Topics

[Applies to:]

Applies to: EUTs with analogue or digital inputs and analogue or digital outputs.
Aim: To measure the output noise of the EUT with no signal present.
Method
If the EUT has a digital input, it is fed with a digital-zero signal; if the EUT has an analogue input it is
terminated with 50R (or 25R if unbalanced).
The RMS amplitude of the output noise of the EUT is measured via the standard weighting filter. The
result is reported for both channels, in dBFS (in the case of an analogue measurement, this is
equivalent to dB below the specified D/A line-up), and designated as 'dBFS CCIR-RMS'.

[Level-dependent logarithmic gain]

Level-dependent logarithmic gain
Applies to: EUTs with analogue or digital inputs and analogue or digital outputs.

Aim: To measure the change in gain of the EUT with signal amplitude, sometimes called 'deviation
from level linearity'.

Method
A 997Hz sinusoidal stimulus is applied at -5dBFS. The output of the EUT is measured after passing
through a 1/3 octave 997Hz band-pass filter. The gain of the EUT is calculated as being the ratio of its
output amplitude to its input amplitude. The narrow-band noise floor of the EUT is also measured
using the same band-pass filter, but with the stimulus muted.
The stimulus is reapplied, and its amplitude is then reduced in 5dB steps, until it is within 5dB of the
measured narrow-band noise floor, noting the output amplitude at each step and calculating the
inferred EUT gain. The results are graphed for each channel, with relativity to the gain measured at â€"
5dBFS expressed in dB, as a function of the applied amplitude. At the end of the test, a worst-case
gain deviation (in dB) is also reported for each channel.
When the A/D and D/A portions are available separately, this test should be repeated for each section
independently.

Conclusion:
The Ultimate Preamplifier exhibits exception linearity all of the way down to -130dBFS yielding at least 22 bits of resolution !

References
http://resources.prismsound.com/tm/dS3_Applications_Manual_A4nc.pdf

This topic has been moved to Performance and Measurements.

https://analog-precision.com/forum/performance-and-measurements-12/measurement-equipment-14/

[Introduction]

Introduction

The Ultimate Pre-amplifier features a soft clipping feature which when enabled, clips in much the same way a push-pull tube amplifier does but does so more accurately and consistently since it is implemented in the digital domain.

Measurements

• Soft Clipping of 1kHz sinewave at 0dBFS.
• Soft Clipping of 1kHz sinewave at -10dBFS.
• Soft Clipping of 1kHz sinewave at -20dBFS.
• Soft Clipping of 1kHz sinewave at -30dBFS.
• Soft Clipping Sweep of THD + N vs Amplitude of 1kHz sinewave.

Conclusion

The soft clipping works extremely well and in textbook fashion according to the measurements below. In fact it works so well there are no more distortion artifacts above 3rd order so even the most ardent of tube aficionados would welcome its tube like characteristics which can be turned on or off at will.

[Introduction]

Introduction

Whilst all of the other tests posted so far are looking for the best possible distortion and noise measurement, in this case we are going in the opposite direction and purposely trying to add distortion in the same way a tube circuit does.

Measurements

In the following tests we operate different tube profiles whilst keeping the grid bias constant. In all of these tests we are mimicking the way a tube Preamplifier behaves which is exposed to a full scale signal all of the way before the volume control which is unlike a Tube Power Amplifier where the distortion increases with increasing volume control !

• "Signal Tube" emulation mode distortion test at -3dBFS
• "Power Pentode" Tube emulation mode distortion test at -3dBFS
• "Power Triode" Tube emulation mode distortion test at -3dBFS

Conclusion

These are just three examples of what can be achieved with Tube Emulation as the grid bias control and amplifier type allows many different permutations of altering the tube linearity and hence the distortion profiles !! For tube and SET aficionados the pure 2nd harmonic distortion should be music to their ears !!

This topic has been moved to Performance and Measurements.

https://analog-precision.com/forum/index.php?topic=11.0

[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

[Hypex NC252MP THD+N vs Power @ 1kHz into 4 ohms, Single Channel Driven]

And now for the Hypex NC252MP Ncore power amp measurements. The Ultimate amplifier uses two of these modules to give 4 channels of additional amplification. The name plate rating of these amps are 250 and 200 watts into 4 and 8 ohms respectively @1% THD. We measured the power to be 306 and 159 watts into 4 ohms and 8 ohms respectively at 1kHz and 1% THD. Whilst the 8 ohm power measurement is down on the manufacturers name plate rating the doubling down of power into 4 ohms shows how well regulated the switch mode power supplies are. 

