Details

Parent Category: Products

Category: Preamplifier Measurements

Created: 15 July 2021

Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on July 15, 2021

General information

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

The Accuphase C-2850 was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The C-2850 (as tested) is an analog line-level preamp offering several balanced (XLR) and unbalanced (RCA) inputs and outputs, and a headphone output (¼″ TRS). The volume control is implemented using a proprietary process Accuphase calls “Accuphase Analog Vari-gain Amplifier (AAVA).” This system works by converting the incoming analog signal from a voltage to a current in 16 weighted steps. Each step is digitally controlled and switched in or out of the circuit depending on the encoded position of the volume knob. The current from each step switched into the circuit is summed and converted back to a voltage. The 16 circuit steps are analogous to on/off bits, and therefore, the volume system allows for 65536 (2¹⁶) discrete positions. Accuphase has configured the volume control to provide 251 steps ranging from -95dB to 0dB. Between -95 and -85dB, step sizes are 5dB; between -80 and -74dB, 3dB; -74 to -60dB, 2dB; -60 to -50dB, 1 dB; -50 to -30dB, 0.5dB; -30 to -8, 0.2dB; and finally between -8 to 0dB, 0.1dB. Considering both the exquisite channel tracking (see table below) and the variable, ultra-fine adjustments, this may be the finest digitally controlled analog volume control available in a consumer product.

The C-2850 also offers three gain settings, both for line-level (12, 18, and 24dB) and for the headphone output (Low, Mid, and High). The preamp gain setting affects the headphone gain, where Low is -10dB relative the preamp setting, Mid is 0dB, and High is +10dB. This means there are nine possible gain settings for the headphone amp: 2, 8, 12, 14, 18, 22, 24, 28, and 34dB. Unless otherwise stated, all measurement data below were taken with the 12dB gain setting for the preamp, and the Mid gain setting for the headphone amp.

When using the unbalanced and balanced inputs and outputs, the C-2850 provides the same gain regardless of combination. That is to say, with the volume set to unity gain, if I fed 2Vrms into the unbalanced input, I measured 2Vrms at the unbalanced and balanced outputs. If I fed 2Vrms into the balanced input, I measured 2Vrms at the unbalanced and balanced outputs. It’s also important to highlight that Accuphase assigns pins 2/3 on their XLR connectors as inverting/noninverting, which is the opposite to what we typically find in North-American or European products. For example, if I fed an unbalanced input and measured phase at the balanced output, it was 180 degrees out-of-phase. To compensate for this, Accuphase provides a polarity-inverting switch on the front panel, which was tested and flips the polarity as advertised.

I found small differences in THD and noise between the RCA and XLR inputs and outputs for the same output voltage. The RCA outputs exhibited about 11dB (unweighted) more noise than the XLR outputs, while the RCA inputs (when measured at the XLR outputs) measured slightly worse in terms of THD compared to the XLR inputs (0.0005% vs 0.0003% at 1kHz). Unless otherwise stated, all measurement data below are with the balanced inputs and outputs, at 2Vrms with volume set to unity gain (-12dB). Signal-to-noise ratios (SNR) were measured with the volume at maximum position.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Accuphase for the C-2850 compared directly against our own. The published specifications are sourced from Accuphase’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 measurement input bandwidth of 10Hz to 90kHz, and the worst case measured result between the left and right channel.

* The discrepancy in balanced output impedance may be due to Accuphase specifying this value for the inverting and noninverting pins separately. Our measurement considers both inputs on the balanced connector together. Treated separately, our measurement would be halved, or 48k ohms.

Our primary measurements revealed the following using the balanced line-level inputs (unless otherwise specified, assume a 1kHz sinewave, 2Vrms output into 200k ohms load, 10Hz to 90kHz bandwidth, 12dB gain setting):

Our primary measurements revealed the following using the balanced analog input and the headphone output (unless specified, assume a 1kHz sinewave at 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth, 12dB and Mid gain setting):

Frequency response

In our measured frequency response plot above, the C-2850 is near perfectly flat within the audioband (20Hz to 20kHz). The blue/red traces are without the 10Hz filter engaged, the purple/green traces with the 10Hz filter. These data do not quite corroborate Accuphase’s claim of 3Hz to 200kHz +0/-3dB (measured down to 5Hz). While at the upper end of the frequency spectrum, the -3dB point was measured at 200kHz, at low frequencies, Accuphase’s claim would imply that the C-2850 is DC coupled, whereas our measurements indicate AC coupling. Nevertheless, at the extremes of the audioband, we measured only -0.35dB at 20Hz (-1dB with filter on) and -0.04dB at 20kHz. The C-2850 can be considered a high-bandwidth audio device. 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.

