absorbing rubber pads dampen the
lower frequencies, thus forming the ideal
complement to the suspensions de-
scribed earlier. In the case where you
would want to use 2, 3, or even 4
stacked darTZeel NHB-108 model one units,
we have designed special pads of differ-
ent diameter to maintain the very same
vibrational absorption behavior. These
pads are easy to install and can be pur-
chased separately.
4.4. Everything is transparent
The darTZeel NHB-108 model one cover is
made of 8mm thick tempered, bronze-
smoked glass. It harmonizes magnifi-
cently with the colors of the housing, and
gives an inimitable touch.
The glass is held by a single, central
screw, and rests on a foam rubber seal,
giving to the darTZeel NHB-108 model one
its dust proof quality. Your machine will
therefore remain as new for years to
come.
The reason why we did not choose a
metal cover is not so obvious as it might
appear. It is true that to look through
the glass of the darTZeel NHB-108 model
one is a pleasure for the eyes. Look at
those Moon-crescent shaped bar busses,
and tell us frankly what you feel…
There are also a couple of technical ad-
vantages that glass has over metal.
Its crystalline, inert structure, combined
with the rubber foam seal, functions as
an internal noise killer.
And contrary to metal, glass is totally
transparent to magnetic fields, thus
avoiding the inherent magnetic loop that
metal would induce over the power sup-
ply transformers. Last but not least, the
internal housing is much less polluted by
magnetic ghosts.
4.5. TIO, Totally Identified Object
It is however the final touch that makes
this darTZeel NHB-108 model one, now in-
stalled in your listening room, your
darTZeel NHB-108 model one:
We refer to the identification plate, fully
described in the Owner's manual.
Made from 24k gold plated brass, it is a
unique handmade piece, just like the
machine on to which it is affixed.
Your name is engraved in gold letters,
thus amalgamating the destiny of the
darTZeel NHB-108 model one with your fu-
ture unforgettable musical moments.
We proudly congratulate you!
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 different kinds
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.
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%
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 1st and 3rd
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.
darTZeel NHB-108 model one Audiophile's technical manual Page 12 of 28
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
technologies coexist in the construction
of transistors, these being in chronologi-
cal order:
-Bipolar, in the early 1950s.
-Field effect, in 1962.
-IGBT, a mix of the previous two, in
the ‘eighties.
At darTZeel, we strongly believe in new
technologies. On the other hand, we
readily admit that nothing can replace
experience.
So, "our" winner is…
Bipolar technology!
5.2.1. 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 contrac-
tion of transfer and resistor.
IGBT stands for "Insulate Gate Bipolar
Transistor". It behaves like a bipolar
transistor at its output, while being
driven like a FET at its input.
IGBTs are mostly used in power applica-
tions, like inverters, switching power
supplies, heart defibrillators, 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, reliabil-
ity and availability.
So for now, and maybe for still a little
while, we will probably not use them di-
rectly in audio applications.
5.2.2. FETs
FET stands for "Field Effect Transistor",
of which there are two main categories.
For small signals, they are called as is,
FETs.
For power applications, their most com-
mon name is MOSFET, which stands for
"Metal Oxide Silicon Field Effect Tran-
sistor". They behave similarly to FETs,
but their internal structure may vary.
FETs are being used more and more, for
several reasons, including the most ob-
vious one:
It is commonly accepted that they per-
form very closely to vacuum tubes (or
electron tubes or valves). Tube lovers
generally like MOSFET amplifiers due to
their similar behavior.
Here, we are forced to say that we do
not share this point of view. Not entirely,
at any rate…
It is true that the sound of MOSFETs
tends to be soft and warm, a bit like
those found in tubes. But their electrical
behavior is not similar, simply because
they are not made of similar materials.
The only true similarity is that MOSFETs
– and FETs of course – are voltage
driven, like tubes. Apart from that, they
are in two different worlds, vacuum for
tube, silicon for FETs.
Conduction in any type of transistor al-
ways acts in a solid, here the metallic
silicon, hence their name of "solid-state"
devices.
In a vacuum tube, electrons move in… a
vacuum.
Coming back to FETs, what is their
sound? For most designs, we would say
that it is only a matter of taste. We are
convinced that one can design a "stan-
dard", good sounding machine in the 3
technologies mentioned above, and even
with the fourth - tubes - with virtually
the same sound.
Obviously, if you like music, you defi-
nitely need a darTZeel NHB-108 model one.
