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 itself.
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.
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.
A Little History
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
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
Why Does Audio Note Choose Double C-Cores
Rather Than I-E Core?
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 direction.
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.
The I-E Core
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.
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 technology.
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 themselves.
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 applications.
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 paramount.
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.
Here the material is generally driven into saturation by a DC polarizing field.