Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 01 November 2022

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on July 15, 2022

General information

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

The DS Audio DS 003 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The DS Audio DS 003 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 Audio DS 003 phono preamp with the APx555 analyzer, certain issues needed to be overcome. To accomplish this, we entered into a dialog with an engineer at DS Audio to ensure that the measurements were performed correctly.

The first issue was that the DS Audio preamp’s RCA input signal leads (left and right) have a -12VDC (no-load) bias to provide power to the LEDs inside the optical cartridge. The second issue was that the ground lead on the right RCA input is biased at +5VDC to provide power for other sections of the optical cartridge. The third issue was that of emulating the output of the optical cartridge in terms of both frequency response and output impedance, using the APx555 signal generator.

To solve the first issue, 2k ohms resistors were wired in series between the DS Audio’s RCA inputs and the APx555’s outputs, thereby limiting the current that the APx555 outputs would need to sink from the DS Audio’s DC voltage sources. The second issue was resolved by lifting the DS Audio right input ground connection (i.e., it was connected to nothing). In order to ensure that both left and right input signals to the DS Audio were referenced to ground, a connection was made between the DS Audio ground post and the APx555 left- and right-input ground posts (note: this wiring configuration is often utilized for conventional phono preamps to reduce noise). In order to ensure that the shield of the RCA cable between the APx555’s output 2 and the right input on the DS Audio were grounded on both ends, a short jumper was wired between the cable’s ground connection and the DS Audio ground post. The third issue was partially resolved at the same time as the first, as the characteristic output impedance of the optical cartridge, which operates as a current source, is 2k ohms. This left the inherent frequency response of the cartridge, and in turn, finding the correct inverted EQ curve to program into the APx555 signal generator for frequency sweeps.

The first component of the EQ curve is the same as for any other phono preamp: an inverted RIAA curve, as these optical cartridges are tracking the grooves from the same records as would an MM or MC cartridge. Records are encoded with an inverted RIAA EQ curve, and phono preamps must implement the RIAA EQ curve to restore a flat frequency response. The inverted RIAA curve is supplied by Audio Precision (AP). The second component of the EQ curve is the inherent response of the optical cartridge. As per our communications with DS Audio, the optical cartridge’s output follows that of a first-order low-pass filter with a 1Hz corner frequency. That is to say that it would be -3dB at 1Hz, -20dB at 10Hz, -26dB at 20Hz, -40dB at 100Hz, -46dB at 200Hz, -60dB at 1kHz, -66dB at 2kHz, -80dB at 10kHz, -86dB at 20kHz, and -100dB at 100kHz. To construct the DS Audio inverted EQ curve, we added the dB values at each frequency point in the supplied AP inverted RIAA EQ curve (44 points in total from 5Hz to 90kHz) to the dB values for a first-order low-pass filter with a 1Hz corner frequency (gain of 0dB) for the same frequency points. We then normalized the curve to 0dB at 1kHz, which required adding 60dB to each data point. The resulting EQ curve is as follows: 

A comparison between the DS Audio phono preamp inverted EQ curve and the conventional inverted RIAA EQ curve used to evaluate conventional phono preamps can be seen below:

The DS Audio DS 003 offers one pair of unbalanced RCA inputs and two pairs of unbalanced (RCA) outputs, the latter labelled Output 1 and 2. Output 1 does not have a high-pass filter applied, but Output 2 does. There is also a switch labelled Cut Off for the high-pass filter that can be toggled between 30Hz and 50Hz. Unless otherwise stated, all measurements were taken using Output 1 and the Cut Off switch set to 30Hz. Using these settings, to achieve the reference output voltage of 1Vrms at 1kHz at the DS Audio DS 003 outputs, 92mVrms was required at the output of the APx555.

Primary measurements

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

Frequency response - Output 1

In our measured frequency-response plots above measured at Output 1, the blue/red traces are with the Cut Off switch to 30Hz, while the purple and green are with the switch set to 50Hz. The DS Audio inverted EQ curve is applied to the input sweep to emulate the output of the DS Audio optical cartridge. From 100Hz to 80kHz, we find a very flat response, with deviations in the +/- 0.5dB range. Below 100Hz, there is a distinct lift in the bass region. With the Cut Off switch at the 30Hz position, there’s 4.5dB of boost at 20Hz (relative to 1kHz), and with the Cut Off switch at the 50Hz position, just under 2.5dB of boost at 20Hz. We would normally refer to this as a deviation; however, in our discussions with DS Audio, they have said that this is intentional, with the following rationale: “as most cutter-heads, except for Neumann's SX74, have a small roll-off in low frequencies below 40Hz, DS Audio implements a small boost in this frequency range.” It is odd, however, that the Cut Off switch has any effect at all on Output 1, since Output 2 has the high-pass filter applied, but Output 1 does not. 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, which 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 - Output 1 (absolute gain with no EQ applied)

Above is the frequency response plot in terms of absolute gain with no EQ applied for Output 1. In terms of the shape of the response curve, we find, as expected, roughly the mirror image of our DS Audio inverted EQ curve. Absolute gain ranges from about 10.5dB at 20Hz, to 21dB at 1kHz, and about 26.5dB at 20kHz with the Cut Off switch at the 30Hz position. In the 50Hz position, we are at 8dB at 20Hz.

Frequency response - Output 2

In our measured frequency-response plots above measured at Output 2, the blue/red traces are with the Cut Off switch set to 30Hz, while the purple and green are with the switch set to 50Hz. The DS inverted EQ curve is applied to the input sweep to emulate the output of the DS Audio optical cartridge. From 100Hz to 80kHz, we find a very flat response, with deviations in the +/- 0.5dB range. Below 100Hz, there is a small lift in the bass response region, followed by a steep cutoff. With the Cut Off switch at the 30Hz position, we are down 2dB at 20Hz, and with the Cut Off switch at the 50Hz position, we are down 4.5dB at 20Hz. Both slopes below 20Hz are approximately 18dB/octave. Output 2 offers the same equalization and gain as Output 1, but with the addition of a high-pass filter to attenuate the lowest frequencies. Given that Output 1 yields intentional bass boost, for those looking for a flatter response, Output 2 should be used—although at the expense of additional phase shift.

Frequency response - Output 2 (absolute gain with no EQ applied)

Above is the frequency response plot in terms of absolute gain with no EQ applied for Output 2. Absolute gain ranges from about 2.5dB at 20Hz, 21dB at 1kHz, and about 26.5dB at 20kHz with the Cut Off switch at the 30Hz position. In the 50Hz position, we are at 0dB at 20Hz.