Yet, and contrary to common belief,
MOSFETs are less linear than bipolar
transistors available today. MOSFETs
often need more sophisticated added
darTZeel NHB-108 model one Audiophile's technical manual Page 13 of 28
circuitry to make the best of their possi-
bilities, and that means a longer and
more complex signal path, and hence a
reduced resolution.
MOSFETs are also slower, in absolute
terms, than bipolars, and are much
harder to drive in pulse mode.
Reproducing very fast transients requires
a lot of instantaneous current drive, not
easy for the power supply to properly
feed, so temporal integrity could suffer.
5.2.3. Bipolars
These have been in existence for more
than fifty years. The industry knows
them very well. Today's bipolars are bet-
ter than ever. Ultra linear and extremely
fast, they are perfectly suited for very
high-end audio applications.
Their excellent linearity makes it possible
to minimize the number of peripheral
components, thus obtaining a straight-
forward signal path, with many less su-
perfluous correction circuits.
In the darTZeel NHB-108 model one, the
audio signal travels through only 6
transistors, from input to output,
maintaining low THD and IMD levels, and
without using any global NFB, the output
stage even operating in a totally open
loop! The slowest transistors used have a
bandwidth of more than 30 MHz, much
higher than for a MOSFET. This extreme
intrinsic speed allows a total phase re-
spect across the whole audio range,
without any static or dynamic deforma-
tion. In brief, no Temporal Distortion.
In terms of music, these breakthrough
advantages bring you closer to the musi-
cians. Now you can share the emotion of
the violinist, the fire of the conductor or
the sweetness of the flutist.
Words are however not enough to de-
scribe what a single bipolar transistor
pair is capable of. The best is still to in-
vite your friends home, making them
understand, share and believe your ears.
5.3. The circuit of darTZeel
5.3.1. Criteria of choice
Simplicity.
The audio circuit of the darTZeel NHB-108
model one uses only 14 transistors in all,
including current sources.
Only three different bipolar transistor
devices are used, all of the same brand.
Purity.
The version Aof the darTZeel NHB-108
model one takes unprecedented care of
the precious musical signal:
No connector, switch, relay or fuse what-
soever is located in the signal path. Even
better, we did not use any of these de-
vices except in the AC mains input.
No current limitation, allowing unbeat-
able dynamic range when in cooperation
with the single output pair devices.
No output DC voltage drift compensa-
tion, offering a truly breathtaking sound,
from whispers to loud shouts.
Only a single, small, local symmetrical
DC NBF, leading to extremely wide
bandwidth, included in the very low fre-
quencies, without addition of any multi
polar phase shift.
The use of a very compact printed circuit
board (PCB) reduces track lengths to the
strict minimum.
Reliability.
The darTZeel NHB-108 model one does not
use any "exotic" or "esoteric" compo-
nent. Each element was chosen for its
intrinsic sound qualities, its long-term
availability, and for its long life. No com-
promise was conceded as to the build
quality of any part or component. Some
of them were tested for 15 years in our
lab before being selected.
5.3.2. darTZeel schematics
For the most curious readers, we give
hereunder the functional diagram of the
darTZeel NHB-108 model one.
Connoisseurs will appreciate how simple
it is, so different from the common belief
of "bigger is better".
darTZeel NHB-108 model one Audiophile's technical manual Page 14 of 28
For once, we give the description in
French, which is, after all, the very origi-
nal text. A translation can be provided
on special request.
This schematic is multi patent protected
Description du circuit:
3 étages, en technologie discrète, se décomposent
ainsi :
1er étage, étage d’entrée. Cet étage, dépourvu de
toute contre réaction, amplifie en courant le signal
d’entrée, de telle sorte qu’il ne subisse aucune
charge susceptible de le déformer. La simplicité
apparente de ce premier étage (1 seul transistor
par polarité) permet des vitesses de commutation
très élevées, sans rotation de phase notable dans
le spectre audio.
2ème étage, étage d’amplification, amplifie en ten-
sion le signal à la valeur nominale requise. Cet
étage comporte 2 demi contre réactions locales,
implantées de manière symétrique. Cette contre
réaction symétrique s’effectue sur tout le spectre
de fréquence, y compris le courant continu, afin
d’apporter une réponse parfaite dans le grave.
La configuration de ce circuit permet une propaga-
tion de groupe homogène sur tout le spectre audio,
grâce à la faible valeur de la contre réaction.
