Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on October 1, 2022
All measurements taken using an Audio Precision APx555 B Series analyzer.
The EMM Labs DS-EQ1 was conditioned for 30 minutes at 2Vrms at the balanced output (1Vrms unbalanced) before any measurements were taken.
The DS-EQ1 phono preamp is designed to operate with DS Audio optical cartridges, and therefore operates differently from a conventional phono preamp designed for moving-magnet (MM) or moving-coil (MC) cartridges. As per DS Audio’s technical information page, these optical cartridges are an “amplitude proportional type” transducer as opposed to a “velocity proportional type” transducer found in record-cutting heads and both MM and MC cartridges, which operate on electromagnetic induction.
In terms of measuring the DS-EQ1 phono preamp with the APx555 analyzer, certain issues needed to be overcome. For a detailed description of these issues, along with test set-up configurations, as well as an explanation of how our DS Audio inverted EQ curve was derived, please see our measurements of the DS Audio DS-003 phono preamp.
The EMM Labs DS-EQ1 offers one pair of unbalanced (RCA) inputs and one pair of unbalanced (RCA) and balanced (XLR) outputs. There is a switch on the front panel that will enable a high-pass filter. Unless otherwise stated, all measurements were taken with the high-pass filter disabled and using the balanced outputs. Aside from the extra 6dB of gain measured at the balanced outputs, no appreciable differences were seen in terms of noise and THD when comparing both outputs. To achieve the reference output voltage of 2Vrms at 1kHz at the DS-EQ1 outputs, 85mVrms was required at the output of the APx555.
The DS-EQ1 uses a switching power supply, which results in a peak at roughly 70kHz (see FFTs below). Our typical bandwidth filter setting of 10Hz-90kHz was maintained for THD measurements; however, for noise and THD+N measurements, a 10Hz-45kHz bandwidth was used, to ignore the 70kHz peak.
Our primary measurements revealed the following (unless specified, assume a 1kHz sine wave, 2Vrms output into a 200k ohms load, 10Hz to 45kHz bandwidth):
|Parameter||Left channel||Right channel|
|Crosstalk, one channel driven (10kHz)||-105.2dB||-107.8dB|
|IMD ratio (18kHz and 19kHz stimulus tones)||<-96dB||<-96dB|
|IMD ratio (3kHz and 4kHz stimulus tones)||<-93dB||<-93dB|
|Maximum output voltage (at clipping 1% THD+N)||22Vrms||22Vrms|
|Noise level (A-weighted)||<118uVrms||<118uVrms|
|Noise level (unweighted)||<212uVrms||<212uVrms|
|Output impedance (XLR)||299 ohms||299 ohms|
|Output impedance (RCA)||151 ohms||151 ohms|
|Signal-to-noise ratio (A-weighted)||83.9dB||83.8dB|
|Signal-to-noise ratio (unweighted)||79.4dB||79.4dB|
In our measured frequency-response plots above, the blue/red traces are with the high-pass filter (HPF) disabled, while the purple and green are with the HPF enabled. The DS Audio inverted EQ is applied to the input sweep to emulate the output of the DS Audio optical cartridge. We find an exceptionally flat response from 20Hz to 20kHz, with only a small bass lift of about 0.7dB at 20Hz with the HPF disabled. With the HPF enabled, it is at -3dB at 20Hz. 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 (absolute gain with no EQ applied)
Above is the frequency response plot in terms of absolute gain with no EQ applied, measured at the unbalanced outputs. The blue/red traces are with the high-pass filter (HPF) disabled, while the purple and green are with the HPF enabled. In terms of the shape of the response curve, we find, as expected, roughly the mirror image of our DS Audio inverted EQ curve when observing the gain response when the HPF is disabled. Absolute gain ranges from about 7.5dB at 20Hz to 21.5dB at 1kHz, and nearly 28dB at 20kHz with the HPF disabled. With the HPF enabled, it is at 3dB at 20Hz.
Above is the phase response of the DS-EQ1, from 20Hz to 20kHz. The right channel has inverted polarity; however, this is intentional, to match the behavior of the optical cartridge. Since the phono preamp must implement a combination of the RIAA equalization curve and a compensation curve for the inherent behavior of the optical cartridge, phase shift at the output is inevitable. Here we find worst-case deviations in the left channel between -140 degrees at 200Hz, down to about -160 degrees at 200Hz and 7-8kHz.
THD ratio (unweighted) vs. frequency
The chart above shows THD ratios as a function of frequency, where the input sweep is EQ’d with our DS Audio inverted EQ curve. The balanced output voltage is maintained at the refrence 2Vrms. THD values are relatively flat, ranging from just over 0.001% at 20Hz, down to as low as 0.0003% at 20kHz.
THD ratio (unweighted) vs output voltage at 1kHz
The chart above shows THD ratios as a function of voltage at 1kHz. THD values at 100mVrms are around 0.02%, then dip as low as 0.0005% between 3 and 5Vrms, then a rise to the “knee” just below 20Vrms, then up to the 1% THD value for both inputs at 22Vrms. 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
Above we can see a plot of THD+N ratios as a function of output voltages at 1kHz. THD+N values at 100mVrms are at 0.2%, then dip as low as 0.003% at 10Vrms, then a rise up to the “knee” just below 20Vrms.
THD+N ratio (A-weighted) vs output voltage at 1kHz
Above we can see a plot of THD+N ratios as a function of output voltage at 1kHz. THD+N (A-weighted) values at 100mVrms are at roughly 0.1%, then dip as low as 0.002% at 7-8Vrms then up to the “knee” just below 20Vrms.
FFT spectrum, 1kHz
Shown above is a fast Fourier Transform (FFT) of a 1kHz input sine-wave stimulus, which results in the reference voltage of 2Vrms (0dBrA) at the balanced outputs. We find an exceptionally clean FFT, with only the second signal harmonic (2kHz) barely visible above the noise floor at -115dBrA, or 0.0002%. On the left side of the signal peak, the 60Hz power-supply fundamental is just barely visible at a very low -115dBRa, or 0.0002%.
FFT spectrum, 50Hz
Shown above is the FFT for a 50Hz input sine-wave stimulus. 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. We find an exceptionally clean FFT, with the second signal harmonic at -110dBrA, or 0.0003%, and the third signal harmonic (150Hz) just barely visible above the noise floor at -120dBrA, or 0.0001%. On the left side of the signal peak, the 60Hz power-supply fundamental is visible at a very low -115dBRa, or 0.0002%.
Intermodulation distortion FFT (18kHz + 19kHz summed stimulus)
The last graph is an FFT of the intermodulation distortion (IMD) products for an 18kHz and 19kHz summed sine-wave stimulus tone at the balanced outputs. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. Once again we see a squeaky-clean FFT, this time, with no visible peaks above the -120dBrA noise floor at the second-order (1kHz) or third-order (17 and 20kHz) IMD locations.
Electronics Measurement Specialist