Simaudio Moon 310LP Phono Preamplifier Measurements

Link: reviewed by Philip Beaudette on SoundStage! Hi-Fi on April 15, 2022

General information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Simaudio Moon 310LP was conditioned for 30 minutes at 1Vrms (2Vrms balanced) at the output before any measurements were taken.

The 310LP offers one pair of unbalanced RCA inputs that can be configured for or a moving-magnet (MM) or moving-coil (MC) cartridge. There are both unbalanced (RCA) and balanced (XLR) outputs. Besides the extra 6dB in gain between the balanced and unbalanced outputs, we found no appreciable differences in terms of THD+N.

There are five gain settings, selectable by moving jumpers on the circuit board: 40, 54, 60, 66dB. Of note, Simaudio specifies all of their gain values for the unbalanced outputs; add 6dB for each gain value if using the balanced outputs. There are five resistive-loading settings: 10, 100, 470, 1000, 47k ohms. There are also capacitive-loading settings: 0, 100, 470 pF. In addition, either RIAA or IEC EQ curves can be selected.

Unless otherwise specified, the unbalanced outputs were used for all measurements, with the RIAA EQ curve, with the following MM settings: 40dB, 47k ohms, 0pF. The MC settings were: 60dB, 100 ohms, 0pF. Using the default settings above, to achieve the reference output voltage of 1Vrms (2Vrms balanced) at 1 kHz, 10mVrms was required with the MM configuration, while 1.2mVrms was needed with the MC configuration.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Simaudio for the 310LP compared directly against our own measurements. The published specifications are sourced from Simaudio’s website, either directly or from the manual available for download, or a combination thereof. With the exception of frequency response, where the Audio Precision bandwidth is set at its maximum (DC to 1MHz), assume, unless otherwise stated, 1Vrms unbalanced output into 100k ohms and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels. For the gain setting measurements, the input impedance was set to 47k ohms.

* Simaudio’s published specification is actually THD (20Hz-20kHz); however, after speaking with someone at the company, it was discovered that this measurement is also bandwidth limited to 20kHz, thereby limiting the highest frequency (where second and third harmonics can be captured) to 6kHz.

Our primary measurements revealed the following using the unbalanced input with the MM configuration (unless specified, assume a 1kHz sine wave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced input with the MC configuration (unless specified, assume a 1kHz sine wave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Frequency response - MM input

In our measured frequency-response plots above for the MM configuration measured at the unbalanced output, the blue and red traces (left and right channels) are with the RIAA EQ curve, while the purple and green (left and right channels) represent the responses with the IEC curve.  An inverse RIAA EQ is applied to the input sweep, so that if a device were to track the RIAA curve perfectly, a flat line would emerge. The 310LP is within +/-0.25dB or so of flat from 20Hz to 20kHz, and about -0.25dB at 20Hz, and +0.1dB (right channel) at 20kHz. These data corroborate Simaudio’s claim of 20Hz to 20kHz +/-0.5dB. With the IEC curve engaged, there is a steep attenuation below 20Hz, with a -3dB point at 20Hz, and -7dB at 10Hz, as advertised. The worst-case channel-to-channel deviation is between 5kHz and 20kHz, where the right channel is less than 0.2dB hotter than the left. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple) and the right channel (red or green). On other graphs, only one trace may be visible. This is because the left and right channels are tracking extremely closely, so as not to show a difference with the chosen axis scales.

Frequency response - MC input

In our measured frequency-response plot above for the MC configuration, the 301LP yields virtually the same results as with the MM configuration above. The IEC curve (not shown), was also essentially the same.

Phase response - MM input

Above is the phase response of the 310LP for the MM configuration, from 20Hz to 20kHz. The 310LP does not invert polarity. Since phono preamplifiers must implement the RIAA equalization curve, which ranges from +19.9dB (20Hz) to -32.6dB (90kHz), phase shift at the output is inevitable. Here we find a worst-case -60 degrees around 200Hz and 5kHz.

Phase response - MC input

Above is the phase response of the 310LP for the MC configuration, from 20Hz to 20kHz. The 310LP does not invert polarity. As with the MM phase response, here we find a worst case of -60 degrees around 200Hz and 5kHz.

THD ratio (unweighted) vs. frequency - MM and MC inputs

The chart above shows THD ratios as a function of frequency, where the input sweep is EQ’d with an inverted RIAA curve. The unbalanced output voltage is maintained at the refrence 1Vrms. The red/blue traces (left and right channels) represent the MM configuration, and purple/green (left and right channels) for the MC configuration. For the MM configuration, THD values are very low, ranging from 0.002% at 20Hz down to 0.0001% around 1kHz, then rise to 0.003% at around 18kHz, then show a steep rise to 0.5% at 20kHz. The MC configuration yielded higher THD ratios, but are still admirably low, ranging from 0.015% at 20Hz, down to around 0.0007% around 1kHz, then back up to 0.015% at 20kHz. It’s important to explain the odd behavior for the MM configuration at high frequencies. The reason for this behavior is input overload at very high frequencies, causing distortion. Because of the RIAA EQ curve, it’s necessary to input roughly 100mVrms of signal amplitude at 20kHz to achieve a 1Vrms output. In the real world, with a real recording on an LP, a MM cartridge would never output anywhere near this voltage at 20kHz. So below . . .

