The Audio Note Transformer Design Philosophy
Article By Andy Grove And Peter Qvortrup
The electronic valve is a high voltage, low current device that is incapable
of driving a low impedance loudspeaker directly. Although
output-transformer-less (OTL) designs have appeared from time to time, with
these types many devices are connected in parallel and a large amount of
negative feedback is used to achieve a workable, but not necessary satisfactory
result. The efficiency of the output transformer less circuit is also always
very, very low due to the severe impedance mismatch, so a large amount of power
must be dissipated within the output valves to achieve a tiny output into the
loudspeaker. The only way to correctly match a valve output stage to a low
impedance loudspeaker is via a step-down output transformer.
The transformer is sometimes seen as a barrier to amplifier performance, and
whilst on a theoretical level this is to an extent is true. A transformer does
have a finite bandwidth, but as will be shown and discussed later in this
article, when properly researched, designed and made the limits achievable in
practice are more than wide enough for what is required by the harmonic envelope
of a musical signal. Most of the problems normally referred to in this regard
relate to problems in amplifiers utilizing negative feedback. The limited
bandwidth (which may still exceed that of the human ear) and associated phase
shifts can make the amplifier unstable, this situation is made considerably
worse if there is a strong high frequency resonance present in the transformer
At Audio Note we have spent many hundreds of hours involved in a
combination of theoretical research and experimental work to develop and combine
proprietary interleaving methods and winding techniques to extend the bandwidth
of our transformers to the point at which they could be considered not only
excellent components from the technical standpoint, but virtually invisible from
a sonic perspective. In some of our designs as many as five wires are wound onto
the bobbin at the same time, using these methods a bandwidth of 5Hz to 200kHz is
achievable with a transformer for single ended operation of a 300B triode. This
extended bandwidth presents the valve with a constant impedance load across the
audio range thereby minimizing distortion, and allows all of the harmonic
overtones and transient events of the music to be accurately reproduced within
the harmonic envelope.
Perhaps not surprisingly, we have found that the materials used within the
transformer greatly affect the both the sound quality and measured performance.
This is an area largely overlooked both now and in the past cost and ease of use
was and still is the primary considerations.
Theoretically speaking, the Interleave Insulation and Primary to Secondary
Insulation acts as the dielectric in a distributed capacitor, therefore it can
be seen that the properties of the dielectric material will affect the
electrical and sonic performance of the transformer. Electrical quantities to be
considered are dielectric constant, which affects the magnitude of the resultant
distributed capacitor and dielectric absorption, which causes distortion by
hysteresis. A vacuum is off course the ideal choice as it has a low dielectric
constant and no dielectric absorption, but a vacuum is as impractical as it is unrealizable
in anything but a laboratory. We have therefore experimented with every man-made
plastic insulating material available, but in the end we found that the best
sounding material is a special type of paper. Paper is a natural material, and
although subject to variations as are all such natural materials, it is more
conducive to creating a natural sound. As with all Audio Note™ products the
ear was the final arbiter as to which material was to be used.
The wire used to wind the transformer is also critical and in this area as in
many others Audio Note was the company that pioneered the finest, Silver and
it was therefore natural to put silver to good use in our best output
transformers. Why silver sounds so superior is still not fully understood, but
it is unlikely to be simply a function of conductivity.
One theory puts forward the notion that the intense AC electrical and
magnetic fields within the transformer interact in some way with the wire
material. Another theory considers the crystalline structure of each material
copper is very sensitive to impurities, in particular oxygen, it is also
possible that the differences are caused by effects that occur on the surface of
the material. Surface chemistry is different to that of the bulk material, the
atoms at the surface are exposed, rather than being enclosed within the crystal
lattice. When the metal is drawn into wire the surface will quickly adsorb
components of the air, particularly oxygen and nitrogen as they are most
prevalent and despite our best efforts (we coat our immediately it leaves the
die), some contamination still takes place. After a while a bulk reaction takes
place producing a layer of oxide and sulphide. Silver and copper compounds are
similar chemically but not identical. Copper oxide is a rather poor
semiconductor compound capable of producing rectification effects whereas silver
oxide is a good conductor and is used in switch contacts and batteries. It may
be possible to draw wire in an inert atmosphere such as argon and then cover the
wire before it reaches the air or to chemically treat the surface before coating
to further improve the wires.
The core of the transformer is vital for it's operation. In our standard
transformers we use good quality silicon steels but in our finest specialist
transformers we make no compromises and use the very best and very expensive
nickel irons such as Radiometal. 3% silicon steel is widely used around the
world and is produced in vast quantities. China, America, Japan, Russia and the
UK are amongst the countries where this material is manufactured. For our
economy transformers we use a material known as M6, in laminations of 0.35mm
thickness. The material is first cold rolled, to align the grain structure, into
a tape then it is punched into laminations. The problem with this is that the
flux runs anti-parallel to the preferred direction at the back of the "E". This
means that at that point the materials full potential is not realized at that
point increasing losses and decreasing effective permeability. M6 steel has
reasonably low hysteresis, good permeability (approximately 10,000) and high
saturation flux density (approximately 2T or 20,000 Gauss). The problem of poor
grain orientation is alleviated if we move from I-E laminations to a C-Core.
