Solid State Power Amp Reviews

To perform measurements on the darTZeel NHB-458, I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It." Before testing one of the monoblocks (serial number TZ-UA1458-538L), I ran it at 1/3 its rated power into 8 ohms for 60 minutes, which thermally is the worst case for an amplifier with a class-A/B output stage. At the end of that period, while the shrouded heatsink on the amplifier's rear was too hot to keep my hand on, at 63.5°C/146°F, the top of the enclosure was only warm, at 40.8°C/107.4°F, according to my infrared thermometer. The THD+noise with the amplifier stone cold was 0.08%; after an hour with the amplifier hot, it had risen slightly, to 0.083%—a much smaller change than I have found with some other solid-state amplifiers.

I wasn't able to test the NHB-458 through its impedance-matched 50-ohm Zeel input; I measured its performance using the conventional unbalanced and balanced inputs, selecting each with the front-panel Menu buttons. (Connecting pin 1 of the XLR input to ground was also selected with the Menu buttons.) As used by Michael Fremer, the amplifier's gain was set to "32dB"; via the unbalanced input, the voltage gain into 8 ohms measured 32.05dB. The balanced input's gain was 6dB lower. Both inputs preserved absolute polarity (ie, were non-inverting), the XLR being wired with pin 2 hot.

The unbalanced input impedance, specified as being >30k ohms, was a usefully high 46k ohms at low and middle frequencies, dropping inconsequentially to 37k ohms at the top of the audioband. The balanced input impedance was 20k ohms at all frequencies, as specified. The output impedance was high for a solid-state design, at 0.3 ohm (including 6' of speaker cable) at all frequencies. Consequently, there was a slight, ±0.25dB variation in frequency response with our standard simulated loudspeaker (fig.1, gray trace), due to the Ohm's Law interaction between this impedance and that of the load. The NHB-458's small-signal frequency was otherwise flat within the audioband with all impedances above 2 ohms; the bandwidth into 8 ohms was wide, the response being down by <1dB at 200kHz (fig.1, blue trace). As a result, the amplifier's reproduction of a 10kHz squarewave into 8 ohms featured very short risetimes (fig.2), with no trace of overshoot or ringing. Good stuff!

The darTZeel amplifier was quiet, the unweighted, wideband signal/noise ratio (ref. 2.83V into 8 ohms) measuring 81.4dB with the RCA input jack shorted to ground. Switching an A-weighting filter into circuit increased the ratio to 92.4dB, as the noise predominantly comprised low levels of the 60Hz AC supply frequency (fig.3), perhaps partly due to magnetic interference from the massive toroidal transformer that is the amplifier's heart.

Figs. 4, 5, and 6 show how the THD+N percentage changes with output power into 8, 4, and 2 ohms, respectively, taken from the unbalanced input. This is a powerful amplifier. Defining the clipping point as the power when the THD+N reaches 1%, the NHB-458 clips at 530W into 8 ohms (27.2dBW), 900W into 4 ohms (26.5dBW), and 1025W into 2 ohms (24.1dBW). (Although all amplifiers under test are powered from a dedicated 20A circuit, I don't hold the wall voltage constant for the power test; measuring 123.4V AC with the darTZeel at idle, the wall voltage had dropped to 119V with the amplifier clipping into 2 ohms.) Usually with a solid-state amplifier, the shapes of the traces in figs. 4, 5, and 6 would indicate that the THD+N reading was dominated by noise at low powers, with the actual distortion rising out of the noise floor at each trace's inflection point. But the NHB-458 is a very quiet amplifier, and these traces reveal that the distortion is, very unusually, higher at lower powers than at higher powers, at least until the amplifier starts to clip. I would have suspected crossover distortion due to a lack of output-stage bias current, except that crossover distortion comprises subjectively irritating high-order harmonics—and, as you will see, the NHB-458's distortion spectrum is dominated by low-order harmonics.

To ensure that noise didn't affect the measurement, I examined how the darTZeel's THD+N percentage changed with frequency at a fairly high voltage, 12.67V (equivalent to 20W into 8 ohms, 40W into 4 ohms, and 80W into 2 ohms). The results are shown in fig.7; while the THD roughly doubles with each halving of the load impedance, it remains constant at all frequencies, which is something I conjecture correlates with good sound quality, provided the spectrum of the distortion consists of low-order harmonics. Which the NHB-458's distortion spectrum does: Fig.8 indicates that it primarily consists of the subjectively innocuous second and third harmonics. This graph was taken at a low power; at high powers, higher-order harmonics can be seen (fig.9), though the spectrum doesn't change significantly when a 4 ohm load (red trace) is substituted for 8 ohms (blue trace), other than an increase in the third harmonic. Despite the higher-than-usual harmonic distortion, the NHB-458 did relatively well on the high-power, high-frequency intermodulation test (fig.10). While the difference component at 1kHz lies at –64dB (0.06%), all the higher-order intermodulation products lie below that level.

Its measured performance reveals that darTZeel's NHB-458 offers extremely wide dynamic range capability. While its distortion is not as low as is usually found in modern solid-state designs, perhaps of greater importance is the fact that that distortion comprises low-order harmonics, and that the distortion doesn't change its harmonic character with frequency—John Atkinson