According to the specifications in the heading, the No.20.5 appears to be a "perfect" amplifier in that its output power almost exactly doubles when the load impedance is halved from 8 to 4 ohms, and again when it drops from 4 to 2 ohms . On the test bench, the measured continuous maximum output power into 8 ohms (actually 8.1 ohms) for a 1% distortion figure was 139.3W (21.4dBW footnote 1). Into 4 ohms, it raised 258.5W (21.1dBW), and into 2 ohms, 410.5W (20.1dBW), which is not far removed from the specification. (That these figures are a little higher than implied by the specification may be due to the wave-shaping taking place as the amplifier approaches clipping, which might allow slightly higher power to be reached for a nominal 1% THD figure.) Certainly this is one powerful amplifier.
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