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AKSA Kitset ~ Design Philosophy


"How can an amplifier produce superbly low measured treble distortion, yet give the aural impression that there's sand in the tweeter?" Martin Colloms, Stereophile, January 1998.


Overview


The comments expressed above by the noted audio critic, Martin Colloms, highlight the dilemma faced by the audio amplifier designer. He vacillates between low distortion and good sound, and is damned if he does, and damned if he doesn't. Within these odd constraints, Aspen Amplifiers developed the AKSA amplifier specifically for a sonic performance not usually available in kitset amplifiers, particularly designs of this apparent simplicity. The design is a novel approach with high quality components which seeks to remove many of the sonic problems which plague Class AB solid state (SS) amplifiers. In particular, the design seeks to emulate that most wonderful of tube amplifier qualities; emotional engagement with the listener.

This paper describes the confusing subjectivity of the hifi golden fleece and explains the beginnings of the AKSA and the ideas behind its evolution. It is fair to say that the vast majority of audio amplifiers over the last thirty years have been designed with engineering considerations in mind, using the meter, cathode ray oscilloscope and distortion analyser. While this has led to some very good amplifiers with impeccable specifications and unmatched stability, it has not always produced the best possible sound and although it would be foolish to insist that bad amplifiers measure well or even that good amplifiers measure badly, it does seem that the distortion specification is not the only criteria for good sonics. It may also be that as the technology progresses and we become ever more aware of the technology, we might well find we should be measuring other parameters.


History


The history of high fidelity audio arguably began with the single ended triode (SET) amplifiers used in the very early 'talking' picture theatres. These were very musical, but they produced tiny output and required huge, efficient horn-loaded loudspeakers. In the 1930's, transformer coupled, class A triode push pull amps for domestic use appeared from the UK company Brook. Limitations at that time were the recording process, and transformer and capacitor quality.

World War II saw great developments in electronics, particularly in the new, more efficient pentode valve. The Williamson amplifier in 1947 saw the beginning of a quest for low distortion and high efficiency which has continued to this day with a marketing emphasis on good specifications. This amplifier employed a push-pull design with heavy global feedback, using a good quality Partridge output transformer, a careful choice of valves and a very carefully arranged feedback network. The seminal Williamson amplifier set specification standards which were probably two decades ahead of speaker technology at the time, but more importantly, it fixed a technical standard in the mind of the modern audiophile, many of whom are still alive. This amplifier is still revered today, despite using more than 15 dB of 'unfashionable' global negative feedback. However, since the writings of the talented Finnish designer Matti Otala were published in the US IEEE journal in 1980, global negative feedback has fallen into disfavour.

In 1948 Bardeen, Brattain and Shockley of Bell Laboratories invented the transistor. The early devices were crude and fragile and offered very low gain, but by the late-fifties the first germanium devices had evolved into something quite useful. In 1956 Harry C. Lin of RCA devised a famous amplifier topology which still bears his name and forms the basis of many commercial realizations of the audio amplifier today, including the AKSA.

Early germanium transistors were not robust devices, exhibiting low gain and adverse thermal and leakage characteristics. The first transistor amplifiers drew heavily on valve design, with transformers frequently used as interstage couplers and in the output stage. This approach was soon abandoned for two reasons: expense, and direct coupling – an easy task with 'P' and 'N' type transistors. Aggressive marketing brought these 'new' amplifiers into many consumer living rooms, and while they did not sound as good as the valve amplifiers they replaced, most listeners failed to notice. Besides, they were much cheaper. By the seventies, a huge variety of silicon transistors had become available, pioneered by companies like RCA, Philips and Fairchild. High current 'P' devices also came onto the market, and while 'P' and 'N' devices were not (and still are not) electrically identical, this development paved the way for the fully complementary design, which was enthusiastically embraced by engineers in the late seventies and has special appeal for many reasons, including its superior distortion characteristics.


The Classic Era


In 1972 Hitachi published a double differential, output transformerless amplifier design in an application note. The new circuit was intended for their new power mosfets which subsequently formed the basis of many high power professional audio designs of today. The approach taken was a differential development of the Lin topology first published in 1956. Professional audio amplifiers, some developing several thousand watts, are a special area of audio design, and they pose a number of important challenges. Hifi designers frequently look to the benchmark designs of Crown Amcron, MacIntosh, Phase Linear and SAE when they ponder reliability, performance and cost.

