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Parent Category: Products

Category: Subwoofer Measurements

Created: 15 February 2018

I measured the frequency response of the Monoprice Monolith THX Ultra 15” (product no. 24458) with an Audiomatica Clio FW 10 audio analyzer and MIC-01 measurement microphone, and in two different ways: the ground-plane technique, with the microphone on the ground 2m in front of the sub, and the result smoothed to 1/6 octave; and the close-miked technique, with the mike placed as close as possible (about 1/4”) to the woofer and ports, and the port responses scaled and summed with the woofer response. I show the close-miked results here because those graphs are clearer; the ground-plane results were within a couple of Hz of them. For the power-compression measurement, I placed the mike on the ground 2m from the front of the sub.

I performed CEA-2010 measurements using an Earthworks M30 mike and M-Audio Mobile Pre USB interface, with the CEA-2010 measurement software running on the Wavemetric Igor Pro scientific software package. Measurements recorded peak output at 2m. (For more information about CEA-2010, see this article.)

The two sets of measurements presented here -- CEA-2010 and the traditional method -- are essentially the same. CEA-2010 mandates that no matter how the sub is measured, the results must be scaled to the equivalent of a measurement at 1m distance using peak values. But the traditional measurement technique used by some audio websites and manufacturers reports results at 2m RMS equivalent, which is -9dB lower than CEA-2010. An “L” next to the result indicates that the output was dictated by the subwoofer’s internal circuitry (i.e., limiter), and not by exceeding the CEA-2010 distortion thresholds. Averages are calculated in pascals.

This chart shows the Monolith THX Ultra’s frequency response with the crossover frequency set to maximum and the sub set to Extended (rather than THX) mode. I’ll show the effects of the crossover and the THX mode in the next graph. You can see that the bass output gradually rises as more ports are opened, and that the response is pretty much flat up to 200Hz. With two and three ports open, the -3dB point (using the peak of the sub’s response curve as that +3dB reference point) is 14Hz. Even in sealed mode, it hits 16Hz.

This chart shows the response of the crossover and the effect of the THX mode, measured with all of the sub’s ports sealed. The crossover frequency was 80Hz, and, as the chart shows, the control is accurately calibrated (not usually the case), and the low-pass function is about -22dB/octave. The THX mode reduces bass output by about 4dB at 20Hz.

This chart shows how the Monolith THX Ultra’s frequency response is affected by increases in volume. I measured this with all three ports open in THX mode, starting at 106dB at 2m, calibrated at 63Hz, then raised the level 3dB for each successive measurement. You can see that the Monolith’s frequency response doesn’t change as it reaches its output limits; with many subs -- especially those with limiters that are set with higher thresholds, which allow greater distortion -- the bass response begins to weaken as the sub reaches the limits of its capabilities. Unfortunately, I had to return the sub in a hurry due to an upcoming trip, and didn’t have time to measure its output in the Extended (non-THX) mode, but based on the frequency-response measurements and what I heard, I expect the Extended-mode output measurements would average about 2dB higher than THX mode.

Please note that if you haven’t seen subwoofer distortion numbers before and are used to looking at amplifier distortion specs, some of these may look high. But in loudspeakers, and especially subwoofers, much higher distortion levels are the norm, and typically are inaudible. The generally accepted threshold for the audibility of total harmonic distortion in subwoofers is 10%, and CEA-2010 thresholds permit a maximum THD of around 30%.

The Monolith THX Ultra’s CEA-2010 output numbers are excellent -- among the best I’ve measured for a sub of this size and configuration.

This chart tracks the CEA-2010 results of the Monoprice Monolith THX Ultra (blue trace) compared with three other ported subs that are somewhat comparable: two 15” models (Hsu Research VTF-15H Mk.2 and Klipsch R-115SW), and one 13” model (SVS PC13-Ultra). The Monoprice doesn’t have quite as much output at higher bass frequencies as some models, but has more deep-bass output than the three other models, and a more consistent maximum output throughout the two octaves covered in the chart.

. . . Brent Butterworth
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Subwoofer Measurements

Created: 15 February 2018

I measured the Adante SUB3070’s frequency response using an Audiomatica Clio FW 10 audio analyzer with the MIC-01 measurement microphone. For the frequency-response measurement I used the close-miked technique, with the mike placed as close as possible (about 1/4”) to one of the woofers. For the power-compression measurement I placed the mike on the floor, 2m in front of the sub.

I performed CEA-2010 measurements using an Earthworks M30 mike and M-Audio Mobile Pre USB interface, with the CEA-2010 measurement software running on the Wavemetric Igor Pro scientific software package. Measurements recorded peak output at 2m.

The two sets of measurements I’ve presented here -- CEA-2010 and the traditional method -- are essentially the same. CEA-2010 mandates that no matter how the sub is measured, the results must be scaled to the equivalent of a measurement taken at a distance of 1m using peak values. But the traditional measurement technique used by some audio websites and manufacturers reports results at an RMS equivalent of 2m, which is 9dB lower than CEA-2010. An L next to the result indicates that the output was dictated by the subwoofer’s internal circuitry (i.e., limiter), and not by exceeding the CEA-2010 distortion thresholds. Averages are calculated in pascals. (For more information about CEA-2010, see this article.)

This chart shows the SUB3070’s frequency response with the crossover frequency set to maximum and the sub set for its Flat, Cinema, Night, and Music modes. Flat mode is indeed almost perfectly flat from 30 to 130Hz. Night mode basically lowers the output by about 3dB. Cinema mode boosts output by a maximum of about 3dB, centered at 80Hz. Music mode does the same, but centers the boost at 40Hz. The -3dB point (using the peak of the sub’s response curve as the +3dB reference point) is 18Hz, and the low-pass function of the SUB3070’s crossover is -24dB/octave.

This chart shows the effects of the auto EQ processing with the SUB3070 placed in the corner of my listening room -- not the best spot for a single subwoofer if you want flat response, but it gives the auto EQ circuit a tougher challenge. The mike was placed near my listening position, about 1’ from my head; I placed the smartphone in the same position when I ran the auto EQ. In this case, the auto EQ processing seems to be making some pretty smart adjustments, flattening the response in general and ignoring the suckout at 73Hz, which is impossible for EQ to fill because it’s a cancellation -- the more energy you pump into it, the more will be canceled. Still, I was able to get a flatter curve using the parametric EQ function.

