Noisy Purifi amplifiers

[SIY]

The feedback in self-oscillating Class D is deep - nevertheless, any first order of astatism feedback loop takes a derivative of distortions thus the hiss is essential. Its level / audibility is a separate issue. The hearing abilities of people are different. If you don't hear anything ... alas, it only means that YOU don't hear anything.

If you claim that there's ANYONE who can hear the hiss levels of good Purifi implementations (like their EVAL system), you are talking absolute nonsense, well contradicted by actual measured data. Ditto other competent Class D implementations, of which there are many.

 

[Deleted member 27948]

Bravo for using an obscure general engineering term to explain linear feedback but completely ignoring the level/audibility of hiss, which is the subject!

A good example on how to derail a thread.

Why? that's not an obscure general term, but a general term from the theory of automatic control which is the theory of feedback. Alas, too many people claim that they know how feedback works... but very, very few have grounds to say so.

As I already noted, the noise level is not enough. you need spectrum, etc.
As shall be well known, male hearing degrades with age for many. There are lots of wealthy babyboomers who finally can afford superb audio equipment but alas, are too old to benefit from it. Females are obviously better designed and age much more gracefully. However, distributions are very wide, and any deterministic conclusion audible/inaudible is inappropriate.

My Naim amp loaded by Focal with 92.5dBW has no audible to me hiss at any distance. The hum is marginally audible in the woofer's Fresnel zone. I do NOT consider it an issue.

 

[sarumbear]

Why? that's not an obscure general term, but a general term from the theory of automatic control which is the theory of feedback. Alas, too many people claim that they know how feedback works... but very, very few have grounds to say so.

Check my bio.

As I already noted, the noise level is not enough. you need spectrum, etc.
As shall be well known, male hearing degrades with age for many. There are lots of wealthy babyboomers who finally can afford superb audio equipment but alas, are too old to benefit from it. Females are obviously better designed and age much more gracefully. However, distributions are very wide, and any deterministic conclusion audible/inaudible is inappropriate.

My Naim amp loaded by Focal with 92.5dBW has no audible to me hiss at any distance. The hum is marginally audible in the woofer's Fresnel zone. I do NOT consider it an issue.

Hiss means audible high frequency noise. You still have not said why a class D amplifier will have hiss, inherently.

We don‘t need to know about your Naim and Focals, nor difference between male and female hearing degradation, which is sexist. Stay on topic, please.

 

[sarumbear]

1) Could you please provide this actual measured data?
2) "Ditto other competent Class D implementations," - prove it.
3) There are young people with very sharp hearing. After a good night's sleep and 3-4 hours in a well soundproof room, they can hear well down to air thermal level.

Here is Rhodos, here you jump.

Please don't put your words into my mouth and don't push any of your beliefs into my throat.
If you have scientific arguments, I'll look forward to hearing them. Otherwise, Iuppiter iratus ergo nefas - your insults prove nothing but your incompetence.

Why do we do that? You are arguing that any class D amplifier will have hiss as it’s inherent to the design because it has linear feedback.

Show us the measurements to prove your point. The onus is on you. It’s not us who needs to jump.

 

[Deleted member 27948]

Please don't put your words into my mouth

 

[Deleted member 27948]

I am a practicing mathematician, adaptive control in complex stochastic systems. People here may not realize that I already know who I am (from a few whose opinions I highly value) and that their jumps to conclusions about my intellect simply make me laugh.

There was a time I was involved in the design of 1kW Class D amp, and used that opportunity to learn more. I would like to learn even more here ... but I get only insults.

 

[Dialectic]

To get rid of the humming transformers in my Cambridge amp I bought 2 Purifi monoblocks. Happy with the way they look and sound. Until I came close to my speakers... Both amps cause (some) noise from my speakers. If I had to describe it, I'd say a slight hiss, with a touch of static. Audible from about 30cm to 40cm (approx. 1ft). It's (obviously and fortunately) not loud enough to be noticeable when music is playing.

I have had some hiss in some but not all amp-speaker combos I've owned. (The Devialet amps, for all their other faults, are dead quiet.) I previously drove myself crazy changing out cables, switching outlets, moving devices, etc. to try to resolve it.

