darTZeel Audio NHB-108 - Manual (Page 20)

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.