Built for Comfort:

7B ST Monoaural Power Amplifier
by Dick Olsher

Where does a solid-state power amp fit in the audio food chain? That's a question every self-respecting loudspeaker in search of a high-calorie meal ought to ask itself. Some amps are shark-like in character; predators waiting to bite off a woofer or two. Others, like Charlie the Tuna, are eager to please. The latter types are likely to drive your speakers to a tweeter-tickling orgasm. But how can you tell? To discern whether an amp might be friend or foe, it's necessary to examine two areas of performance in which transistor amps normally excel: current delivery and damping factor.

The first question out of a prospective buyer's mouth is usually: "How many watts?" To which I would answer that amps deliver current-not watts. A typical power rating of so many watts into 8 ohms is inadequate to describe an amp's performance into real-world loads. Power ratings are established on the basis of test bench measurements with a resistor. Even a monkey knows that a voice coil is not purely resistive, but exhibits an inductance component that for an 8" woofer is on the order of 0.5 mH. Then there are complex crossover networks and driver resonances that are excited simultaneously with a music signal, resulting in a major amplifier headache, It has been estimated that the instantaneous power demand of some 8-ohm loads can resemble that of a 1-ohm resistor. That's getting precariously close to short-circuit conditions, and although mass market receivers and amps look good on paper into an 8-ohm load, I guarantee you that they will choke under these conditions. Their power supplies, output devices, and heat sinking aren't up to the task of sourcing in excess of 10 amps of current-which represents 100 watts into a 1-ohm load - even for short periods of time. As a proud owner of a 7B, you'll sleep well at night with the knowledge that the Bryston can deliver 800 watts into a 1-ohm load.

The subject of damping factors has been generally misunderstood by consumer and professional alike. The ratio of the amp's source impedance to that of the load has been used as a figure of merit to gauge an amp's ability to control the transient behavior of a cone. That's really only an issue with poorly designed drivers; the real significance of a low damping factor is in its implication for amp/load interactions. An amp with a high source impedance (usually a consequence of low-feedback designs) will interact with a speaker's impedance to alter the overall frequency response. The magnitude of the effect depends on whether the source impedance is 0.5 or 2 ohms, and on how uneven the speaker's impedance is as a function of frequency. Worse case, 2-to-4 dB response deviations may be introduced by the amp. And yes, 3 dB alterations in the tonal balance are audible enough that even Joe Sixpack should he able to hear them. But before you jump to the conclusion that these effects are categorically bad, consider that occasionally the result may be synergistic. The amp/load interaction may actually complement or contour the overall balance for the better-much like the action of a tone control. For the record, the 7B's source impedance is well below a tenth of an ohm, which completely eliminates the problem of load interaction.

An audiophile's search for audio nirvana is often frustrated by the naive mixing of components Chinese-menu-style: a class A amp from column A, and a class A speaker from column B. The result is often class D: musical disappointment with a capital D. I firmly believe that many times one component ends up class B and another class A simply because of the particular system context these components are thrown into by a particular reviewer. In some other system, it's quite possible that the relative rankings of these components might be reversed.

Some of the most musical and enjoyable systems I've heard over the years consisted of fairly modest individual components that harmonized together like angels in heaven.

Being in this business for over twenty years. as Bryston has, counts for a lot in my book. It suggests a network of loyal dealers, satisfied customers, and a mature product line. In addition, Bryston's presence in the pro sound area implies a strong commitment to reliability. Take a look at its standard warranty: twenty years, parts and labor, and shipping one-way. That's a warranty for almost a generation of trouble-free music.

The 7B sits on top of Bryston's ST series in terms of power delivery. All ST amplifiers use Bryston's Quad-Complementary output stage that is exceptionally linear while virtually eliminating irritating odd-order harmonic distortion products. The power supply deploys dual custom toroidal power supplies per channel and multiple filter capacitors.

The output stage consists of two amplifier modules, each with its own power supply. There are sixteen power transistors per module for a total of thirty-two devices. The modules may be combined in either of two modes to form a single mono amp. In series mode, the modules are connected in a bridged configuration, which means that the output voltage swing is additive. This mode is recommended for driving loads from 3-to-8 ohms. In parallel mode, the voltage swing is that of a single module, but the current drive is additive. This mode is recommended for driving loads from 1-to-3 ohms. A switch on the back panel allows conversion form one mode to the other, but note that the speaker wiring must also be changed in the process. However, at least with the I-to-8 ohm loads I auditioned the 7B with (the Chario 1000 Reference, the B&W DM601, and the Sam-adhi Acoustics Rocket), I clearly preferred the parallel mode for its smoother textures, better depth perspective and image focus, and slightly more dynamic presentation.

