By plotting a sensi-ble path between the commonplace and the extravagant, quite a few American audio manufacturers have recently been able to establish themselves quite firmly in a grimly competitive international market. Our sampling of their wares - begun in last week's column - continues to examine outstanding power amplifiers. Unlike the popular all-in-one stereo receivers, which combine amplification with control functions and a radio, power amplifiers perform only one task. They boost signal power - usually to a higher wattage than is commonly attained by receivers.
Such components are not intended for casual listeners or those restrained by tight budgets. Power amplifiers are mainly used in conjunction with large loudspeakers in elaborate installations, where they help retain translucent clarity of tonal texture even at extreme volume levels. The operating principles involved in the following instances may seem rather technical, but discussion of them is essential to understanding the merit of these designs.
A basic problem in amplifiers is that the sudden variations in loudness and frequency inherent in the music tends to unsettle their circuits. At one moment deep and powerful sonorities are called for; a fraction of second later, the sound is silvery and filigree. It's not easy to keep up electrically with the constantly changing character of music, and audio engineers are forever trying new ways to fortify their amplifiers against the sonic vicissitudes of a musical score. Compromises must always be countenanced in this respect.
But what sets a top-performing amplifier apart from more ordinary sound gear is that these compromises are very carefully minimized. This is not merely a matter of theoretical nicety. The difference is plainly audible in terms of crispness and transparency of tonal texture.
An outstanding design in this respect is the Citation 22 power amplifier by Harman-Kardon of Woodbury, N.Y. Its $895 list price hardly seems exorbitant in view of its performance. For one thing, it delivers 200 watts per channel, which is far more than enough for every conceivable home listening situation. Moreover, its output circuits are stable enough to be compatible with almost any kind of loudspeaker, including the demanding large models whose deep bass response imposes a strenuous power drain under certain conditions. In fact, this design - unlike others -has a special switch to optimize power transfer from amplifier to speaker for various speaker models with different requirements.
Another uncommon aspect of this design is its frequency response. While many other designs are content to cover the entire audible range from 20 to 20,000 Hertz, this one extends far beyond the limits of human hearing. The figures are spectacular: 0.1 to 250,000 Hertz - far beyond the realm of even the highest-pitched dog whistle - with a maximum deviation of 0.6 decibel at the upper end.
One's first reaction to such technical virtuosity is a shrug. Why bother with all that when you can't even hear it? But, as Harman-Kardon's engineers are quick to point out, you can hear it - at least indirectly. They claim that the extra margin above and below the audible frequency spectrum benefits the sound within the spectrum's natural confines. According to this theory - a basic article of faith in Harman-Kardon's design philosophy - the customary limitation of frequency response to what is audible causes distortion at the limits of the spectrum. This distortion may cause some slight roughness of sound. By extending the response of the amplifier beyond the audible range, this distortion is supposedly avoided.
There has been some rigorous research done at the University of Helsinki that tends to confirm the merits of this approach, although the findings are still a matter of academic debate. In any event, there is no question whatever about the results attained by Harman-Kardon in this amplifier. Skipping superlatives, let's just say it is equal to any.
Another power amplifier that represents notable dollar value is the Parasound D/AS-1000 by Parasound, a company based in San Francisco, which delivers 240 watts per channel and carries a list price of $650. The distinctive feature of this model is what might be called a dual personality. At normal output levels, it operates in what engineers call the Class A mode. In layman's terms, this refers to a circuit that minimizes the audibility of residual distortion. This is done by placing the distortive aspect of the sound in a pure octave relationship to the true sound. Since the human ear cannot readily distinguish true octaves sounding simultaneously (for example, cellos and basses in an orchestra playing the same note an octave apart), the distortion becomes, for all practical purposes, inaudible.
Now for the other side of the Parasound's personality. If peak power is required by loud musical passages, the circuit shifts into another mode, which permits much higher output but abandons the pure-octave masking of distortion. However, since distortion levels are low to begin with, the margin of power is gained at little if any perceptible loss in tonal quality.
Everyone knows about the proof of puddings, and it's the same way with amplifiers. Abandon all theory and just listen.