This chart shows how the SUB3070’s frequency response (measured here in Flat mode from 2m) is affected by increases in volume. I measured starting at 94dB, calibrated at 63Hz, then raised the level 3dB for each successive measurement. You can see that the sub’s internal limiter seems to be most restrictive between 30 and 60Hz.

If you’re used to looking at amplifier distortion specs, some of these may look high. But in loudspeakers, and especially subwoofers, much higher distortion levels are the norm, and typically are not audible. The generally accepted threshold for audibility of distortion in subwoofers is 10% THD; CEA-2010 thresholds permit maximum distortion of around 30% THD.

The output of the SUB3070 at 63Hz is, to the best of my recollection, the highest I’ve measured from a sub of this size. However, from there it falls rather quickly, albeit smoothly. Clearly, the SUB3070 is no home-theater bruiser; it focuses more on fidelity with typical music content, which seldom has much going on below 40Hz.

This chart tracks the CEA-2010 results of the SUB3070 (blue trace) and three other subwoofers that are to some extent comparable, though all are less expensive. While at 63Hz the SUB3070 beats even the mighty SVS PC13-Ultra, its bottom-octave output is more akin to that of a typical, less-expensive 12” model.

. . . Brent Butterworth
brentb@soundstagenetwork.com

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Parent Category: Products

Category: Subwoofer Measurements

Created: 01 August 2017

I measured the BasX S12’s frequency response using an Audiomatica Clio FW 10 audio analyzer with the MIC-01 measurement microphone. For the frequency-response measurement I used the ground-plane technique, with the microphone on the ground 2m in front of the subwoofer, and smoothed the result to one-sixth of an octave. For the power-compression measurement, I placed the mike on the ground 1m in front of the sub.

I performed CEA-2010 measurements using an Earthworks M30 mike and M-Audio Mobile Pre USB interface, with the CEA-2010 measurement software running on the Wavemetric Igor Pro scientific software package. Measurements recorded peak output at 2m.

The two sets of measurements presented in the Maximum Output table are essentially the same, just scaled differently to suit the two different reporting methods in common use for subwoofer output measurements. The CEA-2010 standard mandates reporting at 1m peak output, while the traditional reporting standard used by some audio websites and manufacturers reports results at 2m RMS equivalent. Thus, the CEA-2010 numbers are 9dB higher than the numbers presented under the traditional reporting standard. An L next to the result indicates that the output was dictated by the subwoofer’s internal circuitry (i.e., limiter), and not by exceeding the CEA-2010 distortion thresholds. Averages are calculated in pascals.

This chart shows the BasX S12’s frequency response with its crossover-frequency control set to maximum and to approximately 80Hz. You can see a small peak in the response at about 62Hz. This peak (which also showed up, to a lesser degree, in close-miked measurements) is insignificant; its effects will be swamped by the much larger effects of room acoustics, or possibly eliminated if you use a receiver or surround processor with auto EQ. With the peak taken into account, the ±3dB response is 26-119Hz. If you ignore the peak, the response is 22-155Hz. The crossover rolloff is -17.8dB/octave, -5.0dB at the 80Hz setting, which means that this control is more accurately calibrated than most subwoofers’ crossover-frequency controls.

This chart shows how the BasX S12’s frequency response is affected by increases in volume. This is an excellent result -- the deep-bass output of most subwoofers is greatly reduced relative to midbass output at high levels. I measured this beginning at 100dB at 1m, calibrated at 63Hz, then raised the level 5dB for each successive measurement. Between 40 and 80Hz the level doesn’t increase significantly once it hits 110dB, though it does rise by a few more dB in the bass.

Please note that if you’re used to looking at amplifier distortion specs and haven’t seen subwoofer distortion numbers before, some of these may look high. But in loudspeakers, and especially subwoofers, much higher distortion levels are the norm, and typically are not audible. The generally accepted threshold for audibility of distortion in subwoofers is 10% THD, and CEA-2010 thresholds permit maximum distortion of around 30% THD.

This chart tracks the CEA-2010 results of the BasX S12 (blue trace), compared with three other subwoofers priced in the mid-three-figures: the Outlaw Ultra-X12 (red trace, max output mode, $659), the Rogersound Labs Speedwoofer 10S (orange trace, $399), and the SVS PB-2000 (green trace, $799.99). The BasX S12 has 2-3dB more output than the identically priced (but 25% smaller by volume) Speedwoofer 10S in the second octave of bass (40-63Hz), and about the same output in the bottom octave (20-31.5Hz). Not surprisingly, the larger, more expensive subs outperform the BasX S12, but one could buy two BasX S12s for the price of one PB-2000.

. . . Brent Butterworth
brentb@soundstagenetwork.com

Details

Parent Category: Products

Category: Subwoofer Measurements

Created: 01 May 2017

I measured the Ultra-X13’s frequency response using an Audiomatica Clio FW 10 audio analyzer with the MIC-01 measurement microphone. For the frequency-response measurement I used the ground-plane technique, with the microphone on the ground 2m from the front of the subwoofer, and smoothed the result to 1/6 octave. For the power-compression measurement, I placed the mike on the ground 1m from the front of the sub.

I performed CEA-2010 measurements using an Earthworks M30 mike and M-Audio Mobile Pre USB interface with the CEA-2010 measurement software running on the Wavemetric Igor Pro scientific software package. Measurements recorded peak output at 2m.

The two sets of measurements I’ve presented here -- CEA-2010 and the traditional method -- are essentially the same, but the traditional measurement technique used by some audio websites and manufacturers reports results at 2m RMS equivalent, which is -9dB lower than CEA-2010. An L next to the result indicates that the output was dictated by the subwoofer’s internal circuitry (i.e., limiter), and not by exceeding the CEA-2010 distortion thresholds. Averages are calculated in pascals. (For more information about CEA-2010, see this article.)

