Link: reviewed by Matt Bonaccio on *SoundStage! Hi-Fi* on January 1, 2024

**General information**

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

The Darlington Labs MP-7 and SU-7 were conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The MP-7 and SU-7 each offer one pair of unbalanced RCA inputs and outputs. The MP-7 on its own is designed for a moving-magnet (MM) cartridge with a set gain of 40dB, 47k ohms input impedance, and 100pF of input capacitance. The SU-7 is a step-up amplifier designed to be used in series with and ahead of the MP-7 for low-output moving-coil (MC) cartridges. The SU-7 offers four gain settings: 12, 18, 23, 26dB. It also offers seven input-impedance settings: 47, 100, 220, 470, 1k, 5k, 47k ohms.

Unless otherwise specified, the MP-7 was used on its own for the MM use case, and the SU-7 was used ahead of and in series with the MP-7 with the gain set to 23dB (63dB total) and input impedance set to 220 ohms for the MC use case. Using the default settings above, to achieve the reference output voltage of 1Vrms at 1 kHz, 11mVrms was required for the MM configuration, and 0.9mVrms for the MC configuration.

**Published specifications vs. our primary measurements**

The table below summarizes the measurements published by Darlington Labs for the MP-7 and SU-7 compared directly against our own. The published specifications are sourced from Darlington Labs’ 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 was set at its maximum (DC to 1MHz), assume, unless otherwise stated, 1Vrms output into 100k ohms and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels. For the MC gain setting measurements, the input impedance was set to 47k ohms. The MP-7 and SU-7 specifications were measured separately.

Parameter | Manufacturer | SoundStage! Lab |

MP-7 |
||

Input impedance | 47k ohms | 52.6k ohms |

Frequency response (20Hz-20kHz) | ±0.2dB | +0.5/-0.26dB (30Hz/220Hz) |

THD (1kHz, 500mV out) | <0.08% | <0.056% |

Distortion (1kHz, 500mV out, 3rd harmonic and higher) | <0.01% | <0.0006% |

Signal-to-noise ratio (5mV in, 1kHz, A-wgt) | >78dB | 77.5/75.9dB (L/R) |

Input overload (1kHz, 2% THD) | 140mVrms | 140mVrms |

Maximum output (3% THD) | 17Vrms | 17Vrms |

SU-7 |
||

Gain | 12/18/23/26dB | 11.9/16.7/22.3/29.1dB |

Input impedance | 47/100/220/470/1k/5k/47k ohms | 45/97/212/537/0.95k/4.9k/50.9k ohms |

Frequency response (20Hz-20kHz) | ±0.1dB | -0.01/-0.04dB (20Hz/20kHz) |

THD (1kHz, 5mV out, 23dB gain) | <0.01% | <0.006% |

Signal-to-noise ratio (0.4mV in, 1kHz, A-wgt) | >73dB | 70dB |

Input overload (1kHz) | 140mV | 800mV |

Maximum output | 8Vrms | 9.4Vrms |

Our primary measurements revealed the following using the MM configuration (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -73dB | -51dB |

DC offset | <-12mV | <-11mV |

Gain (default) | 39.3dB | 39.3dB |

IMD ratio (18kHz and 19kHz stimulus tones) | <-26dB | <-25dB |

IMD ratio (3kHz and 4kHz stimulus tones) | <-56dB | <-56dB |

Input impedance | 52.7k ohms | 51.9k ohms |

Maximum output voltage (at clipping 1% THD+N) | 9.1Vrms | 9.1Vrms |

Noise level (with signal, A-weighted) | <55uVrms | <70uVrms |

Noise level (with signal, 20Hz-20kHz) | <260uVrms | <360uVrms |

Output impedance | 271 ohms | 233 ohms |

Overload margin (relative 5mVrms input, 1kHz) | 26dB | 26dB |

Overload margin (relative 5mVrms input, 20Hz) | 4.1dB | 4.1dB |

Overload margin (relative 5mVrms input, 20kHz) | 36.1dB | 36.1dB |

Signal-to-noise ratio (A-weighted) | 83.7dB | 82.3dB |

Signal-to-noise ratio (20Hz-20kHz) | 72.7dB | 69.2dB |

THD (unweighted) | <0.112% | <0.108% |

THD+N (A-weighted) | <0.129% | <0.125% |

THD+N (unweighted) | <0.115% | <0.115% |

Our primary measurements revealed the following using the MC configuration (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 22.4kHz bandwidth):

