NAD C 298 Class D Power Amp Compact Class D

Compact Class D

We first saw Purifi's ET400A Class D module used to great effect in NAD's M33 BluOS streaming amplifier solution [HFN Aug '20]. The technology's lineage can be traced back to designer/inventor Bruno Putzeys' original and innovative 'self oscillating' Hypex UcD module and later Ncore derivative. The original UcD circuit combined an input comparator, a power stage and LC filter with feedback between output and input 'correcting' enough of the LC filter's phase shift to ensure stable operation.


Loop gain was a respectable 30dB but this was boosted to 50dB in the later Ncore modules which derived an improved error signal by comparing the UcD amp's real output with a filter simulation of the same audio. This drove the corrective feedback, breaking the link between stability and loop gain, while offering the same load-tolerant frequency response that still sets UcD apart from plain-vanilla Class D amps.

The Purifi Eigentakt circuit is superior still. It includes a filtered, 'global' feedback regime with a control circuit so complex that all its component values are derived using modelling software. The ET400A offers a full 75dB of feedback, unconditional stability, a vanishingly low 0.005ohm output impedance from the C 298 and a reliably flat, extended response. PM

NAD Electronics International
Supplied by: Sevenoaks Sound and Vision, UK
01732 459555