Hegel Music Systems D50 Digital-to-Analog Converter Measurements

Link: reviewed by George de Sa on SoundStage! Hi-Fi on June 1, 2025

General Information

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

The Hegel Music Systems D50 was conditioned for 30 minutes at 0dBFS (volume set to 2.64Vrms out) into 200k ohms before any measurements were taken.

The D50 offers two coaxial S/PDIF digital inputs (RCA and BNC), two optical S/PDIF inputs (TosLink), one AES-EBU balanced digital input (XLR), and one USB input. There are two sets of line-level outputs (balanced XLR and unbalanced RCA). Comparisons were made between unbalanced and balanced line-level outputs, but no appreciable differences were seen in terms of THD and noise; however, 1kHz FFTs are provided for both balanced and unbalanced outputs.

The analyzer’s input bandwidth filter was set to 10Hz to 22.4kHz for all measurements, except for frequency response (DC to 1 MHz), FFTs (10Hz to 90kHz), and THD vs. Frequency (10Hz to 90kHz). The latter to capture the second and third harmonic of the 20kHz output signal.

Published specifications vs. our primary measurements

The table below summarizes the measurements published by Hegel for the D50 compared directly against our own. The published specifications are sourced from Hegel’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), unless otherwise stated, assume a 1kHz sinewave at 0dBFS (24/96) at the input, 2.64Vrms at the balanced output into 200k ohms, and a measurement input bandwidth of 10Hz to 22.4kHz, and the worst-case measured result between the left and right channels:

Our primary measurements revealed the following using the coaxial input and the balanced line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS, 200k ohms loading, 10Hz to 22.4kHz bandwidth):

* due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value

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

The plot above shows the D50’s frequency response as a function of sample rate. The blue/red traces are for a 16-bit/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. 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 brickwall-type filtering right at half the respective sample rates. The -3dB points for each sample rate are: 21, 46, and 92kHz respectively. 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.

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

Above are the phase-response plots from 20Hz to 20kHz for a 0dBFS input signal as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital, the purple/green traces are for a 24/96 dithered digital input signal, and finally orange/pink represents 24/192 from 5Hz to 96kHz.  The 16/44.1 stream shows -180 degrees of phase shift at 20kHz, the 24/96 data roughly -10 degrees at 20kHz, and the 24/192 shows no phase shift within the audioband.

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 D50. For this test, the digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS and the output was 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 24/96 input data is essentially perfect down to -120dBFS, while the 16/44.1 input data performed well, only over-responding by 2.5/1dB (left/right) at -120dBFS. The 24/96 data yielded such superb results that we extended the sweep down to . . .

. . . -140dBFS. Above we see that even at -140dBFS, the D50 is only over/undershooting by less than 1 dB between -140 and -130dBFS. This is an exemplary linearity test result.

Impulse response

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, fed to the coaxial digital input, measured at the balanced outputs, into a 200k ohm-load for the left channel only. We can see that d50 yields an impulse response with essentially no pre-ringing but sustained post-ringing.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone

The plot above shows a fast Fourier transform (FFT) of the D50’s balanced-line level output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). The steep roll-off above 20kHz in the white-noise spectrum shows the implementation of a brickwall-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 highly suppressed at -100dBrA.

J-Test (coaxial RCA 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 theD50. J-Test was developed by Julian Dunn the 1990s. It is a test signal—specifically, a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24 bits). 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 sinewave (the main peak). In addition, an undithered 250Hz squarewave 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 strong J-Test result, with peaks visible, but only below the -140dBrA level.

J-Test (optical input)

The optical input shows essentially the same J-Test result as with the coaxial RCA input.

J-Test (coaxial BNC input)

The coaxial BNC input shows essentially the same J-Test result as with the coaxial RCA input.

J-Test (AES-EBU input)

The AES-EBU input shows essentially the same J-Test result as with the rest of the inputs.

J-Test (coaxial RCA input, 2kHz sinewave jitter at 10ns)

The plot above shows the results of the J-Test test for the coaxial digital input (the optical input behaved the same) measured at the balanced line-level output, but with an additional 10ns of 2kHz sinewave jitter injected by the APx555, which would manifest as sideband peaks at 10kHz and 14kHz without perfect jitter immunity. The results do show visible sidebands but only below the very low -140dBrA level. This is further evidence of the D50’s strong jitter immunity.

