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

[DuckToller]

1 hour ago, HQ-Sound said:

...  but what is stopping more designers from coming up with competitive solutions?
Why aren't they (the large audio brands) trying to produce their own class D solutions? 

Hi, thank you for your contribution!
Do you want us to include these 2 questions into the Q&A?
Best, Tom

 

 

[marce]

2 hours ago, HQ-Sound said:

Anyone with some basic electronic design knowledge can put together a class A or A/B amplifier in the garage. It might not sound so great but you will get sound. Class D is much more complex and only very few designers are really trying. Mr Putzeys is a great designer with a brilliant mind but what is stopping more designers from coming up with competitive solutions? It appears that we have far more rocket scientists than what we have class D designers. Even the large audio brand buy readymade class D modules and put them into their amplifiers. Why aren't they trying to produce their own class D solutions? I think we would see a faster evolvement of class D amplification if there were more competition.

Outside of the commercial audiophile area, class D is used a hell of a lot, pro gear, equipment amplifiers, sound bars, basic stereo's etc. like all things in the audiophile world it will take time for the technology to trickle down😀, were still arguing over lp's vs digital, another 30 years should see them accepted. While being a lighthearted view there is some truth in it, look at the thread "Class D sucks". I think also as you have said it's more complex, requires more EMC testing and requires rigorous engineering... For advanced DIYers devices like the TAS5630B from TI are a good place to start, plenty of info. including a layout to copy.

[barrows]

3 hours ago, HQ-Sound said:

Anyone with some basic electronic design knowledge can put together a class A or A/B amplifier in the garage. It might not sound so great but you will get sound. Class D is much more complex and only very few designers are really trying. Mr Putzeys is a great designer with a brilliant mind but what is stopping more designers from coming up with competitive solutions? It appears that we have far more rocket scientists than what we have class D designers. Even the large audio brand buy readymade class D modules and put them into their amplifiers. Why aren't they trying to produce their own class D solutions? I think we would see a faster evolvement of class D amplification if there were more competition.

I am pretty sure I know the answer to this, or at least one answer.  while there are a lot of engineers working in class D, most of that development is going into integrated circuit based amps and their development at companies like TI, as this is where the money is.  Making really efficient (think battery life) amplifier chips for portable devices.  Most of the brightest and best young engineers coming out of Universities are not going into high end audio, as there is much less compensation, they are getting recruited into things like telecom.

 

And then you have the old school analog designers who have worked in high end audio for decades, most of them are not about to re-learn their entire field to catch up with what is required to do really high speed circuit design, so they stick to improving what they know.  If an established company wants to try class D, it makes sense that they turn to a modular approach, as they likely do not have the appropriate engineering skills in house to design a really superb class D amplifier module in house.

 

An engineer like Bruno Putzeys is an exception, as he personally was always interested in home audio, and as such has forged his way forward (to all of our benefit) in this area.

 

Although I would not say there is no competition:  There appears to be fierce competition among ICEpower, Hypex, Pascal, now Purifi, and some others, I suspect this is especially true in pro audio and sound reinforcement.  Imagine the difference from the old days of say, Woodstock, where the PA would have been running via tube amplification!

[Matias]

There are some in house developed, proprietary class D on the market. NuPrime and Aavik as far as I know.

[barrows]

47 minutes ago, Matias said:

There are some in house developed, proprietary class D on the market. NuPrime and Saving as far as I know.

And "Digtal Amplifier Company" and their "Cherry" amplifier products... and Merrill Audio as well...

[firedog]

4 hours ago, HQ-Sound said:

Anyone with some basic electronic design knowledge can put together a class A or A/B amplifier in the garage. It might not sound so great but you will get sound. Class D is much more complex and only very few designers are really trying. Mr Putzeys is a great designer with a brilliant mind but what is stopping more designers from coming up with competitive solutions? It appears that we have far more rocket scientists than what we have class D designers. Even the large audio brand buy readymade class D modules and put them into their amplifiers. Why aren't they trying to produce their own class D solutions? I think we would see a faster evolvement of class D amplification if there were more competition.

I think what Barrows said above is correct. Also, my understanding is that it is actually much more difficult to design a high end audiophile SQ class D amp than it is to design a conventional amp for audiophile use. Probably many very good designers in the non Class D area don't really have the knowledge/experience to design a Class D amp to that level. 

[jabbr]

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.

[mfsoa]

Let's please include the Digital Amplifier Company (in good ol' Allentown PA) in discussions of independent (non-commercial module) Class D designers. Tommy O'Brian has been designing and building his own Class D designs for decades.

 

In every case I heard I felt that the DAC amps significantly outperformed the competing module-based amps I heard, either borrowed or owned. 

 

Comparison to apparently highly regarded AB amps was instructive. It was too funny when I had an audio group over and someone swapped in a Modwright KWA 100 SE for one of the old big Cherry amps without me knowing it. I walked into the room to see a dozen really downcast faces and this lifeless sound. Everyone was kind of staring at the ground in disbelief at how much better the DAC amp was. I assumed the SE was defective but apparently it was not.  And now, Tommy's newer amps have surpassed that old Cherry with significant engineering and sound quality improvements.

 

It is stunning when one realizes how much information is not making out of one's previous amp when a DAC amp is installed, the difference is so striking.

 

 

[Rt66indierock]

42 minutes 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.

 

Im going to start looking at active speakers with digital in this summer.

