Pre/Power Amplifiers

For another £1099 many enthusiasts will be tempted by a second 9000P power amplifier and the prospect of a claimed 300W/8ohm in bridged operation. In this mode each 9000P becomes a monoblock using its two internal amplifiers to serve the positive and negative phases of the audio signal (the notional left channel is positive, and ‘right’ is negative in this instance). The available voltage is doubled (gain increases from +23.1dB to +28.9dB in bridged mode here) but power only increases proportionally if the 9000P’s PSU has the current to support this increased voltage across the loudspeaker load. Audiolab rates the 9000P at 100W/8ohm in stereo mode [see PM's Lab Report] which, doubling the voltage, would give 400W/8ohm in bridged mode. Audiolab errs on the side of caution with its 300W rating met, in practice with 370W/8ohm and 520W/4ohm.

Because a bridged mono amplifier effectively ‘sees’ half the load impedance of a single-ended stereo amplifier, distortion typically increases, and dynamic power measurements are safer – here the 9000P delivers 550W into both 8 and 4ohm loads, decreasing to 300W and 170W into 2 and 1ohm, respectively, because it is still governed by a 13A current limit [see above Graph; 8ohm, black; 4ohm, red; 2ohm blue; 1ohm green]. The moral of the story, in this instance, is that bridging the 9000P really comes into its own when paired with 6–8ohm nominal impedance loudspeakers. With tougher loads, 4ohm and below, you may experience less audible headroom. PM