Hypex NC252MP THD+N vs Power @ 1kHz into 4 ohms, Single Channel Driven



Hypex NC252MP Power at 1%THD @ 1kHz into 4 ohms, Single Channel Driven



Hypex NC252MP THD+N vs Power @ 1kHz into 8 ohms, Single Channel Driven



Hypex NC252MP Power at 1%THD @ 1kHz into 8 ohms, Single Channel Driven

[Hypex NC502MP THD+N vs Power @ 1kHz into 4 ohms, Single Channel Driven]

The Hypex NC502MP is the most powerful amplifier used in the Ultimate Amplifier and represents the first two channels (1-2) out of the 8-channels of amplification. Typically it would be used to drive the woofers in a stereo active speaker system so it is imperative that it does this without any problems. With a name plate power rating of at least 500 and 350 watts per channel into 4 and 8 ohms respectively the measurements we made did not disappoint. The following measurements were made using the dScope III by sweeping the power from 10mW to beyond clipping and plotting the THD at 1kHz. The maximum power output is measured at 1% THD @ 1kHz the same as in the Hypex data sheet. The results show that the 4 ohm single channel measurements of 750 watts @ 1% THD showed that the manufacturers power rating is very conservative for these modules. Also there is doubling of power from 8 ohms to 4 ohms which shows that the switch-mode power supplies are extremely well regulated which is a testament to whoever designed the power supplies ! The low actual distortion measurements also confirm the measurements presented in the manufacturers datasheets for this module

Hypex NC502MP THD+N vs Power @ 1kHz into 4 ohms, Single Channel Driven



Hypex NC502MP Power at 1%THD @ 1kHz into 4 ohms, Single Channel Driven



Hypex NC502MP THD+N vs Power @ 1kHz into 8 ohms, Single Channel Driven



Hypex NC502MP Power at 1%THD @ 1kHz into 8 ohms, Single Channel Driven

Measurements are done with a variety of instruments namely :-

• Prism dScope III Audio Analyzer,
• Custom designed LPF for measurement of class-D amplifiers similar to Prism ds-LPF
• Agilent 34401A 6.5 digit Digital Multi-meter
• Tektronix Digital Oscilloscope models, TDS784A, TDS3012, TDS7054
• Tektronix CRO model 2246A
• Leader LDC-824 6.5 digit 500MHz Frequency Counter with Oven Controlled crystal oscillator
• E-MU 0404 external 24 bit 192KHz sound card

Although one of the benefits of class-D amplifiers that has been touted for so long is efficiency, in practice the amps can still generate a sufficient amount of heat and the switch-mode power supplies also contribute to this heat. The Hypex NCore amps are no exception. A look at the data sheet shows that for the NC502MP producing 400 watts a channel into 4 ohms the amps will dissipate about 200 watts which equates to around 80% efficiency. Whilst this looks good on paper, 200 watts of power dissipation will quickly raise the temperature of the small plate heat-sink on the amp module to unsafe levels unless this heat can be channeled away quickly. Then there is the question of dealing with 3 more stereo amps for a total channel count of eight amps each dissipating a given amount of power depending on the frequency response and program content. Although the power output of the amps should diminish the higher the frequency range they are ask to handle, the thermal requirements should not be assumed to be negligible. This a mistake many people make with class-D amps by mounting them on a thread-bare chassis with minimal thermal capacity thus questioning the long-term reliability of the design as a whole.

Our custom designed case works exceptionally well at dissipating huge amounts of thermal energy away from the amp. All of the amps are mounted on a 10 mm thick aluminium heat spreading plate with a large surface area the size of the bottom of the case. This acts as a transient thermal sink with a huge thermal capacity and which can sink a huge amount of transient energy without raising the temperature quickly to unsafe levels. Then the wrap around case with a large amount of surface area acts to slowly dissipate away this energy using convection.

Add to this is our thermal management system which monitors and reports temperature extremes on a channel by channel basis whilst maintaining temperatures inside the case at safe levels.  We deploy a 60mm fan chosen for its low noise level along with a dedicated fan controller which employ both a fan and thermal closed loop control system which carefully ramps up the fan speed over a range of temperatures in order to keep the temperature of the system at a sustainable level and without generating intrusive noise.

The firmware has a diagnostic mode which can continually report the amplifier status over a serial port, including temperatures and fan speed. We can see the first two channels are working hard because the on-board thermistors are reporting higher temperatures than the other channels which are sitting idle. The last two channels are not showing anything because the HF100 modules do not include the thermistors. The temperature of the heat spreading plate has risen to 34 degrees and the fan has already spun up to 1500 RPM to get rid of any excess heat inside the case.  If the thermistor temperature rises to 80 degrees C the respective channel indicator LED will turn white. Further increases above 90 degrees will shut down the amplifier until the temperature is reduced to 80 degrees.

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