Usual caveat: Measurements are merely slices of reality. They can never encompass the full reality of listening experience. Yet when properly interpreted, they do reveal underlying physical mechanisms that correlate, to a limited degree, with listening experience. With the recognition of their limitations and capabilities, they can be a useful guide in evaluating the performance of audio equipment.
FFT Spectrum @ 0 dBFS
FFT (Fast Fourier Transform) spectrum of 1 KHz, 0 dBFS (4 Vrms balanced) signal. The 2nd and 3rd harmonics are below -120 dB (0.0001 %). The higher harmonics are around -130 dB or below, the limit of the test equipment. Also, note that the frequency spectrum is free from power line related noise, i.e., 60Hz and its harmonics.
FFT Spectrum @ -140 dBFS
This measurement with a -140 dBFS signal reveals the DAC's ability to resolve the smallest signal in a 24-bit system. Neither averaging nor dither was used in the FFT in order to expose the raw performance of the DAC. The -140 dBFS signal is clearly resolved, well above the noise floor.
Intermodulation Distortion (IMD)
IMD 19K+20KHz @ 0 dBFS (4 Vrms balanced). The two tones at the upper limit of the audible range push the DAC's ability to track high-frequency signals. The 2nd order difference product falls at 1 KHz and is around -130dB (0.00003%). The 3rd order IMD products produce symmetrical sidebands around 19K/20KHz and are about -120dB.
FFT spectrum of 11.025 KHz signal sampled at 44.1KHz. Very long FFTs, 1024K points averaged 4 times, bring out the smallest frequency-coherent jitters, which would manifest itself as sidebands symmetrically located around the signal. The FFT spectrum is clean, with all the residues around -150 dB. Only one jitter-related double sideband is visible, located +/- 1KHz from the signal at -150 dBFS, For more details about jitter measurements, read "Measuring Jitter".
Crosstalk (Channel Separation)
The D2's channel separation pushes the limit of the test equipment. What is shown here is essentially that of the test equipment. The crosstalk is below -140 dB for most of the audible frequency range, increasing only sightly at 20KHz. This is just one of the signs of care that has gone into the design of the power supplies and the PCB layout.
Headphone Amp FFT
FFT spectrum of the headphone amplifier output, with a 1 KHz, 4 Vrms signal into a 30-ohm load. Its characteristic is similar to that of the line-level output. The harmonic distortion is dominated by the 2nd around -117 dB and the 3rd around -124 dB. The higher harmonics are -130 dB or below.
Headphone Amp IMD
Headphone amp IMD 19K+20KHz, 4 Vrms signal into a 30-ohm load. The two tones at the upper limit of the audible range push the headphone amp's ability to track high-frequency signals. The 2nd order difference product falls at 1 KHz and is around -120dB (0.0001%). The 3rd order IMD products produce symmetrical sidebands around 19K/20KHz and are about -118dB.
Headphone Amp THD+N
Headphone amp THD + N (Total Harmonic Distortion + Noise) with various loads (30, 62, and 300 ohms). The steps around 0.1 and 3.3 Vrms are caused by the level switching inside the test equipment, suggesting that the measurements are limited by the test equipment. The performance is essentially independent of the load impedance, showing only a slight difference at the highest level.
Headphone Amp Frequency Response
The frequency response of the headphone amp is practically flat in the audible range, with only 0.15 dB roll off at 20 KHz. The absence of AC coupling capacitors in the signal path means that the low frequency response extends down to DC.
Headphone Amp Output Impedance
The headphone amp output impedance is under 0.035 ohms in the audible range, rising only slightly at higher frequencies. Such a low impedance output ensures a flat response even under large variations in the headphone impedance.
Notes on Measurement Condition:
- All measurements are performed using the Audio Precision APx525 system. The line outputs of the D2 DAC were terminated with 100Kohms. Unless otherwise indicated, FFTs were 64K points, averaged 4 times, with the AP-equiripple window.
- Download the D2 DAC Manual