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Questions For Interview With Bruno Putzeys & Lars Risbo About Purifi Audio


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7 hours ago, Miska said:

 

Traditional analog PWM class-D amps tend to have aliasing problems, when for example used with DACs that have leaky filters or images due to incomplete reconstruction.

 

Hi Jussi!  Perhaps this in part explains why I prefer non leaky filters in my DAC with my Ncore amp(s).  It is my understanding that you prefer filters with heavy stop band attenuation generally, right?

Also note that Mr. Putzeys has acknowledged this tendency, and has suggested that the new Purifi modules are much less finicky when it comes to use with DACs with high out of band noise.

SO/ROON/HQPe: DSD 512-Sonore opticalModuleDeluxe-Signature Rendu optical with Well Tempered Clock--DIY DSC-2 DAC with SC Pure Clock--DIY Purifi Amplifier-Focus Audio FS888 speakers-JL E 112 sub-Nordost Tyr USB, DIY EventHorizon AC cables, Iconoclast XLR & speaker cables, Synergistic Purple Fuses, Spacetime system clarifiers.  ISOAcoustics Oreas footers.                                                       

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1 minute ago, barrows said:

You cannot fairly directly compare a sim with a working amplifier on the bench.  Simulations are good tools for development, but are not entirely accurate, as I am sure you realize.  This apples to oranges.

Have you ever done a fair comparison, a listening test, of a well implemented Ncore amplifier vs. a comparable class A/B design (approximately the same power output in the same system)?  If not, I would suggest that you do so, as you appear to have a theoretical bias against class D, you might just change your mind if you listened.

Hi,

I am not disputing that class D sounds good, or people like it more than class A/B.

 

The point of referring to the texts is that the simulation reflected closely the real world design. This is how the authors have been able to determine the distortion mechanisms.

 

In addition, class D measurements are restricted to 22kHz measurement bandwidth, and many amplifiers tested are measured using 80kHz bandwidth.

 

I have never claimed anything about the subjective impressions of class D.

 

Regards,

Shadders.

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21 minutes ago, Shadders said:

 

The signal output of the class A/B amplifier is a continuous signal, whereas for class D, it is an average of the discontinuous signal.

 

To get nitpicky with you, the unfiltered Class D signal is continuous as well, just switched quickly from rail to rail.  A digital music file is truly discontinuous so this must really bother you.

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7 minutes ago, Shadders said:

In addition, class D measurements are restricted to 22kHz measurement bandwidth, and many amplifiers tested are measured using 80kHz bandwidth.

 

Not that we hear or tweeters respond to much more than the 22kHz anyway...

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2. LG 77C1 - Marantz SR7005 - Apollon NC502MP+NC252MP - Monitor Audio PL100+PLC150+C265 - SVS SB-3000

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4. Phone - Tanchjim Space - Truthear Zero Red

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3 minutes ago, psjug said:

To get nitpicky with you, the unfiltered Class D signal is continuous as well, just switched quickly from rail to rail.  A digital music file is truly discontinuous so this must really bother you.

Hi,

The switching is not perfect, so this is why there is so much noise in the 20kHz+ band for class D. It is a sharp edge at each switch, so approximates a discontinuity, and is designed to be a discontinuity. The "continuous" aspect is a flaw in the implementation, which can only be reduced, never eliminated.

 

Regards,

Shadders.

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2 minutes ago, Matias said:

 

Not that we hear or tweeters respond to much more than the 22kHz anyway...

Hi,

I have not referred to any subjective aspect of class D. I simply stated that a class A/B design will always outperform class D.

 

Regards,

Shadders.

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4 minutes ago, psjug said:

Just as crossover distortion is a flaw in Class AB.  I don't understand why one is cause to dismiss the technology and the other is not.

Hi,

Crossover distortion is a non-linearity of a continuous function, which can be reduced through feedback, output stage bias, or the use of multiple output devices. The reduction in noise is such that it can be reduced to below the noise floor of a THD measuring device. If you examine the output of a class A/B amplifier, all signals are harmonically related to the input signal.

 

Class D switching is a discontinuity of binary level signal. The noise on the output of a class D amplifier is ALWAYS there - see slide 11 and 12 :

 

www.slideshare.net/shenpei/class-d-and-linear-power-amplifier-testing

 

Here is the simulated output of a basic class A/B with a triple EF stage into 4ohms load, 1kHz signal. Peak power into the 4ohm load is 500watts.

 

image.thumb.png.d63745a801b72f22a1321818469a0729.png

 

 

The peak signal is at 30dB (1kHz), the second harmonic is -135dB down, and third harmonic is -124dB down. The noise profile from 1kHz to 1MHz is non-existent. Compare that to the class D amplifier in slide 11 whose noise is at the -50dB level at least, across the band 100kHz to 700kHz.

 

Whether you can hear it or not, class A/B amplifiers exceed class D in performance. I have made no claim that people can hear above 20kHz.

 

Regards,

Shadders.

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27 minutes ago, psjug said:

Just as crossover distortion is a flaw in Class AB.  I don't understand why one is cause to dismiss the technology and the other is not.

