darTZeel NHB-108 model one
User Manual, version B 1.0
Page
30
of 35
the fuses. The metal in the latter literally evaporates
instantaneously against the glass envelope, as testi-
fied by the latter’s black silver colour.
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 first section of this manual, that is
to say
unplug your
darTZeel NHB-108 model
one
and contact us at
Never, ever, replace faulty fuses by values different
than those originally indicated in the chapter 6.1.
RISK OF FIRE!
T6.2. Supervision system
T6.2.1. Current sensing
Most available amplifiers, if not
all
, use an output
current limitation circuit, protecting 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 protection is
unfortunately located in the very heart of the ampli-
fier, 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 disaster. Any given fuse behaves as
a non-linear resistor. Its resistance is substantial,
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
damaged components.
Purity.
As you might have guessed, in the
NHB-108
model one
we have used a somewhat different
approach.
Rather than limiting the output current, we far pre-
fer to measure in real time the
output power dissi-
pation
. 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 cur-
rent rises too much.
So, our monitoring circuit compares the instantane-
ous 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 mod-
estly 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 de-
vices, it can be done quite easily 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 sensor. 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 10,000
Ω
(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 unlim-
ited peak currents for the time necessary to produce
any transient generated by instruments such as the
piano or drums.
By using such a sophisticated supervision system, a
single output pair
bipolar device can safely manage
the demanding task, ensuring the sonic purity laid
down in the specifications.
T6.2.2. For
its
eyes only
The monitoring circuit not only takes extreme care
of both your amplifier and loudspeakers. It can also
anticipate.
The
darTZeel NHB-108 model one
's Eyes, better
described in the first section are also controlled by
the monitoring circuit.
Apart from their – we hope – aesthetic appeal, they
warn you when limit conditions occur. With some
experience, you will be able to use them as a ther-
mometer. Sorry, this
is
a joke.
T7. Power supplies
T7.1. From mains to loudspeakers
More and more, audio manufacturers are insisting
on the quality of the power supplies. They are quite
right!
After all, the electric energy fed to your loudspeak-
ers comes from the power supply and nowhere else.
The audio circuit itself is really just a sort of regula-
tor for this energy.
