Integrated Amplifiers

In common with most tube amps – triode or pentode – the Rhapsody offers two sets of transformer taps to step-down the high voltage/high impedance output of its 300B power tubes to better match the lower impedance and higher current demand of the attached loudspeaker. Specifically, the Rhapsody has 4mm outputs conceived to 'match' nominal 8ohm and 4ohm loads, even though, in practice, very few speakers provide this linear impedance. Instead the amplifier 'sees' a load that swings up and down with frequency as the various reactive components of crossover and drivers come into play, causing the system response to rise with increasing speaker impedance and fall with decreasing speaker impedance.

The severity of these response variations is determined by the magnitude of the swings in speaker impedance vs. the source impedance of the amplifier (~2ohm from 20Hz-20kHz for the Rhapsody). As the Rhapsody's treble response drops away quite markedly via its 4ohm tap into 4, 2 and 1ohm loads [red, blue and green traces, inset Graph] it is crucial to match it with speakers offering a 'bright' response, high sensitivity and a high, or increasing, HF load impedance.

For Ken's listening we chose the DeVore O/93s [HFN Mar '23] – these offer a useful 90dB sensitivity coupled with a load impedance that's broadly >7ohm (20Hz-20kHz), has acceptable swings in phase angle of +54°/–60° through the bass and a thoroughly 'driveable' worse-case 3.1ohm/99Hz EPDR. The black trace [inset Graph] shows the Rhapsody's extended HF response via its 8ohm tap into a non-reactive 8ohm load while the dashed trace shows the response via the same 8ohm tap into the DeVore O/93. These relatively modest ±1.8dB response variations, in the bass, will not be disruptive but the Rhapsody driving a tougher speaker via its 4ohm tap might look, and sound, very different indeed. PM