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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

Volume position Channel deviation
min 0.9dB
25% 0.246dB
50% 0.200dB
75% 0.137dB
max 0.119dB

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.

Parameter Manufacturer SoundStage! Lab
Maximum input and output voltage 32.5dBu (33Vrms) >26.7Vrms
Input impedance (RCA) 47k ohms 89.1k ohms
Input impedance (XLR) 20k ohms 21.7k ohms
Output impedance 75 ohms 74.3 ohms
Frequency range (-3dB) 4Hz - 300kHz 1Hz(-3dB), 200kHz(-1dB)
Crosstalk (1kHz, ref 0.775Vrms) -108dB -111dB
THD (1kHz, ref 0.775Vrms) 0.000992% <0.00009%
Noise (A-weighted, ref 0.775Vrms) -102.5dB <-100dB
Dynamic range (ref maximum output voltage) 135dB *132dB

*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):

Parameter Left channel Right Channel
Crosstalk, one channel driven (10kHz, analog) -92.9dB -111.9dB
Crosstalk, one channel driven (10kHz, 16/44.1) -97.8dB -111.5dB
Crosstalk, one channel driven (10kHz, 24/96) -97.9dB -111.1dB
DC offset <0.4mV <0.3mV
Dynamic range (A-weighted, 16/44.1) 95.8dB 96.2dB
Dynamic range (unweighted, 16/44.1) 93.0dB 93.4dB
Dynamic range (A-weighted, 24/96) 110.5dB 111.3dB
Dynamic range (unweighted, 24/96) 102.0dB 104.4dB
IMD ratio (18kHz and 19kHz stimulus tones, analog) <-115dB <-117dB
IMD ratio (18kHz and 19kHz stimulus tones, 16/44.1) <-96dB <-96dB
IMD ratio (18kHz and 19kHz stimulus tones, 24/96) <-96dB <-97dB
Input impedance 21.7k ohms 21.4k ohms
Maximum gain -0.115dB -0.234dB
Maximum output voltage >26.7Vrms >26.7Vrms
Output impedance 74.3 ohms 74.2 ohms
Noise level (A-weighted, analog) <8uVrms <8uVrms
Noise level (unweighted, analog) <18uVrms <17uVrms
Noise level (A-weighted, 16/44.1) <71uVrms <70uVrms
Noise level (unweighted, 16/44.1) <106uVrms <98uVrms
Noise level (A-weighted, 24/96) <17uVrms <16uVrms
Noise level (unweighted, 24/96) <42uVrms <30uVrms
Signal-to-noise ratio (A-weighted, analog) 115.1dB 115.0dB
Signal-to-noise ratio (unweighted, analog) 108.6dB 108.7dB
THD ratio (unweighted, analog) <0.00004% <0.00004%
THD ratio (unweighted, 16/44.1) <0.001% <0.001%
THD ratio (unweighted, 24/96) <0.00095% <0.00095%
THD+N ratio (A-weighted, analog) <0.00018% <0.00018%
THD+N ratio (unweighted, analog) <0.0004% <0.0004%
THD+N ratio (A-weighted, 16/44.1) <0.002% <0.002%
THD+N ratio (unweighted, 16/44.1) <0.0027% <0.0025%
THD+N ratio (A-weighted, 24/96) <0.0011% <0.0011%
THD+N ratio (unweighted, 24/96) <0.0013% <0.0012%

Frequency response (analog)

frequency response

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)

frequency response vs sample rate 1644-1 2496 24192

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)

frequency response phono mm

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)

phase response vs sample rate 1644-1 2496 24192.png

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)

digital linearity 1644 1 2496

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)

impulse response 1644 1 2496

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)

jtest coaxial 2448

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)

jtest optical 2448

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)

jtest coaxial 2448 2khz 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)

jtest coaxial 2448 2khz 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)

wideband fft noise plus 19 1khz 1644 1kHz

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)

thd ratio unweighted vs frequency vs load

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)

thd ratio unweighted vs frequency vs load 2496

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)

thd ratio unweighted vs frequency vs sample rate

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)

thd ratio unweighted vs output

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)

thd ratio unweighted vs output 1644-1 2496

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)

thd n ratio unweighted vs output

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)

thd n ratio unweighted vs output 1644-1 2496

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)

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. 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 . . .

FFT spectrum 1khz

. . . 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)

fft spectrum 1khz 1644 1 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)

fft spectrum 1khz 2496 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)

fft spectrum 1khz 1644 1 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)

fft spectrum 1khz 2496 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)

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 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)

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 worst at -120dBrA, or 0.0001%.

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

intermodulation distortion fft 18khz 19khz summed stimulus 1644-1

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)

intermodulation distortion fft 18khz 19khz summed stimulus 2496

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)

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

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