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

[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.

[Shadders]

9 minutes ago, psjug said:

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.

Hi,

Yes - if you can post here - would help, thanks. What i am referring to, is that they do not clip as per class A/B. It seems to be output stage voltage/current capability - based on output device, and not that the output voltage is nearing the voltage rail.

 

Regards,

Shadders.

[Paul R]

The tech about amplifiers is kind of overwhelming, and probably most people like me just want a good sounding amp at an affordable price. So two questions:  

 

1. What amplifier does each of you use at your home, for when you just want to relax with some music? If not one of the Purifi products, do you plan to replace whatever you are using with one of the new Purifi Audio products? Be that a reference implementation, a third party device, or a DIY homebuilt job?   
    
2. What do you recommend the "average Joe" look for (listen for) in an amp that Purifi uniquely provides or provides more of than other amplifiers?  In other words, what should a guy like me look for in an amplifier that would lead me to favor a Purify product over say, Chi-Fi Tubes? (Or fill in anything there you like, I was looking for something that would not start an argument.) Power? Clarity?  

Yours,

-Paul 

 

[Ralf11]

1. a design based on O'scope amps (recently replaced a Carver design); Sure, if it's better and cheap.

 

2. Look for pure fi.

[fas42]

 

3 hours ago, crenca said:

 

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...

 

 

Ummm, they sound like nothing ... or should. If they have a 'sound', then the goal hasn't been met - if you have a rig, and swap the amplfier, and the sound changes, subjectively - then at least one of the two amplifiers is 'flawed'.

[barrows]

@Paul R,

 

1.  Currently I use an Ncore NC-400 based stereo amp (DIY) of my own build, this replaced a Pass Labs X .5 Series amp.  As Bruno Putzeys has suggested that the Purifi module is next level in comparison to Ncore, I am very interested in a DIY Purifi build when modules become available.

 

2.  Clarity/transparency, an amp with no sound of its own with vanishingly low noise and distortions, at a reasonable price, with plenty of power as well.  I am not interested in amplifiers which impose their own sound on the music.

[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/

[fas42]

7 minutes ago, barrows said:

 

2.  Clarity/transparency, an amp with no sound of its own with vanishingly low noise and distortions, at a reasonable price, with plenty of power as well.

 

Translation: one should be able to increase volume to the point of clipping with zero subjective change in the tonality, and sense, of the presentation. This normally doesn't happen, and the speakers are always blamed - but the reality is that it's the amplifier that's nearly always the villain - it takes working with a sorted setup to appreciate this fact.

[Shadders]

3 minutes ago, psjug said:

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/

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.

[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.

[Shadders]

5 minutes ago, psjug said:

It's bridged.

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.

[crenca]

16 minutes ago, psjug said:

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.

 

2 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.

 

Gentleman,

 

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

[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

[barrows]

A question for Bruno Putzeys: Why Purifi?  Why not develop the newest tech with Hypex as before?  why the need to start a new company?

[Matias]

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

[Matias]

Duplicated.

[opus101]

11 hours ago, Miska said:

 

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). 

 

Mathematically it can only be anti-correlated noise. When the signal power is higher the noise power can only be lower since the output stage is delivering constant power at all times.

[Miska]

4 hours ago, opus101 said:

Mathematically it can only be anti-correlated noise. When the signal power is higher the noise power can only be lower since the output stage is delivering constant power at all times.

 

Amount of signal power, percentage-wise of the total noise power is very minuscule, at most ~1.4% for DSD64. When randomly spread and scrambled across 1.3912 MHz wide band goes well hidden.

 

That poses more problem for class-D amps though, on their own with their own modulators (regardless of inputs / with perfect input) than for SDM DACs, because reconstruction output filters on class-D amps are typically not as elaborate as they are on DACs. That is why modulator design is especially important in class-D and use of analog modulators create some additional challenges on design.

 

[Shadders]

8 hours ago, psjug said:

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.

Hi,

Purifi and Neurochrome define clipping differently - that is all.

 

My question is still valid - why does the Purifi amplifier clip or distort significantly at a specific voltage which is much less than the supply rail.

 

How does the output impedance change with current load, or output voltage ?

 

Regards,

Shadders.

[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.

[Shadders]

Hi,

Questions for the list :

 

What power supply was used for testing the module ?.

Are the powers listed RMS powers, or peak sine wave powers ?

What is the maximum current capability of the output devices ?

 

Regards,

Shadders.

[mocenigo]

20 hours ago, Shadders said:

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 ???]

 

All Putzeys designs require an inductive power supply. Hence, divide by Sqrt[2] the input voltage to obtain the one that goes to the actual (i.e. on board) rails. This is Class D power supply design 101.

[Shadders]

39 minutes ago, mocenigo said:

 

All Putzeys designs require an inductive power supply. Hence, divide by Sqrt[2] the input voltage to obtain the one that goes to the actual (i.e. on board) rails. This is Class D power supply design 101.

Hi,

What you have said makes no sense. The specification states 65volts. It does not state it has to be an inductive - some people use linear power supplies which are primarily capacitive.

 

The only reason SQRT(2) is involved is if the power delivery are RMS values on the specification sheet, which would account for the 44volts voltage output, when multiplied by SQRT(2), the 62.2volts - very close to the power supply voltage - as expected.

 

Regards,

Shadders.

[rego22]

As in addition to Bruno Putzeys also Lars Risbo is a co founder, I'm wondering if there is perhaps something coming that would merge the class-D with some of the TI SmartAmplifier technologies Lars has been working on in past. Real time tracking of the loudspeaker voice coil and keeping the bass driver in 'safe zone' could produce some very very potent smallish size HiFi speakers that can do bass on another level than achievable today while maintaining audiophile sound quality overall. As Purifi Audio will also make and sell transducers it would make even more sense to try to build some intelligence in between their own combination of amp and driver. 

 

So a question is if maybe Purifi will Smartifi their amp modules also at some point in future ?