In support of the above popular argument in favour of the supposed beneficial nature of added second-order distortion is the popular myth is that this distortion does not produce intermodulation effects, which manifests itself as a blending of the instruments and a loss of transparency and dynamic range. This myth is supported by classic texts on the subject. In the 1960's, when I was experimenting with single-ended amplifiers, I noticed that as the bass drum sounded, the cymbal sound was distorted, in accordance with the second-order harmonic distortion function. As a tube conducts more current, its transconductance (or amplification) increases, and as current decreases, the transconductance falls. The bass signal was varying the tube current over a wide range, thus subjecting the cymbal signal to varying amplification, or modulation of the gain. This is a situation where one instrument modulates, or distorts, the sound of another instrument. To my mind, this sounds a lot like `intermodulation distortion' which, I suggest is exactly what it is. Consider the possibility that the first classic text author who wrote on this subject got the math right, but didn't understand the application, and that other authors simply repeated the error, assuming the original article to be correct. I have found examples of this type of propagation of incorrect information throughout my study of audio electronics. This has taught me not to blindly accept what I read (or am told), but to conduct my own experiments, and to do original thinking to find out what seems to be the real truth. My finding here is that second-order distortion causes modulation effects, which causes loss of transparency and loss of dynamic range.
An infinite variety of pushpull circuits appear in commercial and specialist amplifiers, and these vary primarily as to which of the myriad of possible flaws have been chosen by the designers, all of whom have been completely ignorant of the ultimate truth in pushpull design.This is not surprising in light of the fact that there is no publication outlining the ideal amplifier, so designers have no idea of the correct goal of their work - and this is assuming natural sound quality is the goal. The goal is usually profit, which means the amplifier must only sound `good enough' in relation to the competition, and this is subject to the entirely arbitrary results obtained with the flawed speakers available. Most designers are happy if the amplifier is electrically stable, and produces good sine waves (a pure tone of one frequency) into a large power resistor. The problem with this type of testing is that music is vastly more complicated that a single pure tone. Music consists of many energy transients, which can be mathematically broken down into spectra of simultaneously occurring sine waves at thousands of frequencies. What happens to the musical information as it passes through the loudspeaker must be understood in order to optimize the sound quality, but this has not been done correctly.
There is the issue of the output transformer. The output transformer couples the output tube signal (at high voltage and low current) to the speaker (at low voltage and high current). Any uncoupled inductance, combined with distributed stray capacitance, forms a low-pass filter, which selectively reduces the higher music frequencies with respect to the lower music frequencies. The phase shift and cutoff frequency of this filter will vary with the permeability of the transformer core. Silicon-steel is commonly used in output transformers as core material. The permeability of steel is low at low applied magnetic force, and is much greater at higher magnetic force levels. The magnetic force is caused by the primary winding magnetizing current, which is determined by the signal voltage across the primary winding impeded by the primary inductance, which in turn depends upon the permeability of the core which depends upon the magnetic force. This means that, as lower music frequencies vary the magnetizing force from negative through zero to positive, the core permeability and hence the characteristics of the low-pass filter will vary, which means that higher music frequencies will be modulated by larger lower music frequencies. This is known as modulation distortion, and it gives rise to loss of dynamic range and loss of transparency.
This problem is alleviated in a single-ended amplifier by magnetically biasing the core by passing the tube quiescent (zero signal) current through the transformer primary winding, and operating at all signal levels in only one magnetic polarity, thus avoiding the region of low permeability around zero magnetic force. A higher quality core material of more consistent permeability will further reduce modulation effects.
In a pushpull output transformer, the quiescent currents of the two output tubes are carried by windings of reverse magnetic polarity, thus there is no quiescent magnetization of the core. The disadvantage appears to be that modulation distortion will occur as signal currents pass the core magnetization through zero, thus causing changes in permeability and hence changes in uncoupled inductance. There is however, another side to this argument: The balanced and magnetically cancelling quiescent currents of pushpull mean that the signal extremes are centred about zero magnetization and therefore the maximum magnetization (of either positive or negative polarity) is half what it would be with single-ended of the same power output. This allows a pushpull transformer to function linearly with a much smaller core than with single-ended. A smaller core means more compact windings which have lower losses, which is an advantage. Pushpull transformers may also be constructed with core materials having near-constant permeability even at very low levels of magnetizing force.
The development of a real-world amplifier whose characteristics closely match those of the ideal amplifier, including the ability to produce essentially undistorted music has required dedication. Serendipity has played an important role in the development of significantly improved amplifier designs.
To use the flying machine business analogy, the Wright brothers airplane of audio has not been developed until recently. The vast majority of existing amplifiers are akin to the flying machines which could not fly. They cannot reproduce the dynamic range which is present in the recorded music, due to a gross lack of understanding of the basic principles of physics, as applied to the art of audio reproduction.
A recent prototype amplifier is reproducing the acoustice dynamic range more accurately than has heretofore been accomplished. This means that, based on subjective observation, the amplifier is enabling there to be a much smaller loss of dynamic range between the reproduced sound and the recorded sound. This is observed as musical instruments sounding more like the real instruments would sound. For example, one often will blink one's eyes upon hearing a snare drum rim-shot at close range, whereas this does not happen when listening to audio systems. This recent prototype does cause one to blink, even when the intervening music is playing at a relatively low level, as is the case when listening to live instruments in close proximity.
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