. . . is the same chart, but with a targeted 0.5Vrms output instread of 1Vrms, to show the 310LP’s behavior without the input overload at high frequencies. Here we see for the MM configuration, THD ratios ranging from 0.003% at 20Hz down to 0.00015% around 2kHz, then a rise to 0.003% at around 20kHz.

THD ratio (unweighted) vs output voltage at 1kHz - MM and MC inputs

The chart above shows THD ratios as a function of output voltage for the unbalanced output at 1kHz. The red/blue traces (left and right channels) represent the MM configuration, and purple/green (left and right channels ) for the MC configuration. For the MM configuration, THD values at 100mVrms are at 0.001%, then dip as low as 0.0001% around 1Vrms, then a slow rise to the “knee” just below 9Vrms. For the MC configuration, THD values at 100mVrms are at 0.005%, then steadily decrease down to near 0.0005% at 1Vrms. Above 1Vrms, for both MM and MC configurations, THD values are lower for the right channel compared to the left channel. The 1% THD values for the both inputs are reached at 9.8Vrms at the output. It’s important to mention that anything above 1-2Vrms is not typically required for most line-level preamps or integrated amps.

THD+N ratio (unweighted) vs output voltage at 1kHz - MM and MC inputs

Above we can see a plot of THD+N ratios as a function of output voltage for the unbalanced output at 1kHz. The red/blue traces (left and right channels) represent the MM configuration, and purple/green (left and right channels) for the MC configuration. For the MM configuration, THD+N values at 100mVrms are at 0.05%, then dip as low as 0.0006% between 5Vrms and the “knee” just below 9Vrms. For the MC input, THD+N values at 100mVrms are at 0.2%, then dip as low as 0.004% just before the “knee.”

THD+N ratio (A-weighted) vs output voltage at 1kHz - MM and MC inputs

Above we can see a plot of THD+N (A-weighted) ratios as a function of output voltage for the unbalanced output at 1kHz. The red/blue traces (left and right channels) represent the MM configuration, and purple/green (left and right channels) for the MC configuration. For the MM configuration, THD+N values at 100mVrms are at 0.01%, then dip as low as 0.0005/0.0002% (left/right) between 5Vrms and the “knee” just below 9Vrms. For the MC configuration, THD+N values at 100mVrms are at 0.07%, then dip as low as 0.003/0.001% (left/right) around the “knee.”

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sine-wave stimulus for the MM configuration, which results in the reference voltage of 1Vrms (0dBrA) at the unbalanced output. Here we see very clean results. Only the signal’s second harmonic (2kHz) is visible at -125dBrA, or 0.00006%. On the left side of the signal peak, there is a small 60Hz power supply fundamental peak at around -105/100dBrA (left/right), or 0.0006/0.001%. There are also very low-level peaks from the higher order odd harmonics of the power supply fundamental (e.g., 300, 420, 540, 660Hz, etc) at -110dBrA, or 0.0003%, and below.

FFT spectrum, 1kHz - MC input

Shown above is an FFT of a 1kHz input sine-wave stimulus for the MC configuration at the unbalanced output. As there is 20dB more gain with the MC setting, predictably the noise floor is elevated compared to the MM FFT above, although only by about 15dB. This is also a very clean FFT for a MC phono preamplifier. The signal and noise peak patterns are essentially the same as with the MM configuration, but about 15dB higher in amplitude.

FFT spectrum, 50Hz - MM input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the unbalanced output for the MM configuration. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The harmonics from the 50Hz signal (100, 150, 200Hz, etc.) are non-existent above the noise floor, and the two power-supply related noise peaks can be seen most predominantly, but still at very low levels of below -110dBRa, or 0.0003%, at higher odd harmonics.

FFT spectrum, 50Hz - MC input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the balanced output for the MC configuration. The X axis is zoomed in from 40Hz to 1kHz, so that peaks from noise artifacts can be directly compared against peaks from the harmonics of the signal. The harmonics from the 50Hz signal (100, 150, 200Hz, etc.) are non-existent above the noise floor, and the two power-supply related noise peaks can be seen most predominantly, but still at low levels of below -90dBRa, or 0.003%, at higher odd harmonics.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MM input

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sine-wave stimulus tone for the MM configuration measured at the balanced output. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 0.5Vrms (reference or 0dBRa) at the output. Here we find the second order modulation product (i.e. the difference signal of 1kHz) at -85/90dBrA (left/right), or 0.006/0.003%. We can also see the third order modulation products (i.e. 17kHz and 20kHz) sitting at roughly -105dBrA, or 0.0006%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus) - MC

The last chart is an FFT of the IMD products for an 18kHz and 19kHz summed sine-wave stimulus tone for the MC configuration. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is at -65/70dBrA (left/right), or 0.06/0.03%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) at just above -100dBRa, or 0.001%.

Diego Estan
Electronics Measurement Specialist