Phase response - Output 1

Above is the phase response of the DS 003, 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 a worst case of about +60 degrees at 20Hz, dipping to +20 degrees at around 100Hz and 5kHz.

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 unbalanced output voltage is maintained at the refrence 1Vrms. THD values are relatively flat, with the right channel outperforming the left by almost 10dB, hovering from 0.01% at 20Hz, down to 0.005% from 100Hz to 10kHz, then up to 0.007% at 20kHz.

THD ratio (unweighted) vs output voltage at 1kHz

The chart above shows THD ratios as a function of Output 1 voltages at 1kHz. Between 300mVrms and about 2Vrms, the right channel outperforms the left channel, again by as much as 10dB. Right-channel THD values at 100mVrms are just under 0.005%, then dip as low as 0.002% around 0.5Vrms, then a slow rise to the 1% THD value for both inputs at 9.8Vrms. 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 1 voltages at 1kHz. THD+N values at 100mVrms are at 0.1% (left/right channels), then dip as low as 0.01% for the right channel at around 1Vrms, then a steady rise up to the 1% mark at 9.8Vrms.

THD+N ratio (A-weighted) vs output voltage at 1kHz

Above we can see a plot of THD+N (A-weighted) ratios as a function of Output 1 voltages at 1kHz. THD+N (A-weighted) values at 100mVrms are just below 0.05%, then dip as low as 0.006% at around 0.5Vrms (right) at 1Vrms, then a steady rise up to the 1% mark at 9.8Vrms.

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 1Vrms (0dBrA) at Output 1. There are two obvious signal harmonics at 2kHz and 3kHz, at -75/-90dBrA (left/right), or 0.02/0.003%, and -90dBrA, respectively. On the left side of the signal peak, the 60Hz power-supply fundamental peak is at -110/-90dBrA (left/right), or 0.0003/0.003%. Subsequent even-ordered (i.e., 120Hz, 240Hz, 360Hz) power-supply-related harmonics are seen at -95dBRa, or 0.002%, and below.

FFT spectrum, 50Hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at Output 1. 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 second and third harmonics from the 50Hz signal (100 and 150Hz) are evident at -80/-90dBrA (left/right), or 0.01/0.003%, and -85dBrA, or 0.006%, respectively. Power-supply-related noise peaks can be seen throughout at levels of -90dBrA, or 0.003%, and below.

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 sinewave stimulus tone at Output 1. The input RMS values are set at -6.02dBrA so that, if summed for a mean frequency of 18.5kHz, would yield 1Vrms (0dBrA) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is at -80/95dBrA (left/right), or 0.01/0.002%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) at just above -95dBRa, or 0.002%.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 April 2022

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

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 March 2022

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on March 15, 2022

General information

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

The Saturn Audio 401 was conditioned for 30 minutes at 2Vrms (1Vrms unbalanced) at the output before any measurements were taken.

The 401 offers two pairs of unbalanced RCA inputs—one pair for a moving-magnet (MM) cartridge, the other for a moving coil (MC) cartridge—selectable by a switch on the rear panel. There are both unbalanced (RCA) and balanced (XLR) outputs.

Besides the extra 6dB in gain the balanced outputs had over the unbalanced outputs, we found no appreciable differences in terms of THD+N. The MM gain is fixed (42dB), while the 401 offers five MC gain settings: 87, 81, 75, 67, and 61dB. Of note, Saturn Audio specifies all of their gain values for the balanced outputs—so, subtract 6dB for each gain value if using the unbalanced outputs. Also included are a grounding post with a ground-lift switch, a rumble filter, and four resistive loading settings for the MC input: 100, 220, 470, and 1000 ohms.

Unless otherwise specified, the balanced outputs were used for all measurements, with the rumble filter off, and with the MC input set to 67dB of gain and a 100-ohm input impedance. The 401 power supply is external—it connects to the main unit using an umbilical terminated with a four-pin XLR connector. Slightly lower noise was achieved with the power supply approximately 3′ away from the main unit, which is what the measurements reflect. Using the default settings above, to achieve the reference output voltage of 2Vrms (1Vrms unbalanced) at 1kHz, 18mVrms was required for the MM input and 1.1mVrms with the MC input.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Saturn Audio for the 401 compared directly against our own. The published specifications are sourced from Saturn Audio’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 was set at its maximum (DC to 1MHz), assume, unless otherwise stated, 2Vrms balanced output into 200k ohms (100k ohms unbalanced) and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels. For the MC gain setting measurements, the input impedance was set to 1k ohm.

Our primary measurements revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 2Vrms output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 2Vrms output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Frequency response - MM input

In our measured frequency-response plots above for the MM input measured at the balanced output, the blue/red traces are with the rumble filter disengaged, while the purple and green represent the responses with the rumble filter.  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 401 is within +/-0.1dB or so of flat from 26Hz to 13kHz, and about +0.5dB up at 20kHz, with a steady rise above the audioband. These data do not corroborate Saturn’s claim of 10Hz to 100kHz +/-0.1dB. With the rumble filter engaged, there is steep attenuation below 18Hz, as advertised. Without the rumble filter, the 401 is at -0.2dB at 20Hz, and about -0.7dB at 10Hz. The worst-case channel to channel deviation is between 2kHz and 20kHz, where the right channel is 0.1dB hotter than the left channel. In the chart above and some of the charts below, we see two visible traces—the left channel (blue or purple lines) and the right channel (red or green lines). On other charts, only one trace may be visible—this is because the left and right channels are tracking extremely closely, so they do not show a difference with the chosen axis scales.

Frequency response - MC input

In our measured frequency-response plot above for the MC input, the 401 yields virtually the same results as with the MM input above. Please note that the rumble-filter plot is not show above, but when measured, was virtually the same as the MM plot.

Phase response - MM input

Above is the phase response of the 401 for the MM input, from 20Hz to 20kHz. The 401 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 5-7kHz.

Phase response - MC input

Above is the phase response of the 401 for the MC input, from 20Hz to 20kHz. The 401 does not invert polarity. Here we find a worst case -60 degrees around 200Hz and 5-7kHz, which mirrors what was found with the MM input.

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 balanced output voltage is maintained at the refrence 2Vrms. The red/blue (left and right channels) traces represent the MM input, and purple/green are the MC input. For the MM input, THD values are very low, ranging from 0.002% at 20Hz, down to 0.00003% at 20kHz. The MC input yielded higher THD ratios, but still admirably low, ranging from 0.01% at 20Hz, down to around 0.0002% at 5kHz, then back up to 0.001% at 20kHz.