J-Test (coaxial RCA input, 2kHz sinewave jitter at 100ns)

The plot above shows the results of the J-Test test for the coaxial digital input (the optical input behaved the same) measured at the balanced line-level output, but with an additional 100ns of 2kHz sinewave jitter injected by the APx555, which would manifest as sideband peaks at 10kHz and 14kHz without perfect jitter immunity. The results do show visible sidebands but only below the low -125dBrA level. This is further evidence of the D50’s strong jitter immunity.

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 differ by only about 10dB throughout the audioband, which is an indication that D50’s outputs are robust and can handle loads below 1k ohms with no difficulty. THD ratios into 200k ohms were extraordinarily low, ranging from 0.00003 to 0.00007% from 20Hz to 20kHz. These values are nearing the limits of what the APx555 can measure.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1, 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. The 24/96 data (right channel) consistently outperformed the 16/44.1 data by 15-20dB from 20Hz to about 1kHz due to the inherently higher noise floor at 16 bits (i.e., the analyzer cannot assign a THD value below the noise floor). THD ratios with 16/44.1 data ranged from 0.0002% from 20Hz to 1kHz, then down to just below 0.0001% past 10kHz.  24/96 THD ratios ranged from 0.00003-0.00007% from 20Hz to 20kHz.

THD ratio (unweighted) vs. output (16/44.1, 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). Once again, the 24/96 data outperformed the 16/44.1 data, with a THD range from 0.3% at 200uVrms to 0.00005% at 1.5-2.6Vrms, while the 16/44.1 ranged from 3% down to 0.0003% at the maximum output voltage of 2.6Vrms.

THD+N ratio (unweighted) vs. output (16/44.1, 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 3% down to 0.0002% at 2.6Vrms, while the 16/44.1 ranged from 30% down to 0.002% at the maximum output voltage of 2.6Vrms.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1; 16/44.1, 24/96)

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. 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 16/44.1 data yields IMD ratios from 2% down to 0.003% at 0dBFS. The 24/96 data yields IMD ratios from 0.06% down to 0.0005% from -15 to 0dBFS.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. The second (2kHz) and third (3kHz) signal harmonics are barely visible above the noise floor at -130dBrA, or 0.00003%. There are also no power-supply noise peaks to speak of to the left of the main signal peak.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. Due to the increased bit-depth, the noise floor is much lower compared to the 16/44.1 FFT at a very low -160dBrA. We see signal harmonics ranging from -130dBrA to below -150dBrA, or 0.00003% to 0.000003%, all the way to 20kHz (and beyond). The second (2kHz) signal harmonic shows a difference between the left (-130dBrA) and right (-140dBrA) channels. Here also, there are no power-supply noise peaks to speak of to the left of the main signal peak.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced output into 100k ohm for the coaxial digital input, sampled at 24/96. Other than a few very low level (-150dBrA) spurious noise peaks to the left of the main signal peak, this FFT is identical to the FFT for the balanced output.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. We see the signal peak at the correct amplitude, and no signal harmonics or noise peaks.

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

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/961 at -90dBFS. We see the signal peak at the correct amplitude, and one very low-level signal harmonic peak (3kHz) at -160dBrA, or 0.000001%.

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 sinewave 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, they would yield 2.6Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz), is not visible above the noise floor at -140dBrA, or 0.00001%, and the third-order modulation products, at 17kHz and 20kHz, are at -125dBrA, or 0.00006%.

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

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave 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, they would yield 2.6Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz), is at -150/-135dBrA (left/right), or 0.000003/0.00002%, and the third-order modulation products, at 17kHz and 20kHz, are at -130dBrA, or 0.00003%.

Intermodulation distortion FFT (line-level input, APx 32 tone)

Shown above is the FFT of the balanced line-level output of the D50 with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBFS reference, and corresponds to 2.6Vrms into 200k ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and below the absurdly low -150dBrA, or 0.000003%, level. This is a very clean IMD result.

Diego Estan
Electronics Measurement Specialist