3ème étage, étage final de sortie, dépourvu de toute
contre réaction. Cet étage amplifie en courant le
signal issu de l’étage précédent, permettant ainsi
d’alimenter un haut parleur. Le courant de repos
des transistors de sortie est ici défini par une ten-
sion de jonction base-émetteur, et non par un cou-
rant de polarisation. Ce système élimine le besoin
d’une régulation thermique. En effet, lorsque les
valeurs initiales ont été fixées, tout échauffement
des transistors de puissance entraîne également un
échauffement des transistors drivers. La tension
VBE chute aussi bien dans les drivers que dans les
éléments de puissance, garantissant un équilibre
thermique stable. Ce montage permet également
des bandes passantes très étendues, sans dépha-
sage important.
The above description is an extract of
the original patent text, and is con-
densed to cover other applications, even
in the non-audio field. The darTZeel pre-
amplifier circuit is directly derived from
the darTZeel NHB-108 model one schemat-
ics.
If you have any specific questions about
this circuit, please do not hesitate to
5.3.3. Component layout
In analog, especially in audio, the layout
of components is of paramount impor-
tance in the signature they can print
onto the musical signal. A transformer
too close to input circuits, or a power
supply located too far from output de-
vices, are just some examples among
others. These parameters are difficult to
cope with, particularly when cost
considerations come into the picture.
The internal volume was exploited down
to the last cubic centimeter. Output
power devices are located less than 10
centimeters from the huge crescent-like
bus bars. All the energy coming from the
capacitor reservoir can then effortlessly
flow to the output bipolar transistors.
Also, the Power Nose – please read the
Owner's manual for more about the ter-
minology we use – does not directly
switch the AC mains, but drives static,
semiconductor relays. This approach al-
lows freedom from any electromagnetic
disturbance from AC, and also to soft
start the amplifier. The switch is also
subject to less wear, since no spark can
appear at its contacts, extending its life-
span close to one million ON-OFF opera-
tions.
We also placed inputs connectors very
close to the entry PCB points, altering as
little as possible the still unamplified in-
coming music.
Dozens of other small things have been
thought about and implemented. We
would need too much space to describe
all of them; furthermore, such a descrip-
tion would be much too boring. Please
believe us, we have done our best to
look after the precious and delicate mu-
sical signal, leaving as little as possible
to chance.
darTZeel NHB-108 model one Audiophile's technical manual Page 15 of 28
5.4. Symmetrically balanced?
The following could seem contradictory
sometimes. It is mainly due to the terms
"balanced" and "symmetrical", or "un-
balanced" and "single ended".
In French, these terms lead to even
more confusion, since both "balanced"
and "symmetrical" are translated by
"symétrique".
So, you will not feel less comfortable
than our French-speaking friends. Feel
you need explanations in greater depth.
It is absolutely fascinating to note that
some technical choices only serve falla-
cious sales arguments, and to our regret,
audio is no exception to the rule…
Most high-end power amplifiers, espe-
cially flagship models, offer balanced
inputs, described as being the very best,
technically and sonically speaking. As
often as not there’s no option as regards
balanced inputs: in fact there’s nothing
else!
For the darTZeel NHB-108 model one, our
approach has been almost the opposite.
To minimize the cost? Could you even
believe that?
Maybe the time has come to refresh our
memory to some extent…
5.4.1. From the microphone…
A balanced line is in fact a transmission
line without reference to ground. Imag-
ine that instead of transmitting the sig-
nal between one wire and ground, we
use two wires. A 3rd cable, earmarked for
ground, is of course also present, even
though not necessary for the signal right
now.
This transmission mode finally leads to
external noise and disturbance cancella-
tion.
In pro audio engineering, balanced lines
are de rigueur and massively used. Un-
balanced links are very rare indeed! The
reason is very easy to understand:
Microphones generate very tiny electric
signals, and very long lengths of cable
run from the studio to the mixing con-
sole. In these conditions, it is primordial
for the signal to be free of any hum
and/or noise.
Also, in concert performances, mixing
consoles are generally located quite far
from the stage, and balanced lines are
welcome. A technical trick, using that
3rd, ground wire, also makes it possible
to carry the phantom power supply feed-
ing electrostatic microphones.
So now you have understood that bal-
anced links are used especially in profes-
sional applications.
5.4.2. To the loudspeakers…
Let us quit the studio and come back to
the high-end, musical world.