Frequency response (Compensator dial 1, 2, and 3 positions)

Above are four frequency response plots for the balanced line-level input, with the Compensator control set to Off (blue/red), 1 (purple/light green), 2 (pink/cyan), and 3 (brown/dark green). We see what appears to be conventional bass-control EQ with various degrees of gain. At position 1, just under +3dB at 20Hz, position 2 yields about +5.5dB at 20Hz, and position 3 about +8.3dB.

Phase response

Above is the phase response plot from 20Hz to 20kHz, with the Phase control disabled (blue/red) and enabled (purple/green). The C-2850 does not invert polarity, while setting the Phase control to Invert does exactly that—it provides -180 degrees of shift. Since these data were collected using the balanced input and output, there is no phase inversion. However, since Accuphase assigns pins 2/3 on their XLR connectors as inverting/noninverting, the opposite to what we typically find in North American or European products, feeding the signal into an unbalanced input and measuring on the balanced output would yield the exact opposite of what is shown above.

THD ratio (unweighted) vs. frequency

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a 2Vrms sine-wave input stimulus. The blue and red plots are for left and right into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values are very low, near 0.0001% around 50-60Hz 20Hz, and around 0.0003-0.0004% through most of the audioband. The worst-case THD values are at 20Hz (0.001%) and 20kHz (0.001% into 600 ohms and 0.0007% into 200k ohms). Overall, the 600 and 200k-ohms load THD data are nearly identical.

THD ratio (unweighted) vs. output voltage at 1kHz

The plot above shows THD ratios measured at the output of the C-2850  as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD values measured around 0.003%, dipping down to around 0.00009% at 0.4Vrms. The “knee” occurs at around 7Vrms, hitting the 1% THD just past 8Vrms.

THD+N ratio (unweighted) vs. output voltage at 1kHz

The plot above shows THD+N ratios measured at the output the C-2850 as a function of output voltage into 200k ohms with a 1kHz input sinewave. At the 10mVrms level, THD+N values measured around 0.05%, dipping down to around 0.0005% from 1.5 to 5Vrms.

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus at 2Vrms, measured at the output into a 200k-ohm load. We see that the signal’s second harmonic, at 2kHz, is at -110dBrA or 0.0003%, while the third harmonic, at 3 kHz, is at -125dBrA or 0.00005%. Below 1kHz, we see some noise artifacts, with the 60Hz peak due to power supply noise visible at -145/-130dBrA (left/right), or 0.000006/0.00002%, and the 120Hz (second harmonic) peak just below -130dBrA.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sinewave stimulus at 2Vrms measured at the output into a 200k-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. Here we find the second harmonic of the signal (100Hz) and the third harmonic of the signal (150Hz) at -120/-125dBrA respectively, or 0.0001/0.00006%. The worst-case power supply peak is at 120Hz measuring just below -130dBrA, or 0.00003%.

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

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the output into a 200k-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 -110dBrA, or 0.0003%, while the third-order modulation products, at 17kHz and 20kHz are at and just above -120dBrA, or 0.0001%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the C-2850’s slew-rate performance. Rather, it should be seen as a qualitative representation of its high bandwidth. An ideal square wave 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 C-2850’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot and overshoot.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Preamplifier Measurements

Created: 15 June 2021

Link: reviewed by Gordon Brockhouse on SoundStage! Simplifi on June 15, 2021

General Information

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

The BR-20 was conditioned for 30 minutes at 0dBFS (4Vrms out) into 200k ohms before any measurements were taken.

The BR-20 offers a multitude of digital and analog inputs, two balanced outputs (XLR) and one headphone output (1/4″ TRS). Comparisons were made between unbalanced and balanced line-level inputs, and aside from the 6dB extra voltage gain seen when using the unbalanced inputs, no difference was measured in terms of THD+N. Comparisons were made between optical, coaxial, and AES/EBU digital inputs; no differences were seen in terms of THD+N. For the measurements below, unless otherwise specified, the coaxial digital input (0dBFS), and the balanced analog input (2 or 4Vrms) were used, with the volume control set to unity gain (0dB). With the volume set to unity, a 0dBFS digital input yields 4Vrms at the output. Signal-to-noise and dynamic-range measurements were made with the volume at maximum (12dB gain).

The BR-20 analog volume control is digitally controlled and offers a range from -67dB to +12dB in 0.5dB steps (except below -30dB, where gain steps range from 4 to 1dB).

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Bryston for the BR-20 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 is set at its maximum (DC to 1MHz), assume a measurement input bandwidth of 10Hz to 90kHz, 200k ohms load, and the worst case measured result between the left and right analog balanced input.