[HQ-Sound]

1 hour ago, barrows said:

And "Digtal Amplifier Company" and their "Cherry" amplifier products... and Merrill Audio as well...

Merrill Audio use Hypex NCore modules. I don't know about all the others but like I wrote the majority of class D amplifiers on the market are based on readymade class D blocks.

[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?”

[crenca]

16 minutes ago, psjug said:

 

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?”

 

 

What exactly is the "power domain"?  A DAC's output is in the "power domain", in that it is a signal of such and such volts and current (though relatively low in output).  What is it about having the signal go through another amplification design and step that would lead (on almost wants to say necessarily) to a better subjective sonic output?  I assume the answer is that by breaking the problem into discrete steps the actual engineering works out better (better judged by subjective sonic output as well as cost, feasibility, etc.), but I wonder if Bruno could not expand on why this is.

[Miska]

3 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.

 

Most class-D these days is analog modulator to run the switching stage (for example Hypex modules). It is simple, but it has many limitations and extra conversion step from D/A to the analog modulator (Mola-Mola and Chord). And things like aliasing problems if the D/A leaks images or such. Also just plain analog limitations of the intermediate stage. I prefer to skip that intermediate step. But doing it right is not easy, otherwise everybody & friends would be doing it already...

 

[HQ-Sound]

7 hours ago, DuckToller said:

Hi, thank you for your contribution!
Do you want us to include these 2 questions into the Q&A?
Best, Tom

 

 

Yes please do, thank you.

[barrows]

2 hours ago, HQ-Sound said:

Merrill Audio use Hypex NCore modules. I don't know about all the others but like I wrote the majority of class D amplifiers on the market are based on readymade class D blocks.

Not anymore, they use their own design class D.

[HQ-Sound]

1 hour ago, barrows said:

Not anymore, they use their own design class D.

Sorry my wrong then but then maybe Merrill Audio should correct the information on their website.

 

"The Mono blocks are a true fully balanced design with both input signals floating for a true balanced differential input, utilizing the Hypex Ncore NC1200."

[barrows]

I have not looked at their site lately,  But my understanding is thta they have dis-continued production of the "Veritas" amplifiers, and now are only producing the "Element" series amplifiers with their GaN based class D output stage:

 

 https://merrillaudio.net/products/

 

I find it interesting though that despite the claimed benefits for GaN by some posting here,the new Merrill amps have worse specifications than their previous (Ncore based) amps.

[opus101]

5 hours ago, crenca said:

 

What exactly is the "power domain"?  

 

In this context its output swings of several tens of volts and currents up to several tens of amps. As opposed to the 'signal domain' where domestic voltage swings are typically under 10V and currents are negligible.

[fas42]

8 hours ago, mfsoa said:

It is stunning when one realizes how much information is not making out of one's previous amp when a DAC amp is installed, the difference is so striking.

 

 

 

This is a general aspect of current audio systems - the amount of detail that is discarded, by one means or another, is quite staggering at times; one would swear that it's a different recording, .

 

The almost unconscious behaviour of audiophiles is that when the distortion is too obvious, too unpleasant, that they then 'tweak' the rig to hide this undesirable part - classic throwing out the baby with the ...

 

 

[jabbr]

On 5/25/2019 at 2:08 PM, psjug said:

 

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?”

 

He he, of course the exact same thing could, and has, been used to criticize Class D from the perspective of Class A.

 

Interestingly he notes that direct digital has the lowest distortion yet he doesn't like the SQ and unless we ever find out the electrical  correlate of his SQ then we have no idea whether the SQ had anything to do with direct digital (power DAC) vs some other unknown explanation. Of course the implementation remains important  

 

I would thus add the question: "Since you've said that you like the SQ of your Class D as opposed to your own direct digital/power DAC ... why? What is the electrical correlate?"

[jabbr]

20 hours ago, opus101 said:

 

In this context its output swings of several tens of volts and currents up to several tens of amps. As opposed to the 'signal domain' where domestic voltage swings are typically under 10V and currents are negligible.

 

Yes, there are many new HF switching transistors. Anyone can buy an IC DAC chip, or even build one from IC logic chips. In the “power domain” you’d most likely need to build your logic from transistors, and “CMOS” circuits aren’t optimal.

[crenca]

4 hours ago, jabbr said:

 

I would thus add the question: "Since you've said that you like the SQ of your Class D as opposed to your own direct digital/power DAC ... why? What is the electrical correlate?"

 

This.

 

On the other hand, as near as I can tell (could be wrong) Bruno does not pretend to be a strict objectivist.  Comes back to a corollary to my question - what is it about the subjective SQ is Bruno after with these new designs as opposed to his previous efforts?  Certainly the answer is not "yet lower THD" or some such...that would be a side step of an answer 😉

[jabbr]

Yes yes, my bias/impression/hunch is that ultimately direct digital will replace Class D — then again what do I know. So just as Class D has evolvted (and details matter) so with direct digital.

[barrows]

So, with "Direct Digital", we are talking about a single bit high rate signal, right?  High rate DSD... which can be thought of as almost an analog signal.  I guess I do not really get it, quiet?  Would the output stage need to switch at MHz rates?

[jabbr]

Yes, but consider a “Power DAC” so could as well be multibit ... but the simplest example would be a “Power flop” that would switch at the DSD rate. 

 

... perhaps the simplest would be a power gate.