Then I remembered that I don't listen to music while sitting 30 to 40 cm from the speakers.

Problem solved.

 

[Deleted member 27948]

Thank you for confirming my hypothesis.

It's a pleasure talking to a nice clever person like you.

 

[mocenigo]

All the more reason to test and publish all amplifiers' residual noise levels in uV. The spectrum of the hiss should be commented upon. It's good see people are finally seeing through the hype, listening for themselves and hearing anomalies in these Class Ds.

There is no anomaly. The SNR results tell us that the top Class D amplifiers have ridiculously low levels of noise and only a few AB amplifiers can top them (Benchmark AHB2, McIntosh MC462). You are right here though:

As @pma states, inputs are shorted and then you measure the residual. Sure, it can be brickwalled at 20kHz for the Class Ds or A-WTD, but if you A-WTD and/or brickwall the measurement, do the same for Class ABs as well or the playing field isn't level.

One should use the same weighting. Regarding the hiss, a properly built NC500 or Purifi amplifier has no hiss. And hiss coming from the sensitive input line stages and them amplified by the power stages can happen in any topology. If an amplifier amplifies a signal, it amplifies also a hiss.

We also know Audiophonics is probably lying about the noise figure (11.5uV) on their amplifier. They have quoted the Purifi spec sheet noise and yet they have a switchable gain stage up front. Note, that is an A-WTD figure, not unweighted. We know the noise will be higher than 11.5uV except on perhaps the "bypass" position. You cannot have 7.2dB or 12.5dB of gain up front without impacting the published module-only noise figure.

It is true that they quote directly form the Purifi spec sheet, in a complete amplifier it is rather difficult, if not impossible to have only 11.5uV of noise. However, if the input buffer's noise is low enough, you can easily have roughly the same total output noise whether unity, 7.2dB or 12.5dB of gain are used. Also because the noise from the buffer does not necessarily depend from the gain.

This said,m even using a state of the art buffer like the Neurochrome Purifi/NCore buffer, we have

The 12.8Db gain of the 1ET400A represent a voltage ratio gain of 4.365. now let us have a look at the noise from the buffer

• 4.0 µV RMS at 13 dB gain -> the contribution from the buffer becomes ~ 17.5 µV RMS
• 1.6 µV RMS at unity gain -> the contribution from the buffer becomes ~ 7 µV RMS

in the first case (assuming Purifi also gives RMS values) the total RMS noise becomes ~ 21 and ~ 13.5 µV RMS.

So giving the value as 11.5uV for the whole amp would be wrong. But also not orders of magnitude wrong.

Of course, it is all to be demonstrated that the Audiophonics amps have such good values – anecdotal evidence seems indeed to point to the contrary, and looking at the poor cable routing would also suggest that.

Roberto

 

[Matias]

"I had to put my ear almost on the tweeter to hear anything at all. The Eigentakt is one quiet amp."
This is what a proper implementation should sound like, Purifi EVAL-1 demo amp with Magico A1 by Doug Schneider.

Note how cables are not routed near the modules.

SoundStage! Hi-Fi | SoundStageHiFi.com - Purifi Audio's Pint-Sized Powerhouses

www.soundstagehifi.com

 

[mocenigo]

Check this first. I thought my NC400's were hissing a bit more than I expected, turned out it was my source. The moment I turned that off or switched to another (better) source they went dead quiet.

It happened to me as well. I had an excellent DAC, and I never noticed its intrinsic thermal noise until I moved from an Abrahamsen amplifier to Apollon NC500 monos.

 

[mocenigo]

Sorry, but you are either trolling or clueless. I have speakers with a nearly 100Db sensitivity and a Purifi based amplifier - no hiss at all even if I put my ear as close as possible to the drivers.

Turns out, I missed a third option: a language barrier? But the 500khz oscillation cannot be described as a hiss, and it is not audible.

 

[mocenigo]

One thing I like about ASR is the lack of personal attacks, but I must admit to thinking the same thing.

As I wrote in a different reply, here we may have missed the option of a language barrier. However 500Khz is not a "hiss" we can hear. True it can intermodulate (and feedback puts the results back into the amplifier, first attenuating and then amplifying them again). But the artefacts in the audio range are still vey low.

