Audio Research D-250 II Servo power amplifier Page 3
Sonically, the D-250 is in most respects a very un-tubelike amplifier. It has virtually none of that (spurious?) richness and warmth through the midbass, or the soft, velvety-sweet high end, that we tend to associate with tubes. The bass is very tight and detailed, albeit a little on the lean side. The highs are crisp, open, and extended, sounding as though they go out to somewhere beyond TV Channel 3. (Actually, the high end is rated at a 60kHz half-power point; there is no frequency response spec as such.) Yet there is no grain or "dryness" to the high end; the treble is exquisitely smooth and sweet, and the overall sound is about as liquidly textureless as any I've heard.
Similarly, the D-250 is less forward-sounding than most tubed amplifiers, resembling in this respect some of the most neutral solid-state designs (such as the Electron Kinetics Eagle 2, which is a shade laid-back by comparison). Yet the D-250 yields to no other amplifier I've heard in the areas of imaging depth and spaciousness. The middle range has truly impressive detail and focus, with an unsurpassed (but occasionally equalled, as by the Threshold SA-1 and the latest version of the Rowland 7) ability to separate individual voices in a large group. On a really well-made symphonic recording, you can readily pick out the individual instruments in the front rows of violins (try Reference Recordings' Church Windows and Sheffield's Firebird).
But the D-250's most overriding virtue is the sheer musicality of its sound. Everything about it has an intrinsic rightness to it that makes it difficult to concentrate on anything but the music. There is nothing to call attention to the fact that one is listening to a reproduction rather than the real thing. Much of this effect can be lost if the speakers aren't up to snuff, but when they are, the D-250 comes through as an almost perfectly neutral device. This, of course, is exactly what we ask of any amplifier, but the Audio Research D-250, like the incredible SP-11 preamplifier, comes about as close to achieving it as any amplifier I've heard.
In short, the D-250 has most of the virtues of the best tube and solid-state amplifiers, with practically none of the weaknesses of either. In fact, it is only in the area of low-frequency impact and "punch" where it appears to be surpassed—and only to a relatively small degree—by a couple of the beefiest high-current solid state power amplifiers.
Tubed amplifiers in general behave poorly into very low-impedance loads, and although I did not have a pair of ribbon speakers such as the Apogee Scintillas to try with the D-250, I doubt that the D-250 would fare much better with them than do other high-powered tube amps. (The problem, usually, is not merely mediocre sound, but limited power output and a tendency to pop fuses.)