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[Noise spectra and RMS voltage of Channel-1/2 Balanced Output]

Noise spectra and RMS voltage of Channel-1/2 Balanced Output



Noise spectra above with expanded scale

[Test Results:]

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 !

[Test Results:]

Test Results:

• Noise performance - Balanced Output
• Noise performance - Unbalanced Output
• Noise performance - Balanced Output - A-weighted
• Noise performance - Unbalanced Output - A-weighted

Conclusions:

The noise floor of the balanced output of the UP/2+ is only 6dB above the noise floor of the dScope III audio analyzer which is approaching -116dBu or 1.2uV  RMS ! Bear in mind that unlike standard DACs we have incorporated a buffer amplifier on each balanced output to boost the output level from the output of the IV converters which will of course add some additional noise of its own. Also in the 2nd image capture below we have expanded the frequency axis from 0-1kHz to show the almost non existent mains components sitting above the noise floor which is a tribute to the balanced design.

The noise performance of the unbalanced output is slightly higher than the balanced out but still does not disappoint ! On the expanded frequency scale one can clearly see the mains artifacts that are not present in the balanced spectrum but are still buried way down below 120dB !! This is caused by leakage flux from the transformer 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 !!

dScope III Noise Performance:

• Unmodified dScope III Noise with inputs shorted

• Upgraded dScope III Noise with inputs shorted

• Upgraded dScope III Noise with inputs shorted (A-weighted)

[References:]

Measurements are done with a variety of instruments namely :-

• Prism dScope III Audio Analyzer,
• Agilent 34401A 6.5 digit Precision Digital Multi-meter
• Tektronix Digital Oscilloscope models, TDS784A, TDS3052, TDS7054
• Tektronix CRO model 2246A
• Tektronix Arbitrary Function Generator model AFG310
• Hewlett Packard AC RMS Voltmeter model 3400A
• Leader LDC-824 6.5 digit 500MHz Frequency Counter with Oven Controlled crystal oscillator

References:

• dScope III Specifications


• Handy online Conversion Calculator


• A description of measurements and procedures by Amir from ASR


• A description of measurements and procedures by Amir from ASR


• Test Methods and Comparison between measurement equipment by the NewAVGuy

We are always on the lookout for new and useful test equipment to add to our lab, and one of the things we have always wanted was a decent audio analyser. Although we have a couple of external sound cards which have been quite useful, nothing beats a dedicated audio analyzer, so when a Prism dScope III came along at the right price we jumped at the chance and purchased it. The Audio-Precision APx555 at $28,000 USD was way out of our budget and save for a few dB less dynamic range, in terms of over all features and customization, the dScope III offers everything we need a plus a whole lot more. We have already used it to chase down a few illusive issues in our existing preamp design and incorporated the changes in our new Preamp designs. Such is the benefit of having access to the right test gear