Even with all five channels driven, the 9B-THX more than met its 120Wpc output power specification, the left-hand trace in fig.7 revealing that it put out approximately 160W (22dBW) into 8 ohms at our standard 1% THD+N clipping point. I didn't have enough high-power resistors to simultaneously test all five channels into 4 ohms, but with two channels driven, 260W (21.1dBW) was available (right-hand trace). (Ignore the sawtooth effect in this graph. With the very low distortion and noise offered by the Bryston, these discontinuities in the traces are due to the Audio Precision's automatic range-changing.)

Fig.7 Bryston 9B-THX, distortion (%) vs continuous output power into (from bottom to top at 2kHz) 8 ohms and 2 ohms.

To test the 9B's ultimate power, I used the Miller Audio Research Amplifier Profiler, which drives one channel of the amplifier under test with a low-duty-cycle 1kHz toneburst (10 cycles on, 400 cycles off). This more closely approximates how the amplifier will behave with a music signal. Fig.8 reveals that, under these dynamic conditions, 227W was available for 1% THD (horizontal magenta line) into 8 ohms (black trace), 410W into 4 ohms (red), 680.5W into 2 ohms (blue), and 967W into 1 ohm (green). The latter is equivalent to a current delivery of 31.1A!

Fig.8 Bryston 9B-THX, distortion (%) vs 1kHz burst output power into 8 ohms (black trace), 4 ohms (red), 2 ohms (blue), and 1 ohm (green).

This excellent set of measurements indicates excellent, reliable engineering.—John Atkinson