Single or multiple pairs of power transistors in a push pull amplifier sit between the low impedance power rails and a loudspeaker voice coil. The audio signal, via the control circuitry, modulates this power supply energy into the speaker. All that stand between the loudspeaker and a beefy, potentially destructive power supply are several large transistors, so the desiigns must guarantee complete reliability and iron-fisted control to avoid incinerating the loudspeaker. There must never be any appreciable DC voltage on the output – offset control is not a trivial matter, and has baffled many designers over the years. This fact points up two quite different requirements of the feedback mechanism in a direct coupled amplifier; feedback must control both the DC offset of the amplifier AND the AC gain of the amplifier. This is a tall order, and has repercussions for good sonics.

During the classic seventies, there was both a high demand for the new technology amplifiers and a genuine desire to improve the 'specification', whatever that meant. An unease had grown among many audiophiles that this new technology was not delivering the sound quality of their old valve amplifiers, and there was a ready willingness to accept learned theories explaining just why this was so. Matti Otala's important 'Slew Rate' article, published in a prestigious US engineering journal in 1980, attempted to explain this conundrum by postulating the existence of Transient Intermodulation Distortion (TIM).

This type of distortion is created when the output voltage is required to change faster than the amplifier can cope; it is also called 'slew rate' distortion, and produces a complex, high order spray of harmonics which the feedback mechanism is too slow to remove. The result of Otala's seminal work has been a ready acceptance of his ideas and a marked improvement in commercial amplifiers over the last twenty years to a slew rate parameter of 25 volts per microsecond or better.


During the seventies and eighties, memorable designers and their products frequently appeared in the market. Examples were Peter Walker (Quad), Bart Locanthi (JBL), Bob Carver (Sunfire and Phase Linear), Jim Bongiorno (Great American Sound, 'Ampzilla'), the Crown Amcron, David Hafler, Susumu Tanaka, Erno Borbely and John Linsley-Hood.  Linsley-Hood (who passed away recently in March 2004) worked in the nuclear industry in the UK and wrote extensively for Wireless World (now Electronics World). He is noteworthy because he effortlessly stood astride both valve and solid state technologies. At regular intervals these audio luminaries introduced new concepts of note: local rather than global feedback, fully complementary circuitry, dual differential input stages, cross coupled drivers, current mirrors, cascoding and sliding bias regimes; and these are all topologies which have been and continue to be used over the last 15 years.  These individuals and their companies slowly advanced modern amplifier design, and most at one time or another produced important designs which still sound good, even by today's standards.

The early valve amplifiers were often characterised by high harmonic distortion, and the celebrated Williamson valve amplifier largely overcame this problem with global negative feedback. At the time, and with distortion figures of the order of 1%, attempts to reduce distortion seemed utterly reasonable. The fact that the spectral distribution of this distortion might be important passed unnoticed until the nineties, when the different orders of distortion, global negative feedback and the sonics of the amplifier began to be investigated. And there were other issues too, such as phase shift and slew rate, both of which continue to attract attention.

Notwithstanding more than forty years of research into solid state amplification, there still remain areas of circuit design which offer promise and yet remain unexplored;  error-correction feedforward, nested feedback loops, single ended push pull, and hybrid circuits, to name four. Recently, we have witnessed the advent of digital amplifiers, two examples being the Bel Canto and the acclaimed Halcro. This are rapidly developing but face a different set of psychoacoustic challenges. Furthermore, at all but the extreme high end the developments of the last thirty years have always been focused upon the engineering, and particularly upon achieving ultra-low distortion specifications.  Sonics have remained an unintended but disappointing last, reflecting a strict engineering approach where measurement dominates design. Digital technology perpetuates this quest. However, there is now strong evidence the sonics are becoming important to close the retail sale, and it is even possible that a new standard, based on psychoacoustic considerations, is taking root in the minds of the consumer.


 Measurement v. Subjectivism


In 1993, a fascinating series of articles on audio amplifiers was written by an English designer, Douglas Self, and published in the UK by the monthly magazine Wireless World.  The series ran for several months, and identified the 'seven great distortion mechanisms' of amplifiers, analysing each in turn with competent mathematical insight.  This article was widely read around the world, and clearly dragged the debate back into the objectivist camp.  Amplifier design had drawn increasing criticism from the subjectivists over the previous decade, 'golden ears' who continued to complain of sterile, mechanical sound – and Self felt that this situation required redressing.

