Purifi Class D

[mocenigo]

9 minutes ago, Jud said:

 

The  first CD players used "brickwall" digital filters without upsampling. It was only a little while later that they began to employ first 4x then 8x upsampling.

 

If am not mistaken this was a filter at 20khz. Which means that the first mirror image started at 24.1Khz, and then they had to use a steep analog filter. They moved quite quickly to 2x upsampling (so with the first image from 68.2khz, allowing for gentler analog filtering) but they started advertising the upsampling with 4x.

[Jud]

14 minutes ago, mocenigo said:

The sigma-delta modulation is not to enable a "simpler, less expensive final analog reconstruction filter", it is a conversion process and in fact it requires a more significant investment analog reconstruction filter. What it does is that it makes the DAC cheaper and at the same time more precise (except the very first iterations which had problems such as potentially unbound settling times and the like).

 

@Miska, writing in 2019:

 

Quote

That is one of the purposes of SDM DACs, that as much as possible things that require accuracy and complexity are done in digital domain using DSP, rather than in analog domain on hardware. This improves reliability and precision while also being cheaper at the same time.

 

It is possible I am confused between less expensive hardware overall and less expensive analog reconstruction filter hardware, though I thought I recalled reading that the latter was true.

[Jud]

4 minutes ago, mocenigo said:

 

If am not mistaken this was a filter at 20khz. Which means that the first mirror image started at 24.1Khz, and then they had to use a steep analog filter. They moved quite quickly to 2x upsampling (so with the first image from 68.2khz, allowing for gentler analog filtering) but they started advertising the upsampling with 4x.

 

Yes, exactly. Upsampling made better filter design easier. (Yes, I do realize that upsampling in itself necessitates filtering.)

[mocenigo]

6 hours ago, Jud said:

 

@Miska, writing in 2019:

 

 

It is possible I am confused between less expensive hardware overall and less expensive analog reconstruction filter hardware, though I thought I recalled reading that the latter was true.

I wrote more investment, i.e. there is some R&D that is more significant. The actual HW, including then analog filter, can be cheaper.

[mocenigo]

On 7/16/2023 at 7:44 PM, PeterG said:

I have not listened to Shokz, but this is an interesting idea.  I think one reason that some people prefer vinyl is that they are feeling certain sounds that are not on a CD and may not be audible

Exactly, like crackles, pops, low frequency rumble, feedback, and a higher noise level overall!

[Miska]

7 hours ago, mocenigo said:

True. But the effect of ringing is overstated, in fact it is essentially absent from "true musical signal," or inaudible, as long as the source does not clip and is properly bandwidth limited. In fact, I never heard of a single study proving it is audible if these conditions are met.

 

It is not absent, it is already created in the ADC as result of that band-limiting. I have demonstrated this also in practice.

 

7 hours ago, mocenigo said:

The sigma-delta modulation is not to enable a "simpler, less expensive final analog reconstruction filter", it is a conversion process and in fact it requires a more significant investment analog reconstruction filter.

 

R2R PCM requires much more complex and expensive analog filter to properly remove the images. And the amount of oversampling you can do is limited by settling time issues, because conversion output needs to settle within 1/2 LSB within fraction of the sample period. Otherwise lot of error (distortion) is generated.

 

7 hours ago, mocenigo said:

So, digital filtering without upsampling will not remove the conversion artifacts, i.e. the images. They are a product of the conversion and they depend on the sampling rate.

 

Yes, there always needs to be analog filter, but digital filter is needed to help the analog filter do it's job.

 

7 hours ago, mocenigo said:

No processing of the signal will reduce them.

 

Yes it does, they get further apart and lower in level due to transfer function of sample-and-hold process. This makes it easier for the analog post-filter to remove what ever remains.

 

We can use Holo Audio NOS DAC combined with oversampling digital filter front-end as an example.

 

Without digital filter, running at 44.1 kHz, 0 - 22.05 kHz sweep output looks like this, you get wide spread of images, reconstruction accuracy about 4 bits:

 

Same source, but with oversampling to 705.6 kHz it looks like this, still one image around 705.6k visible, reconstruction accuracy about 13 bits:

 

And with oversampling to 1.4112 MHz it looks like this, first image would be around 1.4112M at about -110 dB, reconstruction accuracy about 18 bits:

 

 

When we oversample to 11.2 MHz, the first image is at almost 10x higher frequency... If we oversample to 45.1584 MHz (DSD1024) it is way way much further away.