This chart shows the Ultra-X13’s frequency response in sealed and ported modes, and in sealed mode with its internal crossover set to 80Hz. The ±3dB response is 19-161Hz in sealed mode and 24-161Hz in ported mode. The crossover rolloff is -28dB/octave, -4.3dB at the 80Hz setting, which means this control is more accurately calibrated than most subwoofers’ crossover-frequency controls.

This chart shows how the Ultra-X13’s frequency response is affected by increases in volume. This is an excellent result; with most subwoofers, deep-bass output is greatly reduced relative to midbass output at high levels. I measured this starting at 100dB at 1m, calibrated at 63Hz, then raised the level 5dB for each successive measurement. The frequency response remains essentially consistent at all levels, although because of the limiter, output doesn’t increase much above 120dB (green trace) except at very low frequencies.

Please note that if you’re used to looking at amplifier distortion specs and haven’t seen subwoofer distortion numbers before, these may look high. But in loudspeakers, and especially subwoofers, much higher distortion levels are the norm, and typically are inaudible. The generally accepted threshold for audibility of distortion in subwoofers is 10% THD, and CEA-2010 thresholds permit maximum distortion of around 30% THD.

The Ultra-X13’s CEA-2010 output numbers are very good, and competitive with the leading subwoofers in its price range.

This chart tracks the CEA-2010 results of the Ultra-X13 in ported (blue trace) and sealed (orange trace) modes compared with: the Outlaw Ultra-X12 (red trace, maximum output mode), the SVS PC13-Ultra (purple trace, ported mode), and the Hsu Research VTF-15H Mk2 (green trace). The Ultra-X13 is roughly comparable in ported mode to the SVS PC13-Ultra in ported mode, with a couple dB less output in the middle bass and a couple dB more output in the deep bass.

. . . Brent Butterworth
brentb@soundstagenetwork.com

Details

Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 15 April 2024

Link: reviewed by Killain Jones on SoundStage! Solo on April 15, 2024

General information

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

The Bryston BHA-1 was conditioned for 30 minutes at 2Vrms at the output into 300 ohms before any measurements were taken. All measurements were taken with both channels driven.

The BHA-1 offers one set of unbalanced (RCA) inputs, one set of balanced inputs (XLR), and one mini stereo input (1/8″ TRS), which can be selected with a front panel switch. Outputs include one unbalanced headphone output (1/4″ female TRS) and two balanced headphone outputs over left/right three-pin XLR and a single stereo four-pin XLR. In addition, there are line-level balanced outputs on the rear panel so that the BHA-1 can be used as a conventional analog preamp. The front panel is adorned with a power switch, a volume control, a gain switch (high or low), an input selector and a balance control. Unless otherwise stated, measurements were made with the balanced inputs and outputs (four-pin XLR), gain set to high, with a 2Vrms output into a 300-ohm load.

Volume-control accuracy (measured at XLR outputs): left-right channel tracking

Based on the accuracy of the left/right volume channel matching (see table below), the BHA-1 volume control is likely a potentiometer operating in the analog domain.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Bryston for the BHA-1 compared directly against our own. The published specifications are sourced from Bryston’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, a 1kHz sinewave, 2Vrms input and 2Vrms output into a 300-ohm load, 10Hz to 22.4kHz bandwidth, gain set to 0dB, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms input and 2Vrms output into a 300-ohm load, 10Hz to 22.4kHz bandwidth, gain set to high):

Frequency response

In our frequency response (relative to 1kHz) plots above, measured into a 300-ohm load, the BHA-1 is perfectly flat within the audioband (20Hz to 20kHz). At the extremes, the BHA-1 is less than -0.1dB at 5Hz (an indication that it is AC-coupled) and about -0.3dB at 80kHz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue or purple trace) is performing identically to the right channel (red or green trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response

Above is the phase response plot from 20Hz to 20kHz. The BHA-1 does not invert polarity, and it yielded a worst-case 5 degrees or so of phase shift at 20kHz.

Frequency response (600-, 300-, 32-ohm loads)

In the frequency-response (RMS level relative to 0dBrA or 2Vrms at 1kHz) plots above, the blue trace is into a 600-ohm load, purple into 300 ohms, and pink into 32 ohms. Between the 600-ohms and 32-ohm loads, we find about 0.5dB of variation, an indication that the BHA-1 has a low (but not exceptionally low) output impedance.

THD ratio (unweighted) vs. frequency vs. load

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus and 2Vrms at the output. The blue and red plots are for left and right channels into 600 ohms, purple/green (L/R) are into 300 ohms, and pink/orange (L/R) are into 32 ohms. THD values are very low and virtually identical into all three loads. At low frequencies, THD values were slightly higher (0.002% at 20Hz for the left channel), although this may be due to the analyzer’s confounding of signal harmonics with power-supply-related noise harmonics. The evidence for this is that the left channel yielded much higher noise at the power-supply second harmonic (120Hz) than the right channel, and it’s the left channel that yielded the higher low frequency THD in the plots above.  Above 100Hz, THD ratios are flat and consistent for both channels up to 20kHz, hovering at a very low 0.0005%.

THD ratio (unweighted) vs. output power vs. load (low gain)

The plots above show THD ratios measured at the output of the BHA-1 as a function of output power for a 1kHz input sinewave using the low gain setting. The blue and red plots are for left and right channels into 600 ohms, purple/green (L/R) are into 300 ohms, and pink/orange (L/R) are into 32 ohms. The 600-ohm data yielded THD ratios from about 0.005% at 1uW, down to as low as 0.00008% (left channel) at 10mW, then up to 0.0005/0.001% (left/right) at the “knee” at about 300mW, then up to the 1% THD mark at 580mW. The 300-ohm data yielded THD ratios from 0.007% at 1uW, down to as low as 0.0001% at 20mW (left channel), then up to 0.0005/0.001% (left/right) at the “knee” at about 700mW, then up to the 1% THD mark at 1.14W. The 32-ohm data yielded higher THD ratios from about 0.02% at 1uW, down to as low as 0.00005% (left channel) at 500mW, then up to 0.0005% at the “knee” at about 3W, then up to the 1% THD mark at 5.8W.

