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

[Sam Lord]

On 5/18/2019 at 4:43 AM, mocenigo said:

Yes and no. Class D amps have a phase rotation that is a linear function of the frequency. This is just a delay. So in fact this is often much better than irregular phase rotations as in A and AB amps. Luckily, the rotation of the latter are much more contained.

NCore also has much smaller rotations than ICEpower and, it seems, ICEEdge - I expect the same for Eigentakt

Still, it would be VERY INTERESTING to hear Putzeys take on the matter.

Roberto

 

IIRC the phase shift at 20kHz for nCore was not consequential...below 10 degrees.  Bruno is very conscious of phase shift in both amps and speakers, I spoke with him at length about this (many years ago).

 

On 5/20/2019 at 9:54 AM, Shadders said:

...Class D is an approximation since it is a pulse width modulation signal. It requires filtering to average the signal between the PWM samples.

 

Class A/B is a continuous function - there are no discontinuities. The noise is either component noise, or derived from the continuous function (polynomial) that adds the harmonics.

 

If you examine the output of class D, then for a 1kHz signal, there is the signal, the harmonics of the signal, and a lot of non-harmonically related noise at a very high level. (neglecting component noise)

 

If you examine the output of class A/B, then for a 1kHz signal, there is the signal, the harmonics of the signal. (neglecting component noise).

 

Class A/B performance will always exceed class D.

 

Regards,

Shadders.

 

No Class AB amplifier can be modeled analytically: all have numerous discontinuous behaviors.  Crossover notch distortion is often the largest but is far from the only one.  Transistor behavior itself, even within an fairly linear range,  is discontinuous.   

 

On 5/20/2019 at 11:05 AM, Shadders said:

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

 

I can't find any metric which supports that view, when comparing measured performance of Bruno's Class D designs and any Class AB designs.  Note that The Benchmark AB1 is a Class H design, not Class AB.

 

On 5/20/2019 at 9:22 PM, Matias said:

I guess because he was employee at Hypex and now co-owner of Purifi. But sure, let him answer.

 

Bruno was not only the principal designer at Hypex, but the reason the company was created.  No, UcD, no Hypex.

 

On 5/24/2019 at 11:09 AM, barrows said:

@marce, typically, Mr. Putzeys' class D designs use box type polyester capacitors.  As I recall, for example, the NC-400 modules have three Wima polyester caps totaling about 2 µF in the output filter (parallel devices).  Some folks have modded these to slightly larger Wima Polypropylene parts and reported good subjective improvements....   ...Apparently the Mola Mola Kaluga monoblocks use a different type capacitor in the output filter which Bruno only describes as "monolithic"...

 

Interesting!  Monolithic ceramic caps with the C0G/NP0 rating are superb in both coupling and bypass duties.  Also X7Rs are great in spite of microphony: properly potted and/or paralleled they have no faults.  However, many have magnetic leads which are not known to their own manufacturers, whose names I won't state here (Kemet).  

 

On 5/26/2019 at 2:42 PM, jabbr said:

...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?"

 

I think that was a very old statement from him.  Anyway, one explanation is that damping factor of his Class D circuit might have been much higher. 

 

On 5/29/2019 at 12:07 PM, shtf said:

 

Bruno, I'm curious if you've given any consideration in your designs to the digital-like bi-directional noise / distortion inherited in perhaps every Class D amp due to the high-speed switching modules?

 

IMO, Class D amplification can be phenomenal when done right.  But even some of the most naive listeners will notice this digital-like distortion and as a result will often times poo-poo all Class D designs.

 

For example.  I'm a huge advocate for superior line conditioners and in particular passive, dedicated, and bi-directional filtering types of line conditioners that actually cleanse, purify, filter, and/or condition the noisy AC coming in from the street.   My experience with Class D amplification is limited to nuforce and Wyred-4-Sound amps.   Based on my experimenting with Class D mono-blocks sharing a single line conditioner using a cryo-treated audio-grade power strip, this bi-directional noise induced by the switching modules goes up significantly when the 2 channels are sharing their bi-directional noise with each other.

