darTZeel Audio NHB-108 User Manual
Page 12
darTZeel NHB-108 model one
Audiophile's technical manual
Page 12
of 28
If the THD and IMD produced by the
amplifier are kept low enough, say lower
than the audibility threshold, then no
further correction will be needed, and
the audio circuit can remain simple.
At the start of this new century, 3 main
technologies coexist in the construction
of transistors, these being in chronologi-
cal order:
- Bipolar, in the early 1950s.
- Field effect, in 1962.
- IGBT, a mix of the previous two, in
the ‘eighties.
At darTZeel, we strongly believe in new
technologies. On the other hand, we
readily admit that nothing can replace
experience.
So, "our" winner is…
Bipolar technology!
5.2.1. IGBTs
For your own information, we are ready
to tell you an old secret. What does the
word "transistor" mean?
It comes quite simply from the contrac-
tion of transfer and resistor.
IGBT stands for "
I
nsulate
G
ate
B
ipolar
T
ransistor". It behaves like a bipolar
transistor at its output, while being
driven like a FET at its input.
IGBTs are mostly used in power applica-
tions, like inverters, switching power
supplies, heart defibrillators, and… for
some audio power amplifiers.
We do not have any preconceived notion
about the use, and more specifically, the
sound of IGBTs. Results can vary with
the samples used.
Their linearity is close to FETs, which is,
alas, not enough for us.
IGBT technology is by far the youngest,
and we lack time to fairly judge them as
to their long-term sound quality, reliabil-
ity and availability.
So for now, and maybe for still a little
while, we will probably not use them di-
rectly in audio applications.
5.2.2. FETs
FET stands for "
F
ield
E
ffect
T
ransistor",
of which there are two main categories.
For small signals, they are called as is,
FETs.
For power applications, their most com-
mon name is MOSFET, which stands for
"
M
etal
O
xide
S
ilicon
F
ield
E
ffect
T
ran-
sistor". They behave similarly to FETs,
but their internal structure may vary.
FETs are being used more and more, for
several reasons, including the most ob-
vious one:
It is commonly accepted that they per-
form very closely to vacuum tubes (or
electron tubes or valves). Tube lovers
generally like MOSFET amplifiers due to
their similar behavior.
Here, we are forced to say that we do
not share this point of view. Not entirely,
at any rate…
It is true that the sound of MOSFETs
tends to be soft and warm, a bit like
those found in tubes. But their electrical
behavior is not similar, simply because
they are not made of similar materials.
The only true similarity is that MOSFETs
– and FETs of course – are voltage
driven, like tubes. Apart from that, they
are in two different worlds, vacuum for
tube, silicon for FETs.
Conduction in any type of transistor al-
ways acts in a solid, here the metallic
silicon, hence their name of "solid-state"
devices.
In a vacuum tube, electrons move in… a
vacuum.
Coming back to FETs, what is their
sound? For most designs, we would say
that it is only a matter of taste. We are
convinced that one can design a "stan-
dard", good sounding machine in the 3
technologies mentioned above, and even
with the fourth - tubes - with virtually
the same sound.
Obviously, if you like music, you defi-
nitely need a darTZeel NHB-108 model one.
Yet, and contrary to common belief,
MOSFETs are less linear than bipolar
transistors available today. MOSFETs
often need more sophisticated added