Pre/Power Amplifiers

At one time, designer Alberto Guerra worked at Infineon Technologies (IT) and his familiarity with this world leader in semiconductor solutions inspired the core design of his AGD 'GaNTube' Class D amplifiers. The PWM modulator comes from IT's latest Merus range of Class D amplifier solutions. We've seen an early version of this (then International Rectifier) solution before [HFN Sep '15] although that amplifier never benefited from one of IT's more recent innovations – the 'CoolGaN' HEMT or high-electron-mobility transistor. This gallium nitride substrate offers specific advantages over conventional MOSFETs, rendering it more suitable for high power, very high frequency switching applications – such as the output stage of a Class D amp...

AGD has squeezed its ~800kHz switching Class D amp into an illuminated tube 'bottle', but there's neither a vacuum nor filaments inside – this is a purely decorative artifice! Inside each glass tube are two small PCBs, one [top left] hosting the local PSU regulation and protection, the other [top right] combining IT's IRS20957SPBF driver IC with 600V-rated 'CoolGaN' HEMTs.

While noise and spuriae are low in this design the 'traditional' bridged Class D architecture and inductive output filtering retain some drawbacks – output impedance [dashed trace, see Graph, above] rises steeply at HF, as does distortion, while the frequency response varies with speaker load impedance [unloaded/8/4/2/1ohm = grey/black/red/blue/green traces]. Put simply, if the impedance trend of your speakers rises in the treble then the Gran Vivace will likely sound brighter, and vice-versa. PM