Bryston 28B-SST monoblock power amplifier Measurements
I preconditioned the Bryston 28B-SST by running it at 330W into 8 ohms for an hour. The THD+noise was 0.00265% at the start; by the end of the hour, it had dropped slightly, to 0.00218%. The amplifier's heatsinks were way too hot to touch after 60 minutes, but despite the thermal abuse, the amplifier didn't turn itself off.
The amplifier was non-inverting from both its balanced and unbalanced inputs. The voltage gain into 8 ohms was 28.75dB from both inputs with the gain switch set to "29dB/1V," 22.9dB with it set to "23dB/2V." The input impedance was lower than specified, at 7.5k ohms unbalanced, 10.2k ohms balanced. This will result in a slightly lean bass with some tubed preamplifiers.
The output impedance was a very low 0.06 ohm at low and midrange frequencies, rising slightly to 0.13 ohm at 20kHz. The modification of the amplifier's frequency response by the Ohm's Law interaction between its source impedance and that of the loudspeaker will be minimal, therefore. This can be seen in fig.1, where the change in response of the amplifier with our standard simulated loudspeaker remained within ±0.1dB limits. The frequency response was flat up to 20kHz into 8 ohms, but the rising output impedance above the audioband resulted in an output down 0.4dB at 20kHz into 2 ohms. The –3dB point into 8 ohms lay at 120kHz; as a result, the amplifier's reproduction of a 10kHz squarewave into that load showed very short risetimes, without any overshoot or ringing (fig.2).
Fig.1 Bryston 28B-SST, frequency response at 2.83V into (from top to bottom at 2kHz): simulated loudspeaker load, 8, 4, 2 ohms (0.5dB/vertical div., right channel dashed).
Fig.2 Bryston 28B-SST, small-signal 10kHz squarewave into 8 ohms.
Fig.3 plots the THD+N percentage in the 28B-SST's output against its output power into 8, 4, and 2 ohms. The amplifier comfortably exceeded its 1000W/30dBW specification into 8 ohms, delivering no less than 1300W (31.15dBW) at 1% THD+N. (The wall AC voltage dropped from 125.1V to 120V at this power level.) Into 4 ohms, the Bryston clipped at 1800W (29.5dBW, 117.5V wall voltage), though it couldn't maintain its high power into 2 ohms, delivering 1050W (24.2dBW, 116V wall voltage).
Fig.3 Bryston 28B-SST, distortion (%)vs 1kHz continuous output power into (from bottom to top at 100W): 8, 4, 2 ohms.
The downward slope of the traces below 10W in fig.3 indicates that the actual distortion lies beneath the noise floor at these levels, but the Bryston is also a low-noise design. The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms) was an excellent 81.5dB, this improving to 93.6dB when A-weighted. I plotted the manner in which the THD+N changes with frequency at a level of 12.65V, equivalent to 19.5W into 8 ohms, which is where the true THD starts to rise out of the noise. The results are shown in fig.4. The amplifier is superbly linear at all frequencies into 8 and 4 ohms, but is clearly less comfortable driving 2 ohms.
Fig.4 Bryston 28B-SST, THD+N (%)vs frequency at 12.65V into (from bottom to top): 8, 4, 2 ohms.
At low power levels, or at higher powers into 8 ohms and above, the 28B-SST's distortion was almost pure third harmonic in nature (fig.5). The second harmonic made an appearance at higher powers into lower impedances (fig.6), but the Bryston is still very linear in absolute terms, and no power-supply–related components can be seen in this graph, suggesting that the supply is optimally designed for this high-powered an amplifier. Intermodulation distortion was also very low, even with the 28B-SST driving 1kW into 4 ohms (fig.7).
Fig.5 Bryston 28B-SST, 1kHz waveform at 92.5W into 8 ohms (top), 0.0065% THD+N; distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.6 Bryston 28B-SST, spectrum of 50Hz sinewave, DC–1kHz, at 1kW into 4 ohms (linear frequency scale).
Fig.7 Bryston 28B-SST, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 1kW peak into 4 ohms (linear frequency scale).
While it definitely works better with speakers having impedances of 4 ohms or greater, Bryston's 28B-SST joins that select group of very-high-powered amplifiers that have sufficiently low noise and distortion to reproduce high-resolution digital recordings without compromise.—John Atkinson