THD+N ratio (A-weighted) vs. output power vs. load (low gain)

The plots above show THD+N ratios (A-weighted) measured at the output of the BHA-1 as a function of output power for a 1kHz input sinewave with the low gain setting. The blue and red plots are for left and right channels into 600 ohms, purple/green (L/R) are into 300 ohms, and pink/orange (L/R) are into 32 ohms. The left channel consistently yielded higher THD+N ratios (by about 5dB) due to the increased noise (see FFTs below). The 600-ohm right channel data yielded THD+N ratios from 0.03% at 1uW, down to as low as 0.0003% at 20mW. The 300-ohm right channel data yielded THD+N ratios from 0.04% at 1uW, down to as low as 0.0004% at 200mW. The 32-ohm right channel data yielded higher THD+N ratios of about 0.1% at 1uW, down to as low as 0.0004% at 300mW.

THD ratio (unweighted) vs. output power (high gain into 32 ohms)

The plots above show THD ratios measured at the output of the BHA-1 as a function of output power for a 1kHz input sinewave into a 32-ohm load for the high gain setting. The right channel outperformed the left by about 5dB. THD ratios for the right channel range from 0.03% at 1uW, down to as low as 0.0001% at 200mW, then up to 0.0005% at the “knee” at about 3W, then up to the 1% THD mark at 6.8W.

THD+N ratio (A-weighted) vs. output power (high gain into 32 ohms)

The plots above show THD+N (A-weighted) ratios measured at the output of the BHA-1 as a function of output power for a 1kHz input sinewave into a 32-ohm load for the high gain setting. The right channel outperformed the left by about 10dB. THD+N ratios for the right channel range from 0.3% at 1uW, down to as low as 0.0004% at 1-2W, then up to 0.001% at the “knee” at about 3W.

FFT spectrum – 1kHz (balanced in, balanced out, high gain)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 300-ohm load at 2Vrms (0dBrA), for the balanced-in/balanced-out/high-gain configuration (default for these measurements). We see that the signal’s second harmonic, at 2kHz, is at around -120/110dBrA (left/right), or 0.0001/0.0003%, while the third harmonic, at 3kHz, is higher at around -105dBrA, or 0.0006%. On the right side of the signal peak, the power-supply fundamental (60Hz) noise peak is seen at around -120dBrA, or 0.0001%, while the second harmonic (120Hz) dominates at -95/110dBrA (left/right), or 0.002/0.0003%.  Higher-order power-supply-related harmonics can also be seen throughout most of the spectrum at -120dBrA, or 0.001%, and below. There is also a broad unknown peak at 20kHz at -125dBrA, or 0.00006%.

FFT spectrum – 1kHz (balanced in, balanced out, low gain)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 300-ohm load at 2Vrms (0dBrA) for the balanced-in/balanced-out/low-gain configuration. The volume-control position was maintained at the same level, with the input signal on the analyzer increased to compensate. The main differences compared to the default configuration FFT are predictably, slightly lower noise with the left channel 120Hz peak down to -100dBrA, or 0.001%, and higher signal harmonics, with the 3kHz peak dominating at -95dBrA, or 0.002%.

FFT spectrum – 1kHz (balanced in, unbalanced out, high gain)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 300-ohm load at 2Vrms (0dBrA) for the balanced-in/unbalanced-out/high-gain configuration. The volume-control position was maintained at the same level, with the input signal on the analyzer increased to compensate. The main differences compared to the default configuration FFT are lower noise with the right channel 120Hz peak down to -130dBrA, or 0.00003%, and higher signal harmonics, with the 3kHz peak dominating at -95dBrA, or 0.002%.

FFT spectrum – 1kHz (unbalanced in, unbalanced out, high gain)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 300-ohm load at 2Vrms (0dBrA) for the unbalanced in/unbalanced out/high gain configuration. The volume control position was maintained at the same level, with the input signal on the analyzer increased to compensate. The main differences compared to the default configuration FFT are lower noise with the right channel 120Hz peak down to -125dBrA, or 0.00006%, and higher signal harmonics, with the 3kHz peak dominating at -90dBrA, or 0.003%, and the right channel 2kHz peak not far behind at -95dBrA, or 0.002%.

FFT spectrum – 1kHz (unbalanced in, balanced out, high gain)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the output into a 300-ohm load at 2Vrms (0dBrA) for the unbalanced-in/balanced-out/high-gain configuration. The volume-control position was maintained at the same level, with the input signal on the analyzer increased to compensate. The main differences compared to the default configuration FFT are slightly higher signal harmonics, with the 3kHz peak dominating at -100dBrA, or 0.001%, and the 2kHz peak at -110/100dBrA, or 0.0003/0.001%.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sinewave stimulus measured at the output into a 300-ohm load. 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 most predominant non-signal peak is from the power supply’s second harmonic (120Hz) at -95/110dBrA (left/right), or 0.002/0.0003%.  The signal’s second harmonic (100Hz) is at -115dBrA, or 0.0002%, while the third harmonic (150Hz) is at -105dBrA, or 0.0006%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 300-ohm load. 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. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is just above the noise floor for the left channel at -140dBrA, or 0.00001%, while the right channel is at -130dBrA, or 0.00003%. The second-order modulation products are buried amongst power-supply-related upper harmonics at the -120 to -140dBrA level, especially in the left channel. The third-order modulation products, at 17kHz and 20kHz, are at -115dBrA, or 0.0002%.

Intermodulation distortion FFT (APx 32 tone)

Shown above is the FFT of the output of the BHA-1 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBrA reference, and corresponds to 2Vrms into 300 ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier. In this case, most of the visible peaks in the spectrum are due to power-supply noise harmonics (60/120/180/240Hz, etc.). The grassy noise floor where the IMD products would lie are at -130dB, or 0.00003%, and below the reference level.

Square-wave response (10kHz)

Above is the 10kHz squarewave response at the output into 300 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the BHA-1’s slew-rate performance. Rather, it should be seen as a qualitative representation of its high bandwidth. An ideal squarewave can be represented as the sum of a sinewave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The BHA-1’s reproduction of the 10kHz squarewave is very clean, with no ringing or overshoot in the corners.