THD 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 balanced output. The red/blue (left and right channels) traces represent the MM input, and purple/green (left and right channels) for the MC input. For the MM input, THD+N values at 100mVrms are at 0.5%, then dip as low as 0.0006% around 10Vrms, then the “knee” just below 14Vrms. For the MC input, THD+N values at 100mVrms are at 0.2%, then dip as low as 0.003% around 10Vrms until the “knee” just below 14Vrms.

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 balanced output. The red/blue (left and right channels) traces represent the MM input, and purple/green (left and right channels) for the MC input. For the MM input, THD+N values at 100mVrms are at 0.02%, then dip as low as 0.0002% around 10Vrms. For the MC input, THD+N values at 100mVrms are at 0.1%, then dip as low as 0.0007% around the “knee,” at just below 14Vrms.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sine-wave stimulus for the MM input, which results in the reference voltage of 2Vrms (0dBrA) at the balanced output. Here we see exceptionally clean results. Signal harmonics are non-existent above the -130 to -150dBrA noise floor. On the left side of the signal peak, there is only a very small 60Hz power-supply fundamental peak at around -105dBrA, or 0.0006%.

FFT spectrum, 1kHz - MC input

Shown above is an FFT of a 1kHz input sine-wave stimulus for the MC input at the balanced output. As there is 25dB more gain with the MC setting, predictably the noise floor is elevated compared to the MM input FFT, although, only by about 10dB. This is also an exceptionally clean FFT for an MC phono preamplifier. We can just barely see the second signal harmonic (2kHz) above the -120dBrA noise floor. The 60Hz power-supply noise peak is more pronounced due to the higher gain, at -80dBrA, or 0.01%. The third noise harmonic (180Hz) can also be seen at -95dBrA, or 0.002%.

FFT spectrum, 50Hz - MM input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the balanced output for the MM input. 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 just be seen above the low noise floor of -110 to -120 dBrA, or 0.0003% and 0.0001%, at 60Hz and 180Hz.

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 input. 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. Only the signal’s second harmonic (100Hz) can be seen just above the noise floor at -100dBrA, or 0.001%. The 60Hz power-supply related noise peak is clearly seen at -80dBrA, or 0.01%, as is the third harmonic (180Hz) at -95dBrA, or 0.002%.

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 input measured at the balanced output. The input rms values are set so that if summed (for a mean frequency of 18.5kHz), would yield 2Vrms (reference or 0dBRa) at the output. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at a very low -105dBrA, or 0.0006%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) sitting at a vanishingly low -125dBrA, or 0.00006%. This is an exceptional IMD result for a phono preamplifier.

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 input. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBrA, or 0.002%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) at roughly the same amplitude as the MM setting, sitting at or just below -120dBRa, or 0.0001%.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 July 2021

Link: reviewed by Thom Moon on SoundStage! Access on July 15, 2021

General information

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

The AT-PEQ30 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The AT-PEQ30 offers one pair of unbalanced RCA inputs and one pair of unbalanced RCA outputs. The input can be configured for a moving magnet (MM) or moving coil (MC) cartridge by selecting a switch on the front panel. To achieve the reference output voltage of 1Vrms at 1 kHz, 18mVrms was required with the MM setting, and 0.9mVrms with the MC setting.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Audio-Technica for the AT-PEQ30 compared directly against our own measurements. The published specifications are sourced from Audio-Technica’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 output into 100k ohms, a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

*Audio-Technica's SNR specifications (100/74dB for MM/MC) were not given with a reference output voltage, therefore, our SNR measurements are shown with both a 1Vrms output, and Audio-Technica's 250mVrms rated output.

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

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

Frequency response - MM input

Shown above is our frequency-response plot for the MM setting measured at the unbalanced output. 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 AT-PEQ30 is within +/-0.3dB or so of flat from 20Hz to 20kHz, meeting Audio-Technica’s claim of 20Hz-20kHz (+/-0.5dB). It is about -2dB down at 80kHz, and +1/1.5dB (left/right) at about 55kHz. The worst-case channel-to-channel deviation is between 5kHz and 20kHz, where the right channel is almost 0.5dB 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 setting, the AT-PEQ30 is within +0.5dB/-0dB or so of flat from 20Hz to 20kHz, meeting Audio-Technica’s of claim of 20Hz-20kHz (+/-0.5dB). We can see that the MM configuration offers a more extended low frequency bandwidth, where the MC configuration is at -2dB at 10Hz compared to the MM configuration that measured near 0dB at 10Hz. Channel-to-channel deviations can also be seen with the MC setting from 5kHz to 20kHz, but to a lesser degree compared to the MM setting, with about 0.2dB deviation.

Phase response - MM input

Above is the phase response of the AT-PEQ30 for the MM setting, from 20Hz to 20kHz. The AT-PEQ30 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 5-8kHz.

Phase response - MC input

Above is the phase response of the AT-PEQ30 for the MC setting, from 20Hz to 20kHz. The AT-PEQ30 does not invert polarity. Here we find a worst-case -65 degrees around 200Hz and 7-8kHz.

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 (L/R) traces represent the MM configured input, and purple/green for MC. For the MM configuration, THD values at 20Hz are at 0.004%, then dip as low as 0.00007% around 1-2kHz, then up to 0.003% at 20kHz. For the MC configuration, THD values at 20Hz are at 0.1%, then dip down to 0.0002% around 10kHz, then a climb to 0.0007% 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 for the unbalanced output. The red/blue (L/R) traces represent the MM configured input, and purple/green for MC. For the MM configuration, THD values at 100mVrms are at 0.0005%, then dip as low as 0.00007% between 0.8 and 1Vrms, then the “knee” around 2.5Vrms, where THD values reach 0.0002%. For the MC configuration, THD values at 100mVrms are at 0.01%, then steadily decrease down to 0.005% at the 2.5Vrms “knee.” The 1% THD values for the both inputs are reached at around 3Vrms 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. The red/blue (L/R) traces represent the MM-configured input, and purple/green for the MC-configured input For the MM configuration, THD+N values at 100mVrms are at 0.5%, then dip as low as 0.015% between 1 to 2.5Vrms, then the “knee” around 2.5Vrms. For the MC configuration, THD+N values at 100mVrms are at 3%, then dip as low as 0.15% around the 2.5Vrms “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. The red/blue (L/R) traces represent the MM configured input, and purple/green for MC. For the MM configuration, THD+N values at 100mVrms are at 0.006%, then dip as low as 0.0003% at the “knee” around 2.5Vrms. For the MC configuration, THD+N values at 100mVrms are at 0.15%, then dip as low as 0.006% around the 2.5Vrms “knee.”

