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

[shtf]

 

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?

 

 

 

[Matias]

From the 1ET400A product brief:

 

"High loop gain (>75dB) in the entire audio band
o This figure corresponds to an unprecedented 110MHz Gain-Bandwidth Product and produces
consistent ultra-high performance across the audio spectrum unmatched by audio amplifiers of any
technology or operating class."

 

Does this affirmation still stands when looking at the measurements from Benchmark AHB2?

Looks like AHB2 has even lower THD, although some more IMD.

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-benchmark-ahb2-amp.7628/

[Shadders]

20 minutes ago, Matias said:

From the 1ET400A product brief:

 

"High loop gain (>75dB) in the entire audio band
o This figure corresponds to an unprecedented 110MHz Gain-Bandwidth Product and produces
consistent ultra-high performance across the audio spectrum unmatched by audio amplifiers of any
technology or operating class."

 

Does this affirmation still stands when looking at the measurements from Benchmark AHB2?

Looks like AHB2 has even lower THD, although some more IMD.

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-benchmark-ahb2-amp.7628/

Hi,

Given that the amplifier switches at 500kHz, a 110MHz GBW product is really a meaningless statement, since it could never achieve resolution/signal at 200kHz, let alone 110MHz (50x greater).

 

There are class A/B designs using current feedback that have a slew rate of +/-800V/uS, and a DIY version available now that has +/-400V/uS slew rate (1.27MHz bandwidth at 50volts output).

 

Regards,

Shadders.

[barrows]

59 minutes ago, Matias said:

From the 1ET400A product brief:

 

"High loop gain (>75dB) in the entire audio band
o This figure corresponds to an unprecedented 110MHz Gain-Bandwidth Product and produces
consistent ultra-high performance across the audio spectrum unmatched by audio amplifiers of any
technology or operating class."

 

Does this affirmation still stands when looking at the measurements from Benchmark AHB2?

Looks like AHB2 has even lower THD, although some more IMD.

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-benchmark-ahb2-amp.7628/

Firstly, asking an audio manufacturer to compare their products performance with that of a competitor is a non-starter for me.  No audio manufacturer I am aware does this, as they cannot possibly be expected to have all of the possible competitors at hand for testing.  And only testing using the exact same methodology, and equipment is going to yield actually valid comparisons (especially at the levels of noise/distortions we are talking about here).  A manufacturers job is to produce the best performing product they can, and then to present it to the public-comparisons are for reviewers and consumers to do.

 

Secondly, these amplifiers are not equivalent in terms of power output, or, perhaps even more importantly, gain.  Raise the gain of the Benchmark to a similar level, and the noise and distortion are likely to go up correspondingly.  To get the same output power you have to bridge the Benchmark and purchase two of them for stereo use.  Be careful when comparing measurements like this, as a lower gain amplifier can easily look "better" if one does not pay attention to this detail.

[DuckToller]

3 hours ago, Matias said:

From the 1ET400A product brief:

 

"High loop gain (>75dB) in the entire audio band
o This figure corresponds to an unprecedented 110MHz Gain-Bandwidth Product and produces
consistent ultra-high performance across the audio spectrum unmatched by audio amplifiers of any
technology or operating class."

 

Does this affirmation still stands when looking at the measurements from Benchmark AHB2?

Looks like AHB2 has even lower THD, although some more IMD.

https://www.audiosciencereview.com/forum/index.php?threads/review-and-measurements-of-benchmark-ahb2-amp.7628/

Matias,

thank you again for your participation.
I totally agree with @barrows opinion about that question.
As I have forwarded all your (the forists) questions of this thread yesterday morning, I'd prefer not to forward your (Matias) question in the form it is. You may want to rephrase your issue without relating it to the competition?
Thank you for your understanding!
Best regards, Tom

[Matias]

I asked it because that statement was so strong and confident, specially when there is data that (may) contradict it.

But you are right, no need to send them this question.

[Shadders]

3 hours ago, Matias said:

I asked it because that statement was so strong and confident, specially when there is data that (may) contradict it.

But you are right, no need to send them this question.

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.

[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.

[Shadders]

17 hours ago, barrows said:

Firstly, asking an audio manufacturer to compare their products performance with that of a competitor is a non-starter for me.  No audio manufacturer I am aware does this, as they cannot possibly be expected to have all of the possible competitors at hand for testing

Hi,

The question was relevant and should be asked. It is Purifi who claim that

 

"ultra-high performance across the audio spectrum unmatched by audio amplifiers of any technology or operating class.",

 

so the onus is upon them to provide why their amplifier will exceed performance of the Benchmark AHB2.

 

Please see :

https://www.stereophile.com/content/benchmark-media-systems-ahb2-power-amplifier-measurements

 

As you can see, the THD across the audio band is less than 0.0001%, yet the claimed performance of the Purifi is 0.00015%. They should not be making statements that their product out performs every possible amplifier on the planet, without proving this.

 

The statement is marketing promotional material only, and would not withstand scrutiny.

 

Therefore, i ask the question, how can Purifi prove that their product exceeds every other topology/technology/class ?

 

Regards,

Shadders.

[Shadders]

8 hours ago, psjug said:

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.

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.

[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.

 

[Shadders]

30 minutes ago, psjug said:

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.

 

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.

 

They quote loop gain of greater than 75dB and GBW product of 110MHz. Given the value of the gain provided by Purifi, then this has to be open loop gain. In the article you referred to, you can see the open loop gain graph with the value being 125dB (approximately), which extends from 0Hz to 1Hz. From here on, the closed loop gain product declines at 20dB per decade giving a GBW product of 1MHz (graph curve cuts the 0dB axis point).

 

All that Purifi are saying is that the flat bit at the top (open loop gain) starts at 75dB and extends to 20kHz at least, and then begins to decline at 20dB/decade, such that it cuts the 0dB line at 110MHz.