Basing themselves on what they consider
to be the real benchmark, audio compa-
nies equip their flagship machines with
the balanced lines described above, us-
ing the well known XLR connectors.
Most of the time they offer "fully bal-
anced" topology from input to output,
claiming that the sound will remain unaf-
fected since it is immune to external dis-
turbances.
Technically speaking, fully balanced to-
pology is relatively easy to achieve. It
suffices to double the entire electronics,
in a mirroring fashion, assigning a new
channel path for the inverted signal. Of
course this simplicity has a price, in fact
double the price. Finally, the balanced
signal is amplified and routed to the
loudspeakers.
Come to think of it, are loudspeakers
balanced or not balanced? That is the
question!
5.4.3. And into the air…
Once in the air, the musical signal is
traveling on its last trip before delicately
tickling your eardrums.
How exactly does music propagate itself
in the air?
Music is a matter of vibration, and
propagates in the shape of waves. Any
wave, to be propagated, needs a me-
dium. No medium, no wave, no sound.
Just try playing trumpet on the Moon.
Not an easy task, even if your name is
Armstrong…
darTZeel NHB-108 model one Audiophile's technical manual Page 16 of 28
What about radio waves? What medium
do they use, in the vacuum of inter-
sidereal space? Well, write us, and we
will send you the Physicist manual, as
soon as we have got round to writing it…
As for acoustical waves, it is a much eas-
ier phenomenon to describe, and espe-
cially understand, since it is part of our
daily life.
The medium can be water in the case of
mermaids singing… er, sorry, when
whales sing. Or when more human,
technological things such as sonars,
hydrophones and the like generate and
detect acoustical waves.
The medium can also be steel. The
hammer hitting the string, in a piano,
generates a pulse which creates a
propagating wave in the string, making it
vibrate. Then this vibration will be trans-
mitted into the air.
Air: this is the ultimate medium where
man-made music propagates. Music is
spread in wave form, the latter being
described by a physical law, called "wave
propagation theory". We will not enter
into the details, but mention just one
crucial and essential point:
Acoustical waves do not move air.
When we read in some high-end maga-
zine that such and such a flagship loud-
speaker can blow out a candle while re-
producing a trumpet or a saxophone,
this is just metaphorical.
The sound is produced by the vibration
of air molecules, step by step. Yes, you
did read correctly. It is vibration, not
movement.
If you know a friend who plays trumpet
or saxophone, just put your hand on the
bell and you will only feel vibration, not a
single tiny puff of wind. By the way, you
would never think about a piano being
able to stir up air to produce wind, would
you?
These vibrations have a purely single-
ended behavior, since they are produced
around a point of equilibrium, where
vibrations are zero.
To cut a long story short, we can say
that the whole acoustical chain is single-
ended. The only moment when the
acoustical signal could be balanced is
when it travels into the electric wires. In
the air, sound is unbalanced, asymmet-
ric, single-ended, as you prefer.
Why then, this obsession to balance a
naturally unbalanced signal? Is it not
against nature?
Furthermore, where is the real advan-
tage in running the loudspeaker in bal-
anced mode? To our knowledge, there is
no balanced crossover in the market!
Has any manufacturer already told you
that there is no such thing? Okay, now
you’ve been told.
5.4.4. Via the darTZeel
In the version Bof the darTZeel NHB-108
model one, we have also installed bal-
anced inputs. Did we do this just in order
to be “with it”?
First, we want to stress that we use
floating balanced inputs. This means that
rather than doubling the whole electron-
ics, as seen above, we use high quality
input transformers. Of course the use of
transformers is much more expensive,
but the resulting performances are far
superior.
Speaking of external disturbance immu-
nity, transformers are much better than
full balanced topology. The common
mode rejection (this is the name given to
that kind of immunity) can be – wait for
it – no less than five thousand times bet-
ter when using transformers instead of
full balanced circuits. Another, unbeat-
able, advantage is that they offer true
electrical isolation – called galvanic isola-
tion – between the line and the gear,
providing outstanding safety in profes-
sional use. Last but not least is the fact
that all the above-mentioned qualities
are defined at the building stage, mean-
ing that performances will not decrease
over the years or even decades to come.
This is not by any means the case in full
balanced versions.
In conclusion, we cannot resist insisting
on the fact that a full balanced solution
utilizes twice the number of components,
implying a more complex signal path,
less reliability, and furthermore, espe-
cially in power amplifiers, an output im-
pedance twice as high as with single-
darTZeel NHB-108 model one Audiophile's technical manual Page 17 of 28
ended topology, and requires a higher
output stage NFB to compensate.