Our primary measurements revealed the following using the coaxial digital input, the balanced analog input and the balanced line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS or 4Vrms, 200k ohms loading, 10Hz to 90kHz bandwidth):

Our primary measurements revealed the following using the balanced analog input and the headphone output (unless specified, assume a 1kHz sinewave at 2Vrms output, 300 ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (analog)

In our measured frequency response plot above, the BR-20 is perfectly flat within the audioband (20Hz to 20kHz), and only about -0.25dB at 100kHz. These data corroborate Bryston’s claim of 20Hz to 20kHz, +/-0.5dB. The BR-20 can be considered a high-bandwidth audio device. 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 vs. input type (16/44.1, 24/96, 24/192, analog)

The plot above shows the BR-20’s frequency response as a function of sample rate. The blue/red traces are for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally orange/pink represents 24/192 from 5Hz to 96kHz. In addition, for comparison, the analog frequency response is shown in green (up to 80kHz). The behavior at lower frequencies is the same for all plots; 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 rates). The -3dB point for each sample rate is roughly 21, 45 and 58kHz, respectively. It is also obvious from the plots above that the 44.1kHz sampled input signal offers the most “brick-wall” type behavior, while the attenuation of the 96kHz and 192kHz sampled input signals approaching the corner frequencies (48kHz and 96kHz) is more gentle.

Phase response (analog)

Above is the phase response plot from 20Hz to 20kHz for the analog balanced input. The BR-20 does not invert polarity, and the plot shows essentially no phase shift.

Phase response vs. sample rate  (16/44.1, 24/96, 24/192)

Above are the phase response plots from 20Hz to 20kHz for the coaxial input, measured at balanced output. The blue/red traces are for a dithered 16/44.1 input at 0dBFS, the purple/green for 24/96, and the orange/pink for 24/192. We can see that the BR-20 introduces an inversion of polarity (+180 degrees) with digital signals. At 20kHz, the phase shift is at around -80 degrees (from the +180 degree baseline) for the 16/44.1 input data and +70 degrees for the 24/96 input data. The 24/192 input data shows just over +20 degrees at 20kHz.

Digital linearity (16/44.1 and 24/96 data)

The graph above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced line-level output of the BR-20. 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). Both input data types exhibited exemplary linearity. The 16/44.1 data showed a worst-case deviation of only +2dB at -120dBFS, while the 24/96 was essentially perfect (i.e., flat) down to -120dBFS. The sweep was also performed down to -140dBFS to test the limits of the BR-20. Predictably, the 16/44.1 data showed significant deviations below -120dBFS; however, the 24/96 data tracked the input stimuli extremely well all the way down to -140dBFS, showing a worst-case deviation of only -3.5dB at -135dBFS.

Impulse response (16/44.1 and 24/96 data)

The graph above shows the impulse responses for a -20dBFS 16/44.1 dithered input stimulus (blue), and -20dBFS 24/96 dithered input stimulus (purple), measured at the balanced line level output of the BR-20. The implemented filter appears to be designed for minimized pre-impulse ringing. This chart also shows that the BR-20 inverts the polarity of digital input signals.

J-Test (coaxial input)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output of the BR-20. The J-Test was developed by Julian Dunn the 1990s. It is a test signal, specifically a -3dBFS, undithered, 12kHz, 24-bit square wave sampled (in this case) at 48kHz. 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 sinewave (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 sinewave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial input shows a virtually perfect J-test FFT. The -144dBrA 250Hz tone (which is in the file) can just be seen above the noise floor, and, with the exception of a small peak below 6kHz, there are virtually no other artifacts above the noise floor. This is an indication that the BR-20 should not be sensitive to jitter.

To test jitter immunity further, the APx555 was used to artificially inject 2kHz sinewave jitter. Without any jitter rejection by the DAC, this would manifest in the FFT as sideband peaks at 10kHz and 14kHz. However, even with the maximum allowable jitter magnitude of 1592ns, no peaks were seen. This is another indication that the BR-20 is essentially impervious to jitter.

J-Test (optical input)

The optical input shows essentially the same J-test FFT result as the coaxial input. There is a visible peak just above 6kHz, higher in amplitude than for the coaxial input, but still vanishingly low at just below -140dBrA.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone (coaxial input)

The plot above shows a fast Fourier transform (FFT) of the BR-20’s balanced line-level output with white noise at -4dBFS (blue/red), and a 19.1kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). The sharp roll-off above 20kHz in the white noise spectrum shows the implementation of the brick-wall-type reconstruction filter. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is at -100dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone range from the same level up to -80dBrA.

THD ratio (unweighted) vs. frequency vs. load (analog)

The plot above shows THD ratios at the output of the BR-20 as a function of frequency (20Hz to 20kHz) for a sinewave input stimulus of 2Vrms at the analog balanced input. The blue and red plots are for the left and right channels into 200k ohms, while purple/green (left/right) are into 600 ohms. THD values are extremely low: about 0.00005-0.00008% into 200k ohms from 20Hz to 3kHz, climbing to 0.0004% at 20kHz. The 600-ohm data yielded slightly higher THD values, especially at the extremes (20Hz and 20kHz), where THD values were measured at 0.0006% and just above 0.001% (right channel). At 600 ohms, the left channel outperformed the right by about 5dB, starting above 50Hz. It’s important to point out that the BR-20’s analog input THD performance is not too far from the limits of the APx555 analyzer, which is about 0.000015% at this voltage level.