Based almost wholly on objectivism and rigorous engineering, Self's work is an extremely informative engineering text on contemporary amplifier design, and is strongly recommended for the interested reader.  It covers all the engineering and mathematical issues in detail, and even offers a little humour;  but nowhere does it mention actually listening to the amplifier to subjectively assess the sonics.  Indeed, Self lambasts the subjectivists roundly, insisting that all the modern advances in audio are unequivocally the preserve of measurement and mathematics.  This highlights one of the paradoxes of modern amplifier design;  the relentless dominance of measured parameters and a dearth of designers actually prepared to tinker and listen. 

Sound engineering – with sonics largely ignored – has remained pre-eminent, in an area characterised by subjectivity.  No-one has ever suggested that a good amplifier 'makes the listener weep', and yet the ability to convey emotion must surely rate as the definitive test of a good hifi.  On the contrary, these products have always been described in the specifications and oscillographs, an area utterly alien and of dubious relevance to the majority of consumers who buy a hifi to listen to music and derive emotional as well as intellectual enjoyment.

However, a major problem for the objectivists has been the galling knowledge that a lot of very good amplifiers measure very badly, particularly in the single ended tube world.  A reasonable defence of this argument has always been that bad amplifiers measure badly, too – and this apparent contradiction has been the inspiration for the development of the AKSA kitset amplifier.


The AKSA


During the five years commencing in 1993, Printed Electronics' design arm, Aspen Amplifiers, began a series of subjective experiments in an attempt to uncover the design factors which make an amplifier sound 'good'.  It was always appreciated that this was a highly subjective judgment, however, and to that end several sets of ears were asked to judge for consensus.  Since this was an empirical rather than an engineering exercise, it led, some would say inevitably, to valve amplifiers.  Of particular interest was the Single Ended tube (SET) amplifier, which Aspen noted sounded wonderful, particularly at low volume on vocal music. The emotion conveyed by the SET was palpable.  And yet it measured very badly, with generous lashings of 2nd and somewhat lower (around 10-20dB) 3rd harmonic distortion. Clearly there was something beguiling about the sound of this antiquated valve topology from the twenties, something worth investigatingfurther.

A two stage, zero feedback hybrid amplifier of 28 watts was designed and built over a three year period using an octal triode valve as voltage amplifier and two pairs of single ended output power mosfets.  It sounded wonderful, and measured badly, with distortion approaching 1% - an unthinkable level by modern standards.  A second hybrid amplifier in Class AB was developed to a completely different design philosophy which produced four times the power.   It too sounded marvellous, and measured equally badly.  Notable in both cases was an integrated triode, which produced measurable and high levels of second and third harmonic distortion.  It seemed to add a warmth and humanity to the sonics which was difficult to ignore. 

Could it be that by concentrating on the total harmonic distortion of an amplifier, we were in fact missing something?  Might this distortion phenomenon be investigated more carefully, and perhaps turned to advantage?  Why was it that after so many decades of development, audio amplifiers of elegant design and sound engineering still sounded sterile, sibilant, flat and mechanical?  In truth, most gave no passion to a musical performance, and fewer still made the listener weep.


The work of John Linsley-Hood, still designing and now in his mid-seventies, led Aspen to a closer examination of the distortion mechanisms and the subjective aspects of careful listening. Linsley-Hood opined that the higher order, odd harmonics were particularly objectionable in listening sessions, and that the fatiguing sound of many solid state amplifiers might be related to this type of distortion, manifesting as a curious unease during a long listen.  In the late seventies, Matti Otala had proposed that these high order distortion components were created by interactions within the global negative feedback loop of a solid state amplifier. 

These higher order harmonics had long been identified, but the significant issue was the very low level of this distortion.  Often audiophiles would comment on the hard, metallic sound of many solid state amplifiers, and this description seemed somehow related to the notion of listener fatigue. Measuring very low distortion in modern amplifiers is exceedingly difficult, even with single tones;  0.002% is close to the limit, and yet it was not unreasonable to suggest that the sorts of subjective differences noted by audiophiles were showing up despite these very low levels, and might also involve phase shifts within the musical content which were the result of time delays dependent upon frequency, essentially a slew phenomenon.

It was also noted that few audiophiles actually listen to single tones, and that since this was a particularly easy signal for most amplifiers to process, there might be something more to it where complex musical waveforms were involved.  Logically then, the measurement process might itself be masking the problem.
Curiously the Single Ended valve amplifier often measured a total distortion of 2% or even higher, but when examined by order, this distortion seemed almost entirely made up of second and third harmonics, with virtually no higher level, odd harmonics at all.  This was a powerful clue.