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier transform (FFT) of a 1kHz input sine-wave stimulus for the MM setting, which results in the reference voltage of 1Vrms (0dBrA) at the unbalanced output. Here we see exceptionally clean results. Signal harmonics are just below -120dBrA (2kHz), or 0.0001%, and below. The first odd-order harmonic (3kHz) is just barely perceptible above the noise floor at -135dBrA, or 0.00002%. On the left side of the signal peak, there is perhaps just a hint of a 60Hz peak due to power-supply noise at -100dBrA, or 0.001%, and again at the third noise harmonic of 180Hz, at -115dBrA, or 0.0002%.

FFT spectrum, 1kHz - MC input

Shown above is an FFT of a 1kHz input sine-wave stimulus for the MC setting at the unbalanced output. As there is 25dB more gain with the MC setting, predictably, the noise floor is higher, and with this, there are no visible signal or noise harmonic peaks.

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 setting. 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 nonexistent above the noise floor, as are the power-supply noise peaks.

FFT spectrum, 50Hz - MC input

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the unbalanced output for the MC setting. 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 nonexistent above the noise floor, as are the power-supply noise peaks.

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 setting measured at the unbalanced output. 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 -100dBrA, or 0.001%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) sitting just above -110dBRa, or 0.0003%. The fourth- and fifth-modulation products are also clearly visible.  

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 setting. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is at -95dBrA, or 0.002%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) at roughly the same amplitude as the MM setting, sitting just above -110dBRa, or 0.0003%. Unlike the MM setting however, with the MC setting, the fourth- and fifth-modulation products are not visible, likely due to the higher noise floor.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 01 July 2021

Link: reviewed by James Hale on SoundStage! Xperience on July 1, 2021

General information

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

The iFi Audio Zen Phono was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The Zen Phono offers one pair of unbalanced RCA inputs, a pair of unbalanced RCA outputs, and a balanced output via a 4.4mm TRRS connector. The input can be configured for different moving magnet (MM) and moving coil (MC) cartridges via a small DIP switch on the back panel. A small white LED on the front panel indicates the selection: MM, MC High, MC Low, or MC Very Low.  Also included is a subsonic filter, which can be activated using a button on the front panel.

I measured the performance of the Zen Phono for the following input configurations: MM, MC Low, MC Very Low. To achieve the reference output voltage of 1Vrms at 1 kHz, 15mVrms was required with the MM setting, 1mVrms with the MC Low setting, and 0.265mVrms with the MC Very Low setting. Other than the extra 6dB of gain and double the maximum output voltage, we found no differences between the balanced and unbalanced outputs, provided the balanced output was referenced to 2Vrms, and the unbalanced output to 1Vrms (i.e., THD, THD+N and signal-to-noise ratios (SNRs) were identical for the same input voltage).

Published specifications vs. our primary measurements

The table below summarizes the measurements published by iFi Audio for the Zen Phono compared directly against our own. The published specifications are sourced from iFi Audio’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 output into 100k ohms (200k ohms balanced) and a measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channel.

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

Our primary measurements revealed the following using the unbalanced MC Low setting (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC Very Low setting (unless specified, assume a 1kHz sinewave, 1Vrms output into a 100k ohms load, 10Hz to 90kHz bandwidth):

Frequency response RIAA - MM setting

In our measured frequency-response plot above for the MM setting measured at the unbalanced output, the Zen is within +/-0.1dB or so of flat from 5Hz to 50kHz, and about -2dB down at 80kHz, meeting iFi audio’s first claim of 20Hz-20kHz (+/-0.15dB), but not meeting the second claim of 10Hz-100kHz (+/-0.4dB). 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. In our results, the Zen Phono has essentially perfect RIAA accuracy from 5Hz to 50kHz. The purple/green (left/right) traces represent the frequency response with the subsonic filter engaged, where we see -3dB at around 5Hz. In the graph above and some of the graphs below, we see two visible traces: the left channel (blue, purple or pink trace) and the right channel (red, green or orange trace). 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 RIAA - MC Low and MC Very Low settings

In our measured frequency-response plot above for the MC Low/MC Very Low settings (they performed identically) measured at the unbalanced output, the Zen is within +/-0.1dB or so of flat from 20Hz to 20kHz, meeting iFi Audio’s first claim of 20Hz-20kHz (+/-0.15dB). We can see that the MM configuration offers a more extended bandwidth, where these MC configurations are at -1.5dB at 5Hz and -0.7dB at 50kHz. The purple/green (left/right) traces represent the frequency response with the subsonic filter engaged, where see a -3dB point at around 7Hz.

Phase response - MM, MC Low, and MC Very Low settings

Above is the phase response of the Zen for three input settings (they measured effectively identically) measured at the unbalanced output, from 20Hz to 20kHz. The purple/green traces represent the measured phase shift with the subsonic filter engaged. The Zen 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 -55 degrees at 5kHz. With the subsonic filter engaged, there’s a less than a 20 degree difference in phase shift at 20Hz.

THD ratio (unweighted) vs. frequency - MM, MC Low, and MC Very Low settings

The chart above shows THD ratio 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 (left/right) traces represent the MM configured input, purple/green MC Low, and pink/orange MC Very Low. For the MM configuration, THD values at 20Hz are at 0.005%, then dip as low as 0.0004% around 1kHz, then up to 0.003% at 20kHz. For the MC Low configuration, THD values at 20Hz are at 0.01%, then dip as low as 0.003% around 100Hz, then a steady climb to 0.3% at 20kHz. For the MC Very Low configuration, THD values at 20Hz are at around 0.04%, then dip as low as 0.004% between 500Hz and 1kHz, then up to 0.025% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz - MM, MC Low, MC Very settings

Above we can see a plot of THD ratios as a function of output voltage for the unbalanced output. The red/blue (left/right) traces represent the MM configured input, purple/green MC Low, and pink/orange MC Very Low. For the MM configuration, THD values at 100mVrms are at 0.003%, then dip as low as 0.0003% between 1 and 3Vrms, then the “knee” around 8-9Vrms, where THD values reach 0.003%. For the MC Low configuration, THD values at 100mVrms are at 0.005%, then steadily increase up to the 1% mark at 10Vrms, where THD vs. output values for all input configurations meet. For the MC Very Low configuration, THD values at 100mVrms are near 0.02%, then dip as low as 0.003% around 1Vrms, then the “knee” around 8-9Vrms, where THD values reach 0.03%. 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, MC Low, and MC Very Low settings

Above we can see a plot of THD+N ratios as a function of output voltage for the unbalanced output. The red/blue (L/R) traces represent the MM configured input, purple/green MC Low, and pink/orange MC Very Low. For the MM configuration, THD+N values at 100mVrms are at 0.15%, then dip as low as 0.002% between 6 to 8Vrms, then the “knee” around 9Vrms. For the MC Low configuration, THD+N values at 100mVrms are at 0.2%, then dip as low as 0.03% around 1Vrms, then rise steadily to the “knee” near 10Vrms, where the 1%THD+N values can be seen. For the MC Very Low configuration, THD+N values at 100mVrms are at around 0.7%, then dip as low as 0.02% between 3 to 5Vrms, then the “knee” around 9Vrms, where THD+N sits at 0.03%.