Both balanced and unbalanced inputs are accepted. The choice may be made with the flick of a switch. Note that I did all of my listening via the unbalanced inputs.

Although its personality was never offensive nor especially assertive, the Bryston managed nonetheless to leave its imprint on the music. Harmonic textures were portrayed with a distinctly Canadian touch: cool and clear. Imagine a wintry Canadian vista. The air is fresh, and the snow is deep underfoot. You can see for a hundred miles. It almost feels like you could reach out and touch the mountains in the distance. The feel for the music's immediacy was superb, but the balance projected from the lower mids on down was diametrically opposed to that of classic tube sound. I'm thinking here of a Mac 275 or a vintage Marantz. There was none of the romantic glow or warmth that classic glass is famous for. The fire in the Bryston's hearth was subdued. It became obvious to me that its furnace needed some stoking. It was time to alter the system context. So I put on my chef's hat and added tube sound to the mix.

The Presence Audio line stage, one of my favorite line-level preamps, was introduced into the front end. Voilà! Much of the lean and bony character of the lower mids vanished. At last a decent measure of quintessential heart and soul suffused the orchestral power range. The basic point is that almost no amp can be expected to perform optimally by being blindly dropped into an existing system context. Once I was willing to recognize the Bryston's special needs, I was beginning to enjoy its considerable sonic talent.
I was particularly surprised by its level of refinement through the upper octaves. Treble transients were nicely textured and controlled. The grit and hash ubiquitous to transistorized designs were not in evidence here. Neither did I pick up the sort of bright tinge that I've come to associate with odd-order harmonic distortion products. On the other hand, the Bryston's sonic verity at the top of the spectrum leaves bright sounding speakers with no place to hide. This is not a rolled-off and soft-sounding amp, hence care in system building is required to avoid antagonizing a hostile tweeter. I had no trouble, however, with either the B&W DM601 and its metal dome or with the Chario Reference 1000 and its soft dome. In fact, there was considerable synergism between the Bryston and the B&W DM601. It was pure joy to hear the B&W puppys sing sweetly through the upper mids and presence regions. Violin overtones -a challenge for most amps- were served up with their sheen and sugar content fully intact .

Midrange textures were fairly liquid and smooth; more so when the Presence Audio line stage was added to the chain. The microstructure of the music was very much in evidence. I'm referring to the nuances and modulations in frequency, volume, and time that imbue music with its personality and emotions. Rhythmic drive was unrestrained. Harmonic colors were quite accurate, especially with the Presence Audio preamp. Clarity abounded: the reverb signature of a hall was nicely fleshed out, allowing me to follow transient decay into the noise floor of the recording. Unquestionably, the Bryston not only got my attention but also managed to push my buttons.

Bass performance was every bit as good as I had expected it to be from a high-current drive amp. Bass lines were tightly controlled with the sort of definition most tube amps can only dream about. In particular, detail through the upper bass range was clearly exposed. Reproduction of cello and double bass benefited from the Bryston's ability to delineate detail and timing nuances. Starker Plays Kodaly (Delos DE-1015) exploded onto the soundstage with rare intensity. Starker attacks the cello like a man possessed. His visceral string technique came alive in no uncertain terms. Kick drum was reproduced with plenty of punch. The Bryston could really rock 'n' roll with the greatest of ease when pushed hard by the program material.

In matters of spatial definition and image focus, I didn't expect much tube magic, but actually the Bryston did well. It could crisply focus and resolve instrumental outlines, but felt a bit short of 3-D palpability in its portrayal of same. And depth perspective was a bit curtailed.

It's indeed rare for any power amp to successfully blend brawn and finesse. The truth is that high-power amps usually don't have much else going for them. The realities of the marketplace are such that too many times we're faced with a choice between the big and dumb and the sweet and small. Providing it is properly integrated into a system, the Bryston 7B delivers sledgehammer bass, engaging dynamics, and sweet harmonic textures in one soundly engineered package.

The Bryston is very listenable. But unlike a number of solid-state amps that get there with rolled-off highs and soft textures, the Bryston does it by avoiding irritating distortion products. It's not often that Toobman recommends a solid-state amp. Toobman says: a mandatory audition at this price point.

We invite you to experience the Bryston SST² Series amplifiers

20 Year Warranty - A Generation of Music