THE AUDIO PROBLEM
Years ago someone said that perception is more important than reality. This applies equally to the audio industry, so much of the equipment designed for domestic audio reproduction focusses on the music source, the amplifier and the speakers. Speakers have electro-mechanical issues of course, but they are not so objectionable as the electronic sins of sources and amplifiers.
The human ear seeks ‘natural’ sound in everything it hears. This is simply primordial, our evolution; arising from the need to avoid danger and to communicate with others. With good hearing we can avoid most of the many dangers of modern life. As we walk around our cities, our hearing tells us the approach and direction of fast moving vehicles and people. While most sounds in the natural world are created by wind, people, animals, moving objects and rain, there are sounds created by chemical and electrical processes, such as controlled explosions, lightning, firearms, electrical hum and other man-made machines. Most humans can identify many sources from the sound, just as most can perceive the sound differences between a gasoline and a diesel reciprocating engine. When we perceive sound which is man-made, we generally recognise its source from our experience. We easily differentiate the sounds of aeroplanes, trains and cars. We recognise the ‘machine’ sounds immediately, even an even, slow mechanical sounds like a steam engine. But this skill to differentiate natural from man-made noises is pivotal, and it is very much the domain of the serious audiophile who strains to hear ‘unnatural’ sounds from his system from his wide experience of music and instrument sounds. This focusses us on the subjective; perhaps a preferred sound system could simply be choice, the way we select clothing, food, cars, or entertainment?
THE AUDIO DESIGN CONUNDRUM
So how do we make sense of this evident skill our ears deliver? Are there measureable markers which differentiate natural from ‘non-natural’ sounds? What is ‘distortion’? Is low THD essential, or is it not the whole story?
As it happens, there are some markers which most people agree on. ‘Warm’ is one of them, and people agree that tube amplifiers are warmer than SS amps. Resolution is another; most can hear the rain right through The Doors ‘Riders of the Storm’ in a good system, yet we do not hear the level of clarity on an iPod. Natural sound is more difficult, but most recognise a hard, brittle sound with digital compressed music. But more subtle aspects, like musical engagement and image depth, are much more subjective and mark the wonderful from the ‘competent’. So, how does distortion alter the perception of sound? Can we analyse this, and what are Fourier harmonics, and what is harmonic profiling? Why does a tube amp with 1% THD sounds very good, yet a SS amp with 0.005% can sound sharp, strident, non-engaging and unmusical?
SIGNIFICANT ISSUES IN AUDIO AMPLIFIER DESIGN
THD of the buffer is 0.0213%, but more than 99.96% of this THD is attributed to H2, H3 and H4, all of which are found in the natural environment and which are musical. The second harmonic is at -74dB, and this confers slight ‘warmth’ to the sound, and frequency is extended to 10MHz to indicate that nothing appears above the noise floor over about 5KHz. This is an excellent profile and very like the profile created by a good single ended triode amplifier.
Next graphs the FFT of a good quality SS Class AB PP amplifier (the AKSA 55 created by Aspen in 2000), at 12.5W (+20dB) into 8R. The profile is very different to the above single transistor amplifier. THD 0.028%, H2 -75dB, H3 -111dB, and H4 and H5 -98dB (see over page):
You see the higher orders beyond 10Khz, the train of artefacts created by feedback. These are below -100dB, but there is some concern about ‘fatigue’.
Finally, here is the harmonic profile of my flagship amplifier, the Maya, which closely demonstrates a monotonic decrease of the harmonics, with H2 at -71dB for 12.5W (+20dB) output into 8R 1KHz. THD is 0.028%. Note the high, odd order harmonics are well below their contiguous, even order harmonics:
For best sound, and since we love a ‘sweet, warm sound’, we should design so that we start with highest H2, reducing evenly down to H5 and no higher orders. This is a ‘natural’ profile which the ear ‘seems’ as ‘real’, originally noted by Jean Hiraga of France in the sixties. If a higher harmonic is higher than the previous harmonic, it will not sound natural. The AKSA has H4 and H5 will higher than H3, and while it is an excellent amp, it is not as ‘natural’ as the previous single transistor buffer above. These are all simulated, but they are recreated on the testbench with fourier analysis. In subjective terms, humans perceive tone changes, so the harmonic of a profile is profoundly important to subjectivity of music. Tube amplifiers follow the Hiraga observations, but start at H2 around -42dB below fundamental. This is very high ‘distortion’, creating very high THD, but many audiophiles prefer the tube sound to SS, and their preference cannot be ignored.
Here is the 1KHz crossover point at 0V magnified to a tiny range of 1.4V total:
You can see that the voltage applied to the speaker is completely linear and free of crossover glitches; the transfer of power from the outputs is accurate and free of distortion. Crossover is not the bogey man it is painted; PP amplifiers do produce odd order distortion, but it is large signal distortion, not small signal distortion around the 0V crossover point.
Distortion is inevitable in amplifier design and operation. Whilst it should be minimised, we need to pay attention to the type of distortion, in particular, the harmonic profile. We need to go more into Fourier analysis, which reveals the type and amplitude of the harmonics. From our subjective understanding of tube amplifiers, high levels of THD do not relate to poor sound quality, but the corollary that low THD does not ensure high sound quality. It depends on the harmonic profile, which can only be tailored by careful choice of topology, operating points, feedback regimes and component choice. Design for very high sound quality remains a black art and explains why people like John Curl, Charles Hansen, Julian Verecker and Nelson Pass are demi-gods in the audio community. Few can create a killer amp which everyone loves to listen to!