Solid State Preamp Reviews

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") to measure the Simaudio Moon Evolution 850P. With the 850P's volume control set to "80," the maximum gain for balanced input and balanced output was 5.95dB, which is 3dB lower than the specified 9dB. For single-ended input/output, the maximum gain was the same: 5.95dB. The volume control operated in accurate steps of 0.1dB for the first 30dB, meaning that the unity-gain setting was "74."

At low and middle frequencies, the single-ended input impedance was twice the specified 11k ohms, dropping to 12k ohms at the top of the audioband. The balanced input impedance was twice these figures, and all inputs preserved absolute polarity; ie, were non-inverting (the XLRs are wired with pin 2 hot). The single-ended output impedance was to specification, at 50 ohms at all frequencies; the balanced impedance was twice that value, as expected.

The 850P's frequency response with the volume set to its maximum is shown in fig.1. The two channels are perfectly matched, and the preamplifier's output extends at full level an octave above the audioband; it is not affected by reducing the load impedance to 600 ohms or by adjusting the volume control. Channel separation was superb, at >135dB in both directions below 2kHz, and was still 130dB (L–R) and 117dB (R–L) at 20kHz. The preamplifier was also superbly quiet. The unweighted, wideband, balanced signal/noise ratio—ref.1V, taken with the volume control set to "80" and the input shorted to ground—was 93.5dB. When A-weighted, this increased to 109.6dB.

Figs. 2 and 3, taken into 100k ohms, indicate that the 850P clips (defined as 1% THD+noise) at 22V balanced and 11V unbalanced, both well above the level required by any power amplifier to reach its maximum power and higher than specified. Reducing the load to just 600 ohms resulted in clipping levels of 19 and 10V for, respectively, the balanced and unbalanced outputs. This preamp has a bombproof output stage. You can see from these graphs that the traces are dominated by noise up to 5V or so. Even though the noise floor is very low, any distortion is buried beneath that floor. I measured how the percentage of THD+N changed with frequency at 2V into 100k ohms and 600 ohms (fig.4). Though there is a slight rise in actual THD between 20 and 30kHz, the graph shows noise only at lower frequencies.

Even into 600 ohms, the Simaudio's distortion is primarily second and third harmonic in nature (fig.5), albeit at very low levels: below –116dB. You can also see a hint of 120Hz in the right channel (red trace), but at –128dB, this is close to the roots of the universe! Intermodulation distortion, also measured into 600 ohms (fig.6), was likewise extraordinarily low.

No doubt about it—Simaudio's Moon Evolution 850P is one of the best-measuring preamplifiers I have encountered.—John Atkinson