NAD M23 Class D Power Amp Lab Report

Lab Report

This, to all intents and purposes, is the power amp stage from NAD's fully-fledged digital/analogue BluOS-equipped M33 integrated [HFN Aug '20], the Purifi ET400A Class D module delivering the raw specification of the more affordable C 298 power amp [HFN Oct '21] but with 'Masters series refinement'. Both the M33 and M23 are rated at 200W/8ohm and while the M23 nudges ahead with 237W/8ohm and 455W/4ohm, it is not any 'beefier' than the 223W/430W, respectively, achieved by the M33. The uplift under dynamic conditions to 268W, 543W, 730W and 420W into 8, 4, 2 and 1ohm loads shows the same progressive increase in THD to 'soften' the blow near clipping into 8/4ohm loads [see Graph 1] but the M23's output into the lowest 2/1ohm is actually slightly lower than we measured for both the M33 and C 298. Incidentally, the three gain settings of +19dB, +23.9dB and +29.2dB (balanced input) do not influence power output or frequency response which holds true to –0.07dB/20kHz and –10.8dB/100kHz into all loads from 1-8ohm.

Distortion is extremely low through bass and midrange frequencies – fractionally lower than achieved by the C 298 – at 0.00025%/1W, 0.0002%/10W and 0.0003%/100W, the inevitable 'Class D' rise at higher treble frequencies delayed here to above 10kHz, if only reaching a mere 0.002%/1W, 0.004%/10W and 0.025%/100W at 20kHz [see Graph 2, below]. Again, these Purifi Eigentakt modules buck the trend of more traditional Class D solutions whose high frequency response and distortion are markedly influenced by speaker load. Residual noise is about 3dB lower than that from the C 298, yielding an above-average 88.4dB A-wtd S/N ratio (re. 0dBW). PM


Dynamic power output versus distortion into 8ohm (black trace), 4ohm (red), 2ohm (blue) and 1ohm (green) speaker loads. Max. current is 20.5A


Distortion versus frequency versus power output (1W/8ohm, black; 10W, pink; 100W, red)

Power output (<1% THD, 8/4ohm) 237W / 455W
Dynamic power (<1% THD, 8/4/2/1ohm) 268W / 543W / 730W / 420W
Output imp. (20Hz–20kHz/100kHz) 0.006–0.007ohm / 0.001ohm
Freq. resp. (20Hz–20kHz/100kHz) +0.0dB to –0.07dB/–10.8dB
Input sensitivity (for 0dBW/200W) 184mV / 2599mV
A-wtd S/N ratio (re. 0dBW/200W) 88.4dB / 111.4dB
Distortion (20Hz-20kHz, 10W/8ohm) 0.00012–0.0045%
Power consumption (Idle/Rated o/p) 22W / 422W (1W standby)
Dimensions (WHD) / Weight 435x133x396mm / 9.7kg
Price £3399

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