FFT spectrum, 1kHz - MM setting

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MM setting, which results in the reference voltage of 1Vrms at the unbalanced output. Here we see exceptionally clean results. Signal harmonics are at -120dBrA or 0.0001% and below. The odd order harmonics (i.e., 3kHz, 5kHz) are slightly higher in amplitude than the 2kHz even order second harmonic. On the left side of the signal peak, there are no peaks due to power supply noise above the noise floor, which ranges from -90dBrA, at 20Hz to -120dBrA at 1kHz.

FFT spectrum, 1kHz - MC Low setting

Shown above is an FFT of a 1kHz input sinewave stimulus for the MC Low setting at the unbalanced output. The 2kHz second-order signal harmonic peak is at -75dBrA, or 0.02%%, while the 3kHz third-order harmonic is at -85dBrA, or 0.006%. On the left side of the signal peak, we see the power-supply noise fundamental peak (60Hz) at -90/-85dBrA (L/R) or 0.003/0.006%, and just barely above the noise floor, we see the second- and third-harmonic noise peaks (120Hz, 180Hz), just above -100dBrA, or 0.001%.

FFT spectrum, 1kHz - MC Very Low setting

Shown above is an FFT of a 1kHz input sinewave stimulus for the MC Very Low setting at the unbalanced output. Despite the extra gain compared to the MC Low setting, this setting yields far less distortion, although predictably, with a higher noise floor. The 2kHz second-order signal harmonic peak is at -90dBrA, or 0.003%, while the 3kHz third-order harmonic is practically nonexistent. On the left side of the signal peak, we see the power-supply noise fundamental peak (60Hz) at -75/-70dBrA (left/right), or 0.02/0.03%, and just barely above the noise floor, we see the second and third harmonic noise peaks (120Hz, 180Hz), right around -90dBrA, or 0.003%.

FFT spectrum, 50Hz - MM setting

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the unbalanced output for the MM setting. 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 nonexistent above the noise floor, as are the power-supply noise peaks.

FFT spectrum, 50Hz - MC Low setting

The chart above is the FFT for a 50Hz input sinewave stimulus measured at the unbalanced output for the MC Low setting. The second harmonic from the 50Hz signal (100Hz) is at -90dBrA, or 0.003%, so right around the same level as the 60Hz noise peak (left channel). Subsequent signal harmonics cannot be seen above the noise floor.

FFT spectrum, 50Hz - MC Very Low setting

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the unbalanced output for the MC Very Low setting. The harmonics from the 50Hz signal cannot be seen above the noise floor, while the 60Hz noise peak is again visible at -75/-70dBrA (left/right), or 0.02/0.03%.

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

Above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM setting measured at the unbalanced output. 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 -85dBrA, or 0.006%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) sitting at -110/-105dBRa (left/right), or 0.0003/0.0006%. The fourth and fifth modulation products are also clearly visible.  

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

This chart is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC Low setting. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) is quite high at -40dBrA, or 1%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are also high, at -50dBRa, or 0.3%. These IMD FFTs are reflected in our simplified IMD results (which only account for the sum of the second- and third-order modulation products) in our primary measurement table, where the MM setting measured a respectable -76dB, or 0.015%, but the MC Low setting yielded only -33dB, or 0.02%.

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

This chart is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC Very Low setting. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at around -55dBrA, or 0.2%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are much lower, at -75dBRa, or 0.02%.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 May 2021

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on May 15, 2021

General information

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

The iPhono3 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The iPhono3 has one switch and one unbalanced pair of RCA outputs on one end, while the other end has two pairs of unbalanced RCA inputs for connection of a moving-magnet (MM) or moving-coil (MC) cartridge. The switch allows the user to select between RIAA, Columbia, and Decca EQ settings. There are a number of DIP switches underneath, allowing the user to alter MC resistive loading, MM capacitive loading, gain, and variations on EQ (i.e., enhanced RIAA and standard with and without subsonic filter).

For the MM input, capacitive loading was set to 100pF and the gain set to 36dB, which required a 16.2mVrms 1kHz sinewave to achieve the reference output voltage. For the MC input, resistive loading was set to 100 ohms and the gain set to 60dB, which required a 1.18mVrms 1kHz sinewave to achieve the reference output voltage

Published specifications vs. our primary measurements

The table below summarizes the measurements published by iFi Audio for the iPhono3 Black Label compared directly against our own. The published specifications are sourced from iFi Audio’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 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.

Our primary measurements revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 1Vrms output in 100k ohms, 10Hz to 90kHz bandwidth):

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

Frequency response RIAA - MM input

In our measured frequency-response plot above for the MM input, the iPhono3 is at worst +0.7dB (left channel) or so of flat from 20Hz to 20kHz, not quite meeting iFi’s claim of +/-0.2dB. 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 claim of +/-0.3dB from 10Hz to 100kHz was also not corroborated by our measurements, where at 80kHz, the MM input was at about +2dB. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, which 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 RIAA - MC input

In our measured frequency-response plot above for the MC input, the iPhono3 is at worst +0.7dB (left channel) or so of flat from 20Hz to 20kHz, not quite meeting iFi’s claim of +/-0.2dB. The claim of +/-0.3dB from 10Hz to 100kHz was also not corroborated by our measurements, where at 80kHz, the MC input was at about -2.5dB.

Frequency response for RIAA, Columbia, and Decca settings - MM input (no EQ applied to input sweep)

Above is the raw (no EQ applied in the Audio Precision generator) frequency response of the iPhono3 using the RIAA (blue), Columbia (green) and Decca (burgundy) EQ settings for the MM input with a 0.5mVrms sinewave input swept from 10Hz to 80kHz (the results for the MC input were effectively identical). We find deviations of about 5 dB at 20Hz, and 3dB at 20kHz between all three EQ settings.

Phase response - MM and MC inputs

Above is the phase response of the iPhono3 for both the MM and MC inputs (they measured effectively identically), from 20Hz to 20kHz. 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 6kHz. Because the worst case is -60 degrees, that indicates that the iPhono3 does not invert polarity. 

