User Manual

darTZeel NHB-108 model one Audiophile's technical manual Page 22 of 28
pedance, and hence the need for a
greater NFB at output stage. This
also induces TD.
- Much greater physical volume and
area, limiting the high frequency re-
sponse because of higher RFI sensi-
tivity.
And this list is unfortunately not exhaus-
tive.
Without special precautions, the use of
speakers with nominal impedance of 4Ω
or less on a single output paired stage
can cause excessive dissipated heat,
outside the safe working range of the
output devices.
We gave great thought to this and finally
came up with a solution, allowing the
use of 2Ω, or even 1Ω loudspeakers, if
you can find some, without loss of power
or quality.
Power transformers have 4 identical sec-
ondary windings. Depending on whether
they are connected in series or parallel,
the result is a big voltage or a big cur-
rent.
For loads between 4Ω and 8Ω, the cur-
rent remains fairly low, around 7A RMS,
corresponding to about 200 watts under
4Ω, but the voltage needed is relatively
high, in the region of ±50 to ±60 volts
for delivering a comfortable 150 watts
under 8Ω.
Conversely, 200 watts under 1Ω need an
RMS current of 14 amperes, but with
only 14 volts RMS as an output voltage,
allowing power supply rails of about ±25
to ±30 volts.
By switching the secondary windings ac-
cordingly, the power transfer is opti-
mized and the power transistors in the
output stage remain within their safe
working range.
So we can only use one single output
transistor pair per channel. The signal
path is shorter, uniform, and only passes
though one silicon junction at a time.
Music is thus reproduced with unprece-
dented ease, purity and fidelity, simply
impossible with more complex designs.
7.3. Received idea
This chapter devoted to power supplies
would not be complete without the fol-
lowing little addition, small in size but of
great significance…
It is usually admitted in the audio world,
particularly in the high-end manufactur-
ers' community, that output current
availability is of paramount importance,
to the point where one praises machines
capable of outputting dozens, or even
hundreds, of amperes.
What nonsense!
Until the cont rary is proven, as far as we
know voltage and current are in a certain
relationship, as clearly stated by Mr.
Ohm's law:
IRV
•
=
R
V
I =
I
V
R =
It is of course the same equation, just
written in its 3 most usual forms.
Now, let us take an ideal amplifier, ca-
pable of delivering exactly 250 watts/8Ω,
500 watts/4Ω, 1000 watts/2Ω, and a
whopping 2000 watts/1Ω.
The currents needed to deliver these
abundant and generous powers are, re-
spectively, of 5.6A, 11.2A, 22.4A and
44.8A.
Yes, we have to admit that the last fig-
ure is quite high… So, this amplifier can
deliver roughly 45 amps under 1Ω.
Now say that you own speakers having
an impedance of 6Ω, much easier to
drive than 1Ω. How many amperes will
this imposing amplifier deliver to your
loudspeakers? 45 amperes with a big
smile? Not at all!
Given Mr. Ohm's law, the amplifier will
deliver at its best 333 watts, say 7.45A
under 44.7volts.
So you can see that in the real world,
you just need 7.5 amperes, not 45. Let
us confess that for 333(!) watts, this is
quite reasonable…
Do you now better understand why "Lots
of Amperes" does not necessarily mean
"Superb Bass"?