Now that you have read these simple but
demonstrative explanations, do the
words "full balanced" still mean "abso-
lute superior sound" for you?
All this explains our choice for using, as
a matter of course, transformers of the
highest quality for our XLR inputs in the
version B.
We said above that by very nature, mu-
sic is part of a single-end world. More
than 100 years ago, designers chose
floating balanced lines – full balanced
was not ready yet – for long distance
links for the sole purpose of minimizing
external disturbances.
Electric signals were therefore transmit-
ted in a balanced way, the equipment
working in single-ended mode.
The darTZeel NHB-108 model one version B
offers this very same possibility to pro-
fessional users wanting to link their re-
mote consoles to the NHB-108 model
one, without having to use poor quality
Balun-DI devices.
Despite what all our esteemed competi-
tors might think, we assert and corrobo-
rate that the one and only means of
processing, amplifying and broadcasting
a musical signal without altering it, even
in the slightest, is simply to use the sin-
gle-ended mode.
But only in a special way, though…
With short cables, say less than 10 me-
ters, symmetrical - balanced - transmis-
sion does not have any justification but
marketing. A given gear "singing" better
in balanced mode only reveals poor de-
sign in some part of the circuit, which
can be partially masked by internal dis-
turbance cancellation.
Over longer lengths, external distur-
bances like hum, RFI and so forth take
on a greater degree of importance.
The disturbances' intensity can be fig-
ured out by computing the RMS signal to
noise ratio, in decibels.
A balanced, high quality, floating line can
reach more than 120 dB of rejection,
or -120dB relative to the signal, which
represents one part per million, 1 ppm, a
truly remarkable performance.
In full balanced mode, the result is far
inferior, and in practice is barely better
than -60dB, 0.1%. This may seem small,
but just keep in mind that this implies
component tolerances tighter than 0.5%,
a truly demanding task. Professional
consoles capable of such results are truly
high-end by their 6+ figures price.
It is very important to point out that bal-
anced lines, whatever they may be, are
not impedance matched. This means
that even though they are fairly immune
to external disturbances, they alter the
musical signal proportionally to their
length.
Just ask a sound engineer if he is happy
to use a cable 100 meters long when 10
meters are plenty enough.
Just ask him if high frequencies do not
suffer from very great lengths, of the
order of 100+ meters.
The 50Ωlinks used in the darTZeel NHB-
108 model one, and described hereunder,
behave differently…
5.5. darT to Zeel 50Ω
The darTZeel NHB-108 model one is
equipped with 50ΩBNC connectors.
After a lot of research, we concluded that
the one and only means for transmitting
an electrical musical signal with no al-
teration or losses over a long distance is
impedance matched lines, from end to
end.
We have already mentioned that the
darTZeel NHB-108 model one was thought
up without any compromise in mind, es-
pecially regarding its cost price.
We confirm this once again, of course,
but the purpose here is just to say we
pursued this quest of sound purity
simply because no other amplifier could
bring us what we were looking for. So we
designed the darTZeel NHB-108 model one.
As for electric transmission lines, we
didn’t want to reinvent the already exist-
ing wheel. Perfect impedance-matched
lines have been in use for almost a cen-
tury. And so have coaxial cables.
Impedance matched links are utilized
everywhere when high tech performance
is needed. Radio applications, radar, mi-
crowaves, computers, and all such preci-
darTZeel NHB-108 model one Audiophile's technical manual Page 18 of 28
sion technologies use impedance-
matched links. So why not audio?
The great advantage of impedance mat-
ched-links is their virtual absence of
losses, whatever their length.
For those of you who want to know eve-
rything about matching impedance in
audio links, do not hesitate to contact us
Propagation time delay is preserved in
DC up to several GHz in such lines, and
no other link from any make can claim
this, unless perfect impedance matching
is achieved.
So the darTZeel NHB-108 model one is fit-
ted with such inputs, here called "Zeel
50Ω", while the darTZeel preamplifier has
"50ΩdarT" outputs.
These inputs/outputs use 50Ωcoaxial
cables fitted on BNC connectors.
External disturbance immunity of a coax-
ial link depends on the cable itself. It can
vary from -50dB to -100dB or more, the
latter being greatly superior to the full
balanced mode, and all this, please bear
in mind, without any sonic alteration.
