Pre/Power Amplifiers

Purifi's sophisticated Eigentakt amplifier technology was all but debuted in NAD's M33 BluOS streaming solution [HFN Aug '20] before being trickled down to the 'entry-level' C 298 power amp [HFN Oct '21], but the underlying principles of designer/inventor Bruno Putzeys' innovative 'self oscillating' Class D principle have their roots in the earlier Hypex UcD and Ncore modules. This original Class D circuit employed just enough feedback between the switching (PWM) output, LC filter and the input comparator to offset the filter's phase shift and ensure stable operation, but loop gain was low at 30dB. This was improved to 50dB in the later Ncore modules by refining the feedback network with a hybrid error signal – a combination of the UcD amp's real output with a filter simulation.

The use of this partially-synthesised error signal severed the link between stability and loop gain. And it achieved this without compromising the load-tolerant frequency response that has always set the Hypex/Ncore amps apart from old-school Class D amps whose output filters are not encompassed within their feedback networks. The Purifi Eigentakt circuit takes another step, combining a filtered, 'global' feedback regime with a control circuit of such precision that all its component values are derived using computer modelling software. The ET400A module used in the M23 offers a full 75dB of feedback, unconditional stability, a vanishingly low 0.006ohm output impedance and a flat, extended response regardless of speaker load. PM