SCOBIE CUSTOM AUDIO

Types of Amplifiers

The two extremes in amplifier design are known as single-ended and pushpull. Single-ended amplifiers have a single output tube which amplifies the signal in one phase, while pushpull amplifiers have two output tubes, one of which amplifies the signal in positive phase, while the other amplifies the signal in negative phase. Single-ended amplifiers produce harmonic distortion of all orders (for example, second harmonic distortion is present when the amplifier adds frequencies that are double the program material frequencies), while pushpull amplifiers, at least in theory, cancel out the even order distortions present in identical output tubes working in opposite phase. Opposite phase means that as one tube draws more current, the other tube draws less current and vice versa, in equal amounts as the signal varies. The Gods, in their infinite wisdom in creating nature, have chosen one of these types of amplifier configurations to be a distillation from the theoretically ideal amplifier. Both cannot aspire to absolute perfection. Plato described beauty as `the one', which is enduring.

The Single-Ended Amplifier

There is a popular myth regarding single-ended amplifiers. It is that an amplifier will sound `more musical' if it produces even-order harmonic distortion along with odd-order harmonics, as opposed to the pushpull case of reduced even-order harmonics. The theory goes that pushpull , in eliminating the even harmonics, creates a sort of harmonic imbalance that is less pleasing to the ear. It is also argued that, as octaves in music are based on a doubling of the frequency of the fundamental tone, adding even-order harmonics to the music will be just like playing the music simultaneously on other octaves, which is somehow going to improve the music over the original form, which in good music is based on hundreds of years of development of musical instruments and musicianship. I wish to contribute to this discussion the revolutionary concept that the music will sound most musical if it is faithfully reproduced without any distortions at all. According to this view, both single-ended and pushpull amplifiers can be made to sound musical if the amplifier is designed according to careful considerations, which are generally unknown.

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. There is no substitute for the truth that no distortion is best.

The Pushpull Amplifier

In the pushpull amplifier, the signal is simultaneously amplified by two tubes, working in opposite phase to provide a single output signal to the speaker. As the current rises by a given increment in one tube, it falls by the same increment in the other tube. Thus the total current drawn by the two tubes is constant. I recently explained this to a friend who had little understanding of electronics, and she remarked: "that is a beautiful concept" - which I agree with. I have discussed this point with high-end audio designers who pride themselves on building natural sounding amplifiers, but the sense of beauty seemed to be lost on them. Not all people have a gift for recognizing beauty. To those who do, there is the possibility of seeing truth, as truth is the ultimate beauty. Technical understanding of an innovative concept always follows in the wake of discovery, which is often fuelled by a search for beauty.

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.

The Phase-Inverter Circuit

One of the complications of pushpull circuits over single-ended is that, as two phases of signal are needed to drive the output stage, a phase inverter circuit is needed to create the out-of-phase signal.This added complexity, as stated above, gives pushpull designers a much greater opportunity to introduce flaws into the design. This is popularly considered the "Achilles Heel" of pushpull amplifiers. Over the years, I have tested many phase inverter circuits, extensively studied the classic texts on this, and have become familiar with literally dozens of phase inversion circuits. None of these circuits proved to be even remotely acceptable to my standard of absolute perfection: that the inverted signal created by the phase inverter must be absolutely free of phase errors and harmonic distortions, and that this be true over a very wide range of frequencies. I developed several unique phase inverter circuits of high performance, and one of these proved to provide extreme high performance, as evidenced by superior transparency in the music.

Designs Falling Between Single-Ended and Pushpull

A number of designers, in a search for a better single-ended amplifier, have tried such things as shunt regulation of total output stage current, and shunt loading of the output stage, with some sort of current regulation device or impedance coupling the output stage to the power supply. What these techniques all have in common is this: there are two active devices (tubes or transistors) in the output stage (instead of one device in pure single-ended), and, as one device draws more current, the other device draws less current. These designs are effectively functioning in pushpull mode, but without the considerable design advantages that come with purely symmetrical pushpull circuits. The advantage here is that no phase inverter circuit is used.These designs represent a middle ground between single-ended and pushpull.

Single-Ended Versus Pushpull in Classic Theory

The main theoretical advantage of pushpull over single-ended is the fact that even-order harmonic distortion created by any tube non-linearity is cancelled out in a balanced circuit, whereas with single-ended, all orders of harmonic distortion are fed to the loudspeaker. This would appear to give pushpull a clear advantage, but as pushpull amplifiers are almost never designed according to theoretically sound principles (these are not in any book), the baby is consistently thrown out with the bath water, creating an opportunity for single-ended to sound most natural.

Single-Ended Versus Pushpull in the Real World

I feel that the real reason that single-ended amplifiers are preferred by people who care about the quality of the sound is the following: No designer, until now, has understood the ideal amplifier, and much less how to build a real-world version of it. Thus every amplifier is full of flaws, so the simpler the circuit, the fewer flaws, on average, will be made. As single-ended amplifiers can be made quite simple, they have this law of `fewer likely flaws' working in their favour. I have designed, built, and listened to many pushpull amplifiers, which has given me a healthy appreciation of the vast increase in possible ways to go wrong that come with a more complex amplifier. It was partly this appreciation of amplifier design flaws that fuelled my quest for the theoretically ideal amplifier, from which all optimized real amplifiers would be distillations in their various levels of sophistication.

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 through the origin. I think it is a false argument that the biased core is an advantage of single-ended that cannot be offset by the lower inherent uncoupled inductance of pushpull transformer designs. Pushpull can also be constructed using two independent biased core transformers, each one coupling one of the two output tubes to the speaker. This type of circuit could be used to demonstrate that a magnetically biased core is an overall advantage, but this has not been done.

Advanced Design Pushpull

The theoretically perfect amplifier, in its purest form is a pushpull amplifier with a number of ideal conceptual characteristics. This is a theoretical concept that has never been published, and is known only to its discoverer, Duncan Scobie. This ideal was conceived only in the wake of many years of dedicated research and development, which was continuously fuelled by a quest for beauty and truth, both in circuitry and in reproduced music.

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 proven to be an extremely difficult problem. The solution to this depended on a lifetime of dedicated and comprehensive study of amplifiers, and the seemingly serendipitous realization of a number of subtle twists.This goal has been recently achieved with a very satisfying level of success by Duncan Scobie, proprietor of Scobie Custom Audio.

This is the first time in the 90-year history of the electronic audio art that an amplifier with the capability of recreating reality has been constructed. To use the flying machine business analogy, the Wright brothers machine of audio first flew successfully in April 1999.

The amplifiers offered for sale here are essentially FREE OF ALL DISTORTIONS, both harmonic and modulation, as they appear in the SOUND EMANATING FROM THE LOUDSPEAKER. This fact has been demonstrated repeatedly in our listening room, and has consistently induced a FEELING OF RAPTURE AND ECSTASY in all music lovers who have had the exquisite pleasure of witnessing this true miracle of human creativity. References may be provided upon the request of interested individuals.

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Scobie Custom Audio
Duncan Scobie, Proprietor
1156 Union Road, Victoria, BC, V8P 2J3, Canada
E-Mail: scobie@crunchers.bc.ca
URL: www.crunchers.bc.ca/scobie

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