Solid State Preamp Reviews

Operated with either balanced input and output or unbalanced input and output, the Simaudio Moon Evolution P-7 offered a maximum gain of 5.5dB rather than the specified 9dB. This is sensible design I feel, and in this respect the P-7 behaved identically to the twin-chassis Moon Evolution P-8, which Kalman Rubinson reviewed in November 2006 (see its measured performance here). The volume-control setting that gave unity gain was "74.5," as expected (ie, 5.5dB lower than the gain at the control's maximum setting of "80"). The preamplifier preserved absolute polarity—ie, it was noninverting—in all conditions, the XLR jacks being wired with pin 2 hot, the AES-recommended practice. The input impedance was 22k ohms single-ended and 44k ohms balanced at low and midrange frequencies, though this dropped to 5.3k and 15k ohms, respectively, at 20kHz. The output impedance was as specified, at a low 50 ohms single-ended and 100 ohms balanced, both figures consistent across the audioband.

The P-7's frequency response for fully balanced operation into 100k ohms (top, blue and red traces) and 600 ohms (bottom, green and magenta traces) is shown in fig.1. The preamp offers excellent channel matching and has a wide bandwidth, its output not reaching –3dB until above the 200kHz limit of this graph. Commendably, the response was not affected by the volume-control setting or by unbalanced operation.

Fig.1 Simaudio Moon Evolution P-7, balanced frequency response with volume control set to maximum at 1V into: 100k ohms (left channel blue, right red), 600 ohms (left channel green, right magenta). (0.25dB/vertical div.)

The wideband signal/noise ratio, with the input shorted but the volume control set to its maximum, was 78dB (ref. 1V), this improving to 93.9dB when the measurement bandwidth was restricted to the audioband, and to 96.6dB when A-weighted. Channel separation (not shown) was superb, at >120dB below 5kHz in both directions, and still >112dB at 20kHz. Simaudio's engineers obviously know a thing or two about circuit-board layout. I did notice some slight inter-input high-frequency crosstalk, a 20kHz tone at 2V fed to Balanced Input 1 generating a signal down an almost negligible 62dB in Single-ended Input 1 when the latter was left unterminated by a source component. In practice, this will not be an issue, but I mention it both for completeness's sake and because I found it difficult to find anything amiss in the P-7's measured behavior.

Fig.2 plots the THD+noise percentage in the Simaudio's balanced output voltage into 100k and 600 ohms. The preamp clips (defined as 1% THD) at 16V into 100k ohms and at 12V into 600 ohms. The unbalanced figures (not shown) were 8V into 100k ohms and 7V into 600 ohms. All these figures are well above what will be required in practice. The downward slope below 7V of both traces in fig.2 indicates that the P-7's actual distortion under all practical conditions will be buried in the already low noise floor ("unmeasurable," claims Simaudio in the P-7's specifications). Plotting the preamplifier's THD+N percentage against frequency at 2V (fig.3) really shows only the effect of the noise, therefore, though the punishing 600 ohm load (green and magenta traces) did reveal some actual distortion in the top octave; still, at a maximum value of 0.0027%, this is negligible in absolute terms. (This graph was taken for balanced operation; unbalanced was identical.)

Fig.2 Simaudio Moon Evolution P-7, balanced distortion (%) vs 1kHz output level into (from bottom to top at 8V): 100k, 600 ohms.

Fig.3 Simaudio Moon Evolution P-7, balanced distortion (%) vs frequency at 3.5V into: 100k ohms (left channel blue, right red), 600 ohms (left channel green, right magenta).

That the Moon P-7 is an extraordinarily linear design is revealed by fig.4, which shows the spectrum of the preamplifier's output at a level close to the maximum it would be required to deliver in a real-world system: 2V. The only harmonics of the 1kHz tone that raise themselves above the noise-floor components at –130dB are the third, fifth, and seventh, but even the highest in level, the third harmonic at 3kHz, lies below –120dB (0.0001%). This graph was taken with a high 100k ohms load. Reducing the load to a ridiculously low 600 ohms and repeating the spectral analysis shows (fig.5) that the third harmonic rises to –110dB (0.0003%), and that the second harmonic now makes an appearance, at –120dB. This increase in distortion is academic, however. High-frequency intermodulation is similarly extremely low, even into 600 ohms (fig.6).

Fig.4 Simaudio Moon Evolution P-7, balanced spectrum of 1kHz sinewave, DC–10kHz, at 2V into 100k ohms (linear frequency scale; left channel blue, right red).

Fig.5 Simaudio Moon Evolution P-7, balanced spectrum of 1kHz sinewave, DC–10kHz, at 2V into 600 ohms (linear frequency scale; left channel blue, right red).

Fig.6 Simaudio Moon Evolution P-7, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 2V peak into 600 ohms (linear frequency scale; left channel blue, right red).

It may lack the more expensive P-8's twin chassis, but the Moon Evolution P-7 measures almost equally as well—which is about as well as a preamplifier can.—John Atkinson