 

 

In addition, combining oversampling with suitable word length and noise-shaper designed to linearise conversion stage, we can correct low level linearity errors inherent to R2R...

 

Here's output at oversampled to 705.6k, 24-bit TPDF dithered output:

 

Then when with engage suitable noise-shaper and set word length to 20-bit, we can see the distortion components are gone:

 

[Miska]

47 minutes ago, mocenigo said:

I wrote more investment, i.e. there is some R&D that is more significant.

 

Yes, I've been doing my current project since 1998. So 25 years of R&D investment has been going to it.

 

But it is still more economical per playback system.

 

[mocenigo]

5 hours ago, Miska said:

it is not absent, it is already created in the ADC as result of that band-limiting. I have demonstrated this also in practice.

 

Do not misquote me. I did not write that it is absent. It is for audibility purposes.

 

 

 12 hours ago,  mocenigo said: 

No processing of the signal will reduce them.

5 hours ago, Miska said:

Yes it does, they get further apart and lower in level due to transfer function of sample-and-hold process. This makes it easier for the analog post-filter to remove what ever remains

 

Please do not reply out of context. I was referring to a NOS context. In that case at most you filter at 20khz and then you eliminate the components of the image between 22.05khz and 24khz.  Not much. 

[Jud]

54 minutes ago, mocenigo said:

 

Do not misquote me. I did not write that it is absent. It is for audibility purposes.

 

 

 

 12 hours ago,  mocenigo said: 

No processing of the signal will reduce them.

 

Please do not reply out of context. I was referring to a NOS context. In that case at most you filter at 20khz and then you eliminate the components of the image between 22.05khz and 24khz.  Not much. 

 

To eliminate any confusion, you're now agreeing (as shown in the graphs Miska provided) that oversampling can make it easier to do good digital filtering (though oversampling itself necessitates digital filtering steps)?

[Jud]

7 hours ago, mocenigo said:

I wrote more investment, i.e. there is some R&D that is more significant. The actual HW, including then analog filter, can be cheaper.

 

I believe it is rather common for businesses to invest in technologies that will create better economic return in the end.

[mocenigo]

8 hours ago, Jud said:

To eliminate any confusion, you're now agreeing (as shown in the graphs Miska provided) that oversampling can make it easier to do good digital filtering (though oversampling itself necessitates digital filtering steps)?

we can say this. I would phrase it differently but I can agree that we agree.

 

(Miska however interpreted some data in a misleading way. You are not reducing the resolution of a system to just 4 bits if the artifacts are not in the audible band. And even a 10khz artifacts does not mask a less loud 440hz sound.)

[Allan F]

On 11/22/2022 at 4:56 PM, Jud said:

 

Grateful thanks for this.  During the interview, as noted previously in the thread, Bruno says the TI OPA 1656 op amp will sound better than the TI OPA 1612 op amp he includes with his eval board (and that ships with various Purifi-based amps, including my Apollon).  Barrows tried the OPA 1656 and told me he liked it, so I got hold of a couple (at least for the Apollon, you want the DIP8 modules - readily available on eBay in the US for less than $20 the pair; don't know about availability in other countries).  Surprise, I think Bruno was right; I feel that I like the sound better.  Subjective impression (so to be taken with a grain of salt) is richer, more reverberant, more musical, sweeter, without being overly warm or less accurate.

 

I notice that, according to the Apollon Audio website, the Purifi 1ET400A Mini Stereo Amplifier - also referred to as the Purifi 1ET400A ST Lux Stereo Amplifier - now comes equipped with the OPA 1656 op amp as standard instead of the OPA 1612. This presumably is in response to both reviews suggesting that the OPA 1656 provides better sound quality and the recommendation of Bruno Putzeys himself. FWIW, boXem also uses the OPA 1656 op amp as standard for its Purifi 1ET400A based amplifiers.

[DuckToller]

53 minutes ago, Allan F said:

 

I notice that, according to the Apollon Audio website, the Purifi 1ET400A Mini Stereo Amplifier - also referred to as the Purifi 1ET400A ST Lux Stereo Amplifier - now comes equipped with the OPA 1656 op amp as standard instead of the OPA 1612. This presumably is in response to both reviews suggesting that the OPA 1656 provides better sound quality and the recommendation of Bruno Putzeys himself. FWIW, boXem also uses the OPA 1656 op amp as standard for its Purifi 1ET400A based amplifiers.

