Bryston 3B-ST power amplifier Measurements
I made a full set of measurements of the Bryston 3B-ST using its unbalanced inputs, with selected measurements repeated in the balanced mode, as noted below.
Following its 1/3-power, one-hour preconditioning test, the 3B-ST's heatsinks were very hot, though not outside the normal range of temperatures usually encountered in this test. The 3B-ST is non-inverting when driven from its positive, unbalanced input; at the balanced, XLR input terminal, pin 2 is positive as specified. DC offset measured 5.5mV in the left channel, 4.1mV in the right.
The Bryston 3B-ST's input impedance measured 49.7k ohms (16.2k ohms, balanced). The output impedance measured under 0.03 ohms at 20Hz and 1kHz, increasing to a maximum of 0.09 ohms at 20kHz. Voltage gain measured 29.2dB (23.1dB, balanced). The unweighted signal/noise (ref. 1W into 8 ohms) measured 92dB over a 22Hz-22kHz bandwidth, 83dB over a 10Hz-500kHz bandwidth. The A-weighted figure was 95dB. The corresponding figures for balanced drive were all just under 1dB worse (higher) in all cases.
Fig.1 shows the small-signal frequency response of the 3B-ST (the unbalanced result is shown, the balanced was a virtual overlay). There is little worthy of comment here. The same is true of the 10kHz squarewave response (fig.2), which is virtually textbook, with good risetime, and no overshoot or ringing (the 1kHz squarewave looked like it came directly from a squarewave generator, and is not shown).
Fig.1 Bryston 3B-ST, frequency response at (from top to bottom at 20kHz): 2W into 4 ohms, 1W into 8 ohms, and 2.83V into simulated speaker load (right channel dashed, 0.5dB/vertical div.).
Fig.2 Bryston 3B-ST, small-signal 10kHz squarewave into 8 ohms.
Fig.3 shows the 3B-ST's crosstalk. The difference between the two channels should be of no audible consequence at such high absolute separation levels.
Fig.3 Bryston 3B-ST, crosstalk (from top to bottom): L-R, R-L (10dB/vertical div.).
The manner in which the Bryston's THD+noise varies with frequency is shown in fig.4. The THD of the Bryston is so low that I used 10 times our normal output power for this measurement to get results which were not obscured by noise. Note that for the simulated real load, I measured at an output of 8.9V—a small change from our recent practice. (Since the impedance of that load varies with frequency, stating a wattage for this reading is not particularly relevant, as the wattage will vary with frequency for a constant voltage. 8.9V was chosen here because that would be the output voltage for an output of 20W if the load were a pure 4 ohms).
Fig.4 Bryston 3B-ST, THD+noise vs frequency at (from top to bottom at 10kHz): 40W into 2 ohms, 20W into 4 ohms, 8.9V into simulated speaker load, and 10W into 8 ohms (right channel dashed).
Fig.5 compares the THD+noise in unbalanced and balanced modes; note that the unbalanced is lower—though both are very low).
Fig.5 Bryston 3B-ST, THD+noise vs frequency at (from top to bottom at 10kHz): 10W into 8 ohms, balanced; and 10W into 8 ohms, unbalanced (right channel dashed).
The waveform of the distortion at 25W into 2 ohms is shown in fig.6. It is heavily third harmonic, with some noise. The waveforms (not shown) into 4 and 8 ohm were similar, though with higher powers required to get a significant reading above the low levels of noise.
Fig.6 Bryston 3B-ST, 1kHz waveform at 25W into 2 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
The 3B-ST's output spectrum reproducing 50Hz at 154W into 4 ohms is shown in fig.7. The distortion products are all extremely low in level—below -90dB or 0.003%. At an output of 31.2V into our simulated real load (footnote 1), I obtained the result shown in fig.8. Only the third harmonic (at -68dB or 0.04%) is in any way relevant, though other artifacts at less than -80dB are visible.
Fig.7 Bryston 3B-ST, spectrum of 50Hz sinewave, DC-1kHz, at 154W into 4 ohms (linear frequency scale).
Fig.8 Bryston 3B-ST, spectrum of 50Hz sinewave, DC-1kHz, at 31.2V into simulated loudspeaker load (linear frequency scale). Note that the third harmonic is the highest in level at -70dB (0.03%).
Figs.9 & 10 shows the output spectrum resulting from the amplifier driving a combined 19+20kHz signal at 70.5W into 8 ohms and 117W into 4 ohms, respectively (visible clipping is present above this output with this input signal). While clearly more IM products were produced into the 4 ohm load, in all cases the artifacts are very low in level—below -80dB (0.01%).
Fig.9 Bryston 3B-ST, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 70.5W into 8 ohms (linear frequency scale).
Fig.10 Bryston 3B-ST, HF intermodulation spectrum, DC-22kHz, 19+20kHz at 117W into 4 ohms (linear frequency scale).
The way in which the 3B-ST's THD+noise varies with output power (at 1kHz) is shown in fig.11. The discrete clipping levels for the 3B-ST are shown in Table 1. Into 8 ohms, bridged, the 3B-ST reached clipping (1% THD+noise at 1kHz) at 453W (line voltage 114V). Note that the 3B-ST's power supply is starting to give up into a 2 ohm load. It will drive the load, but at considerably reduced power.
Fig.11 Bryston 3B-ST, distortion (%) vs output power into (from bottom to top at 100W): 8 ohms, 4 ohms, and 2 ohms.
The Bryston 3B-ST produced a solid set of measurements, especially notable for its low distortion, noise, and crosstalk. I would not choose this amplifier to drive a loudspeaker which hovered around 2 ohms for most of the audible range, but fortunately such loudspeakers are rare.—Thomas J. Norton
Footnote 1: Why 31.2V?. As is noted above, the impedance of the simulated load varies across the frequency range, making interpretation of the result difficult if we relate the output to power. I therefore measured the voltage at which the amplifier clipped at 1kHz into the simulated real load (1% THD+noise), 38V in this case, and took the measurement at 82% of this. Why 82%? Because we normally take the reading at 67% of rated power, and power is proportional to the square of the voltage—0.82 squared is 0.67.—Thomas J. Norton