Solid State Power Amp Reviews

I performed a full set of measurements on the Krell Solo 575 (serial no. 30315060125) using my Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It"). Before measuring it, I preconditioned the amplifier by running it at one-third the specified power, 190W, into 8 ohms for 30 minutes. (I usually run an amplifier for an hour, but my test load was getting uncomfortably hot after half that time.) The amplifier's cooling fans turned on immediately at this power level and were noisier than I was anticipating. At the end of 30 minutes, the top panel was warm; only the slotted area where the fans vent was hot, at 137.4°F (58.6°C). The percentage of THD+noise at this power level was 0.009% with the amplifier cold, 0.0088% with it fully warmed up.

The Solo 575's voltage gain at 1kHz into 8 ohms was 26.5dB with both balanced and unbalanced input signals. (I couldn't measure its performance via the CAST input; with unbalanced drive, I inserted the supplied shorting plug between pins 1 and 3 of the input XLR jack.) Unusually, the Krell inverted absolute polarity for both its balanced and unbalanced inputs. Although the specified input impedance was very high, at 100k ohms unbalanced and 200k ohms balanced, the measured impedance was very much lower, at close to 4.9k ohms for both inputs across the audioband. This is not an amplifier that should be used with capacitor-coupled tube preamplifiers, though it would not have been a problem with MF's darTZeel preamp. The output impedance was very low, at 0.09 ohm at 20Hz and 1kHz, rising slightly to 0.11 ohm at 20kHz. (All figures include the series resistance of 6' of speaker cable.)

As a result, the modification of the amplifier's frequency response by the Ohm's law interaction between this impedance and that of our standard simulated loudspeaker (fig.1, gray trace) was very low. The Solo 575's small-signal bandwidth was wide, with a –3dB frequency greater than 100kHz. The amplifier's reproduction of a 10kHz squarewave was therefore essentially perfect (fig.2), with short rise times and no overshoot or ringing. The unbalanced frequency response (not shown) was identical at high frequencies, but had a small (0.25dB) rise at 10Hz. Measured with the balanced input shorted to ground, the unweighted, wideband signal/noise ratio was very good, at 78.2dB ref. 2.83V into 8 ohms. With an A-weighting filter, the S/N improved to 89.1dB. Considering its very high output power (see below), this is a high–dynamic-range amplifier. Looking at the low-frequency noise floor as the Krell drove a 1kHz tone at 2.83V into 8 ohms: While some power-supply–related spuriae can be seen (fig.3), even the highest in level, at 120Hz, lies at just –100dB (0.001%).

Figs. 4 and 5 respectively plot the THD+N percentage in the Solo 575's output against power in watts into 8 and 4 ohms. Although the Krell's specified maximum power into these loads is very high, at a respective 575W (27.6dBW) and 900W (26.5dBW), the amplifier exceeded those powers, clipping at 625W into 8 ohms (28dBW) and 910W into 4 ohms (26.6dBW). (We specify clipping as when the THD+N reaches 1%.) Although I don't hold the wall voltage constant during these tests, the Solo 575 clipped at 1050W into 2 ohms (24.2dBW). This is a powerhouse of an amplifier!

It also offers very low distortion. I plotted how the THD+N percentage changes with frequency at a level—12.65V, equivalent to 20W into 8 ohms, 40W into 4 ohms, and 80W into 2 ohms—at which I could be sure that I was measuring actual distortion, not noise. Even so, the level of distortion was very low (fig.6), especially into 8 ohms, and rose only slightly at the top of the audioband.

And that distortion was almost purely third-harmonic in nature (fig.7), with some higher-order odd harmonics appearing at high powers and low frequencies (fig.8). Even so, other than the third harmonic at –80dB (0.01%), these all lay at or below –90dB (0.003%). Intermodulation distortion was similarly low (fig.9).

It is difficult not to be impressed by how Krell's Solo 575 measured. Amplifiers don't get much better.—John Atkinson