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

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96, dithered, 1kHz 0dBFS signal at the coaxial digital input. The 200k and 600 ohms data are very close from 50Hz to 1kHz, hovering around a very low 0.0007%. At the frequency extremes, THD increased into 600 ohms vs 200k ohms, where we see 0.003% vs 0.0007% at 20Hz, and 0.015% vs 0.01% at 20kHz.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

The chart above shows THD ratios at the balanced line-level output into 200k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz 0dBFS signal at the coaxial digital input. Both data input types performed almost identically. We see THD values around 0.0007% from 20Hz to 1kHz, then a climb to 0.01% at 20kHz.

THD ratio (unweighted) vs. output (analog)

The chart above shows THD ratios measured at the output the BR-20 as a function of output voltage into 200k ohms with a 1kHz input sinewave at the balanced analog input. For this sweep, the volume was set to maximum. At the 1mVrms level, THD values measured around 0.2%, dipping down to around 0.00006% at 4-5Vrms. The “knee” occurs at around 10Vrms, hitting the 1% THD just past 14Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial digital input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD range from 0.3% at 1mVrms to 0.00015% at 3Vrms, while the 16/44.1 ranged from 4% at 1mVrms down to 0.0005% at 7-9Vrms.

THD+N ratio (unweighted) vs. output (analog)

The plot above shows THD+N ratios measured at the output the BR-20 as a function of output voltage into 200k ohms with a 1kHz input sinewave at the balanced analog input. At the 1mVrms level, THD+N values measured around 4%, dipping down to around 0.0005% at 10Vrms.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial digital input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD+N range from 6% at 1mVrms down to 0.0008% at 7-10Vrms, while the 16/44.1 data ranged from 35% at 1mVrms down to 0.003% at the 10Vrms “knee.”

FFT spectrum – 1kHz (analog at 2Vrms)

Shown above is the fast Fourier transform (FFT) for a 1kHz 2Vrms input sinewave stimulus at the balanced analog input, measured at the output into a 200k-ohm load. We see that the signal’s second harmonic, at 2kHz, is at a vanishingly low -140dBrA, or 0.00001%, while the third harmonic, at 3kHz, is just slightly above at -135dBrA, or 0.00002%. Below 1kHz, we don’t see any noise artifacts above the noise floor.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a dithered 1kHz 0dBFS input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see signal harmonics at -110dBrA, or 0.0003%, at 2kHz, and -100dBrA, or 0.001%, at 3kHz.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a dithered 1kHz 0dBFS input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. We see signal harmonics at -110dBrA, or 0.0003%, at 2kHz, and -100dBrA, or 0.001%, at 3kHz, as well as lower level signal harmonics at 4/5/6/7 kHz at -130dBrA, or 0.00003% and below.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a dithered 1kHz input sinewave stimulus, measured at the balanced output into 200k ohms for the coaxial digital input, sampled at 16/44.1 at -90dBFS. We see no signal harmonics above the noise floor within the audioband.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a dithered 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96 at -90dBFS. We see no signal harmonics above the noise floor within the audioband.

FFT spectrum – 50Hz (analog at 2Vrms)

Shown above is the FFT for a 50Hz 2Vrms input sinewave stimulus at the balanced analog input measured at the output into a 200k-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. We find the second and third harmonic of the signal (100/150Hz) just peaking above the -140dBrA noise floor, and once again, no power-supply noise peaks are visible.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone for the balanced analog input measured at the output into a 200k-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 -125dBrA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz are at around -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 16/44.1)

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial digital input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -115dBRA, or 0.0002%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at about -100dBrA, or 0.001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 24/96)

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial digital input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -115dBRA, or 0.0002%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at above -100dBrA, or 0.001%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response for the balanced analog input at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this chart should not be used to infer or extrapolate the BR-20’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very high bandwidth. An ideal square wave 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 BR-20’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot and overshoot.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Preamplifier Measurements

Created: 15 April 2021

Link: reviewed by Doug Schneider on SoundStage! Hi-Fi on April 15, 2021

General information

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

The LINEb was conditioned for 30 minutes at 2Vrms at the output before any measurements were taken. All measurements were taken with both channels driven.

The LINEb offers two sets of line-level unbalanced (RCA) inputs, four sets of line-level balanced (XLR) inputs, and two sets of balanced (XLR) outputs. The volume control is implemented using relays and a discrete high-precision resistor ladder. The RCA inputs yield 6dB more gain than the XLR inputs, with a range from –51.4dB (volume position 1 on the display) to +11.8dB (volume position 64). The XLR inputs range from -57.4dB to +5.8dB. The volume control offers 1dB steps from 1 to 56, 0.5dB from 56 to 57, 1dB from 58 to 61, 1.5dB from 61 to 63, and 2dB from 63 to 64. Unity gain (+0.1dB) is achieved at position 60 for the XLR inputs, 54 (-0.1dB) for the RCA inputs. Channel volume tracking is superb (see table below).