Parameter | Left channel | Right channel |

Crosstalk, one channel driven (10kHz) | -68.4dB | -51.3dB |

DC offset | <-12mV | <-12mV |

Gain (default SU-7) | 22.3dB | 22.3dB |

IMD ratio (18kHz and 19kHz stimulus tones) | <-26dB | <-26dB |

IMD ratio (3kHz and 4kHz stimulus tones) | <-56dB | <-56dB |

Input impedance | 212 | 214 |

Maximum output voltage (at clipping 1% THD+N, SU-7) | 9.4Vrms | 9.4Vrms |

Noise level (with signal, A-weighted) | <126uVrms | <122uVrms |

Noise level (with signal, 20Hz-20kHz) | <1.7uVrms | <0.5uVrms |

Output impedance (SU-7) | 95 ohms | 94 ohms |

Overload margin (relative 0.5mVrms input, 1kHz) | 24.1dB | 24.1dB |

Overload margin (relative 0.5mVrms input, 20Hz) | 2.92dB | 2.92dB |

Overload margin (relative 0.5mVrms input, 20kHz) | 35dB | 35dB |

Signal-to-noise ratio (A-weighted) | 77.4dB | 77.6dB |

Signal-to-noise ratio (20Hz-20kHz) | 55.2dB | 65.6dB |

THD (unweighted) | <0.109% | <0.105% |

THD+N (A-weighted) | <0.127% | <0.122% |

THD+N (unweighted) | <0.20% | <0.12% |

*SU-7 measured on its own without MP-7

**Frequency response - MM input**

Above are our measured frequency-response plots (relative to 1kHz) for the MM configuration. 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 MP-7 is within +/-0.5dB or so of flat from 20Hz to 20kHz. The worst-case deviation can be seen at 30Hz, with a +0.5dB rise. At 20kHz, the response is at -0.1dB. These data do not quite corroborate Darlington Labs’ claim of 20Hz to 20kHz +/-0.2dB. The worst-case channel-to-channel deviation is roughly 0.1dB, from 100Hz to 300Hz and 3kHz to 6kHz. 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. When one trace is visible it is because the left and right channels are tracking extremely closely, so they do 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 MP-7 and SU-7 yields virtually the same results as with the MM configuration (MP-7 on its own) above. Of note is that only the MP-7 implements the RIAA equalization curve.

**Phase response - MM and MC inputs**

Above is the phase response for the MM and MC configuration, from 20Hz to 20kHz. The MP-7 and SU-7 do 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 -90 degrees at 20kHz.

**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 output voltage is maintained at the refrence 1Vrms. The red/blue (L/R) traces represent the MM configuration (MP-7), and purple/green for the MC configuration (SU-7 + MP-7). THD ratios are essentially identical for both MM and MC configurations, ranging from 0.15% from 20Hz to 200Hz, down to 0.1% at 1-2kHz, then up to 0.3% at 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. The red/blue (L/R) traces represent the MM configuration, and purple/green the MC configuration. THD ratios are essentially identical for both MM and MC configurations, ranging from 0.01% from 50mVrms to 100mVrms, then a steady rise to just past 2% at roughly 15Vrms. The 1% THD mark was reached at 9.1Vrms. At an output of 0.5Vrms, where the input for the MM configuartion is very close to the standard 5mVrms (and 0.5mVrms for the MC configuratiuon), THD ratios are 0.05%.

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

The chart above shows THD+N ratios as a function of output voltage. The red/blue (L/R) traces represent the MM configuration, and purple/green the MC configuration. THD+N ratios are essentially identical for both MM and MC configurations above 2Vrms, where THD dominates, ranging from 0.2% up to 2% at 15Vrms. At low output voltages (50mVrms to 1Vrms), where noise is more dominant, the left channel for the MC configuration exhibited higher THD+N ratios, ranging from 3.5% at 50mVrms down to 0.2% at 1Vrms. The right-channel MC configuration THD+N ratios ranged from 1% at 50mVrms down to 0.1% at 1Vrms. The left and right channels for the MM configuration THD+N ratios ranged from 0.5% at 50mVrms down to 0.1% at 1Vrms.

