Solid State Power Amp Reviews

To measure the Soulution 710, I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com). I ran the 710 at one-third power into 8 ohms for an hour, which maximally stresses a class-A/B amplifier's heatsinking. At the end of this preconditioning period the amplifier's top panel was only slightly warm, at 90°F (32.2°C).

The voltage gain into 8 ohms was 25.9dB from both the balanced and unbalanced inputs, and both inputs preserved absolute polarity (ie, were non-inverting). The XLR jacks are wired with pin 2 hot. The unbalanced input impedance was the specified 10k ohms at all frequencies. However, the balanced input impedance was 2860 ohms, which is both low in absolute terms and significantly lower than the specified 4.8k ohms. Tube preamplifiers with a high balanced source impedance are best avoided, though it is fair to say that such preamps are rare.

The output impedance, including the series resistance of 6' of speaker cable, was very low, at 0.06 ohm at 20Hz and 1kHz, rising to 0.075 ohm at 20kHz. The variation in the amplifier's frequency response, due to the interaction between this impedance and how that of the speaker varies with frequency, is therefore also very low, at ±0.07dB (fig.1, gray trace). Into 8 ohms (blue and red traces), the Soulution 710 offered a very wide small-signal bandwidth—note the superb channel matching in this graph—which correlated with the very short risetimes of the amplifier's reproduction of a 10kHz squarewave (fig.2), though there was the slightest hint of overshoot. The ultrasonic output rolled off a little faster into lower impedances, but even into 2 ohms, the output was still flat within the audioband and down 3dB at 160kHz (fig.1, green trace). The 1kHz squarewave into 8 ohms was about as good as it gets (fig.3).

Channel separation (not shown) was superb, at >110dB below 1kHz and still 84dB in both directions at 20kHz. The Soulution is also a very quiet amplifier, its unweighted, wideband signal/noise ratio measuring 83dB ref. 1W into 8 ohms. Restricting the measurement bandwidth to the audioband increased the S/N ratio to 94dB, and switching in an A-weighting filter further increased it, to 96.8dB. The Soulution 710 joins that select group of amplifiers that has sufficient dynamic range to match the best high-resolution recordings.

Despite the very low noise level, any distortion present in the 710's output remains below that noise almost to the point where the amplifier clips. Figs. 4 and 5 plot the percentage of THD+noise against output power at 1kHz into 8 and 4 ohms, respectively; the constant downward slope of the traces in these graphs indicates that the THD+N percentage is dominated by noise. If clipping is defined as when the percentage of THD+N reaches 1%, the Soulution 710 with both channels driven clips at 130Wpc into 8 ohms (21.1dBW) and 218Wpc into 4 ohms (20.4dBW). With just one channel driven, the clipping power was 135W into 8 ohms (21.3dBW), 260W into 4 ohms (21.1dBW), and 490W into 2 ohms (20.9dBW). The amplifier meets or exceeds its specified 130W into 8 ohms and 260W into 4 ohms (both 21.1dBW), but only with one channel driven. This is because I don't hold the wall voltage constant during these tests; it was 121.5V AC with the amplifier quiescent, dropping to 120.1V AC with the amplifier clipping both channels into 4 ohms or one channel into 2 ohms.

Soulution specifies the THD at 50W into 8 ohms, which is equivalent to 20V, so that is the level at which I plotted the THD+N percentage. The results are shown in fig.6; the THD+N in the midrange and bass into both 8 and 4 ohms is an astonishingly low 0.0006%. This does rise at higher frequencies and into 2 ohms, but not to any level that would cause concern.

With a 1kHz signal, I had to raise the output level close to clipping in order to see the distortion spuriae (fig.7). These are predominantly third-harmonic in nature, but even after averaging 64 waveform captures, the trace is still overlaid with noise. (The THD+N percentage was just 0.00056%!) The spectrum of the signal is shown in fig.8. The third harmonic is the highest in level, at –114dB (0.0002%), with the second harmonic in the left channel and the fifth in the right lying at –120dB (0.0001%), along with a power-supply component at 120Hz. The Soulution 710 performed equally well on the high-frequency intermodulation test. With an equal mix of 19 and 20kHz tones at a peak level of 85W into 8 ohms (fig.9), the 1kHz difference components lay at –120dB!

The following sentence appeared in a review of the Soulution 710 published in October 2005 in the German magazine Stereo: "Lab chief technician Rolf Hähle has hung the test certificate for the Soulution 710 in a golden frame over his measuring desk." I wouldn't go quite that far, but the Soulution 710 is definitely one of the best-measuring amplifiers I have encountered. Color me impressed.—John Atkinson