There is an Audio Gnd switch on the LINEb back panel. Presumably, this switch disconnects audio ground from chassis/earth ground. I found no differences in noise performance with the switch in the off or on position. It was left on for the measurements.

I found effectively no difference in THD+N values between the RCA and XLR inputs for the same output voltage. I attempted to optimize the volume position to achieve the best signal-to-noise (SNR) and THD+N measurements; however, I found only small differences with the volume at various positions (for the same output voltage). Most measurements were made with the volume set to unity gain (60) using the XLR inputs.

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Karan Acoustics for the LINEb compared directly against our own. The published specifications are sourced from Karan Acoustic’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 measurement input bandwidth of 10Hz to 90kHz, and the worst-case measured result between the left and right channels.

* The discrepancy in balanced input/output impedances may be due to Karan specifying this value for the inverting and noninverting pins separately. Our measurement considers both inputs/outputs on the balanced connector together. Treated separately, our measurement would be halved, or, respectively, 28.5k ohms and 90 ohms for the input and output impedances.

Our primary measurements revealed the following using the balanced line-level inputs (unless specified, assume a 1kHz sine wave, 2Vrms output into 200k ohms load, 10Hz to 90kHz bandwidth):

Frequency response

In our measured frequency-response chart above, the LINEb is perfectly flat within the audioband (20Hz to 20kHz) and beyond. These data partially corroborate Karan Acoustics’ claim of 20Hz to 20kHz +/-0dB, 1.5Hz to 3MHz (-3dB). However, since the Audio Precision can only sweep to just past 200kHz, we cannot verify the -3dB at 3MHz claim portion. The LINEb is at 0dB at 5Hz, and at about -0.2dB at 200kHz. To state that the LINEb is a high-bandwidth audio device would be an understatement.

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 overlap perfectly, indicating that the two channels are ideally matched.

Phase response

Above is the phase-response chart from 20Hz to 20kHz. The LINEb does not invert polarity, and the plot shows essentially no phase shift within the audioband.

THD ratio (unweighted) vs. frequency

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sine-wave input stimulus. The blue and red plots are for left and right channels into 200k ohms, while purple/green (L/R) are into 600 ohms. THD values are very low, about 0.00004% into 200k ohms from 20Hz to 1kHz, and, most impressively, even lower at about 0.00003% into a 600-ohm load. There is a rise in THD values above 1kHz, where at 20kHz, the 600-ohm data are about 0.0003%, and the 200k-ohm data are lower at about 0.0002%, which are still extremely low figures.

THD ratio (unweighted) vs. output voltage at 1kHz

The chart above shows THD ratios measured at the output of the LINEb as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD values measured around 0.007%, dipping down to around 0.00003% at 3Vrms. The “knee” occurs at around 18Vrms, hitting the 1% THD just past 20Vrms. It’s important to note here that the LINEb’s extraordinarily low THD values are approaching the limits of the Audio Precision analyzer, which, when measured in loopback mode (generator feeding analyzer) measures about 50% lower than the LINEb (at 3Vrms), at about 0.000015%. It’s also important to mention that anything above 2-4Vrms is not typically required to drive most power amps into full power.

THD+N ratio (unweighted) vs. output voltage at 1kHz

The chart above shows THD+N ratios measured at the output the LINEb as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 10mVrms level, THD+N values measured around 0.1-0.2%, dipping down to around 0.0002% at 10Vrms.

FFT spectrum – 1kHz

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load. The red is the right channel, the blue the left. We see that the signal’s second harmonic, at 2kHz, is at a vanishingly low -140dBrA, or 0.00001%, while the third harmonic, at 3kHz, is just slightly above -140dBrA. Below 1kHz, we see some noise artifacts, with the 60Hz peak due to power supply-noise barely perceptible on the left channel below -140dBrA, and the 180Hz (third harmonic) peak just above -140dBrA. The right channel does not appear to show any noise peaks above the very low -150dBrA noise floor.

FFT spectrum – 50Hz

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output into a 200k-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. Here again, there are barely any noticeable peaks. We find the third harmonic of the signal (150Hz) just peaking above the -150dBrA noise floor, or 0.000003%, and the left channel showing the third-harmonic noise peak (180Hz) just above -140dBrA, or 0.00001%.

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 200k-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 -125dBrA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz are at -130dBrA and -125dBrA, or 0.00003% and 0.00006%, respectively. These extraordinarily low harmonic peaks are reflected in the IMD values in our primary table of -113/-115dB, which represent the sum of the second- and third-order intermodulation product peaks.

Square-wave response (10kHz)

Shown above is the 10kHz square-wave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this chart should not be used to infer or extrapolate the LINEb’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very high bandwidth. An ideal square wave 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, in the square-wave representation. As mentioned above, the LINEb is a very high-bandwidth component. Correspondingly, the LINEb’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot or overshoot.