THD ratio (unweighted) vs. frequency - MM input

Above is the THD ratio as a function of frequency chart for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the reference 1Vrms. Since iFi provides specs for 600-ohm loading, here we show data for a typical 100k ohms load (blue/red), and for a 600 ohms load (purple/green). The iPhono3 performed identically with either load. The THD values vary from 0.01% at 20Hz, down to below 0.001% from 100Hz to 200Hz, then back up just above 0.02% from 2kHz to 20kHz.

THD ratio (unweighted) vs. frequency - MC input

Above is the THD ratio as a function of frequency chart for the MC input. The output voltage is maintained at the refrence 1Vrms. Since iFi provides specs for 600 ohm loading, here we show data for a typical 100k ohms load (blue/red), and for a 600 ohms load (purple/green). The THD values vary from 0.1% at 20Hz, down to 0.004% at around 100Hz, then a steady near linear climb to 0.3% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above is the chart of THD ratio as a function of output voltage for the MM input. We can see very low THD ratio values, ranging from as low as 0.002% at 150mVrms, up to about 0.06% at 4Vrms. Beyond this point there is sharp rise in THD. The 1% THD ratio value is reached at 6Vrms at the output.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms) - MM input

Above we can see the plot of THD+N ratio as a function of output voltage for the MM input. We can see THD+N ratio values ranging from 0.2% at around 50mVrms, down to about 0.02% at 1Vrms.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 12mVrms) - MC input

Above is the THD ratio as a function of output voltage for the MC input. The MC input behaved almost identically to the MM input, with the lowest THD values recorded also around 150mVrms, at around 0.004%.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 12mVrms) - MC input

Above we can see the plot of THD+N ratio as a function of output voltage for the MC input. We can see THD+N ratio values ranging from 1.5% at around 50mVrms, down to about 0.05% between 2 and 3Vrms.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier Transform (FFT) of a 1kHz input sinewave stimulus for the MM input, which results in the reference output voltage of 1Vrms. Here we see that the third signal harmonic at 3kHz is close to -75dB below the reference signal, or at -75dBrA, equivalent to 0.02%. The second harmonic at 2kHz is non-existent. The frequency components on the left side of the 1kHz peak are mostly due to power supply noise, where the typical 60Hz and 120Hz peaks can be seen. The worst noise peak (60Hz) is at around -80dBrA, or 0.01%, with the subsequent harmonic (120Hz) at around -90dBrA, or 0.003%. The third (180Hz) and fourth (240Hz) harmonics are just above -100dBrA, or 0.001%.

FFT spectrum, 1kHz - MC input

Shown above is the FFT of a 1kHz input sinewave stimulus for the MC input. Here we see that the second and third signal harmonics (2 and 3kHz) are close to -75dBrA, or 0.02%. The 60Hz peak is at -60dBrA, or 0.1%; the second noise harmonic (120Hz) is close to -70dBrA, or 0.03%; and the third (180Hz) and fourth (240Hz) harmonics are just above -80dBrA, or 0.01%.

FFT spectrum, 50Hz - MM input

Shown above is the FFTs for a 50Hz input sinewave stimulus measured at the output for the MM input. 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 second harmonic of the 50Hz signal (100Hz) is non-existent. What dominate are the power supply noise peaks, which were described in the 1kHz FFT above.

FFT spectrum, 50Hz - MC input

Shown above is the FFTs for a 50Hz input sinewave stimulus for the MC input. The second harmonic of the 50Hz signal (100Hz) is at -90dBrA, or 0.003%. What dominate are the power supply noise peaks, which were described in the 1kHz FFT above.

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

Shown above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM input. 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 almost -100dBrA, or 0.001%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) measure at around -85dBrA, or 0.006%.

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

This chart shows an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC input. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) at around -40dBrA, or 1%. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are also considerably different between the MM and MC inputs, where the MM peaks measure at around -85dBrA, or 0.006%, and the MC peaks are just under -50dBrA, or 0.3%. These differences are reflected in our simplified IMD results (which only account for the sum of the second- and third-order modulation products) in our primary measurement table, where the MM input outperformed the MC input by 36dB.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 15 January 2021

Link: reviewed by Jason Thorpe on SoundStage! Hi-Fi on January 15, 2021

General information

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

The V10 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The V10 offers one pair of unbalanced RCA outputs and one pair of balanced XLR outputs, as well as two pairs of unbalanced RCA inputs, one pair for moving-magnet (MM) and the other for a moving-coil (MC) cartridge. There are a number of DIP switches on the rear panel of the V10, allowing the user to alter MC resistive loading, MM capacitive loading, and gain, as well to turn on or off a subsonic filter. Other than the 6dB difference in gain between the balanced and unbalanced outputs (+6dB for balanced), there were no mentionable differences between both outputs types in terms of THD+N, as long as the gain was kept constant. Settings were left at the manufacturer’s default positions (see specs below). To achieve the reference output voltage of 1Vrms at 1kHz at the balanced output, 10mVrms was required at the MM input and 1mVrms at the MC input.

Published specifications vs. our primary measurements

The tables below summarize our primary measurements performed on the V10. Here we can compare directly against Hegel’s own published specifications for the V10, which are stated as follows:

• Gain (XLR): MM, 40/45/50/52dB; MC, 60/65/70/72dB (default = 40/60dB for MM/MC)
• Gain (RCA): MM, 34/39/44/46dB; MC, 54/59/64/66dB (default = 34/54dB for MM/MC)
• MC load impedance: 100/300/47k ohms (default = 100 ohms)
• MM load capacitance: 100/147/200/220/247/320/420pF at 47k ohms (default = 100pF)
• Subsonic filter: on, -3dB at 20Hz; off, -18dB/octave
• RIAA accuracy: +/-0.1dB 20Hz to 20kHz
• Output noise: -84dB (MM, A-weighted, ref 1Vrms), -81dB (MC, A-weighted, ref 1Vrms)
• Output impedance: XLR and RCA, 200 ohms
• Channel crosstalk: -84dB @ 1kHz (ref 1Vrms)
• Frequency response: 2Hz to 20kHz
• THD: MM, <0.005%; MC, <0.009% at 1kHz ref 1Vrms
• Maximum output voltage: 23Vrms

Our primary measurements revealed the following using the unbalanced MM input (unless specified, assume a 1kHz sinewave, 1Vrms at the balanced output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the unbalanced MC input (unless specified, assume a 1kHz sinewave, 1Vrms at the balanced output into a 200k ohms load, 10Hz to 90kHz bandwidth):

Gain was measured at 40 and 60dB (Balanced, MM/MC), the same as Hegel’s spec. The MC load input impedance was measured at 124 ohms, closely matching Hegel’s spec and setting of 100 ohms. The input impedance for the MM input was measured at 47 and 46k ohms (L/R), very close to the industry standard 47k ohms.

