Solid State Power Amp Reviews

I measured a different sample of the Naim NAP 250, serial number 518784, than that auditioned by Martin Colloms. I performed the measurements with my Audio Precision SYS2722 system after preconditioning the amplifier by following the CEA's recommendation of running it at one-eighth the specified power into 8 ohms for 30 minutes. At the end of that time, the cooling fan on the rear panel had turned on but the sidemounted heatsinks were very hot, at 127.1°F (52.9°C), and the top panel was only slightly cooler, at 116.4°F (46.9°C).

The Naim NAP 250 preserved absolute polarity, ie, was noninverting, and its voltage gain at 1kHz into 8 ohms was close to the specified 29dB, at 28.6dB. The Naim's balanced input impedance is specified as 47k ohms. I measured 47k ohms at 20Hz and 1kHz, but 24.4k ohms at 20kHz.

The NAP 250's output impedance at low and middle frequencies was 0.31 ohms, rising slightly to 0.33 ohm at the top of the audioband. The amplifier offered a wide small-signal bandwidth into resistive loads, the output at 100kHz into 8 ohms lying at –1dB (fig.1, blue and red traces). Into our standard simulated loudspeaker, the variation in response in the audioband, due to the Ohm's law interaction between the amplifier's source impedance and the impedance of the simulated speaker, was ±0.3dB (fig.1, gray trace). With its wide bandwidth, the Naim's reproduction of a 10kHz squarewave was superbly square, with no overshoot or ringing (fig.2).

The NAP 250's channel separation (not shown) was excellent in both directions, at >90dB below 2kHz, though it decreased to 70dB at 20kHz due to the usual capacitive coupling between the channels. The unweighted, wideband signal/noise ratio (ref. 1W into 8 ohms and measured with the input shorted to ground) was good, at 60.5dB (average of the two channels). This ratio improved to an excellent 89.4dB when the measurement bandwidth was restricted to 22Hz– 22kHz, and to 92dB when A-weighted. Spectral analysis of the low-frequency noisefloor while the Naim amplifier drove a 1kHz tone at 1Wpc into 8 ohms (fig.3) revealed that AC-related spuriae were present at odd-order, integer multiples of the 60Hz AC frequency. These will be due to magnetic interference from the power supply transformer, but the spuriae are all very low in level, lying below –104dB ref. 1W into 8 ohms.

The NAP 250's output power is specified as 100Wpc into 8 ohms (20dBW) and 190Wpc into 4 ohms (19.3dBW) (footnote 1). With our usual definition of clipping as being when the THD+noise reaches 1%, I measured clipping powers with both channels driven of 110Wpc into 8 ohms (20.4dBW, fig.4) and 195Wpc into 4 ohms (19.9dBW, fig.5). Fig.6 shows how the percentage of THD+noise varies with frequency into 8 and 4 ohms at 12.67V (equivalent to 20W into 8 ohms and 40W into 4 ohms). The distortion into both impedances was extremely low in the bass and midrange, but rose a little in the treble. The level of the distortion was also a little higher in the left channel (blue and green traces) than the right (red and gray traces).

The THD+N waveform (fig.7, bottom trace) is dominated by the second harmonic, though there is a hint of crossover distortion. Spectral analysis (fig.8) confirmed that the second harmonic was the highest in level, but even at 40Wpc into 4 ohms, it lay at just –86dB in the right channel (red trace) and –80dB in the left channel (blue trace). However, this graph indicates that higher-order harmonics are also present, albeit at even lower levels. With the NAP 250 driving an equal mix of 19 and 20kHz tones at 40Wpc peak into 4 ohms (fig.9), the 1kHz difference product lay at –80dB (0.01%), with the higher-order products at 18 and 21kHz almost 10dB lower in level.

Naim's NAP 250 offers relatively high power with low noise and very low, predominantly second-harmonic distortion in a modest-sized chassis. But it needs to be well ventilated.—John Atkinson