Diego Estan
Electronics Measurement Specialist

Details

Parent Category: Products

Category: Preamplifier Measurements

Created: 15 August 2021

Link: reviewed by Aron Garrecht on SoundStage! Ultra on August 15, 2021

General Information

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

The SPL Director Mk2 was conditioned for 30 minutes at 0dBFS (4.3Vrms out) into 200k ohms before any measurements were taken.

The Director Mk2 offers a multitude of digital and analog inputs, including one set of balanced outputs (XLR), a tape loop (single-ended RCA inputs and outputs), and a fixed single-ended line-level output (RCA). Comparisons were made between S/PDIF optical (TosLink), S/PDIF coaxial (RCA), and AES/EBU (XLR) digital inputs; total harmonic distortion plus noise (THD+N) was the same for all of them. For the measurements below, unless otherwise specified, the coaxial digital input (0dBFS) and the balanced analog input (2 or 4.3Vrms) were used, with the volume control set to maximum (-0.1dB). With the volume at maximum, a 0dBFS digital input yields 4.3Vrms at the output.

The Director Mk2 volume control appears to be a traditional potentiometer offering a range of attenuation from about -90dB to -0.1dB.

Whereas most preamplifiers offer at least 6dB of gain, one interesting design aspect of the Director Mk2 is that it offers no gain. In fact, in the table where we have our primary measurements, the gain for each channel is a little less than 0dB. As a result, potential users should ensure compatibility with whatever power amplifier and/or source component(s) the Director Mk2 will be partnered with. 

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

Published specifications vs. our primary measurements

The table below summarizes the measurements published by SPL for the Director Mk2 compared directly against our own. The published specifications are sourced from SPL’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 a measurement input bandwidth of 10Hz to 90kHz, 200k ohms load, and the worst-case measured result between the left and right analog balanced input.

*The maximum input voltage available with the Audio Precision APx555 is 26.66Vrms. Since the SPL has no gain, roughly the same voltage is available at the output. At 26.66Vrms, the SNR is 130.2dB. The 132dB figure was calculated based on an assumed maximum output voltage of 33Vrms.

Our primary measurements revealed the following using the coaxial input, the balanced analog input and the balanced line-level outputs (unless specified, assume a 1kHz sine wave at 0dBFS or 4.3Vrms, 200k ohms loading, 10Hz to 90kHz bandwidth):

Frequency response (analog)

In our measured frequency-response plot above, the Director Mk2 is perfectly flat within the audioband (20Hz to 20kHz), and only about -0.25dB at 100kHz. SPL’s claim of a frequency range (-3dB) of 4Hz to 300kHz can be corroborated at 4Hz (we measured -0.1dB at 5Hz), but due to the limitations of the Audio Precision’s maximum 200kHz upper limit for a frequency sweep, the 300kHz figure can only be inferred. Since we measured -0.5dB (left) and -0.7dB (right) at 200kHz, it’s fairly safe to assume that the Director Mk2 makes or comes close to making the company’s -3dB 300kHz spec. The Director Mk2 can definitely be considered a high-bandwidth audio device. 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 vs. input type (16/44.1, 24/96, 24/192, analog)

The chart above shows the Director Mk2’s frequency response as a function of sample rate. The blue/red traces are for a 16bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally orange/pink represents 24/192 from 5Hz to 96kHz. In addition, for comparison, the analog frequency response is shown in green (up to 80kHz). The behavior at low frequencies is the same for all plots—near perfectly flat down to 5Hz. There is an oddity at high frequencies, however, where the right channel showed a softer attenuation around the corner frequency at all sample rates compared to the left channel. All three sample rate data for the right channel were at -0.5dB at 20kHz, while the left channel at all sample rates was at -0.1dB at 20kHz. The behavior of the left channel at high frequencies for all three digital sample rates is as expected, offering sharp filtering around 22, 48, and 96kHz (half the respective sample rate). The -3dB point for each sample rate (left channel) is roughly 21, 46, and 90kHz, respectively. It is also obvious from the plots above that the 44.1kHz sampled input signal (left channel) offers the most “brick-wall”-type behavior, while the attenuation of the 96kHz and 192kHz sampled input signals approaching the corner frequencies (48kHz and 96kHz) is gentler.

Phase response (analog)

Above is the phase response plot from 20Hz to 20kHz. The Director Mk2 does not invert polarity, and the plot shows less than -10 degrees of phase shift at 20kHz.

Phase response vs. sample rate  (16/44.1, 24/96, 24/192)

Above are the phase response plots from 20Hz to 20kHz for the coaxial input, measured at the balanced output. The blue/red traces are for a dithered 16/44.1 input at -20dBFS, the purple/green for 24/96, and the orange/pink for 24/192. Here again we see the differences between the left and right channels. Since the left channel exhibits sharper attenuation than the right for all sample rates, predictably, there is more phase shift at 15-20kHz than the right channel. At 15kHz, the phase shift is at around +144/+128 (left/right) degrees for the 16/44.1 input data, +45/+30 (left/right) degrees for the 24/96 input data, and +24/+8 (left/right) for the 24/192 input data.

