darTZeel Audio NHB-108 User Manual
Page 19
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 4Ω nominal 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
Volts
p-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!