You can use very affordable off the shelf
RG58U cable, and will be very surprised
by the result. Many shorter but much
more expensive cables do not do better!
And when there is a big length increase,
there is no shadow of doubt, darT to Zeel
50Ωis simply unbeatable. Trying and
hearing is just believing.
The optional coaxial cable delivered with
the darTZeel NHB-108 model one is fur-
thermore of the high-end grade, silver
plated pure copper, designed for hyper
frequency applications. You will at least
discover what resolution really means…
As stated above, one of the main advan-
tages of darT to Zeel links is that you can
locate the power amplifier as far as you
want from the preamplifier. No more
treble roll off, harsh or fuzzy. No more
sluggish low end.
The theoretical length limit is… infinite!
In practice, we recommend not to use
lengths greater than 10 kilometers (we
are not joking here, for once.)
This new way of linking is still in its in-
fancy as regards audio, even if some of
our esteemed competitors disagree.
darTZeel will also offer in the near future,
for professional applications, darT to Zeel
floating balanced links.
Are you looking for darTZeel sound integrity
in concerts and pro audio studios? You
can have it just for the asking.
6. Onboard safety
A power amplifier like the darTZeel NHB-
108 model one cannot but offer the high-
est quality level when it comes to moni-
toring and protection.
Purity.
But we did not call this part of the darT-
Zeel NHB-108 model one the "protection
circuit", by far preferring "supervision
system", or "monitoring circuit", as you
prefer. Indeed, this very sophisticated
module is kept totally outside the signal
path, electrically and physically, again so
as not to disturb the delicate musical
message.
Reliability.
The supervision system is based on a
100% analog design, making it inde-
pendent from any problem or bug com-
ing from a microprocessor.
The vital passive components have been
selected for their extremely long life,
greater than 40 years in continuous use.
Your loudspeakers – and yourself – can
rest on both ears, for quite a while.
6.1. Crowbar circuit
You will have read this odd name several
times in the Owner's manual, maybe
without having a clue about what it
really is.
This circuit has been well known for dec-
ades in industrial power electronics. By
power electronics, we mean powers
ranging from 50 to 200 kilowatts, like in
on-line inverters used for mainframe
computers or in hospital surgery "white"
rooms.
On these powerful machines it is not al-
ways easy to suddenly cut off the power
supply without causing electrical dam-
age. Inductive loads can release huge
energy transients which need to be
properly directed.
darTZeel NHB-108 model one Audiophile's technical manual Page 19 of 28
Generally the best solution is to insert a
fuse – rather a big one of its kind – be-
tween the supplies and the loads. In
case of emergency, you only need to
short circuit the power at the load termi-
nals. But this is easier said than done:
the sudden, huge current peak gener-
ated, well supported by the power in-
verter, immediately melts the fuse, shut-
ting the load down. If the fuse is located
at the input of the inverter, the entire
supply system will be stopped.
It is such a circuit that we have installed
in the darTZeel NHB-108 model one. When
an anomaly or a faulty condition is de-
tected, the Crowbar circuit, consisting
mainly of a power thyristor, is activated.
The Crowbar shorts the power supply,
melting – actually evaporating – the
mains fuse of the channel concerned.
The principal advantage of such a Crow-
bar circuit is that it can be kept totally
outside the audio signal path, thus com-
pletely avoiding any influence on the
music.
Its caveat? The price. Thyristors like
those we use in the darTZeel NHB-108
model one can short peak currents of
around 1,400 amperes. They cannot be
considered as being "cheap" parts.
6.1.1. Crowbar activation
In the Owner's manual, we often men-
tion that the Crowbar can be triggered,
in cases where you have not followed the
instructions for use.
To dissipate any remaining doubt from
your mind, we would like to mention
hereunder the faulty conditions which
will trig the Crowbar:
-Using less than 4Ωnominal loud-
speakers, while the speaker's imped-
ance selector is on Hi position.
-Short-circuit at speaker terminals.
-Output DC voltage drift greater than
2 volts at speaker terminals.
-Powering ON the darTZeel NHB-108
model one while speakers are not
hooked to the speaker terminals.
