Integrated Amplifiers

HFN broke news of Devialet’s inaugural D-Premier amplifier, and its remarkable ADH – Analogue/Digital Hybrid – tech, some 15 years ago [HFN Apr ’10] and while the flexibility of subsequent ranges has improved almost out of recognition, that proprietary ADH core, and equally astonishing switchmode PSU, has been retained. Naturally there have been upgrades to the SHARC DSP and the ADH’s ‘mapping’ of the output stage, but it’s the faster/higher resolution (now 12-bit) control loop between the Class D stage and Class A preamp that has realised the biggest change, significantly boosting power output at high frequencies. Otherwise, Devialet’s ‘concept amplifier’ is still a uniquely ingenious twist on the ‘feedforward’ Class AB Current Dumping technique seen in Quad’s 405 power amplifier a half century ago [HFN Apr ’76].

In Devialet’s version, digital volume and all other DSP correction is applied prior to a current-mirror I-to-V stage (the ‘Magic Wire’) that converts the current output of the Astra’s PCM1792 DACs into a voltage. A high quality analogue Class A (voltage) preamplifier takes this to speaker level while a unity-gain Class D amp provides the current to maintain this voltage across the speaker load. Hence the term ‘ADH’ or ‘Analogue/Digital Hybrid’.

Importantly, this really is a digital Class D stage because the controlling PWM signal is synchronised to the amplifier’s 192kHz sample rate. Nevertheless, it’s the Class A voltage preamp – not the rugged Class D dumpers – that defines the linearity of the amplifier as a whole. Moreover, unlike old-school Class D amps, the Astra has no invasive LC filter network on its output so its high frequency response remains unaffected by speaker load impedance [see PM's Lab Report].

Devialet rates the Astra at 2x150W/8ohm and 2x300W/4ohm, a theoretical doubling (+3dB) that’s rarely possible except with a PWM-style amplifier equipped with a very ‘stiff’ PSU. In practice it achieves 160W/8ohm and 320W/4ohm under either continuous or dynamic conditions where it doubles again to 640W/2ohm [blue trace, Graph above]. Because the performance envelope of the Class D stage is explicitly mapped in the DSP, it will never allow a signal to pass that would cause the switching transistors to run out of road. That limit is 696W/1ohm [green trace] or 26.4A (re. 1kHz/10msec at <1% THD). Devialet claims a maximum 1kW but this is for 1msec at an unspecified THD.

This power and control, reinforced by the vanishingly low 0.002ohm output impedance, is rendered all the more impressive by the very low 34psec jitter [Graph above]. This is superior to many high-end DACs operating in a benign low current environment, so to achieve this in what is effectively a ‘power DAC’ at 10-100W is truly amazing! PM