Pre/Power Amplifiers

The past decade has seen a lot of engineering effort expended on developing efficient (cool-running) amplifier technologies, avoiding the severe crossover distortion incurred by a ‘cold’ (zero-bias) Class B output stage, while competing on sonic grounds with the best ‘traditional’ Class A and A/B designs [see Investigation, HFN Feb ’15]. Class D amplifiers are one solution – representing the audio signal as a series of very high frequency pulses – and they have improved in leaps and bounds in respect of load tolerance, response flatness, low noise and distortion. Alternatives include Class H, which is the US designation of Europe’s Class G. These employ two or more PSU rails, each of higher voltage, with the lowest voltage always selected to accommodate the prevailing signal. Excess dissipation in the output stage is thus minimised, and efficiency improved.

For the headphone amplifier in its HPA4, Benchmark uses THX’s ‘Class H Achromatic Audio Amplifier’ (THX AAA) technology – a further refinement of the breed that uses two or more pairs of DC rails of increasing voltage. Here, however, any rail-switching glitches or crossover distortion in its final Class B output stage are largely eliminated by the use of feedforward error correction. Of course, supreme ‘green’ efficiency is not vital for an AC-powered headphone amp of limited output and the specific THX AAA-888 module used here still has a quiescent (idle) consumption of a few watts. In practice, Benchmark is using THX AAA tech on grounds of performance, and so the HPA4 still runs fairly warm! PM