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[Introduction:]

Introduction:

The THD+N vs Amplitude measurements are good for testing a devices linearity and noise performance and see whether there is any dependence of noise with signal level. In this test a stimulus in the form of a 1kHz sinewave is fed to the EUT (equipment under test) to see how it responds. In an ideal test what is fed into the EUT should also be present at its output in proportion and without any other frequency components or artifacts present if the EUT is perfectly linear. However in the real world some non linearity is always present and we expect to see some frequency artifacts in the form  harmonics of the original fundamental test frequency. The random uncorrelated noise is also added to this measurement.

We also use this test to show up a flaw in some Esstech Sabre DAC implementations in the form of a hump in THD measurements at a certain band of signal amplitudes.

[Introduction:]

Introduction:

The IMD vs Amplitude measurements are good for testing a devices linearity. In this test, usually two or more tones are mixed together and fed to the EUT (equipment under test) to see how it responds. In an ideal test what is fed into the EUT should also be present at its output at the same relative amplitudes and without any other frequency components or artifacts present if the EUT is perfectly linear. However in the real world some non linearity is always present and we expect to see some frequency artifacts in the form of difference or summation of the two frequencies which show up on a frequency spectrum as side bands around the fundamental test frequencies.

We also use this test to show up a flaw in some Esstech Sabre DAC implementations in the form of a hump in IMD measurements at a certain band of signal amplitudes.

Test Methodology

From Amir's excellent website:-

Intermodulation Distortion

When an ideal linear system is fed two tones, it produces two tones. But when fed to a system with linearity errors, we get modulation frequencies above and below our two tones. This is called intermodulation distortion. There are many dual tone tests. For this, I have picked the SMPTE test which combines a low frequency (60 hz) with a high frequency (7 kHz) in a 4:1 ratio. Here is the explanation from Audio Precision:

The stimulus is a strong low-frequency interfering signal (f1) combined with a weaker high frequency signal of interest (f2). f1 is usually 60 Hz and f2 is usually 7 kHz, at a ratio of f1_f2=4:1. The stimulus signal is the sum of the two sine waves. In a distorting DUT, this stimulus results in an AM (amplitude modulated) waveform, with f2 as the “carrier” and f1 as the modulation.

In analysis, f1 is removed, and the residual is bandpass filtered and then demodulated to reveal the AM modulation products. The rms level of the modulation products is measured and expressed as a ratio to the rms level of f2. The SMPTE IMD measurement includes noise within the passband, and is insensitive to FM (frequency modulation) distortion.​

[Introduction:]

Introduction:

The Ultimate-Preamplifier has a built in precision digital waveform generator which is useful for doing spot checks and distortion measurements. When generating waveforms such as sinewaves it uses 32bit precision at 192kHz sample rate so one would expect the distortion of this generator to be very low and way below the distortion and noise of anything else in the chain such as the DAC.

Test Methodology:

To verify the performance of the built in generator we need to exclude anything else in the signal path such as the DAC in the preamp and the ADC in the measurement analyzer which will only swamp the measurements with their own distortions. We can do this in the same way we tested the on-board ADC by feeding the resultant digital signals words directly to the dScope III analyzer via the S/PDIF output on the back of the Preamp since the Coax output can handle the native 192kHz resolution of the test generator.

Conclusions:

From the test results below what can we say except to note that any noise and distortion artifacts are way below anything else in the chain and are so low they don't even indicate on the distortion meters ! So indeed the built in test generator can be used as a reference standard waveform generator !!

[Introduction:]

Introduction:

The Ultimate Preamplifier features a magnetic cartridge input that also incorporates the requisite inverse RIAA equalization curve. The phono amplification stage is a hybrid design and is a mixture of both analog and digital gain cells along with the inverse RIAA curve implemented in the digital domain exclusively by the DSP. This makes its frequency response extremely accurate to within 0.1dB with no inherent drift over time. What we are looking at when we measure the frequency response of the Phono stage should look something like this from  WikiPedia .