Output impedance vs. frequency (unbalanced output, 20Hz to 20kHz)

The final chart above is the output impedance as a function of frequency. Both channels show a nearly constant and identical low output impedance across the audioband, between 2.22 and 2.15 ohms. The balanced outputs have the same output impedance as seen above but on each of the positive and negative portions, combining for a summed output impedance of 4.3.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 01 January 2023

Link: reviewed by James Hale on SoundStage! Xperience on January 1, 2023

General information

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

The Helm Audio DB12 AAAMP was conditioned for 30 minutes at 2Vrms at the output into 300 ohms before any measurements were taken. All measurements were taken with both channels driven.

The DB12 AAAMP offers one unbalanced input (1/8″ male TRS) and one unbalanced output (1/8″ female TRS). There is a volume control, but it does not control the amplifier gain or provide onboard attenuation; rather, it sends volume control signals to the source device (e.g., smartphone). There is also a Bass+ (Bass Boost) switch. Unless otherwise stated, measurements were made with the Bass+ switch disabled, the DB12 fully charged but unplugged from the USB charging port, with a 2Vrms output into a 300-ohm load.

One noteworthy attribute of the DB12 AAAMP is that if the unit is off (or the battery were to die), signals are passed through at unity gain.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Helm Audio for the DB12 AAAMP compared directly against our own. The published specifications are sourced from Helm 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, a 1kHz sine wave, 0.5Vrms input, and 2Vrms output into a 300-ohm load, 10Hz to 90kHz bandwidth, and the worst-case measured result between the left and right channels.

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sine wave, 0.5Vrms input and 2Vrms output into a 300-ohm load, 10Hz to 90kHz bandwidth):

Frequency response

In our measured frequency-response plots above, the blue/red traces are with the Bass+ (Bass Boost) function disengaged, while the purple and green represent the responses with the bass-boost engaged. The DB12 is essentially perfectly flat within the audioband, into a 300-ohm load. Worst-case deviations are +/- 0.03dB. The DB12 also appears to be DC coupled, as it is ruler flat down to 5Hz. The DB12 also offers an extended bandwidth, only down about 0.4dB at 80kHz. With Bass Boost engaged, there’s a +5.5 to 6dB bump in the response between 5Hz and 100Hz. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple or pink trace) is performing identically to the right channel (red, green or orange trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response vs. load

In the frequency-response plots above, the blue/red traces are into a 600-ohm load, purple/green into 300 ohms, and pink/orange into 32 ohms. The 600- and 300-ohm data are essentially identical, but into a 32-ohm load, there is a roll-off a high frequencies: -0.2dB at 20kHz, and nearly -2.5dB at 80kHz. This corroborates Helm Audio’s claim of +0.01/-0.2dB from 20Hz to 20kHz into 32 ohms.

Phase response

Above is the phase response plot from 20Hz to 20kHz. The DB12 AAAMP does not invert polarity, and yielded a worst-case 40 degrees or so of phase shift at 20kHz.

THD ratio (unweighted) vs. frequency vs. load

The plot above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sine-wave input stimulus and 2Vrms at the output. The blue and red plots are for left and right channels into 600 ohms, purple/green (L/R) are into 300 ohms, and pink/orange (L/R) are into 32 ohms. THD values are very low and almost identical into 600 and 300 ohm loads. These ranged from 0.0003% from 20Hz to 5kHz, them up to 0.0005% at 20kHz. The 32-ohm data yielded higher THD ratios, from 0.0004% from 20Hz to 200Hz, then a steady rise to 0.003% at 10kHz to 20kHz.

THD ratio (unweighted) vs. output power vs. load

The plots above show THD ratios measured at the output of the DB12 as a function of output power for a 1kHz input sine wave. The blue and red plots are for left and right channels into 600 ohms, purple/green (L/R) are into 300 ohms, and pink/orange (L/R) are into 32 ohms. The 600-ohm data yielded THD ratios from 0.0002% at 0.2mW, down to as low as 0.0001% at 1 to 2mW, then up to 0.0005% at the “knee” at about 12mW, then up to the 1% THD mark at 13.6mW. The 300-ohm data yielded THD ratios from 0.0003% at 0.2mW, down to as low as 0.0001% at 2 to 3mW, then up to 0.0005% at the “knee” at about 22mW, then up to the 1% THD mark at 27mW. The 32-ohm data yielded higher and relatively flat THD ratios of about 0.001% from 0.2mW through to the “knee” at about 120mW, then up to the 1% THD mark at 164mW.

THD+N ratio (A-weighted) vs. output power vs. load

The plots above show THD+N ratios (A-weighted), measured at the output of the DB12 as a function of output power for a 1kHz input sine wave. The blue and red plots are for left and right channels into 600 ohms, purple/green (L/R) are into 300 ohms, and pink/orange (L/R) are into 32 ohms. The 600-ohm data yielded THD+N ratios from 0.001% at 0.2mW, down to as low as 0.0003% at 3 to 5mW, then up to 0.0005% at the “knee.” The 300-ohm data yielded THD+N ratios from 0.0015% at 0.2mW, down to as low as 0.0003% at 5 to 10mW, then up to 0.0005% at the “knee.” The 32-ohm data yielded higher THD+N ratios of about 0.005% at 0.2mW, down to as low as 0.001% from 10mW to the “knee.”

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 300-ohm load at 2Vrms (0dBrA). We see that the signal’s second harmonic, at 2kHz, is at around -120dBrA, or 0.0001%, while the third harmonic, at 3 kHz, is higher at -110dBrA, or 0.0003%. Higher-order even harmonics (4/6/8/10kHz) can be seen below the -130dBrA, or 0.00003%, level. On the right side of the signal peak, only a very small peak at the power-supply fundamental (60Hz) can be seen at a vanishingly low -140dBrA, or 0.00001%.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output into a 300-ohm load. 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 most predominant non-signal peak is from the signal’s third harmonic (3kHz) at -110dBrA, or 0.0003%. The second signal harmonic (100Hz) is at -130dBrA, or 0.00003%. There are no visible power-supply-related noise peaks.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the output into a 300-ohm load. 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. We find that the second-order-modulation product (i.e., the difference signal of 1kHz) is at -125/-130dBrA (left/right), or 0.00006/0.00003%, while the third-order modulation products, at 17kHz and 20kHz, are just above -120dBrA, or 0.0001%.

