Rotel RB-956AX Power Amplifier Review

The growing popularity of surround sound, for a home theater or simply for enhanced enjoyment of recorded music, has been accompanied by a proliferation of multichannel receivers as well as a number of preamplifiers with surround-sound capability. A surround-sound system normally has five or six channels of amplification (sometimes as many as eight or more), including two for the main stereo speakers, one for a center speaker, a pair for surround speakers, and possibly one or more for subwoofers.

An A/V receiver has most or all necessary amplifier channels built in, but an A/V preamplifier, or control amplifier, typically requires several external stereo power amplifiers to drive the various speakers. Aside from cost and space considerations, that can entail a considerable amount of signal and power wiring.

A logical alternative would be a single six-channel power amplifier, essentially three stereo amplifiers in a single package. Such products are now making their appearance, and we have had the opportunity to put one through its paces.

The Rotel RB-956AX contains six nominally 30-watt (but conservatively rated) amplifiers in a single unit somewhat smaller than a typical A/V receiver. The channels are grouped in three stereo pairs, with independent signal-input jacks and speaker-output terminals. Each stereo pair also has a small switch on the rear apron, near its input and output connections, that converts it to bridged mono operation providing triple the rated power of one of its stereo channels. Another option is bi-amping in a conventional stereo system, with two of the amplifier pairs separately driving the tweeter and woofer sections of speakers whose low- and high-frequency sections have separate inputs; the third amplifier pair could be used for another purpose, such as driving a subwoofer or speakers in another room.

The RB-956AX’s front panel contains a pushbutton power switch and three small knobs for separate level adjustment of each channel pair. Colored lights above the knobs indicate whether each amplifier pair is operating in its stereo or bridged mode.

In a surround-sound system, it is virtually impossible for all channels to be driven to their maximum output simultaneously, and we have had unhappy experiences when AC line fuses blew in such operation. Although the RB-956AX is very well protected, the fuses are internally mounted (replacing the fuses in the speaker outputs requires some amplifier disassembly). Therefore, we tested only one pair of channels at a time.

The amplifier’s frequency response was quite flat from 20 Hz to 20 kHz, falling off to -2 dB at 80 kHz and -6 dB at 190 kHz. Rated at 30 watts per channel into 8 ohms, the RB-956AX o actually clipped at 48 watts, 2 dB better than specified. At 30 watts, its distortion (including noise) was between a 0.05 and 0.06 percent from 20 Hz to 20 kHz. The minimum distortion reading of 0.03 percent occurred at 40 watts. Into 4-ohm loads, the clipping power was 70 watts, and the minimum distortion reading was 0.05 percent at 55 watts.

Although the RB-956AX’s power rating is relatively low compared with those of most stereo components, it had very adequate clipping and dynamic headroom and never ran out of reserves during our listening tests in a conventional stereo system. The amplifier was totally free of thumps or clicks when being turned on or off, and it ran cooler than any other amplifier we have used in some time. Even during full-power testing its top became only slightly warm; in normal listening its warmth was barely detectable to the touch.

A neat and workmanlike assembly may not have much to do with an amplifier’s sound, but it says a lot about the care that went into its design and construction. The inside of the Rotel RB-956AX is as neat as the proverbial pin. A single large circuit board contains all the amplifier components except for three sets of internal heat-sink fins along the sides and rear of the unit. Two smaller boards carry the power-supply components, and the large toroidal power transformer is at front and center, well away from the signal circuits.

The Rotel RB-956AX should be an excellent complement to an A/V tuner/preamplifier, making possible a full-featured home theater without visual domination by a large A/V receiver or the complication and expense of installing the equivalent in separate stereo amplifiers.

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