Here the metal tape, after being cold rolled, is wound into a loop and then cut,
now the magnetic flux always travels in the preferred direction in the steel,
this alone gives a significant increase in performance. When we move up to a
C-Core we change the material's specifications to M0 or HiB silicon steel a
material that has slightly lower losses and higher permeability than M6, the
permeability of HiB can be 40,000 or more. HiB is processed in a different way
to M6 giving it a different grain structure this special material is
manufactured in Japan and America only. Our finest transformers use two versions
of Radiometal core in the form of a C-Core. Radiometal is a 36% Nickel iron and
Superradiometal a 48% Nickel iron alloy of excellent magnetic properties the
permeability is similar to that of HiB but it's saturation flux density is lower
at 1.6T or 16000 Gauss. Radiometal has a much lower hysteresis loss than silicon
steel and is far more sensitive to small signals. If one is to firstly listen to
a transformer with the best silicon steel core and then change to one with the
Radiometal core, one experiences more colours and texture in the performance and
more low level details are present. The high frequencies are so much clearer. It
is like the difference between an artificial light and sunlight.
A Little History
Traditional transformer designers still use winding calculations and
technology that were established in the 1940's, just after WWII, these
calculations are designed to yield the best results from a standard M6 type
C-core and companies like Partridge, Savage, Parmeco and Gardners made excellent
examples of these types of conventional Push Pull output transformers in the
1950's and 1960's, however, when the new highly permeable magnetic materials
such as the Radiometals emerged in the early 1950's no-one realized that they
require a quite different approach to winding technique to get the best magnetic
coupling between the windings and the core possible and thereby utilizing the
capabilities of these fine magnetic materials fully.
Over 40 years later Audio Note™ is so far the only company in the world to
conduct such work. Work, which is further enhanced by the advantage of having
both in-house transformer and circuit design capabilities side by side,
something which allows Audio Note™ to design our transformers specifically for
a specific circuit thus maximizing the harmonic envelope and dynamic transfer
and utilizing the best combination of both, because we can always check the
sonic properties of any given combination during the prototype stages.
No other audio manufacturer have this in-house facility, and they therefore
have to source standard designs from transformer manufacturers who do not have
the ability or necessary understanding of electronic circuitry to test and
design the best possible transformer for each specific application, but will
always supply a compromise.
In contrast Audio Note designs its best transformers practically without
cost restraints a fact which has resulted in a transformer quality undreamed of
even 20 years ago, the completely "invisible" transformer is a goal so far
unattainable, the Audio Note™ silver wired Super Perma 55% nickel C-core
transformer is the closest alternative!
The Single-Ended Transformer
One final point of interest with a S.E. transformer is the air gap. This is
necessary in order to bring the operating point of the core to the correct
region on its B-H curve. It does not seem that anyone has ever experimented with
anything other material than paper or plastic for use as a spacer between the
core limbs. At Audio Note we have discovered that the use of a metallic spacer
reduces the distortion produced by the transformer and the improvement in the
sound of the transformer is considerable provided the correct material is used
and it is applied in the correct way.
Overall a transformer could be described in a similar way to a culinary dish.
To get the best flavor one must use the best ingredients and cook them in the
correct way and as new ingredients emerge and are developed, be sure that we at
Audio Note™ will be the first cooks to write the new recipes...
Audio Note Group C
Double C-Core Output Transformers
Shortform Introductory Article
Why Does Audio Note Choose Double C-Cores
Rather Than I-E Core?
This is an area of very little understanding in the modern audio industry and
as a result much controversy, so Andy Grove and I would like to give at least
some background as to why Audio Note's best transformers use C-cores and always
will do, despite cost and availability problems.
Basic (very) domain theory tells us that magnetic steels function by two main
processes, domain growth and domain rotation.
Under low magnetization, the field domains, which are oriented in the
direction of the applied field, grow at the expense of their non-oriented and
anti-parallel neighbors, this low field domain growth is generally reversible if
the field is removed.
Under a medium applied field again domain growth is the predominant factor.
However there will be some non-reversible growth of the domains, and a reverse
field is required to return them to their original state.
Under a large magnetization field those domains, which were not oriented in
the direction of the applied field start to rotate towards that direction.
Eventually all of the domains are pointing in the direction of the applied field
and saturation is reached.
This neatly explains the familiar shape of the B-H curve and hysteresis loop.
Essentially iron crystallizes in a body cubic form and the domains are
oriented parallel to the edges of the crystal, therefore an iron crystal will be
easier to magnetize if the applied field is parallel to an edge, and will be
most difficult to magnetize in a diagonal direction across the cube.