 

Most power amplifiers have a closed loop gain of 26dB to 30dB. So, for a 2volt input signal, the output voltage is 39volts to 63volts respectively (approximate). A loop gain of 75dB means that 2volts input signal produces 11kV. So, it has to be open loop gain they are referring to.

 

Regards,

Shadders.

[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. 

[Shadders]

2 minutes ago, psjug said:

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. 

Hi,

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

Regards,

Shadders.

[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.

[Shadders]

2 minutes ago, psjug said:

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.

Hi,

The specifications state that the voltage gain is 12.8dB (most power amplifiers have 26dB to 30dB). This is very low - to obtain the full output power, they indicate a 9.6volts (RMS) input signal (differential). It does not state whether this is a differential input, or single ended input to the power amplifier module. Maybe this is why there is such a large loop gain, as the opamp has significant feedback ?

 

Regards,

Shadders.

[Shadders]

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 ?

 

Regards,

Shadders.

[Shadders]

3 hours ago, Shadders said:

Yes - open loop gain is where there is feedback applied, and open loop gain is with no feedback applied

Need to correct : closed loop gain is where there is feedback applied...

[DuckToller]

1 hour ago, Shadders said:

A few more questions

Thank you guys & @Shadders

l'll send the addendum with the late entries on Monday. Pls feel free to continue.

Enjoy your weekend, best, Tom

[crenca]

3 hours ago, Shadders said:

Hi,

The specifications state that the voltage gain is 12.8dB (most power amplifiers have 26dB to 30dB). This is very low - to obtain the full output power, they indicate a 9.6volts (RMS) input signal (differential). It does not state whether this is a differential input, or single ended input to the power amplifier module.

 

For the non EE, what your talking about is an opamp between the input and the gain stage of the amplifer correct?  So input (typical 2v RMS single ended for example), then opamp with gain to 9.6v RMS, then the Purifi Class D gain stage, then speaker terminal outs...?

[Shadders]

2 hours ago, crenca said:

 

For the non EE, what your talking about is an opamp between the input and the gain stage of the amplifer correct?  So input (typical 2v RMS single ended for example), then opamp with gain to 9.6v RMS, then the Purifi Class D gain stage, then speaker terminal outs...?

Hi,

Yes - that would be sensible - else the amplifier would have great difficulty in being used with a passive preamp, or an preamp with limited voltage output. 9.6volts RMS is a lot of volts for a preamp to output to obtain 425watts into 4ohms.

 

If the THD figures have been based on the 9.6volts RMS input signal, then it will affect the S/N and THD measurement. No opamp in the path means lower THD. But if the module for the consumer has to use the opamp, then will this affect the THD.

 

Regards,

Shadders.

[jabbr]

7 hours ago, 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 ?

 

Regards,

Shadders.

 

Isn't this supposed to have a PCM 24/96 input which is converted to  PWM ... in that case gain is in the digital domain involving typically both voltage and current from ? CMOS i.e. 3.3V

 

In this case "passive preamp" or any analog volume control prior to the amplifier is not relevent.

 

Or is that discussion only for the analog input section?

[Matias]

3 hours ago, Shadders said:

Hi,

Yes - that would be sensible - else the amplifier would have great difficulty in being used with a passive preamp, or an preamp with limited voltage output. 9.6volts RMS is a lot of volts for a preamp to output to obtain 425watts into 4ohms.

 

If the THD figures have been based on the 9.6volts RMS input signal, then it will affect the S/N and THD measurement. No opamp in the path means lower THD. But if the module for the consumer has to use the opamp, then will this affect the THD.

 

Regards,

Shadders.

 

In my opinion this is quite clever from Benchmark: make DACs with higher output levels than normal but also with the option of normal output levels, and make your power amplifier have less gain than normal but also normal gain levels. This way customers who use their complete system have the benefit or higher SNR as a whole, but each product used individually is also compatible with others in the market.

[Shadders]

11 hours ago, jabbr said:

 

Isn't this supposed to have a PCM 24/96 input which is converted to  PWM ... in that case gain is in the digital domain involving typically both voltage and current from ? CMOS i.e. 3.3V

 

In this case "passive preamp" or any analog volume control prior to the amplifier is not relevent.

 

Or is that discussion only for the analog input section?

Hi,

The data sheet does not specify that the design converts the signal to PCM 24bit/96kHz. All it states is that it is an Analog Input Class-D amplifier module.

 

All the datasheet states is that it requires 9.6volts RMS input (differential) to obtain 425watts RMS into 4ohms load - which should be 4.8volts RMS (6.79volts peak) for a single ended signal. Therefore a standard output of a maximum 2.1volts RMS (2.97volts peak), the maximum power would be 84watts RMS into 4ohms, which is less than 1/4 power. So a passive preamp is not feasible, and some active preamps may not be suitable either.

 

This assumes that the opamp is in circuit. If the figures quoted in the specification are for the module without the opamp in circuit - then this is not an issue, as the opamp will be used to lower the required input voltage from 4.8volts RMS to some other lower value, based on the gain of the opamp circuit. It would be unusual for this amplifier to require 4.8volts RMS input signal for the maximum output power.

 

Which is why i asked the questions - are the THD figures based on the opamp in circuit, or not in circuit ?

 

Regards,

Shadders.

[jabbr]

On 6/1/2019 at 3:58 AM, Shadders said:

The data sheet does not specify that the design converts the signal to PCM 24bit/96kHz. All it states is that it is an Analog Input Class-D amplifier module.

 

You are correct. This seems to me at second glance to be a Class D to;pology, not direct digital, which Risbo as a founder of Toccata was a pioneer of... I shouldn't have assumed ... as they say