FEW COMPANIES attempt to straddle domestic and professional markets with, ostensibly, the same products, but Bryston is one of the successful examples. Bryston sees itself as a purist company, catering for the sonic expectations of the hi-fi market while maintaining the rugged build and bomb-proof reliability demanded by the professional sector. It is distributed in the UK through PMC Ltd. who implement Bryston's 20-year warranty, By all accounts, its most popular product is the 4B ST power amplifier, a "250W" model that's sandwiched between the budget 3B and rather more substantial 7B/8B models And at just 6 l,495 (UK, ex-VAT), the 4B clearly services an important sector of the pro market.

A 17-inch domestic version is provided without handles while the professional iteration of the 4B is distinguished by two silver trimpots mounted alongside a pair of dual-colour LEDs on the fascia, The latter flash red if the amp is driven hard into clipping (they remained resolutely "green" during my power tests to 1% THD) while the former are provided to trim gain and balance. Turned fully clock-wise, the 4B's full +29.3dB of gain is realised, falling to +l6dB if the pots are reversed anticlockwise. This trim is independent of whether the unbalanced (RCA) or balanced (Neutrik) inputs are deployed.

Bryston uses a dual-mono construction, right down to the use of seperate toroidal mains transformers for each channel and, indeed, the "sag" from 305W to 300W (one channel versus two channels/8ohms) is minimal. This said, a stereo separation of 74dB (Re. 20kHz), while perfectly adequate, still implies a degree of coupling somewhere along the line. Meanwhile the two-channel 4ohm power output is equally generous at 430W, clearly besting its 400W rating. Do note, however, that full power at 20kHz drops by 0.9dB (8ohms) and 0.6dB (4ohms) though its response stretches from 3Hz-80kHz (-1 dB) at a nominal OdBW (1W/8ohm). Incidentally, where available I have noted Bryston's own specification in brackets alongside its measured performance through the Test Table. In all respects, save input sensitivity, the 4B is seen to meet or exceed its minimum performance levels.

Under dynamic conditions, the 4B is capable of sustaining 395W, 625W, 784W (19.8A) and 762W (27.6A) into 8ohm, 4ohm, 2ohm, and 1ohm loads, respectively, with one channel driven. Into 8ohms, this is equiva1ent to an IHF headroom of +1.0d8. The voltage profile provides a measure of the 4B's load tolerance. In a perfect world, the 56Vrms output achieved across 8ohms (black trace) would be sustained across 4ohms, 2ohms and 1ohm (red, blue and green traces, respectively) The gradual relaxation in voltage witnessed here is preferable to an abrupt collapse in the regulation of the PSU into 2ohm or 1ohm.

This is further reflected in the almost uniform increase in current available into 8ohms, 4ohms, 2ohms and 1ohm 1oads. The 28A maximum (29.6A over 5ms) is comfortably accommodated by Bryston's quad-complementary output stage, comprising some eight TO#-style bipolar devices per channel mounted on 3200sq/cm of heatsinking. As a result, there's no need for internal fan cooling nor, if the amplifier is mounted in a l9-inch rack bay, is it strictly necessary to use external air-conditioning. Control cupboards might be a different issue, however. Otherwise, and aside from an excusable loss in composure above 24Arms, the highly consistent distortion profile of the 4B demonstrates little or no progressive VI-limiting.

This, together with the low 0.014ohms output impedance, suggests the 4B will behave very consistently with all manner of speaker loads, assuming a decent gauge of cable is employed.

Distortion, too, remains exceptiona11y low under these taxing conditions and is very close to the limit of measurement (~-90dB) using a Short-Term Fourier Transform and just 10ms of data! Under continuous conditions, THD hovers around 0.0013% midband and 0.003% at HF across 90% of the amplifier's dynamic range. The increase from mid-to-high frequencies is visible on the 3D plot beyond 5kHz, but is primarily composed of innocuous 2nd and 3rd harmonics. The higher 4th-9th harmonics are indicative of crossover distortion, but, at typically<0.001%, are of little issue. Indeed, Bryston prides itself on the 1inearity of its amplifiers, specifying IM products at <O.00009%. This is confirmed by Fig.5 where both 2nd-order (1kHz) and 3rd-order (18kHz and 2l kHz) products are at or below -100dB.

Fig.5 also highlights some PSU residual (an earth-lift is provided), but the A-wtd S/N ratio of 88.6dB (re 0d8W) is still 1dB or 2dB above average for a product in this class. Relative to two-thirds output, a figure of 110dB suggests the 4B will not prove the limiting noise factor in a typical rig! All in all, the Bryston 4B ST emerges with a clean bill of health - a powerful, value- oriented amplifier with no significant flaws and, significantly, the renaissance of a 20-year warranty.

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20 Year Warranty - A Generation of Music