Audio Research D-250 II Servo power amplifier
The appearance alone of the D-250 inspires respect. The amplifier is huge, with most of its 138-lb weight coming from massive power and output transformers. This is not, if you'll pardon the observation, a product to be taken lightly. Neither is it something that we recommend buying by mail-order, partly because it is so deucedly awkward to manhandle, and partly because it is not delivered ready to plug in and turn on. All the tubes are packed separately (for maximum protection), and must be installed prior to fire-up. Believe me when I say you'll be much happier having a local dealer install them prior to delivery for final location of the amplifier in your system. (If there isn't an ARC dealer in your area, you might consider opting for a different amplifier.)
Like all ARC products, this one has a rack-mountable front panel. And even though there appears to be enough bracing between the panel and the main chassis to support the whole thing from the front (where most of the weight is concentrated), not many home-type racks will take this kind of weight without a catastrophic cave-in. (Some racks have provision for supporting the back of the amplifier as well as the front, which would be fine.) If in doubt, I advise putting the amplifier at the very bottom of the rack, or simply placing it on the floor. (If your floor is carpeted, the amp should be set on a 17" by 19" plywood panel, to maintain ventilation access to the bottom of the chassis.)
A Floating Common
Also, like all Audio Research tubed amplifiers—yes, they make solid-state ones too, but we reviewers rarely hear about them—the D-250's outputs use a "floating zero" output. The 4-ohm tap is actually the output Ground or Common connection, while the 0 tap is actually a "hot" connection, having reverse phase with respect to the other Hot taps. The reason for this unique arrangement has to do with ARC's push-pull-throughout (footnote 1) circuit topology; the antiphase output tap allows inverse feedback to be returned to both sides of the amplifier's first push-pull amplifying stage, rather than just to one point near the amp's input.
The floating Common is not something one needs to think about in most installations, where 0 will always be one side of the speaker line, regardless of which impedance taps are used. It is only of significance in systems utilizing loudspeaker switching (which I frown on anyway, because of the potential for signal degradation in the switch contacts), or "servo control" boxes such as are supplied with Infinity's SS-1B speaker system; in such cases it is critically important.
Tying the Common tap to the system Ground will upset the signal balance between the two halves of the push-pull circuitry, drastically reducing the amp's overall gain and increasing its distortion. If switching is absolutely necessary to serve several remote speakers, the floating Common is easily coped with; it just requires greater design complexity in the switcher, which must usually be custom-designed. An alternative is to use the 8 and/or 16-ohm taps as the Hot connections, with the 4 as Ground, but since this won't sound as good, it would waste much of the D-250's potential quality, not to mention cost. (I would seriously advise that you use a pair of low-current AC-rated power knife switches to connect the amp to the distribution switch or directly to the main speakers, for serious listening.)
For connections to a servo controller like that supplied with Infinity's RS-1B speakers, it is only necessary to make sure the D-250's 4-ohm output tap is connected to the servo box's Speaker Ground terminal, and that the other output tap used is connected to the servo box's Hot Speaker terminal. The D-250 is normally non-inverting, so the proper setting of the servo box's Polarity will depend on which output tap is used for the "Hot" side. With the 8 or the 16 as Hot, polarity should be set to normal. With the 0 as the "Hot" connection, polarity should be set for Inverting. (Audio Research recommends using 0 and 4 to drive the Infinity RS-1B woofers.) For the upper-range drivers, which are not involved with the servo box, the tap marked 0 is Ground and any other tap is Hot. No polarity reversal is required or, in fact, desirable.
Design & Technology
Audio Research has never been known for the simplicity of its designs, and the schematic diagrams at the back of the D-250's instruction manual are further proof that this reputation is well justified. Each channel requires 13 tubes, including eight 6550s for output. All of the others are dual triodes (two "tubes" per envelope). There are not, however, 13 amplifying "stages" in each channel; the output tubes operate in two push-pull banks of four per side, and the entire amplifying chain is push-pull from the input stage to the output transformer. There are, nonetheless, four gain stages per channel, not counting the power output stage and phase inverter, although the push-pull configuration and cross-coupling both act to reduce distortion to a much lower level than would normally result from this many gain stages. The amplifier's <0.005% harmonic distortion rating (1 watt output) says it all.
Footnote 1: In push-pull operation, a complementary pair of devices amplify signals of opposite electrical polarity and equal amplitude. When these signals are combined at the amplifier's output (with tubes, in a center-tapped transformer), the circuit's symmetry acts to cancel spurious even-order harmonics introduced by the amplifying devices.