Hi,

Not sure why you are indicating that i am dismissing class D technology. I never said that.

 

All i have said is that class A/B will outperform class D, and always will do.

 

I stated "flaw" in that class D implementation can never achieve the perfect implementation, and as such your statement that the switching is continuous, is what i am stating is a flaw of class D implementation.

 

I am following your lead here. I would not state that class D switching is continuous, it is a discontinuous signal, although it does have a finite rise and fall time.

 

Regards,

Shadders.

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10 minutes ago, Shadders said:

Hi,

Crossover distortion is a non-linearity of a continuous function, which can be reduced through feedback, output stage bias, or the use of multiple output devices. The reduction in noise is such that it can be reduced to below the noise floor of a THD measuring device. If you examine the output of a class A/B amplifier, all signals are harmonically related to the input signal.

 

Class D switching is a discontinuity of binary level signal. The noise on the output of a class D amplifier is ALWAYS there - see slide 11 and 12 :

 

www.slideshare.net/shenpei/class-d-and-linear-power-amplifier-testing

 

Here is the simulated output of a basic class A/B with a triple EF stage into 4ohms load, 1kHz signal. Peak power into the 4ohm load is 500watts.

 

image.thumb.png.d63745a801b72f22a1321818469a0729.png

 

 

The peak signal is at 30dB (1kHz), the second harmonic is -135dB down, and third harmonic is -124dB down. The noise profile from 1kHz to 1MHz is non-existent. Compare that to the class D amplifier in slide 11 whose noise is at the -50dB level at least, across the band 100kHz to 700kHz.

 

Whether you can hear it or not, class A/B amplifiers exceed class D in performance. I have made no claim that people can hear above 20kHz.

 

Regards,

Shadders.

Likewise, feedback can reduce Class D THD+N to extremely low levels in the audible band.  If you don't want the ultrasonic noise, I think that is a valid reason to prefer Class AB.  But it would be much easier just to say so and leave it at that.

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58 minutes ago, barrows said:

Hi Jussi!  Perhaps this in part explains why I prefer non leaky filters in my DAC with my Ncore amp(s).  It is my understanding that you prefer filters with heavy stop band attenuation generally, right?

 

Yes, I tend to aim for >= 192 dB stop-band attenuation... ext2 and xtr filters do >= 240 dB.

 

1 hour ago, barrows said:

Also note that Mr. Putzeys has acknowledged this tendency, and has suggested that the new Purifi modules are much less finicky when it comes to use with DACs with high out of band noise.

 

It is still good to pay attention to proper DAC design and not just hope things later in the chain will cope... :) IMO, many DAC designs don't pay enough attention to this.

 

Nice thing with something like low rate DSD is that even if there would be out of band leakage, it is uncorrelated random noise (when coming from a good modulator). While leaks from digital filters or PCM images are directly correlated with the music. When audible, former is perceived as constant white noise (like tape hiss) while latter is perceived as distortion...

 

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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8 minutes ago, Miska said:

 

Yes, I tend to aim for >= 192 dB stop-band attenuation... ext2 and xtr filters do >= 240 dB.

 

But isn't 24 bit noise floor of the recordings at 144dB? And even the best DACs have noise floor of about 160dB, so why so much digital stop-band attenuation?

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2. LG 77C1 - Marantz SR7005 - Apollon NC502MP+NC252MP - Monitor Audio PL100+PLC150+C265 - SVS SB-3000

3. PC - RME ADI-2 DAC FS - Neumann KH 80 DSP

4. Phone - Tanchjim Space - Truthear Zero Red

5. PC - Keysion ES2981 - Truthear Zero Red

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22 minutes ago, Miska said:

It is still good to pay attention to proper DAC design and not just hope things later in the chain will cope... :) IMO, many DAC designs don't pay enough attention to this.

Agreed!  Of course if one is a commercial amp designer, as is the case with Purifi here, one cannot control what DAC might be at the input of the amp so...  Same is true of what speaker might be connected to the amp's output...

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36 minutes ago, Matias said:

But isn't 24 bit noise floor of the recordings at 144dB? And even the best DACs have noise floor of about 160dB, so why so much digital stop-band attenuation?

 

Why not? Maximum PCM output resolution is 32-bit -> 20 * log10(2^32) = 192.66 dB. I want to have output reconstruction accuracy at least matching the output resolution range.

 

With high rate DSD outputs however, it is possible to exceed this, that's why there are filters with even more attenuation.

 

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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45 minutes ago, barrows said:

Agreed!  Of course if one is a commercial amp designer, as is the case with Purifi here, one cannot control what DAC might be at the input of the amp so...  Same is true of what speaker might be connected to the amp's output...

 

It easily becomes excessive in terms of adding high-order active analog filters in inputs of power amp, just based on assumption that anything could appear in the input.

 

I rather go the route of using well designed devices that are technically compatible with each other...

 

Signalyst - Developer of HQPlayer

Pulse & Fidelity - Software Defined Amplifiers

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Perhaps we can tack back to the point of this thread, and away from a high level debate about the relative design merits of class design, which are of course related but after 3 pages what is the question for Bruno?