In a non-oriented material the crystals and the domains are oriented
randomly, therefore it will magnetize much the same in any direction. However no
direction is aligned with the preferred direction of all of the crystals, and a
lot of the crystals will be oriented in the worst direction.
Therefore permeability is low and losses are high. The hysteresis loop will
be wide and rounded. In a singly oriented steel (M4 etc, there are cubic
oriented types which we use as well) the crystals are oriented so that two of
the faces are perpendicular to the strip rolling direction, two of the edges are
parallel to it and the other edges are at 45 degrees to the strip surface. In
other words the plane of the strip cuts the diagonal of the faces, which are
perpendicular to it. This means that the material is very easy to magnetize by a
field parallel to the strip rolling direction as the domains are facing in that
This makes for a material with a high permeability, low losses and a narrow
rectangular hysteresis loop when the field is in the strip direction. But it
also means that a field in any other direction in the plane of the strip will be
trying to magnetize the crystal in it's worst possible mode. The highest losses
always occur at approximately 45 degrees to the rolling direction, in the plane
of the strip.
This diagram shows the magnetization behavior of GOSS and
non-GOSS strip relative to rolling direction
The I-E Core
Now, laminations are punched out of a steel strip such that the "arms" of the
E point in the strip direction, however the back of the E is perpendicular to
the rolling direction. The I is punched so that it's longest side is parallel to
the rolling direction. This of course means that with an I-E laminated
transformer the flux has to curve round, across the grain at the corners, both
at the junction of E and I and at the back of the E, and travel perpendicular to
it across the back of the E.
In addition I-E laminations generally have whopping great holes punched just
where you don't want them. This means that a stack of grain oriented laminations
ends up with better properties than a non-oriented stack but not by much. Quite
serious curvature of the B-H curve starts to appear at around 1.2T to 1.3 T
(Tesla) even though true saturation doesn't occur until about 1.6T to 1.8T.
Enter the C-Core, as the C-Core is wound out of the strip the flux always
traverses the preferred direction. This means that a C-Core remains linear
almost to saturation and then hits a brick wall around 1.8T maybe a bit more.
The losses are much lower as well, and that translates into lower distortion as
the hysteresis loop is narrow and straight sided. Let's say that 1.3T peak is as
far as one would like to go on lams, and 1.7T peak for a C-Core that is a ratio
of 1:1.3. So for a given number of turns on a core of equal dimension the C-Core
could sustain a 30% higher voltage across that winding.
That is an increase in power of 70% for a given level of core distortion.
Or translated into the realm of mains power transformers it explains why
strip wound cores, especially toroidal transformers are so small for their power
rating. Of course increasing the cross sectional area of a stack of lams can equalize
the power rating but that brings about an increase in winding length and hence
an increase in leakage inductance and capacitance.
In the Audio Note™ high quality output transformers we use 50% or 55%
nickel iron alloys, both oriented and non-oriented through a carefully
developed, customized and proprietary heat treatment processes depending upon
the application. These materials offer greatly reduced distortion at low signal
levels, due to the very narrow hysteresis loop of these materials. The downside
is their expense and lower saturation flux density, which in single-ended low
power amplifier applications is not an issue. A small word about winding
Traditionally, transformer design has focused on achieving the widest
possible frequency response and this has been the main tenet of design priority
for the past 80 or so years, in our research into the behavior and interaction
between coil and core we have discovered that when dealing with the highly
permeable nickel cores, purely looking at frequency as the main arbiter of
transformer quality is woefully inadequate and as a result we have spent years
developing and refining the best way of winding the coil with special focus on
improving the low level linearity and bandwidth, the end results speak for
From a purely practical standpoint, thin materials in the 0.1mm range are
impossible to handle as large laminations, especially in the very mechanically
soft nickel irons, and the C-Core format allows their use. Very thin laminations
or strips are more important to very high permeability materials because eddy
currents in thicker material, greatly reduces the effective permeability.
But remember that flux density is inversely proportional to frequency so at
1kHz the flux density in an output transformer will only be 2% of that at 20Hz,
and 0.1% of it at 20kHz. Assuming 1.3T peak at 20Hz (for lams of M6) that gives
26mT peak at 1kHz and 1.3mT peak at 20kHz.
A high frequency power transformer such as used in a switch mode supply
transformer would run the core at maybe 0.5T or more peak at 20kHz and then
losses would become very significant. This is where ferrites with their very
high intrinsic resistance become important, but useless for wideband audio
Cobalt irons offer high saturation flux densities but they have a very wide
hysteresis loop, not far from a semi-hard material and they don't lend
themselves to audio output transformer work where low level resolution is
This fact does not relegate cobalt based materials from other audio
applications, for example permendur (49% cobalt) has uses in pole pieces for
magnets in phono cartridges, loudspeakers and for electromagnets as the high
saturation flux density allows for a greater density in the gap.
the material is generally driven into saturation by a DC polarizing field.
Audio Note (UK) Ltd.
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