B&K ST-140 power amplifier

I must admit that even before I connected up this amplifier I was put off by the accompanying literature. B&K makes some persuasive points about the validity (or rather the lack thereof) of some traditional amplifier tests, but the literature was so loaded with flagrant grammaticides, syntactical ineptitudes, and outright errors that I could not help but wonder if the same lack of concern had gone into the product itself (eg, the term "infrasonic" is used throughout to mean "ultrasonic"). Good copy editors aren't that hard to find; B&K should have found one.

There were other off-putters. Although the unit is commendably well-protected by fuses, four power-supply fuses, which the instructions say may blow if the amplifier is "severely overdriven," are located inside the chassis. And the Owner's Manual (one typewritten sheet of paper) states that only "qualified personnel" should open up the unit. This is ridiculous! Although a 70Wpc amplifier with a reasonable load is not likely to blow a 4-amp fuse, it is very likely to be overdriven on occasion, and if the load happens to be a nasty one, fuse popping is well within the realm of possibility.

The ST-140 has speaker fuses located on the rear panel, and these (if properly chosen) should pop before any power supply fuses do. But insisting that the owner trot the amp down to his friendly repairman (who will gleefully charge him $25 to remove six screws and replace a 20-cent fuse) if a fuse should blow, that's a bit much. These fuses should be accessible from the rear panel, the way fuses having a similar function are on most other high-quality amplifiers. Or the user should be instructed how to change the internal ones himself.

This may seem like a trivial point, but it could cost the owner of an ST-140 some unnecessary expense and make him afraid to play his system at much above pussyfoot volume levels. I promised myself in advance that if I blew any of these fuses, I would consider myself a qualified personnel and do the damn thing myself. None ever blew, even though I often ran the amplifier to the point of overload.

The term "dynamic headroom" has been around for a while (I think the first company to use it was APT), but this is the first amplifier I've tested that specified its dynamic headroom: 3.2dB. Apparently, this refers to the amount of power that can be delivered beyond rated power for some short period of time, and is relevant to the observation that many amplifiers sound much more powerful than their rating would suggest—it used to be called "peak power output." Unfortunately, some of the relevance of this is lost by the failure to cite the length of time for which this power can be delivered, If accurate, that modest-looking 3.2dB translates into an instantaneous peak output capability of a respectable 147 watts. B&K has also invented a measurement—"phase noise"—the explanation of which I found deliriously incomprehensible.

Sound Quality
The following observations about the B&K's sound are based on the performance of the second sample received. The first was about to get an exceedingly bad report because of an intolerably shrill, glassy high end. But when that sample suddenly quit on us, it raised a serious question as to whether it might have been defective all along, with the shrillness merely reflecting a circuit problem that finally caused it to fail. This was evidently the case, as the second sample sounded quite different from the first. In fact, the ST-140 turned out to be a real sleeper!

Before auditioning the ST-140, I had been using a recently upgraded (to a Series II) Threshold Stasis S/500 power amplifier, having just returned our Electron Kinetics Eagle 7A (my favorite solid-state amp to date) for modernization. We'll have followups on both of these in a future issue, but suffice it to say that the Threshold now leads the solid-state field, with (among other things) the smoothest, sweetest, most open high end I have yet heard from any non-tube amplifier. When I disconnected the Threshold to try out the B&K, I expected the difference to be laughable, but much to my surprise it wasn't!

Amazingly, here was a $395 70Wpc amplifier that could hold its own against the best I have heard! This amp is detailed, beautifully sweet and airy at the top, capable of reproducing remarkable depth and spread, and all that with a truly authoritative low end that can compete with the best. Of course, it doesn't deliver the beef that a 200-watter can. Those last vestiges of control and impact at the extreme bottom are missing, it can't make most speakers shake the walls the way a truly high-powered amplifier can, and it tends to exacerbate acoustic feedback through the LP player more than does a very high--powered amp when the system is played at very high levels. But if 70 watts looks puny on paper, it sure doesn't sound puny from this amplifier! That dynamic headroom factor, perhaps?

Its high end sounds more naturally balanced through good dynamic systems than through electrostatics, but it is one of the growing number of amplifiers that are quite tolerable on either kind of system. In short, if this had cost $550 it would have gotten a very favorab1e review. At $395, the B&K ST-140 is the amplifier of choice for the perfectionist on a tightish budget. It's a veritable triumph of design, and perhaps the most cost-effective amplifier I know of.—J. Gordon Holt

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