 

I have a question but I think it is too broad so perhaps you folks can help me out.  It is an expeansion of what @firedog and @Em2016were asking on the first page.

 

Now that THD measurements have been driven so low, what else is Bruno hoping to achieve in this and future designs that relate to the unmeasured quality of the sound of Hi Fi amplification gear?  Not looking for a high level debate about measurements, but I am assuming that not everything related to SQ has been measured and/or is not seen in the standard measurements.  Instead, I am looking for a specific inteligible question for Bruno around this:  How do these new amps sound?  What is your goal around sound?  and the like...

 

 

Hey MQA, if it is not all $voodoo$, show us the math!

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1 hour ago, crenca said:

Now that THD measurements have been driven so low, what else is Bruno hoping to achieve in this and future designs that relate to the unmeasured quality of the sound of Hi Fi amplification gear?  Not looking for a high level debate about measurements, but I am assuming that not everything related to SQ has been measured and/or is not seen in the standard measurements.  Instead, I am looking for a specific inteligible question for Bruno around this:  How do these new amps sound?  What is your goal around sound?  and the like...

Hi,

Yes, to some extent for class D. If you examine the preliminary data sheet - figure 10, shows that THD of 1% for various supply voltages. It shows that for 65volts power supply voltage, that for 4ohm load, the power output is 475watts. The THD is quite high at 1%, but very low at lower powers.

 

For a class A/B design, using a +/-65volt power supply, triple EF output, three power transistors per positive and negative swing, and two pre drivers in parallel, then for a 4ohm load with peak output power of 500watts, the THD is 0.000083%.

 

So, class D is 1% THD for a 4ohm load, and class A/B is 0.000083% THD for a 4ohm load.

 

This specific class D module does not seem to perform very well for the given power supply voltages. So my question is, why for such a high power supply voltage, does this unit perform so badly at high power levels.

 

To be noted, 475watts peak into 4ohms is 43.59volts peak across the 4ohm load, so if 65volts is the power supply voltage, is there a stepping down of the voltage in the design, or does the design achieve low output impedance at the expense of THD and output voltage ???

 

Regards,

Shadders.

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2 minutes ago, Matias said:

Shadders, all amplifiers eventually start to clip. Industry standard defines that a given power spec is when THD reaches 1%. You want more power, see another module, like NC1200 for example. All of them eventually THD graph rises to 1% and above, even your simulated amp would too in real life.

Now back to Bruno's questions...?

Hi,

Class D does not clip. The peak output voltage is ALWAYS being applied to the loudspeaker, even when there is no input signal. The peak output voltage for 475watts is 43.59volts, and the module is using 65volts power supply.

 

That is why i asked the question - why the degraded performance ?

 

Despite the stated low output impedance of 0.05miili-ohms (1kHz, 1A current), there is an issue for higher currents - which indicates that for 475watts peak output power, that the output impedance rises significantly.

 

This is why i asked the questions.

 

Regards,

Shadders.

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16 minutes ago, Shadders said:

Hi,

Yes, to some extent for class D. If you examine the preliminary data sheet - figure 10, shows that THD of 1% for various supply voltages. It shows that for 65volts power supply voltage, that for 4ohm load, the power output is 475watts. The THD is quite high at 1%, but very low at lower powers.

 

For a class A/B design, using a +/-65volt power supply, triple EF output, three power transistors per positive and negative swing, and two pre drivers in parallel, then for a 4ohm load with peak output power of 500watts, the THD is 0.000083%.

 

So, class D is 1% THD for a 4ohm load, and class A/B is 0.000083% THD for a 4ohm load.

 

This specific class D module does not seem to perform very well for the given power supply voltages. So my question is, why for such a high power supply voltage, does this unit perform so badly at high power levels.

 

To be noted, 475watts peak into 4ohms is 43.59volts peak across the 4ohm load, so if 65volts is the power supply voltage, is there a stepping down of the voltage in the design, or does the design achieve low output impedance at the expense of THD and output voltage ???

 

Regards,

Shadders.

I really feel that you are being disingenuous.  Since you looked at the product info I'm sure you saw Figure 2 that shows low THD+N to over 300W into 4 ohms.

image.thumb.png.226383efc3d24065fabdb0e2e37a72f3.png

 

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1 minute ago, psjug said:

I really feel that you are being disingenuous.  Since you looked at the product info I'm sure you saw Figure 2 that shows low THD+N to over 300W into 4 ohms.

Hi,

This is not being disingenuous. There is no clipping in class D as per the clipping mechanism of class A/B (which is the voltage rails, assuming max current can be supplied from power supply). What Figure 10 shows is that for a 4ohm load, that the power output of 475watts generates 1% THD.

 

The table on page 3 for the output impedance, shows a very low output impedance value.

 

My questions are fair - you would not expect a class A/B to begin to clip at 43.59volts for an amplifier that has +/-65volts power supply.

 

What seems to be happening is that there is an output voltage versus current capability restriction. 475watts into 4ohms generates 1% THD, yet 500watts into 2ohms generates approx 0.00045% THD. The 500watts into 2ohms generates more current.

 

Regards,

Shadders.

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