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

[psjug]

You (Bruno) once described your pursuit of a power DAC as "folly".

https://www.soundandvision.com/content/bruno-putzeys-head-class-d

 

Do you still feel that a power DAC cannot hope to compete with your analog designs?  What about direct digital designs with feedback?

[psjug]

The latest Qualcomm DDFA can run at 800kHz.  I think that is the frequency that the Sonos Amp uses.  I have one and there is a lot of ripple at 800KHz, so I think the frequency needs to be much higher for switching noise to be tamed by a simple LC output filter.

[psjug]

12 minutes ago, Matias said:

 

I refer to class A and AB with linear power supply, which together should account for something like 90% of the high end market.

 

As for references, check out the THD x power graphs, or 19+20kHz x frequency graphs, that Stereophile, SoundStage and Audio Science Review forums have, and compare nCore against similar prices or even ultra expensive amps.

 

Not that I am a hardcore objectivist but still.

If you are comparing THD you should compare the best against the best.  I think the Class AB from Benchmark and Neurochrome  might beat nCore in this regard, or at least they all measure extremely well.

[psjug]

2 minutes ago, Matias said:

Agree, but they are 2 amps out of a million class A and class AB models. And not only nCore, but also other class D modules from ICEpower or Pascal measure better than the majority of ABs out there.

I don't think that is a valid reason to regard one class superior to another.

[psjug]

18 minutes ago, Shadders said:

.

Class A/B will always outperform class D, when comparing the best designs of both.

 

I don't see how you can know that either.  And anyway at what point do you say THD+N is good enough and start looking at performance in other areas?

[psjug]

16 minutes ago, Shadders said:

Hi,

Since class A/B is continuous - class D which is discontinuous will always be an approximation.

 

The measurements of class D amplifiers restrict the bandwidth to 22kHz, yet many class A/B amplifiers have a measurement bandwidth of 80kHz. Maybe this is why people state class D is better than the majority of class A/B........

 

Regards,

Shadders.

I think you really need to find a better word than "approximation".  I get it that you don't like the discoontinuous nature of Class D - why don'y you leave it at that?  And  you did not answer my question in the other thread: do you also dislike DSD and sigma delta converters?  Do you only like analog source for your music?

[psjug]

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.

[psjug]

1 minute ago, Shadders said:

The "continuous" aspect is a flaw in the implementation, which can only be reduced, never eliminated.

 

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.

[psjug]

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.

 

 

 

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.

[psjug]

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.

 

[psjug]

3 minutes ago, Shadders said:

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.

Class D amplifiers clip.  If you'd like I will capture a clipped waveform from a little 20W T amp and send it to you tomorrow.

 

I don't know the specifics but in general amplifiers clip at a voltage below the nominal rail voltage, so there needs to be some allowance there.

[psjug]

1 hour ago, Shadders said:

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.

 

Here are specs and measurements of a Neurochrome Modulus 686 Class AB amplifier, running on +/- 36V rails, and bridged.  Why does it start clipping at just over 300W into 4 ohms?

https://www.neurochrome.com/modulus-686/

[psjug]

6 minutes ago, Shadders said:

Hi,

300watts peak into 4ohms is equivalent to 34.64volts peak across 4ohms. The amplifier performs very well, at within 1.36volts of the supply rail.

 

Regards,

Shadders.

It's bridged though.  Maybe it is simpler to look at the 286 which is not bridged.  But at lower power the issue is less severe I think.

[psjug]

3 minutes ago, Shadders said:

Hi,

OK - it is a chip amplifier - but the LM3886 specification states that the clipping voltage is about 4.5volts below 36volts. The specification from the integrators page states onset of clipping at 225watts into 8ohms, but the THD is 0.01% for 240watts into 8ohms. I think the integrators definition of clipping is different to the Texas Instruments definition.

 

Regards,

Shadders.

I think it is complicated but also consider that the power supply is not going to behave perfectly.  My point was that the Purifi amplifier does not strike me as atypical with regard to clipping at the stated rail voltage.

[psjug]

4 minutes ago, crenca said:

 

 

Gentleman,

 

What is the question for Bruno around all of this?!??

Sorry - it is like many threads go.  If Chris wants to clean it up my feelings won't be hurt.  I did submit a question a few pages back

[psjug]

A question to Purifi:

 

Can you explain how your clipping recovery is better than with typical amplifier designs as stated in your literature:

• Clips cleanly and recovers immediately without “overhang”. Current limiting is equally instantaneous and glitch free. This guarantees the smallest amount of audible artefacts when pushed into clipping or overload protection.

