DAC - THD + Noise @ 1kHz

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Test Results:

• THD + Noise (Balanced Output)
• THD + Noise (Unbalanced Output)
• THD + Noise (Balanced Output - 80kHz bandwidth)

Conclusions:

The Ultimate-Preamp 2/+ has vanishing low levels of distortion and noise, so much so that it is approaching the limits of our dScope III audio analyzer and most likely exceeding it !! Even at much higher output voltages compared to your typical DAC the UP2/+ does not disappoint as shown by the following screen shots of the THD+N measurements 😁 The difference in distortion between the two channels is most likely due to the differences in the analyzer ADC and not the Preamp because reversing the channels does not appear to have any effect on this 😉 To get better measurements we would need access to an analyzer such as the Audio Precision APx555 which nulls out the fundamental so that it can more accurately read the harmonics from the output of the Preamp rather than the added distortion from its own ADC. However, we are not complaining about these results because it looks like all of the work we have done on the DAC and post DAC circuitry in this new Preamp has payed off

The THD + Noise performance of the unbalanced output is a whisker higher than the balanced output but still does not disappoint ! Below 1kHz one can clearly see the mains artifacts that are not present in the balanced output spectrum but are still buried way down below the threshold of hearing of -120dB and so of no concern !! These artifacts are caused by leakage flux from the transformer used in the linear power supply setting up earth loops in the internal wiring from the DSP board to the Unbalanced output board. Yes folks, balanced always wins out simply because common mode noise is cancelled out !! Above the 1kHz fundamental test signal we can see the usual harmonic distortion spikes mainly attributable to the distortion of the ADC used inside the analyzer. There are no spurious noise spikes from an external switch mode power suppliy simply because we don't use one and the on-board switching regulator used to power all of the digital circuitry runs at 500kHz which is well outside the audio bandwidth !

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Output level of 25dBU = 13.7 VRMS on Channels-1/2

Note the differences between the two THD values is probably due to the measurements dwarfing the lower limits of the analyzer. (Anyone care to lend me an Analog-Precision APx555 ??)

Output level of 3 VRMS on Channels-1/2

Output level of 1.9 VRMS on Channels-1/2

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THD + Noise spectra and RMS voltage of Channels-1/2 unbalanced output

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In this test we increased the analyzer bandwidth to 80 kHz and increased the sampling rate from 48kHz to 192kHz to view the ultrasonic spectrum above the audio bandwidth just to see if there was any surprises. As can be seen from the spectrum below it is fairly clean with some artifacts centered at 55kHz at extremely low level of around -120dB which could be some slight peaking in the IV circuit so nothing to be worried about because it is at such low level and outside the audible bandwidth. Running a simulator on the implimentation of the IV converter suggest no peaking at all so something else at play because the actual peaking is dependent on audio level. Further thoughts are that the peaking may be some beating with the on-board switching regulator but at 120dB down we have done a pretty good job at suppressing it. The broad peaking indicates some spread spectrum which is typical of a switching regulator that continuously regulates itself by varying it's pulse width.