Output noise (A-weighted) was measured at -85dB (MM) and -79dB (MC), either exceeding or approaching Hegel’s specs of -84/-81dB (MM/MC). Our measured output impedance of 200 ohms confirmed Hegel’s spec of the same value.

Our measured crosstalk values at 10kHz were close to Hegel’s spec of -84dB (at 1kHz), where we measured -83dB for both channels for the MM input and -88/-95dB (left and right channels) for the MC input. For a direct comparison, we also measured crosstalk at 1kHz, and found -98/-100dB (L/R channels, MM input), and -88/-90dB (L/R channels, MC input), bettering the -84dB Hegel spec.

Hegel’s claim of better than 0.005% (MM) and 0.009% (MC) THD was also verified. There are many ways to measure THD, and manufacturers are often shy on showing all of the parameters used for the measurement. Here, we assume that Hegel is referring to THD (not THD+N), unweighted, against a 1Vrms 1kHz output signal. The bandwidth for Hegel’s measurement is unknown, but we use 10Hz to 90kHz for our measurements. Under those conditions, we found the V10’s THD to be below 0.0008% (MM) and 0.001% (MC). Even if Hegel’s THD spec is actually THD+N (A-weighted), they still meet spec for the MM input, as we measured 0.005%, and come very close for the MC input at 0.01%.

Hegel’s maximum output voltage (1% THD+N) claim of 23Vrms (balanced) was also confirmed, where we measured 24Vrms.

Frequency response - MM input

The blue (left channel) and red (right channel) traces represent the frequency response without the subsonic filter turned on. In our measured frequency-response plot above for the MM input, the V10 is within +/-0.2dB or so of flat from 20Hz to 20kHz, just about meeting Hegel’s RIAA accuracy claim of +/-0.1dB. 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 V10 has nearly perfect RIAA accuracy from 5Hz to 80kHz. The purple (left channel) and green (right channel) traces represent the frequency response with the subsonic filter engaged, where Hegel’s claim of -3dB at 20Hz is confirmed. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). On other graphs, only one trace may be visible, which 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 input shown with the blue and red traces (left and right channels), the V10 is within +/-0.2dB or so of flat from 20Hz to 20kHz, just about meeting Hegel’s RIAA accuracy claim of +/-0.1dB. The worst-case deviation was seen at around 8Hz, were the V10 over-responded by 0.5dB. The purple and green trace (left and right channels) represent the frequency response with the sub-sonic filter engaged, where Hegel’s claim of -3dB at 20Hz is confirmed.

Phase response - MM and MC inputs

Above is the phase response of the V10 for both the MM and MC inputs (they measured effectively identically), from 20Hz to 20kHz. The V10 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 -55 degrees at 5kHz.

THD ratio (unweighted) vs. frequency - MM input

The chart above shows THD ratio as a function of frequency for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The output voltage is maintained at the refrence 1Vrms. The THD values vary from 0.01% at 20Hz, down to below 0.0001% at 10kHz, then back up just above 0.0003% at 20kHz.

THD ratio (unweighted) vs. frequency - MC input

The chart above shows THD ratio as a function of frequency for the MM input, where the input sweep is EQ’d with an inverted RIAA curve. The THD values vary from 0.04% at 20Hz, down to around 0.0004% at 5kHz, then a climb to 0.001% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 30mVrms) - MM input

Above we can see a plot of THD ratios as a function of output voltage for the MM input. We can see very low THD ratio values, ranging from as low as 0.0001% at 5Vrms, up to about 0.0006% at the “knee”, just past 20Vrms, and about 0.005% at the lowest output voltage of 100mVrms. Beyond the “knee” there is sharp rise in THD as the V10 reaches its maximum output voltage. 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 (input voltage from 1mVrms to 100mVrms) - MM input

Above we can see a plot of THD+N ratios as a function of output voltage for the MM input. We can see THD+N ratio values, ranging from 0.2% at 100Vrms, down to about 0.0015% between 15 and 20Vrms.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 30mVrms) - MC input

Above we can see a plot of THD ratios as a function of output voltage for the MC input. The MC input behaved similarly to the MM input, with 0.007% THD at 100mVrms, dipping to the lowest THD value of 0.0005% at 2Vrms, then up just past 0.003% at the knee at 20Vrms. The 1% THD ratio value is reached at 24Vrms at the output.

THD+N ratio (unweighted) vs. output voltage at 1kHz (input voltage from 0.1mVrms to 30mVrms) - MC input

Above we can see a plot of THD+N ratios as a function of output voltage for the MC input. We can see THD+N ratio values ranging from around 0.5% at 100Vrms, down to about 0.005% between 15 and 20Vrms.

FFT spectrum, 1kHz - MM input

Shown above is a fast Fourier transform (FFT) of a 1kHz input sinewave stimulus for the MM input, which results in the reference output voltage of 1Vrms. Here we see exceptionally clean results. Signal harmonics are non-existent (i.e., cannot be seen above the -120dB noise floor at 2kHz). The frequency components on the left side of the 1kHz peak are mostly due to power-supply noise, where the typical 60Hz and 120Hz peaks can be seen. The worst noise peak (60Hz) is below -80dBrA, with the subsequent harmonic (120Hz) at around -90dBrA. The third harmonic (180Hz) is just below -90dBrA, and the subsequent harmonics are below -100dBrA.

FFT spectrum, 1kHz - MC input

Shown above is the FFT of a 1kHz input sinewave stimulus for the MC input. Signal harmonics are virtually non-existent, only a hint of a peak can be seen below -110dB at 2kHz. The 60Hz and 120Hz peaks are just below -70dBrA, and the third and fourth noise harmonics are just below -80dBrA.

FFT spectrum, 50Hz - MM input

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output for the MM input. 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. The power supply noise peaks can clearly be seen, which were described in the 1kHz FFT chart above.

FFT spectrum, 50Hz - MC input

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output for the MC input. Like the MM FFT, here again, the harmonics from the 50Hz signal (100, 150, 200Hz, etc.) are non-existent. The power-supply noise peaks can clearly be seen, which were described in the 1kHz FFT above.

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

The chart above is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MM input. 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) just above -100dBrA. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are extremely low, below -120dBRa.