Digital linearity (16/44.1 and 24/96 data)

The chart above shows the results of a linearity test for the coaxial digital input (the optical input performed identically) for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced line-level output. 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). Both input data types exhibited exemplary linearity. The 16/44.1 and 24/96 data showed a worst-case deviation of only +2dB around -120dBFS. At -100dBFS, both input data yielded essentially perfect results down to 0dBFS. The sweep was also performed down to -140dBFS (not shown) where both input data showed significant deviations below -120dBFS.

Impulse response (16/44.1 and 24/96 data)

The chart above shows the impulse responses for a 16/44.1 dithered input stimulus at -20dBFS (blue), and a 24/96 dithered input stimulus at -20dBFS (purple), with both measured at the balanced line-level output. The implemented filter appears to be designed for minimized pre-impulse ringing.

J-Test (coaxial input)

The chart above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output. The J-Test was developed by Julian Dunn in the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz square wave sampled (in this case) at 48kHz (24 bit). Since even the first odd harmonic (i.e., 36kHz) of the 12kHz squarewave 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 coaxial input shows obvious peaks in the audioband from -90dBrA to just below -130dBrA. This is an indication that the Director Mk2’s DAC may be susceptible to jitter through the coaxial input.

J-Test (optical input)

The optical input shows close to the same but slightly worse J-Test FFT result compared to the coaxial input. The peaks adjacent to the primary signal reach almost -85dBrA.

J-Test (coaxial input, 2kHz sine-wave jitter at 10ns)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output, with an additional 10ns of 2kHz sine-wave jitter injected by the APx555. The results are very clear, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at near -70dBrA. This is a clear indication that the DAC in the Director Mk2 has poor jitter immunity. For this test, the optical input yielded defectively the same results.

J-Test (coaxial input, 2kHz sine-wave jitter at 100ns)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output, with an additional 100ns of 2kHz sine-wave jitter injected by the APx555. The poor jitter-immunity results are further corroborated, as we see the sidebands at 10kHz and 14kHz (12kHz main signal +/-2kHz jitter signal) manifest at near -50dBrA. For this test, the optical input yielded similar results.

Wideband FFT spectrum of white noise and 19.1kHz sine-wave tone (coaxial input)

The chart above shows a fast Fourier transform (FFT) of the Director Mk2’s balanced-line level output with white noise at -4dBFS (blue/red) and a 19.1 kHz sine-wave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). The sharp roll-off above 20kHz in the white-noise spectrum shows the implementation of the brick-wall type reconstruction filter. There are minor imaged aliasing artifacts in the audioband between -100 and -110dBrA. The primary aliasing signal at 25kHz is just below -100dBrA, while the second and third distortion harmonics (38.2, 57.3kHz) of the 19.1kHz tone range from -90 to -100dBrA.

THD ratio (unweighted) vs. frequency vs. load (analog)

The chart above shows THD ratios at the output as a function of frequency (20Hz to 20kHz) for a sine-wave input stimulus of 2Vrms. The blue and red plots are for left and right channels into 200k ohms, while purple/green (left and right) are into 600 ohms. THD values are extremely low: about 0.00005-0.0002% into 200k ohms from 20Hz to 3kHz, climbing to 0.0005% at 20kHz. The 600-ohm data yielded higher THD values, especially at frequencies above 2kHz, where THD values were measured as low as 0.00007% (100Hz) and as high as 0.005% (20kHz). The Director Mk2’s analog THD values are extremely low, and in most cases, the signal harmonic peaks that the Audio Precision is “looking” for to calculate THD are buried amongst noise peaks, which may cause errors in the measurements, exhibited as peaks in the data above. For example, there is a sample point just above 1kHz in the plots above, where the Audio Precision would look for signal harmonics just above 2kHz and 3kHz. Unfortunately, the Director Mk2 has a noise peak at 3.02kHz, which causes a false and unnaturally high THD rating at 1kHz. See FFT charts below for a full explanation.

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

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the coaxial input. The 200k and 600 ohms data are very close from 20Hz to 6kHz, hovering around 0.001%. At 20kHz, THD increased into 600 ohms vs 200k ohms, where we see 0.005% vs 0.002%.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1 and 24/96)

The chart above shows THD ratios at the balanced line-level output into 200k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the coaxial input. Both data input types performed almost identically. We see THD values around 0.001% from 20Hz to 10kHz, then a climb to 0.002% at 20kHz.

