Investigation

Despite employing a very similar Class D architecture with GaN FET output devices, the RA280 is not simply a stripped back, two-channel version of the four-channel 'steampunk' RA180. The main PWM switching module [pictured] has been refined – the RA180's 'CoolGaN' enhancement-mode power transistors from Infineon Technologies are replaced here by a pair of Transphorm (TP65H050G4BS) 'SuperGaN FET' devices [top of PCB]. The Infineon IRS20957S MOSFET driver chip is retained and it's in here that the 'dead time' – the period between one FET switching off and the other switching on – is managed 'for better audio performance... lower distortion and lower audio noise floor'.

HiFi Rose discusses 'dead time' on its website as part of what it calls 'Class AD'. In practice, the increased switching speed, lower 'on' resistance and improved thermal stability of these 'SuperGaN FETs' gives them a performance advantage over silicon FETs in this analogue PWM circuit. However, the architecture remains similar to other Class D amplifiers tested in HFN where the inductive output filter's sensitivity to the speaker load gives rise to variations in the final amplifier/system frequency response. The longstanding Hypex NCore [HFN May '22], Purifi Eigentakt [HFN Oct '22] and more recent ICEpower Edge [HFN Jan '21] Class D solutions do not employ GaN FETs but do include elegant solutions to load sensitivity. PM