darTZeel NHB-108 model one
User Manual, version B 1.0
Page
31
of 35
The better the quality of the source energy, the
easier the task of modulating it into sound waves.
This modulation is truly the audio signal you listen
to. It is this same signal that will deliver the energy
supply to your loudspeakers, which in turn will
excite the air molecules to vibration, producing that
magical feeling we audiophiles call "music".
In your
darTZeel NHB-108 model one
, the
power supplies are not really standard ones. The
amplifier is a
true
dual mono. We apologize about
stressing the word "true", but much too often this
description is abused.
The
NHB-108 model one
has 2 fully independent
power supplies, one for each channel. The two
channels are fully insulated, respective ground pa-
ths included, from each other. The crosstalk figure
speaks for itself, at more than 90dB separation
across the entire audio spectrum.
Toroidal transformers, each of 300VA, are wound
on 450VA cores. Magnetic fields are thus reduced
to the point that no core saturation can occur, ensur-
ing clear power output under any dynamic condi-
tions, without induced hysteresis distortion.
Cores are grain oriented, and primaries are electro-
statically shielded from the secondaries, keeping
RFI away. The entire units are impregnated in ep-
oxy resin, eliminating possible winding vibration.
As seen above, the transformers are also suspended.
Their residual mechanical noise is so low that even
in
very
quiet listening rooms, you will not be dis-
turbed anymore.
Immediately after the rectifier bridges, the DC
sources are filtered by 6 paralleled, 22mF capaci-
tors, which corresponds to 132,000 µF per channel,
totalling a whopping storage energy of 230 joules
per channel. Not so bad for a 100 watter…
The copper bus bars, CNC water-jet cut in 5mm-
thick blocks, connect the filtering capacitors' leads
together, creating as it were a low impedance power
supply “on the spot”. The output transistors are
located only a few centimetres away from the
power supply: hence no problem in case of high
current demand.
Our power supplies are filtered only, avoiding any
dynamic limitation for which regulated supplies are
often responsible. Fully regulated supplies have
very low output impedance through high feedback
regulation (NFB everywhere!). When huge dy-
namic changes arise, the NFB is in a state of over-
flow and the output impedance suddenly increases
dramatically, causing dynamic compression. Does
this remind you of something?
T7.2. Voltage or current?
For purity reasons invoked earlier, the output stages
of
darTZeel NHB-108 model one
only have a
single bipolar pair of output transistors.
The vast majority of amplifiers of over 50wpc use
paralleled transistors, from 3 to 48(!) – or even
more – pairs. The purpose of this parallelism is to
obtain a greater output current, as required by low
impedance loudspeakers.
This method is much cheaper than the solution used
in the
NHB-108
, power transistors being much less
expensive than in the past.
But parallelism has numerous drawbacks, as fol-
lows:
-
The need to match components for even heat
spreading.
-
The signal path is divided into multiple parallel
paths, leading to TD (Temporal Distortion) by
degradation of propagation time delay uni-
formity, each path not being of the very same
length.
-
Much longer mean path length, considerably
increasing the output impedance, and hence
the need for a greater NFB at output stage.
This also induces TD.
-
Much greater physical volume and area, limit-
ing the high frequency response because of
higher RFI sensitivity.
And this list is unfortunately not exhaustive.
Without special precautions, the use of speakers
with nominal impedance of 4
Ω
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 2
Ω
, or even
1.5
Ω
loudspeakers, if you can find some, without
significant loss of power, yet with the same sonic
quality attributes.
Power transformers have 4 identical secondary
windings. Depending on whether they are con-
nected in series or parallel, the result is a big volt-
age or a big current.
For loads between 3
Ω
and 8
Ω
, the current remains
fairly low, around 7A RMS, corresponding to about
200 watts under 4
Ω
, but the voltage needed is rela-
tively high, in the region of ±50 to ±60 volts for
delivering a comfortable 130 watts under 8
Ω
.
Conversely, 200 watts under 1.5
Ω
need an RMS
current of 11.5 amperes, but with only 17 volts
RMS as an output voltage, allowing power supply
rails of about ±25 to ±30 volts.