Squarewave response (10kHz)

Above is the 10kHz squarewave response at the output into 300 ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the DB12’s slew-rate performance. Rather, it should be seen as a qualitative representation of its relatively high bandwidth. An ideal squarewave can be represented as the sum of a sine wave and an infinite series of its odd-order harmonics (e.g., 10kHz + 30kHz + 50kHz + 70kHz . . .). A limited bandwidth will show only the sum of the lower-order harmonics, which may result in noticeable undershoot and/or overshoot, and softening of the edges. The DB12’s reproduction of the 10kHz squarewave is clean, with only mild ringing in the corners.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 10 August 2022

Reviewed on: SoundStage! Solo, August 2022

I measured the KZ AZ09 using laboratory-grade equipment: a GRAS Model 43AG ear/cheek simulator/RA0402 ear simulator and an Audiomatica Clio 12 QC audio analyzer. This is not the kind of comprehensive measurements SoundStage! would normally do with audio electronics; it’s merely an attempt to gauge the basic operating parameters and performance limits of the device. All measurements involving earphones are “flat”; no diffuse-field or free-field compensation curve was employed. If you’d like to learn more about what our measurements mean, click here.

This chart shows the AZ09’s frequency response measured straight into the Clio 12 QC analyzer, with no load. The measurement level was at a “medium” level of 178mV (the drive level required to reach 1mW into a 32-ohm load). Bass rolloff is about -0.1dB at 10Hz; treble rolloff is about -0.3dB at 20Hz—pretty good for a $30 device, and not enough to cause noticeable colorations. There’s a roughly 0.02dB level mismatch between the left and right channels, which will not be audible. The glitches at 5 and 7kHz appeared consistently in both channels; I suspect they are Bluetooth artifacts. They are of too high a Q and too low in magnitude to be audible.

This chart shows how the AZ09 affected the response of the TinHiFi T3 Plus and CCA C10 earphones versus a cabled connection from those earphones to a Musical Fidelity V-CAN headphone amp. With both earphones, the AZ09 produced a tiny bit of extra bass compared with the wired connection: about +1dB at 10Hz for the T3 Pluses, and +1.5dB for the CCA C10s. There are also some tiny differences in the treble, but these are likely due to ordinary measurement-to-measurement variation.

This chart shows the total harmonic distortion plus noise (THD+N) of the AZ09 at gradually increasing output levels, measured into a 32-ohm load. (I would normally measure this into 250- and 600-ohm loads as well, but I have never encountered earphones with such high impedances so the measurement would be irrelevant.) As we see with almost all amplifiers, the signal-to-noise ratio falls as the output increases. Measured at 1kHz, the AZ09 reaches its minimum THD+N at its maximum output of 4.07mW, so at that frequency, it seems impossible to make the AZ09 clip. Wondering if the AZ09 could perform as well in the bass, I reduced the test frequency to 20Hz, and at that frequency, I was able to make the AZ09 clip—although THD+N rose only to 0.73% at the maximum 20Hz output of 4.175mW. That’s not an audible level of distortion at such a low frequency.

Note that the above chart also shows that the AZ09’s output peaks at about 4mW into 32 ohms. That’s not much, but with a typical set of earphones rated at 105dB sensitivity with a 1mW signal, that’ll get you up to 111dB SPL peaks, which is plenty, and enough to damage your hearing if you run the AZ09 at max volume for long periods of time.

Best-case latency I measured from the AZ09 was about 80ms, which is excellent for a true wireless product, although the latency sometimes jumped up into the 300ms area with some of the measurements I did; that’s more typical for a true wireless product. This is probably irrelevant, though, because latency only matters when you’re watching videos or playing games, and it seems unlikely this product would be used for those purposes.

Output impedance of the AZ09, measured at 1kHz, is 9.1 ohms—which is fairly low compared with other headphone/earphone amps, and probably not high enough to alter the frequency response of any earphones the AZ09 would be used with.

Bottom line: Obviously, the AZ09 has performance limitations compared with a conventional headphone amp or DAC-amp, but it seems well-designed overall, and is unlikely to present any audible problems.

. . . Brent Butterworth
brentb@soundstagenetwork.com

Details

Parent Category: Products

Category: Headphone Amplifier Measurements

Created: 01 July 2022

Link: reviewed by James Hale on SoundStage! Xperience on July 1, 2022

General Information

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

The iFi Audio Go Bar was conditioned for 30 minutes at 0dBFS (2Vrms out) into 300 ohms before any measurements were taken.

The Go Bar allows for a USB input only. There are two headphone outputs: one unbalanced over 3.5mm TRS: and one balanced over 4.4mm TRRS.  Comparisons were made between unbalanced and balanced outputs for a 24/96 0dBFS input, in terms of noise, THD, and dynamic range. Other than the extra 6dB of gain and output voltage, the two outputs were virtually identical.

The Go Bar offers an in-ear-monitor (IEM) matching selector, which lowers output voltage for sensitive IEMs and increases the output impedance; this switch was left in the Off position for the measurements. There is also a Turbo Mode associated with the use of volume. Engaging Turbo Mode allows for the Go Bar full output-voltage potential. Turbo Mode was left engaged for the measurements. There are also four user selectable digital filters. All measurements below, unless otherwise stated, are for the balanced output, using the standard STD filter. The four filters are described as follows in the Go Bar manual:

• Bit-Perfect (BP): no digital filtering, no pre- or post-ringing
• Standard (STD): modest filtering, modest pre- and post-ringing
• Minimum Phase (MIN): slow roll-off, minimum pre- and post-ringing
• Giggs-Transient-Optimized (GTO): upsampled to 352kHz or 384kHz, minimum filtering, no pre-ringing, minimum post-ringing

Because the Go Bar exhibits considerable noise above 20kHz (see FFTs below), our typical input bandwidth filter setting of 10Hz-90kHz was necessarily changed to 10Hz-22.4kHz for all measurements, except for frequency response and for FFT measurements. In addition, THD versus frequency sweeps were limited to 6kHz to adequately capture the second and third signal harmonics with the restricted bandwidth setting.