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

The next chart is an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone for the MC input. Here we find the second-order modulation product (i.e., the difference signal of 1kHz) just at -80dBrA, which is very low. We can also see the third-order modulation products (i.e., 17kHz and 20kHz) are extremely low, below -120dBRa. These clean IMD FFTs are reflected in our simplified IMD results (which only account for the sum of the second- and third-order modulation products) in our primary measurement table, where the MM input measured at -92dB for both channels and the MC input at -76/-73dB, for, respectively, the left and right channels. Incidentally, the 3dB difference between left and right channels for the MC input can be seen in the 1kHz peak in the FFT chart above. The fourth-order modulation products are non-existent in this FFT and the one for the MM input.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Phono Preamplifier Measurements

Created: 01 January 2021

Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on January 1, 2021

General information

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

The Bellari VP549 was conditioned for 30 minutes at 1Vrms at the output before any measurements were taken.

The VP549 is designed for moving-magnet (MM) cartridges only and has one set of single-ended RCA inputs and outputs. There are two switches and one knob on the front panel of the VP549. One switch selects between 120pF, 220pF, and 330pF cartridge-loading capacitance. The other switch, Rumble Filter, enables a low-frequency filter that provides attenuation below 20Hz. The knob controls the preamp’s overall gain. The center position is labelled “0,” the minimum position “-10dB,” and the maximum position “+4dB.” Unless otherwise stated, measurements were performed with the cartridge load capacitance set to 220pF, the Rumble Filter switch disengaged, and the gain set to the default “0” position. An 8mVrms 1kHz sinewave was required to achieve the reference output voltage of 1Vrms.

Published specifications vs. our primary measurements

The table below summarizes our primary measurements performed on the VP549. Here we can compare directly against Bellari’s own published specifications for the VP549, which are stated as follows:

• Gain: 41.51dB gain @ 1kHz at the "0" setting, 31.64dB of gain at the -10 setting, 45.66dB of gain at the +4 setting
• Input Impedance: 47k ohms
• Output Impedance: 470 ohms
• THD: 0.005% @ 1kHz
• S/N Ratio: >94dB unweighted
• Equalization: RIAA +/-1dB, 14Hz to 23kHz

Our primary measurements revealed the following (unless specified, assume a 1kHz sinewave, 1Vrms output into 100k ohms, 10Hz to 90kHz bandwidth, gain set to 0):

Our measured input (47.8/46.9k ohms, L/R) and output (460.5/460.8 ohms, L/R) impedances match very closely Bellari’s specs of 47k ohms and 470 ohms. Our measured gain values at all three settings also corroborate the published specifications, with maximum deviations in the order of 2-3%.

For the remainder of the specifications, as is often the case with manufacturer supplied measurements, not enough information is given to directly reproduce their results. With respect to Bellari’s THD of 0.005%, the company has not specified a reference output voltage, gain setting, weighting, or bandwidth filter. Our THD measurement shows an even lower <0.0012% (unweighted) against a 1Vrms output voltage, whereas our THD+N figure (A-weighted) is <0.0063% for both channels.

Bellari’s signal-to-noise ratio (SNR) of >94dB (unweighted) seems generous, however, again, we have very little information provided with respect to measurement parameters. The most charitable SNR measurement we could achieve was with the gain setting at -10dB and the output voltage at 1.35 Vrms (1 kHz), where we measured 95dB (A-weighted). Our reference SNR measurement (1Vrms output, gain set to 0) is 83dB (A-weighted), or 60dB (unweighted, input bandwidth filter set from 20Hz to 20kHz).

Frequency response

The chart above shows our frequency response measurement. 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 blue and red curves are the left and right channel without the Rumble Filter switch engaged, the green and purple curves with the Rumble Filter switch engaged. The measurement was also made with the gain set to its minimum and maximum values, and with every cartridge load capacitance setting, which had no effect on the frequency-response results. We can see here that Bellari’s claim of +/-1dB is valid at the top end of the spectrum (up to 23kHz as stated), but the +/-1dB claim down to 14Hz was not corroborated by our measurement. We find a -5.3dB dip at 30Hz, and at 14Hz, down -5.5dB again. In the graph above and some of the graphs below, we see two visible traces; the left channel (blue or purple trace) and the right channel (red or green trace). 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.

Phase response

Shown above is the phase response from 20Hz to 20kHz. The VP549 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 +20 degrees at 20Hz, and just below -60 degrees between 5 and 10kHz.

THD ratio (unweighted) vs. frequency

The chart above is the THD ratio 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 THD values vary from 0.05% at 20Hz, down to below 0.001% at 1kHz, then back up to 0.007% at 20kHz.

THD ratio (unweighted) vs. output voltage at 1kHz (input voltage from 1mVrms to 100mVrms)

Above we can see a plot of THD ratio as a function of output voltage. We can see very low THD ratio values, ranging from 0.005% at 100mVrms down to below 0.001% at around 1Vrms at the output, then a sharp rise in THD at the “knee” at around 3.5Vrms at the output. Beyond this output voltage, the VP549 distortion begins to rise exponentially. The 1% THD ratio value is reached at around 4Vrms 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 (input voltage from 1mVrms to 100mVrms)

Above we can see a plot of THD+N ratio as a function of output voltage. At 100mVrms, THD+N values are at around 3%, then down to between 0.1 and 0.2% at 3Vrms. There is a considerable difference in the THD+N vs. THD plots for the VP549, where it’s clear that distortion products are low, but noise, as shown in this chart, is relatively high.

FFT spectrum, 1kHz

Shown above is a fast Fourier transform (FFT) of a 1kHz 8mVrms input sinewave stimulus, which results in the reference output voltage of 1Vrms with the gain set to 0. Here we see that the second harmonic at 2kHz is <-100dB below the reference signal (dBrA). The frequency components on the left side of the 1kHz peak are mostly due to power supply noise. Of note is the peak at 60Hz at a level near -70dBrA.

FFT spectrum, 50Hz

The chart above is an FFT of a 50Hz input sinewave stimulus, which results in the reference output voltage of 1Vrms with the gain set to 0. 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 no perceptible signal harmonic peaks (e.g., 100Hz, 150Hz) above the relatively high noise floor. The highest noise peak is from the primary source (60Hz) at about -75dBrA, while the next highest peak is from the third harmonic noise peak (180Hz) at about -85dBrA.

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

This final chart shows an FFT of the IMD products for an 18kHz and 19kHz summed sinewave stimulus tone. 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. Although it appears that the second-order modulation product (i.e., the difference signal of 1kHz) is sitting at -105dBrA, the peak is actually at 1020Hz, which is the 17th harmonic from the 60Hz power-supply noise. The 1kHz IMD peak is imperceptible above the noise floor. The third-order modulation products (i.e., 17kHz and 20kHz) are just above -110dBra, and the subsequent modulation products are not noticeable.

Diego Estan
Electronics Measurement Specialist