I understand that in the eaarly design of a powerful amplifier (which mt42 claims to have done and I have no reason to doubt that) these artifacts may be audible, but this is the reason some designs like the Purifi have multiple feedback loops (and even a feed forward path) – even the much more minimalist Orchard Audio have two loops. If one goes just for power, which is perfectly valid application, then artefacts may be audible. But it is not the type of amplifiers we talk about here.

 

[SIY]

1) Could you please provide this actual measured data?
2) "Ditto other competent Class D implementations," - prove it.
3) There are young people with very sharp hearing. After a good night's sleep and 3-4 hours in a well soundproof room, they can hear well down to air thermal level.

Here is Rhodos, here you jump.

Please don't put your words into my mouth and don't push any of your beliefs into my throat.
If you have scientific arguments, I'll look forward to hearing them. Otherwise, Iuppiter iratus ergo nefas - your insults prove nothing but your incompetence.

Read my reviews (with full noise spectrum measurements) of the Purifi, Orchard Starkrimson, and others. Also about a zillion measurements here from Amir.

You are completely incorrect. Stop digging.

 

[Matias]

With your permission, @SIY, I quote below. If not let me know and I will delete this post.

"The no-signal noise spectrum is shown in Figure 8, and indicates a 500 kHz switching frequency at about 0.2 V, which is inconsequential.
The ultrasonic noise floor is likewise quite low with the largest component being a spur at -93 dBV at 90 kHz. This is low enough not to disorient bats passing by."
Audioxpress 07-2020, Purifi EVAL-1 review.

 

[sarumbear]

I am a practicing mathematician, adaptive control in complex stochastic systems. People here may not realize that I already know who I am (from a few whose opinions I highly value) and that their jumps to conclusions about my intellect simply make me laugh.

There was a time I was involved in the design of 1kW Class D amp, and used that opportunity to learn more. I would like to learn even more here ... but I get only insults.

Then do please tell us how Purifi managed to build an amplifier using an inherently hissy design and achieved 11.5 micro volt noise at its output? That’s 108dB below 2.83V that generates 1W at a nominal 8 ohm 86dB/W sensitivity speaker.

How can anyone hear that?

PS. This is a very strange statement: “People here may not realize that I already know who I am.” Good that you know yourself but maybe you consider updating your profile so we know you too.

 

[Walter]

I've deleted my last 3 posts in this thread. I decided they add no value and do not reflect the spirit of the "Wanted" message at the top of this and every other ASR forum page. My apologies.

 

[Killingbeans]

I would like to learn even more here ... but I get only insults.

It would help if you dialed down the smugness a bit. You can have all the brains in the world, but it won't do you much good if you come off as a d##k.

I think we can all agree that Class-D amps are "noisy", but that good designs also push the vast majority of the noise into the ultrasonics.

The only thing we disagree about is whether the remainder is enough to cause audible hiss.

 

[Deleted member 27948]

May I please suggest looking at Amir's review of Purifi DIY block 1ET400A,

Review and Measurements of Purifi 1ET400A Amplifier

This is a review and detailed measurements of the Purifi 1ET400A Class-D amplifier module. Company was kind enough to send me a complete, assembled kit consisting of two modules, power supply and case, ready to go. They have not disclosed the cost of the unit. Modules will be available to DIY...

www.audiosciencereview.com

picture of 1kHz wide spectrum: https://www.audiosciencereview.com/...hz-fft-spectrum-audio-measurements-png.28895/

The original complaint of the thread starter was that some hiss was [barely] audible when pressing an ear directly to twitter. My answer was - that's fine, don't worry, that's not an indication that something is [inherently] wrong with the amp. You seem to have problems with it... and you also have other problems, like with objective scientific finding that females' hearing degrades less than males' with age. Excuse me ... these are NOT my problems.

Also, may I please ask you to refrain from any foul language as it pollutes ASR?

 

[BrokenEnglishGuy]

I have the vtv Purifi EVAL1, very clean power amp.
The only chance to hear the hiss is to have the ear pointed at the tweeter at 5 cm in the night, very difficult to notice.