This was discussed lately from Apollon in the asr-Forum. While the OPA1612 tests slightly better, the OPA 1656 may offer better compatibility with upstream devices, iirc.

[Jud]

1 hour ago, DuckToller said:

While the OPA1612 tests slightly better, the OPA 1656 may offer better compatibility with upstream devices

 

Bruno Putzeys of course is very big on measurements, so one wonders what he's testing that varies from ASR.

[DuckToller]

3 minutes ago, Jud said:

 

Bruno Putzeys of course is very big on measurements, so one wonders what he's testing that varies from ASR.

Jud, could you explain that for me?

He had a video interview where he mentioned the OPA1656, which may have changed the tide, and Apollon had added some flesh on his opinion, if I understood correctly.

 

[Jud]

2 minutes ago, DuckToller said:

Jud, could you explain that for me?

He had a video interview where he mentioned the OPA1656, which may have changed the tide, and Apollon had added some flesh on his opinion, if I understood correctly.

 

 

Hi. 🙂

 

Yes, Bruno said in the interview OPA1656 was superior, while you mentioned the discussion at ASR where the OPA1612 was said to test better. Since Bruno's MO is to test rather than rely completely on subjective impressions, I wondered whether he had done some different tests in which the OPA1656 excelled.

[barrows]

One consideration of the 1612 vs 1656 is that the 1656 has higher output current availability.  Theoretically, the 1612 has "enough" output current to properly drive the Purifi modules from a technical perspective.  IMO though, one often will experience better sound quality when a driving circuit has a higher amount of current headroom available-I have experienced this a few times with my DIY DAC builds, and I also preferred the 1656 (to the 1612) driving the Purifi modules in my own DIY Purifi amplifier.

The 1612 has slightly better specs when it coms to distortion than the 1656, but if one looks closely, both of these ICs have  distortion specs which are comfortably below levels anyone would consider audible, so there are no worries there...

 

While technically the 1612 should be powerful enough to drive the Purifi modules, it does appear to be pushing pretty close to its maximum levels for both output voltage (considering +/- 12 VDC rails) and current when driving the Purifi modules at high levels, perhaps the greater headroom of the 1656 is responsible for the (subjectively) better sound quality.

[Allan F]

3 hours ago, barrows said:

The 1612 has slightly better specs when it coms to distortion than the 1656, but if one looks closely, both of these ICs have  distortion specs which are comfortably below levels anyone would consider audible, so there are no worries there...

 

Bingo!

[davide256]

Anyone auditioned this Class D amp?

 

https://parttimeaudiophile.com/2023/10/20/emerald-physics-600-2-se-stereo-amplifier-review/

 

Ice modules switchable between solid state or tube input buffer, 600 watts claimed at $1999

[watts]

On 9/11/2023 at 12:57 PM, Jud said:

Yes, Bruno said in the interview OPA1656 was superior, 

 

On 9/11/2023 at 1:35 PM, barrows said:

and I also preferred the 1656 (to the 1612) driving the Purifi modules in my own DIY Purifi amplifier.

 

I had the 1656's installed in my Apollon NCx500. Tibor said he preferred these as well. When I received the amp last year I went back and forth between these and some Sonic Imagery 994's I had from my Nord's. I prefer the SI's. They sound smoother and less congested to my ears. Maybe there is a little more treble information with the 1656 but I feel it is not as well resolved. The one exception was brass/horns- I feel with just these instruments the 1656 I prefer, for whatever reason.

 

After reading this thread yesterday (quite a few twists and turns in between the Purifi discussions :) I just put them in again today as I have changed my front end, and was curious if they would be any different, but no, I came to same conclusion. Bass remained the same of course as this is done with a different amp in my system. 

[mocenigo]

On 12/1/2023 at 8:05 PM, davide256 said:

Anyone auditioned this Class D amp?

 

https://parttimeaudiophile.com/2023/10/20/emerald-physics-600-2-se-stereo-amplifier-review/

 

Ice modules switchable between solid state or tube input buffer, 600 watts claimed at $1999

how to ruin a perfectly valid amplifier module :-)

[watts]

We would never judge sound quality on looks, of course, but which of these op amps look like they might sound better?