Conclusions:

The frequency response measurement of both channels of the Inverse RIAA response as implemented in the Ultimate Preamplifier shows a text book response with both channels displaying identical responses as you would expect from the accuracy of the digital domain

[Test Methodology:]

Test Methodology:

In this test we exclusively test the frequency response of the ADC by not including the the DAC in the measurements. We do this ny measuring the ADC output from an S/PDIF serial stream generated by the Preamp which we then feed directly into the digital input of the dScope. Since the Preamp samples at 192kHz by default. the Toslink output of the Preamp will not handle anything above 96kHz so we have to use the Coax out for this test. NB it is possible to switch the preamp into sampling at 48 or 96 kHz but by default the Preamp is shipped at its highest sampling rate so we use this for the test.

dScope Analog Output -> UP Analog Input

UP S/PDIF or Toslink output -> dScope S/PDIF or Tosiink Input

Conclusions:

Only 0.3dB down at 20Hz !!

[Test Methodology:]

Test Methodology:

There are a variety ways of measuring jitter but the single single RMS or peak jitter metric is not very revealing since it does not reveal the nature of the jitter, so a more detailed analysis such as the J-test method is used to test the susceptibility and tolerance of the device under test. The J-test methodology is described on the ASR forum J-test methodology 1 and J-test methodology 2

Test Results:

• J-test SPDIF input (Direct DAC mode)
• J-test Toslink input (Direct DAC mode)
• J-test USB input (Direct DAC mode)
• J-test dScope III internal generator test
• J-test S/PDIF Input (Preamplifier mode using the SHARC ASRC)
• J-test S/PDIF Input 16-bit (Direct DAC mode)

Conclusion:

As one can see from the detailed spectra in the following tests any side bands from the J-test are buried way into the noise floor below the threshold of hearing and should not pose any audible issues at all ! In fact, because all three test results are very similar it is possible that we are measuring the jitter of the analyzer itself !! All in all an exceptional result and shows the effectiveness of the high quality oscillator used, the clock distribution network, the jitter reduction hardware in the Sabre DAC as well as the careful attention to detail that we placed in the design of the DAC and pcb layout

Compared with the DEQX Pre-mate as measured by Stereophile we can see that the Ultimate-Preamplifier indeed has way superior jitter performance

This thread is about improving the performance of an already great bit of test gear. If you don't have the necessary skills to do these mods or your instrument is still under manufacturers warranty then we don't suggest you perform these mods.

According to the specs the Prism dScopeIII has an equivalent input noise voltage of about 1.2uV which is equivalent to about -116dBu and which was also confirmed by NewAvGuy on his blog. We also confirmed this on our own dScope III by running a similar test by shorting the inputs and measuring the noise level over a 20kHz bandwidth and we also got close to -116 dBu which is only a few dB lower than the Audio-Precision analyzers which sell for a lot more than the dScope III



Is it possible to improve on this ? The ADC converter use in the dScopeIII is a AKM AK5394A which has a S/N 123dB so there should be some room for improvement. Removing the top cover of the dScope III reveals that all of the ADC and DAC circuitry is contained on a riser board that plugs into the switched attenuator board. On further observation we note some of the IC's are plugged into sockets which would indicate that the manufacturer was contemplating replacing the chips with better devices as they became available or simply they may have been trying different chips from a batch until they found the ones that met their specs. Bear in mind the design of this analyzer dates back to 2001 so it is quite remarkable that they were able to achieve these specs with the silicon they had at the time ! We note that the two IC's closest to the ADC are standard NE5532 dual opamps which would be part of the buffers/level-shifters used to drive the ADC. As these devices have an equivalent noise voltage of 5nV/rtHz it would be a simple exercise to replace these with devices with better noise an distortion performance. As high quality opamps with DIP packaging are slim pickings these days we ended using the tried and true LME49720 which is a drop in replacement with much improved characteristics over the venerable NE5532. The LME49720 has an equivalent noise voltage of 2.7nV/rtHz and an order of magnitude lower distortion so we could expect at most an improvement of noise reduction of 5.35dB but in reality it would be somewhat less. Lets see what happens



And from the new noise measurements we see an improvement of about 3dB from -116dBu down to -119dBu so well worth the few dollar investment of these opamps The question arises could we improve on this yet again ? Since the DIP style of package is becoming very rare for audio opamps, to get a lower noise floor in a dual opamp configuration would mean that we'd have to go to an SMD package and use some sort of SMD to DIP adapter so probably not worth it for chasing down a few extra dB's. Maybe an experiment for a rainy day but the current mod is quite easy to achieve for an hours work and a few extra dollars for the opamps

[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