6.1.2. Crowbar cycle
For those who want to know better when
and how the Crowbar acts, here are
some further explanations…
When one of the following elements
and/or signals, or a combination of them
is detected, namely:
-An output DC voltage drift greater
than 2 volts at speaker terminals;
-A permanent output current greater
than 5 amperes, while the power
supply impedance is on Hi and output
voltage swing is no greater than 60
Voltsp-p;
-A permanent output current is
greater than 12A;
-A peak, transient output current
greater than 25A and longer than
25ms;
Then the power thyristor, paralleled with
the total supply voltage, is trigged, i.e.
put into conduction. The huge, abrupt
short-circuit current peaks to around 350
amperes for 6.5 ms, completely releas-
ing the filtering capacitors' storage en-
ergy, through an appropriate, custom
designed choke absorber.
This sudden current rise at the trans-
former's secondary windings also implies
a very high current at its primary wind-
ings, proportional to the inverse ratio of
the primary to secondary voltages. This
current is at least 20 times greater than
the nominal value of the fuses. The
metal in the latter literally evaporates
instantaneously against the glass enve-
lope, as testified by the latter’s black
silver color.
While you are replacing the melted fuse
with a new one, the Crowbar circuit will
have reset itself in the meantime, and
will now be ready to work again, just in
case. It is important to note, however,
that if a newly replaced fuse is blown at
power ON, you must follow the Owner's
manual instructions, that is to say un-
plug your darTZeel NHB-108 model one and
contact our customer service at
Never, ever, replace faulty fuses by val-
ues different than those originally indi-
cated in the Owner's manual.
RISK OF FIRE!
darTZeel NHB-108 model one Audiophile's technical manual Page 20 of 28
6.2. Supervision system
6.2.1. Current sensing
Most available amplifiers, if not all, use
an output current limitation circuit, pro-
tecting the output stages against any
possible overload.
This current limitation generally takes
the form of one or more transistors that
will shunt the incoming signal to ground
when the current has reached a defined
value. The current sensing is taken
through one of the emitter resistors of
the output stage.
Although very efficient, this type of pro-
tection is unfortunately located in the
very heart of the amplifier, and cannot
be kept apart from the signal path.
Other manufacturers, in order to avoid
such an intrusive protection circuitry,
just insert some fuses either in the rail
supplies, or even worse, directly in series
with the output speaker terminals. Of
course this solution is by far the least
expensive, but is sonically a true disas-
ter. Any given fuse behaves as a non-
linear resistor. Its resistance is substan-
tial, and is thus not very compatible with
high quality reproduction. Last but not
least, fuses inserted in this way do not
react quickly enough, leading to dam-
aged components.
Purity.
As you might have guessed, in the
darTZeel NHB-108 model one we have used
a somewhat different approach.
Rather than limiting the output current,
we far prefer to measure in real time the
output power dissipation. It is finally the
output stage’s temperature that defines
its working range. An output device does
not fail because the current is too high,
but simply because the temperature
caused by this current rises too much.
So, our monitoring circuit compares the
instantaneous power dissipation with the
value that the output stage can handle.
There is no thermal inertia here, since
we just measure the right thing at the
right place. A premiere in the audio field,
we can modestly say…
We then measure voltage and current
passing through the output devices.
Without adding any disturbance in the
signal path? Yes, indeed.
As for measuring the voltage across the
output devices, it can be done quite eas-
ily without altering the signal. Good
news. For the current, however, it’s quite
another story.
Keeping in mind not to harm the audio
signal, which is music after all, we use a
special Hall effect electromagnetic sen-
sor. The speaker wire goes through it,
coupled magnetically.
This sophisticated current sensor has the
enormous advantages of presenting an
extremely light load, in the region of
10,000Ω(compared to the 8 ohms of a
speaker), of not interrupting the signal
path, and of being truly linear from 0 Hz
to more than 150kHz, well above what is
needed for music.
Its main drawback? The price, once
again.
Simplicity.
To put it briefly, the monitoring circuit
allows unlimited peak currents for the
time necessary to produce any transient
generated by instruments such as the
piano or drums.
By using such a sophisticated supervi-
sion system, a single output pair bipolar
device can safely manage the demanding
task, ensuring the sonic purity laid down
in the specifications.
6.2.2. For its eyes only
The monitoring circuit not only takes ex-
treme care of both your amplifier and
loudspeakers. It can also anticipate.
The darTZeel NHB-108 model one's eyes,
better described in the Owner's manual,
are also controlled by the monitoring
circuit.
Apart from their – we hope – aesthetic
appeal, they warn you when limit condi-
tions occur. With some experience, you
will be able to use them as a thermome-
ter. Sorry, this is a joke.
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