Based on the accuracy and repeatable results at various volume levels of the left/right channel matching (see table below), and the lack of analog inputs, the Go Bar volume control is likely applied in the digital domain. The volume control offers 53 steps in 2dB to 0.5dB increments.

Volume-control accuracy (measured at headphone output): left-right channel tracking

Published specifications vs. our primary measurements

The table below summarizes the measurements published by iFi Audio for the Go Bar compared directly against our own. The published specifications are sourced from iFi’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, the coaxial digital input (1kHz sine wave sampled at 96kHz at 0dBFS/2Vrms out), the balanced line-level output into 300 ohms, using a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels.

* into 300 ohms, THD for the left channel was the same as the right, at 0.0026%

Our primary measurements revealed the following using the coaxial input and the balanced output (unless specified, assume a 1kHz sine wave sampled at 96kHz at 0dBFS/2Vrms out, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

Frequency response (16/44.1, 24/96, 24/192)

The plot above shows the Go Bar frequency response as a function of sample rate. The blue/red traces are for a 44.1kHz sampled dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 96kHz sampled dithered digital input signal from 5Hz to 48kHz, and, finally, orange/pink represents 192kHz sampled data from 5Hz to 96kHz. The behavior at low frequencies is the same for the digital input—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is as expected, offering sharp filtering around 22k, 48k, and 96kHz (half the respective sample rate). The -3dB point for each sample rate is roughly 21.1, 45.9 and 89.7kHz, respectively. This corroborates iFi’s claim of a frequency response of 20Hz-45kHz (-3dB). It is also obvious from the plots above that all three responses offer “brick-wall”-type behavior. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple or orange trace) is performing identically to the right channel (red, green or pink trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Frequency response (96kHz, xBase on/off)

The plot above shows the Go Bar frequency response with and without xBass engaged, for 96kHz dithered sampled data. The blue/red without xBass and the purple/green traces are with xBass engaged. We can see that xBass applies about 8dB of boost at 20Hz, and is flat by about 300Hz.

Frequency response (44.1kHz, all four filters)

The plots above show frequency-response for 44.1kHz sampled input data for filters 1 through 4 into a 300 ohm-load for the left channel only. The Standard (STD) filter is in red, Giggs-Transient-Optimized (GTO) in purple, Bit-Perfect (BP) in green, and Minimum Phase (MIN) in pink. The graph is zoomed in from 1kHz to 22kHz, to highlight the various responses of the filters around the “knee” of the response. At 20kHz, the STD filter is at +0.1dB, the GTO filter is at -0.3dB, the BP filter is at -3dB, and the MIN filter is at 0dB. The STD and MIN filters have nearly identical frequency response behaviors.

Note: the filter characteristics are described under General Information section above. Our measured frequency responses generally match the descriptions provided by iFi Audio.

Digital linearity (96kHz)

The graph above shows the results of a linearity test for 96kHz sampled input data. The digital input is swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output is analyzed by the APx555. The ideal response is a straight flat line at 0dB (i.e., the amplitude of the digital input perfectly matches the amplitude of the measured analog output). The data is perfectly linear down to -100dBFS and only -1dB or better at -120dBFS.

Impulse response (44.1kHz, STD and BP filters)

The graph above shows the impulse responses for the first two filter types (STD and BP), for 44.1kHz sampled dithered data, using the Audio Precision’s Transfer Function/Impulse Response function. STD is in blue and BP in pink.

Note: the filter characteristics are described under General Information above. Our measured impulses responses generally match the descriptions provided by iFi Audio.

Impulse response (44.1kHz, GTO and MIN filters)

The graph above shows the impulse responses for the two other two filter types (GTO and MIN), for 44.1kHz sampled dithered data, using the Audio Precision’s Transfer Function/Impulse Response function. GTO is in purple and MIN in green.

Note: the filter characteristics are described under General information above. Our measured impulses responses generally match the descriptions provided by iFi Audio.

J-Test

The plot above shows the results of the J-Test test for the USB input measured at the balanced line level output of the Go Bar. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bits). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz square wave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sine wave (the main peak). In addition, an undithered 250Hz square wave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle, which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g., 250Hz, 750Hz, 1250Hz, etc.). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA (fundamental at 250Hz) down to -170dBrA for the odd harmonics.  The test file can also be used in conjunction with artificially injected sine-wave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The FFT shows some of the alternating 500Hz peaks in the audio band but at low levels—below -130dBrA. This is an indication that the Go Bar should not be sensitive to jitter.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (STD filter)

The plot above shows a fast Fourier transform (FFT) of the Go Bar balanced output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sine wave at -1dBFS sampled at 44.1kHz (purple/green), using the STD filter. The sharp rolloff above 20kHz in the white-noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are two imaged aliasing artifacts in the audio band above the -135dBrA noise floor, at around 6kHz (-110dBrA) and 13kHz (-115dBrA). The primary aliasing signal at 25kHz is at -115dBrA, with subsequent harmonics of the 25kHz peak near -80 and -70dBrA.

Note: the filter characteristics are described under General information above. Our measured FFTs generally match the descriptions provided by iFi Audio.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (BP filter)

The plot above shows a fast Fourier transform (FFT) of the Go Bar balanced output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sine wave at -1dBFS sampled at 44.1kHz (purple/green), using the BP filter. As advertised, the BP filter uses no filter at all, which is evident in the very slow rolloff in the noise spectrum. As a consequence, there are obvious imaged aliasing artifacts in the audio band above the -135dBrA noise floor, as high as -90dBrA at 6kHz and 13kHz. The primary aliasing signal at 25kHz is hardly suppressed at all, as is expected, and sits at -5dBrA.  