THD ratio (unweighted) vs. output (analog)

The chart above shows THD ratios measured at the output as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 1mVrms level, THD values measured around 0.06%, dipping down to nearly 0.00002% at 3-5Vrms. It’s important to highlight just how low the Director Mk2’s THD values are, as they are flirting with the inherent THD performance of the Audio Precision of 0.000015% at these voltage levels. Also important to note here is that it was not possible to sweep the input voltage high enough to see the 1% THD point. This is because the Director Mk2 can handle up to 33Vrms (input or output), while, the AP can only output 26.7Vrms. Also, the Director Mk2 has a maximum gain of -0.1dB, thereby limiting the output to around 26Vrms.

THD ratio (unweighted) vs. output (16/44.1 and 24/96)

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD range from 0.3% to 0.0002%, while the 16/44.1 ranged from 2% down to 0.0005%.

THD+N ratio (unweighted) vs. output (analog)

The chart above shows THD+N ratios measured at the output as a function of output voltage into 200k ohms with a 1kHz input sine wave. At the 1mVrms level, THD+N values measured around 2%, dipping down to around 0.0002% at 20Vrms.

THD+N ratio (unweighted) vs. output (16/44.1 and 24/96)

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green). The 24/96 outperformed the 16/44.1 data, with a THD+N range from 5% down to 0.001% (right channel), while the 16/44.1 ranged from 20% down to 0.003% at 4Vrms. For the 24/96 data, the right channel outperformed the left by about 1-2dB.

FFT spectrum – 1kHz (analog at 2Vrms)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the output into a 200k-ohm load. Below 1kHz, we see peaks due to power-supply noise at 60Hz (-135dBrA, or 0.00002%), 120Hz (-135dBrA), 180Hz (-125dBrA, or 0.00006%), and beyond. Above 1kHz, at first glance, it appears that there’s a peak at 3kHz (third signal harmonic) at -115dBrA. However, when zoomed in . . .

. . . we find that this is actually a noise peak at 3.02kHz, and that the signal harmonic is at a vanishingly low -149.5dBrA, or 0.000003%. All signal-harmonic peaks are extremely low for the Director Mk2, and buried below and between a multitude of noise peaks.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see clear signal harmonics at -110dBrA, or 0.0003% (2kHz), and -100dBrA, or 0.001% (3kHz).

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. We see signal harmonics at -110dBrA, or 0.0003% (2kHz), and -100dBrA, or 0.001% (3kHz), as well as lower-level signal harmonics at 4/5/6kHz at around -130dBrA, or 0.00003%, and below. Power-supply noise peaks are just visible to the right of the main signal peak, at 60Hz (-140dBrA, or 0.00001%) and 180Hz (-140/130dBrA, or 0.00001/0.00003%, for the left and right channels).

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. The primary signal peak is at the correct amplitude and there are no visible signal harmonics. The peak that appears to be at 3kHz is actually just above 3kHz and is a noise artifact.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sine-wave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96 at -90dBFS. The primary signal peak is at the correct amplitude. The peak that appears to be at 3kHz is actually just above 3kHz and is a noise artifact. Power-supply noise peaks are clearly visible to the right of the main signal peak, at 60Hz (-140dBrA, or 0.00001%) and 180Hz (-140/130dBrA, or 0.00001/0.00003%, for the left and right channels).

FFT spectrum – 50Hz (analog at 2Vrms)

Shown above is the FFT for a 50Hz input sine-wave stimulus measured at the output into a 200k-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. Here we can clearly see how vanishingly low the signal harmonics are, where we see the second harmonic (100Hz) at -145/-150dbRA, or 0.000006/0.000003% (left/right), and the third harmonic (150Hz) at -140dBrA, or 0.00001%. The worst-case power-supply-noise peaks are at 180Hz (third harmonic) and 300Hz (fifth harmonic), both around -130dBrA, or 0.00003%.

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

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 200k-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 -125dBrA, or 0.00006%, while the third-order modulation products, at 17kHz and 20kHz, are at worst at -120dBrA, or 0.0001%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 16/44.1)

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 200k ohms for the coaxial input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4.3Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is near -115dBRA, or 0.0002%, and the third-order modulation products, at 17kHz and 20kHz, are slightly higher, at or above -110dBrA, or 0.0003%.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 24/96)

Shown above is an FFT of the intermodulation (IMD) products for an 18kHz + 19kHz summed sine-wave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 4Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -120dBRA, or 0.0001%, while the third-order modulation products, at 17kHz and 20kHz, are higher, at just above and below -110dBrA, or 0.0003%.

Square-wave response (10kHz)

Above is the 10kHz square-wave response at the output into 200k ohms. Due to limitations inherent to the Audio Precision APx555 B Series analyzer, this graph should not be used to infer or extrapolate the Director Mk2’s slew-rate performance. Rather, it should be seen as a qualitative representation of its very high bandwidth. An ideal square wave 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 Director Mk2’s reproduction of the 10kHz square wave is squeaky clean, with very sharp edges devoid of undershoot and overshoot, confirming its high bandwidth.

Diego Estan
Electronics Measurement Specialist