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

The chart above shows THD+N ratios (A-weighted) as a function of output voltage. The red/blue (L/R) traces represent the MM configuration, and purple/green the MC configuration. THD+N ratios are essentially identical for both MM and MC configurations above 0.5Vrms, where THD dominates, ranging from 0.06% up to 2% at 15Vrms. At low output voltages (50mVrms to 0.5Vrms), where noise is more dominant, the MC configuration exhibited higher THD+N ratios, ranging from 0.2% at 50mVrms down to 0.05% at 1Vrms. The MM configuration THD+N ratios ranged from 0.1% at 50mVrms down to 0.04% at 0.2-0.3Vrms.

**FFT spectrum, 1kHz - MM input**

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MM configuration, which results in the reference voltage of 1Vrms (0dBrA) at the output. Signal harmonics are dominated by the second harmonic (2kHz) at a high -60dBrA, or 0.1%, while the third harmonic (3kHz) can be seen at -90dBrA, or 0.003%. No further signal harmonics can be seen above the -120dBrA noise floor. On the left side of the signal peak, the dominant peak is from the power supply’s second harmonic (120Hz) at -70dBrA, or 0.03%, while higher-order even-order harmonics can also be seen at -80dBrA, or 0.01%, and below.

**FFT spectrum, 1kHz - MC input**

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MC configuration, which results in the reference voltage of 1Vrms (0dBrA) at the output. Signal harmonics are dominated by the second harmonic (2kHz) at a high -60dBrA, or 0.1%, while the third harmonic (3kHz) can be seen at -90dBrA, or 0.003%. No further signal harmonics can be seen above the -120dBrA noise floor. On the left side of the signal peak, the dominant peak is from the power supply’s primary (60Hz) frequency at -60dBrA, or 0.1%, for the left channel, while the right channel is at -80dBrA, or 0.01%. Even-order power-supply related harmonics can also be seen at -70dBrA, or 0.03%, and below, all the way out to 5-6kHz.

**FFT spectrum, 50Hz - MM input**

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the 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. Signal harmonics are dominated by the second harmonic (100Hz) at a high -60dBrA, or 0.1%, while the third harmonic (150kHz) can be seen at -90dBrA, or 0.003%. No further signal harmonics can be seen above the -120dBrA noise floor. The dominant power-supply-related peak is at 120Hz at -70dBrA, or 0.03%.

**FFT spectrum, 50Hz - MC input**

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MC configuration, which results in the reference voltage of 1Vrms (0dBrA) at the output. Signal harmonics are dominated by the second harmonic (100Hz) at a high -60dBrA, or 0.1%, while the third harmonic (150Hz) can be seen at -90dBrA, or 0.003%. No further signal harmonics can be seen above the -110dBrA noise floor. Power-supply-related noise is dominated by the 60Hz peak at -60dBrA, or 0.1%, for the left channel, while the right channel is at -80dBrA, or 0.01%. The 120Hz power-supply-related harmonic can also be seen at -70dBrA, or 0.03%.

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

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM configuration. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (Reference or 0dBRa) at the output. Here we find the second-order modulation product (*i.e.*, the difference signal of 1kHz) at a very high -30dBrA, or 3%. We can also see the third-order modulation products (*i.e.*, 17kHz and 20kHz) sitting at a -80dBrA, or 0.01%. This is a very poor IMD result for a phono preamplifier.

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

The last graph is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC configuration. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 1Vrms (Reference or 0dBRa) at the output. Here we find the second-order modulation product (*i.e.*, the difference signal of 1kHz) at a very high -30dBrA, or 3%. We can also see the third-order modulation products (*i.e.*, 17kHz and 20kHz) sitting at a -80dBrA, or 0.01%. Once again, this is a very poor IMD result for a phono preamplifier.

*Diego Estan*

Electronics Measurement Specialist