Aspen concluded that perhaps the hifi holy grail was less about reducing distortion to vanishingly low levels than about removing, or perhaps masking, the adverse effects of high order distortion.  The low order distortion of the single ended amplifier might explain why the zero feedback, valve amplifier sounded so full and rich;  and perhaps also explained the lean, almost surgical sound of the low distortion, negative feedback, solid state amplifier.  Of course, this explanation is anathema to the objectivist, who logically strives to faithfully reproduce the signal as it was originally laid down.  But if high order distortion can be somehow eliminated or reduced, it may be possible for a solid state amplifier to sound benign, perhaps even something like a tube amplifier.  If we were to place our faith in the ear, then it might prove a most accurate measuring instrument!
This later proved an acute observation, and was the primary focus of the AKSA.


A New Approach


The thoughts expressed here lay dormant for some years, but eventually crystallised a different approach to audio design. Aspen had noted in 1997 that a low distortion, zero feedback, solid state, single ended output stage was close to transparent, and when preceded by a valve voltage amplifier, sounded similar in ambience and sound stage to a top quality valve amplifier – but with much more drive and arguably better resolution.
Was there a way of negating Otala's ruinous effects of global feedback?  Could the designer inexpensively simulate the valve using entirely solid state devices?  After all, triodes are very expensive and valve power supplies are killers, both literally and financially!

In late 1999 ago Aspen examined an inexpensive kitset SS amplifier rated at 40 watts.  The owner asked if there might be a way to improve the sound to the level of his $3000 15W Sugden Class A push pull amplifier.  While the kitset amplifier sounded passable, it suffered most of the usual solid state problems; a shallow sound stage and a sharp sibilance.  Surprisingly, there was an element of warmth, a quality of tube amplifiers.  This attracted immediate attention; why?  So with the special appeal of the Class AB amplifier's high efficiency as a design target, the investigation began.

The simplicity of the original amplifier was exceptional.  It was even simpler than the original Lin amplifier design proposed back in 1956.  One of its features was a simple resistive feed to the input differential pair; while this seemed simple, when it was replaced with a 'proper' current source the warmth vanished.  It seemed that a resistive feed conferred a 'warmth' to the sound owing to second and third harmonic distortion – just like a valve! However, further development was required to solve the other sonic shortcomings.  It became apparent that there were a number of key areas of potential improvement.


Six Design Rules


It emerged that there were essentially five critical design rules to a greatly improved solid state, push-pull, Class AB amplifier. They were subtle, but simple to implement. They are:
* Prevent Interstage Crosstalk – Properly decouple the supply rail for the low current stages.
* Foster Voltage Amplifier Linearity – Operate the voltage amplifier at constant current, but avoid the bland CCS.
* Minimise amplitude/phase intermodulations – Split DC offset and AC feedback control.
* Eliminate Switching Transients – Implement accurate and progressive charge suckout on the output stage drivers.
* Specify the highest quality (silver mica) and lowest value lag compensation capacitor.
* Choose Semiconductors with care, focusing on speed and current linearity.


Subjectives


Comparisons with truly high end amplifiers revealed that the AKSA design using carefully chosen semiconductor devices gave astonishing synergy.   The realism, dynamics, sound stage and transparency of the sound was impressive, and several listeners assessed the sound as 'valve-like'.  Some of the clear advantages it held over a 35W valve amplifier with which it was compared were resolution and layering;  it was possible to identify each and every sound in the performing space clearly, and no one sound ever seemed to interfere with another, no matter how high the level.  A surprising quality was the sense of space in a room; this was definitely a tube quality.  Since initial listening tests this quality has been confirmed, and over a period it was felt that the goal had been reached.  The amplifier was duly subjected to exhaustive trials, the production prototypes progressively refined, and finally, after almost a year of development, marketed across the Internet as an inexpensive kitset.  Reports from constructors are describing this amplifier as something very special, regularly besting Class A design costing orders of magnitude more. 

    
Summary


Further testing in carefully structured listening environments has shown that the AKSA is a remarkable amplifier with extraordinary, unexpected qualities.  It is unlike any solid state amplifier ever tested by one audiophile we know, an accomplished speaker designer, who feels it outperforms almost every tube amplifier he has ever heard.  Since those early days, the amplifier has undergone further refinement, particularly in the area of board design, and the final product is both easy to construct and very durable.  With reasonable care the AKSA will give many years of long, pleasurable service, and hold its head high in the company of some of the best amplifiers available in this 21st century world.
I invite you now to review what others say who have built and auditioned the AKSA amplifier.  Their impressions may be read here.

Hugh Dean


© Copyright Hugh R. Dean 2000, 2001
All rights reserved

Last modified 2nd September 2001