 

Having forgotten all about the bi-directional noise issue some years ago I had purchased a high-powered Class D integrated amp and though it was connected to a superior line conditioner, I noticed very audible distortions that was causing the presentation to be very unmusical though the int. amp was clearly doing its job otherwise quite well.  Then I recalled this bi-directional noise issue with Class D and swapped out the int. amp for a pair of mono-blocks (same mfg'er, same wpc, same Class D modules) with seemingly no such digital-like distortions.

 

What I realized from this Class D int. amp experience was that not only were the 2 Class D modules sharing their switch module digital-like bi-directional noise at the IEC inlet but the distortions were significantly compounded over and above due to the fact that the active pre-amp gain stage had to borrow AC power from one of the channels so now the pre-amp section was getting a double dose of this bi-directional noise and then amplifying it at the gain stage.

 

My lesson learned then and there was to never ever purchase a stereo 1-chassis 1-IEC inlet Class D amp or integrated amp because the consumer can do nothing to reduce this internal sharing of this bi-directional noise.  On the other hand, Class D mono-block amps with superior bi-directional filtering line conditioners are for me the cat's meow. 

 

For 20 years now, all my playback systems have included superior line conditioning because of what they do to treat the noisy AC coming in from the street.  But even with Class D mono-block amps, I find superior line conditioners absolutely mandatory to keep the digital-like bi-directional noise from going back into the AC outlet, some say even back to the AC service panel, and then induce its digital-like noise into other components.

 

That said, I see this very real digital-like bi-directional noise as perhaps the single greatest shortcoming for Class D for most consumers so my question to you is this.  Since few are aware of this distortion and perhaps even fewer are aware of superior line conditioning, what if anything, has Purifi done to address this very real distortion with Class D amplification?

 

Bruno has stated that his nCore circuits have *very* low HF components either radiated *or* transmitted onto the AC line.  His extremely adept use of differential signals in both the SMPS and amp module are the reason.  But still, any garbage on the line is bad and minimizing it is important.  Recently Bruno demonstrated with Shunyata, with whom I worked under contract from my old company for a couple years (no financial ties now), and their non-reactive, environmental HF reduction is a great antidote to line- and component- generated noise.

 

On 5/31/2019 at 9:16 AM, Shadders said:

Hi, A few more questions: Is the voltage gain specified (12.8dB) based on the opamp being in circuit, or out of circuit ?

Are the THD figures also based on the opamp being in circuit or out of circuit ?

Can the module be used without the opamp in circuit ?

What opamp is provided with the module ?

Is the loop gain based on the opamp being in circuit ?

 

Really good point Shadders!  Many of the measurement improvements disappear when one adds another stage to replace the lost gain!  Are the Purifi modules really an improvement over nCore?  Another 13dB of gain would knock S/N down by about 4-5dB, right where the NC500 measured.   

 

However, the concept of lowering amp gain makes sense, given the excessive output of many high-end DACs.  On the third hand, 13 dB is too low.  On the fourth hand, you'll *never even nearly* clip the amp when your DAC sends a maximum signal.  But fifthly, I'm guessing that an opamp *is* meant to be added to the front of this circuit.

[Sam Lord]

Since all of you good folks are captive now, waiting for Bruno's response, I'll spew some debris about my experience with class D, in fact an older and lesser variety: 1st gen Icepower in Bel Canto amps.

 

1) Class D has very low output impedance.  If you want to hear them at their best, USE IT!  Don't use a stereo amp, use monoblocks with very short speaker cables. Or do what I do, vertically biamp with a stereo amp on each side.  This prevents cabling from halving your amp's output impedance, or worse!

 

2) As above, use that low output Z to maximum benefit: bi-amp if possible, or at least bi-wire, because the difference between 2ft and  6ft of shared speaker cable makes a measurable and audible difference in performance.  Separate circuits make an even bigger jump.  Bruno emphasizes biwiring for the same reasons, but any IMD between transducers is eliminated with multi-amping.

 

3) Class D circuits, moreso than Class AB, ramp up in distortion in an abrupt way, usually around 1/2 of the power at clipping (1% THD).  If you listen at very high levels, as I do (I have very inefficient ATC monitors), Get at least 2x the amp power you expect to need.  Then your loudest passages will truly be pristine.