Note: the filter characteristics are described under General information above. Our measured FFTs generally match the descriptions provided by iFi Audio.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (GTO filter)

The plot above shows a fast Fourier transform (FFT) of the Go Bar balanced output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sine wave at -1dBFS sampled at 44.1kHz (purple/green), using the GTO filter. The GTO filter offers a softer rolloff above 20kHz in the white-noise spectrum compared to the STD and MIN filters. There are two imaged aliasing artifacts in the audio band above the -135dBrA noise floor at around 6kHz (-100dBrA) and 13kHz (-100dBrA). The primary aliasing signal at 25kHz is only mildly suppressed at -20dBrA, with subsequent harmonics of the 25kHz peak below this level.

Note: the filter characteristics are described under General information above. Our measured FFTs generally match the descriptions provided by iFi Audio.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (MIN filter)

The plot above shows a fast Fourier transform (FFT) of the Go Bar balanced output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sine wave at -1dBFS sampled at 44.1kHz (purple/green), using the MIN filter. The sharp rolloff above 20kHz in the white-noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are two imaged aliasing artifacts in the audio band above the -135dBrA noise floor, at around 6kHz (-110dBrA) and 13kHz (-115dBrA). The primary aliasing signal at 25kHz is at -65dBrA, with subsequent harmonics of the 25kHz peak near -80 and -70dBrA. The behavior of the MIN filter is similar to that of the STD filter.

Note: the filter characteristics are described under General information above. Our measured FFTs generally match the descriptions provided by iFi Audio.

THD ratio (unweighted) vs. frequency vs. load (24/96)

The chart above shows THD ratios at the balanced output into 600 ohms (blue/red), 300 ohms (purple/green) and 32 ohms (pink/orange) for a constant 2Vrms output as a function of frequency, for 96kHz sampled dithered input data. Into 600 ohms, the right channel clearly outperforms the left by almost 10dB. Into 300 ohms, THD ratios are between 0.002 and 0.003%, the same as the right channel into 600 ohms. At 6kHz, THD ratios into 300 ohms are nearing 0.005%. Into 32 ohms, THD ratios are a bit higher, between slightly above, and below 0.005%.

THD ratio (unweighted) vs. output (600, 300, and 32 ohms) at 1kHz

The chart above shows THD ratios at the balanced output into 600 ohms (blue/red), 300 ohms (purple/green), and 32 ohms (pink/orange) at 1kHz as a function of output voltage, for 96kHz sampled dithered input data. Up to about 50mVrms, all three data sets track, with a THD ratio of about 0.005%. From 50mVrms to about 2Vrms (125mW), the 32-ohm data is fairly flat between 0.005% and 0.01%. At 200mVrms, the 600 and 300-ohm data still track closely, at just above 0.001%. From 200mVrms to about 2Vrms (13mW), the 300-ohm data is fairly flat between 0.001% and 0.002%. The 600-ohm data reaches a THD low of about 0.0006% at 400mVrms, then up to 0.002% at 2Vrms (6.7mW). The 1% THD mark is close to the same into 600 ohms (95mW) and 300 ohms (177mW), just over 7Vrms. Into 32 ohms, the 1% THD mark is just over 5.5Vrms, at 1W.

THD+N ratio (unweighted) vs. output (96kHz for 600, 300, and 32 ohms) at 1kHz

The chart above shows THD+N ratios at the balanced output into 600 ohms (blue/red), 300 ohms (purple/green), and 32 ohms (pink/orange) at 1kHz as a function of output voltage, for 96kHz sampled dithered input data. All data sets track closely up to about 200mVrms, where THD+N ratios measure 0.015%. Beyond 200mVrms, the 32-ohm data reaches a low of 0.005% at 2Vrms, while the 300 and 600-ohm data track almost perfectly, and reach a THD+N low of just over 0.002% at 2Vrms.

Intermodulation distortion vs generator level (SMPTE, 60Hz:4kHz, 4:1 for 600, 300, and 32 ohms)

The chart above shows intermodulation distortion (IMD) ratios measured at the balanced output into 600 ohms (blue/red), 300 ohms (purple/green), and 32 ohms (pink/orange) from -60dBFS to 0dBFS, for 96kHz sampled dithered input data. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 600-ohm data yielded the highest IMD ratios, from 2% down to 0.06% at -30dBFS, then up to 0.3% (left) at -18dBFS, then back down to roughly 0.02% at 0dBFS. The 300- and 32-ohm data track perfectly down to -30dBFS, where IMD ratios measured 0.02%. Beyond this threshold, the 32-ohm data flattens out and reaches 0.01% at 0dBFS. The 300-ohm data reaches an IMD low of nearly 0.002% just shy of 0dBFS.

FFT spectrum – 1kHz (44.1kHz data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 300 ohms sampled at 44.1kHz. We see the third signal harmonic (3kHz) dominating at -95dBrA, or 0.002%, while the second harmonic (2kHz) is between -100 and -110dBrA (left/right), or 0.001 and 0.0003%. Subsequent odd harmonics (3, 5, 7, 9kHz) can be seen at and below -110dBrA, or 0.0003%. There are no power-supply noise peaks to speak of to the left of the main signal peak.

FFT spectrum – 1kHz (96kHz data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 300 ohm sampled at 96kHz. Within the audio band, we see essentially the same FFT as with the 44.1kHz sampled data above.

FFT spectrum – 1kHz (44.1kHz data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 300 ohms, sampled at 44.1kHz at -90dBFS. We see the main peak at the correct amplitude, surround by an elevated noise floor at around -100dBrA.

FFT spectrum – 1kHz (96kHz data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 300 ohm, sampled at 44.1kHz at -90dBFS. We see the main peak at the correct amplitude, surround by an elevated noise floor at around -100dBrA. The noise floor is identical to the 44.1kHz FFT above because as a USB DAC, bit depth could not be altered and is held at 32 bits for all sample rates. 

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 44.1kHz)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 300 ohms sampled at 44.1kHz. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -95/100dBRa (left/right), or 0.002/0.001%, while the third-order modulation products, at 17kHz and 20kHz, are also at -95dBrA.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 96kHz)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 300 ohms sampled at 44.1kHz. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. Within the audioband, we see essentially the same IMD FFT as with the 44.1kHz sampled data above.

Diego Estan
Electronics Measurement Specialist