darTZeel NHB-108 model one
User Manual, version B 1.0
Page
29
of 35
T6. Onboard safety
A power amplifier like the
darTZeel NHB-108
model one
cannot but offer the highest quality
level when it comes to monitoring and protection.
Purity.
But we did not call this part of the
NHB-108 model
one
the "protection circuit", by far preferring "su-
pervision 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 independent from any problem or
bug coming 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 your-
self – can rest on both ears, for quite a while.
T6.1. Crowbar circuit
You will have read this odd name several times in
the first section, maybe without having a clue about
what it really is.
This circuit has been well known for decades in
industrial power electronics. By power electronics,
we mean powers ranging from 50 to 200
kilowatts
,
like in on-line inverters used for mainframe com-
puters or in hospital surgery "white" rooms.
On these powerful machines it is not always easy to
suddenly cut off the power supply without causing
electrical damage. Inductive loads can release huge
energy transients which need to be properly di-
rected.
Generally the best solution is to insert a fuse –
rather a big one of its kind – between the supplies
and the loads. In case of emergency, you only need
to
short circuit
the power at the load terminals. But
this is easier said than done: the sudden, huge cur-
rent peak generated, well supported by the power
inverter, immediately melts the fuse, shutting 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
NHB-108
. When an anomaly or a faulty condition
is detected, 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 Crowbar circuit
is that it can be kept
totally
outside the audio signal
path, thus completely avoiding any influence on the
music.
Its caveat? The price. Thyristors like those we use
in the
NHB-108 model one
can short peak cur-
rents of around 1,400 amperes. They cannot be
considered as being "cheap" parts.
T6.1.1. Crowbar activation
In the first section of this manual we often mention
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 con-
ditions which will trig the Crowbar:
-
Using less than 3
Ω
nominal loudspeakers with
dramatic impedance dips of less than 2 ohms,
while the speaker's impedance 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
.
T6.1.2. Crowbar cycle
For those who want to know better when and how
the Crowbar acts, here are some further explana-
tions…
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 7 am-
peres, 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
14A;
-
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 energy, through
an appropriate, custom designed choke absorber.
This sudden current rise at the transformer's secon-
dary windings also implies a very high current at its
primary windings, proportional to the inverse ratio
of the primary to secondary voltages. This current
is at least 20 times greater than the nominal value of