[psjug]

@The Computer Audiophile  when do you plan to do this?  I'm looking forward to reading what they have to say.

[psjug]

2 hours ago, jabbr said:

I would be interested in hearing their impression of "Class D" which involves analog feedback, and A/D conversion at the input, to a "Direct Digital" approach in which the digital input is directly amplified and then LPF at the output. I believe @Miska is now involved with the Estelon folks "Pulse & Fidelity" which I am assuming is a "Direct Digital" approach ... an advantage is that there is no A/D conversion at the input ... essentially a zero feedback class A design in the digital space.

 

I wonder if his thinking has changed on this at all.  Here is what he said a couple years ago: https://www.soundandvision.com/content/bruno-putzeys-head-class-d

 

I was just as confused and as a result concocted a power DAC that directly converted DSD to analog in the power domain. The thought process that made me embark on this folly is a prime example of how not to start an engineering task but the result is still the most efficient (97 percent) and lowest distortion (0.007 percent) zero-feedback power amp ever made. Sonically, it was immediately killed by the first UcD prototype, which got me thinking. I decided that an aesthetic precept like “digital all the way” or “no feedback” can’t hold a candle to the simple question, “What problem are we trying to solve here?”

[psjug]

1 hour ago, Shadders said:

Hi,

You were right to ask the question. There is an Elektor electronics design, called Titan 2000 from 1999, which has an open loop gain of 8600x, which euqates to 78.7dB, which exceeds the Purifi figure of 75dB. Elektor is an electronics magazine.

 

The claim of ultra high performance across the audio spectrum, unmatched by other audio amplifiers of any technology or operating class, is just not true. The operating bandwidth of the class D designs can easily be excelled by a current feedback amplifier, and voltage feedback amplifiers too.

 

Them you have to ask the question, why do they make this statement of 110MHz GBW product, when they have to reduce the measurement bandwidth from 80kHz to 22kHz to ensure better THD performance figures, and the class D amplifier cannot replicate a signal with frequency at 200kHz (550x less than 110MHz).

 

Regards,

Shadders.

That bullet does not say the loop gain or GBW product are unmatched.  It says performance is unmatched, at least that's how I read it.  Meaning overall amplifier performance.  Obviously the bandwidth of the amplifier is nowhere close to MHz, they say it is 60KHz.  Since the idea of 110MHz GBW Product for this amplifier strikes you as nonsensical, maybe you should ask Purifi to explain why high GBW Product matters in their design.

[psjug]

39 minutes ago, Shadders said:

Hi,

The marketing statement lists the GBW product as >75dB and then proceeds to state "This figure corresponds to an unprecedented 110MHz Gain-Bandwidth Product and produces consistent ultra-high performance "

 

Therefore i ask the questions :

 

Why does the open loop gain produce a performance that is unmatched by any other audio amplifier technology or class ?

 

What parameters/measurements are being used to make this claim on the performance of the amplifier, when compared to voltage feedback amplifiers, and current feedback amplifiers ?

 

Regards,

Shadders.

Ah, you are right that I missed the "unprecedented.  I apologize.

Purifi mentions "loop gain" and you are mentioning "open loop gain".  Honestly "loop gain" is not terminology I am familiar with, but note that there is a difference, explained here.

https://www.analog.com/en/technical-articles/loop-gain-and-its-effect-on-analog-control-systems.html

 

I am glad you posted questions, however, which seems more useful than picking apart the product brief.

 

[psjug]

7 minutes ago, Shadders said:

Hi,

Yes - open loop gain is where there is feedback applied, and open loop gain is with no feedback applied, as per the article you have referenced.

 

You are still missing that "loop gain" is different than "open loop gain".  Loop gain is a measure of excess gain available in the amplifier circuit, as described in the linked article:

So the gap between the open loop gain curve and the closed loop gain curve is βA₀ (about 105dB at DC). Referring to Figure 1, we can see that A₀ • β is the gain going through the amplifier and the feedback loop, so βA₀ is our loop gain and is a representation of the excess gain available in the system. 

[psjug]

3 minutes ago, Shadders said:

Hi,

Apologies, yes i stand corrected. But then, does this high loop gain include the opamp in the signal path ?

Regards,

Shadders.

Honestly, I understand this when applied to op amps, but not so much when applying this stuff to Class D.  So for me it would be educational to hear Bruno explain this stuff.