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On Class D Amplifiers Measurements

solderdude

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electrocuted or electrified ?

Of course one can run a 100kHz to 1MHz sweep into any amp while monitoring the audible band while one is at it.
Question is what good does it do for non sampling amps/active speakers and how time consuming would such a test be.
Who would invent the test and standardize it and incorporate it in a standard measurement suite and convince AP to include a valid test for this.
 

solderdude

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Just for info (and fun), this is what I get from speaker cable connected to both amplifier and speaker, in the place where I live. I know that it is individual and location dependent ;).

View attachment 39211
howz about with the amplifier not connected ?
and does it make it to the amplifier input via the feedback loop ?
When measuring without a speaker cable, just on the binding posts what does one get ?
 

audimus

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Why limiting this test to class D? Demodulating out of band noise in audible band can be done in many ways.
Moreover, I believe the issue highlighted here is not general to class D, but associated to self oscillating amplifiers.
For me testing should be agnostic.
Yes, let us be PC and not do class profiling. ;)

Only if you can show that many dacs output a sufficient level of RF garbage to create a problem. I would suggest that is the first place to start. Take a look at the output of a bunch of dacs to see whats coming out and if there is something to be concerned about or if you are chasing ghosts.
Were you chasing ghosts when you decided to put a RF input filter in your amps? :p

Not sure why that is a condition since we can never guarantee there is no DAC or a SACD player or whatever source in the future might do without testing all of them. Seems like given the potential behavior and that you think it important enough to put a RF filter, we should expect this of any Class D amp and should ensure that such filters do what they do by a measurement. To give it a clean bill of health, so to speak.
 

FrantzM

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Lucille Ball hearing strong radio signals via her temporary lead dental fillings has never been confirmed or disproven, but one thing that she did is appreciated by me and millions of other science fiction fans - and she had no idea what the show was about. A very "strange but true" tale.
WoW!!!
 

March Audio

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Were you chasing ghosts when you decided to put a RF input filter in your amps? :p

Not sure why that is a condition since we can never guarantee there is no DAC or a SACD player or whatever source in the future might do without testing all of them. Seems like given the potential behavior and that you think it important enough to put a RF filter, we should expect this of any Class D amp and should ensure that such filters do what they do by a measurement. To give it a clean bill of health, so to speak.
TBH really yes. I did it as good design practice to try and account for all eventualities. It wasn't actually because I had experienced a problem with external RF or created a problem by attaching any dac I had available.

That isn't to say there are no dacs out there that could spew out enough crap to create a problem, but it hasn't happened with anything I have tried.

RF filters are good design practice on any class of amp. This is not about class d. I would do exactly the same on a class A or A/B
 
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March Audio

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Just for info (and fun), this is what I get from speaker cable connected to both amplifier and speaker, in the place where I live. I know that it is individual and location dependent ;).

View attachment 39211
So about 100uV. So what would end up inside a screened input cable?
 

audimus

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TBH really yes. I did it as good design practice to try and account for all eventualities. It wasn't actually because I had experienced a problem with external RF or created a problem by attaching any dac I had available.

That isn't to say there are no dacs out there that could spew out enough crap to create a problem, but it hasn't happened with anything I have tried.

RF filters are good design practice on any class of amp. This is not about class d. I would do exactly the same on a class A or A/B
Is there any other class of amp where audible effects are known to happen IF such an RF input came in at sufficient levels as shown by the OP test? This is not a rhetorical question. I really don’t know.

If not, then it would be a documented Class D peculiarity and so worth making sure that it was protected against it. If there is, then it would be worth establishing the outcome in the same way OP did. And if that is done, then they can be tested as well. I don’t really see a problem with doing such a test. But if I was worried about the optics of such a test on a class of amp I was selling, I would be. It could lead to people with vested opposing interests making it a talking point in their narrative.

Adcom amps always had a bad rap for not having speaker protection although getting a reliable report of an Adcom amp failure actually taking out a speaker were hard to come by and it wasn’t because the amps never failed, just didn’t fail in a mode that would fry the speakers while theoretically possible. They just didn’t realize how insecure people would feel with even a suggestion that their expensive speakers could be taken out by an amp failure.

So, I always thought Adcom messed up by not putting some protection as a precautionary measure rather than letting that narrative grow from a marketing point of view while trying to dismiss such concerns. And making that protection circuitry a matter-of-fact bullet in their brochures.

Seems to me, from a marketing point of view, Class D amp manufacturers would be better off just owning this possibility and simply saying it is a non-issue given the RF filters as you did (and making the effectiveness of the filter a talking point) but not protest so much about whether it is chasing ghosts because it cannot happen. Don’t need the latter. It does feel like it is over-defensive even if technically correct and that can create the kernel of doubt that lets opposing narratives grow.
 

March Audio

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Is there any other class of amp where audible effects are known to happen IF such an RF input came in at sufficient levels as shown by the OP test? This is not a rhetorical question. I really don’t know.

If not, then it would be a documented Class D peculiarity and so worth making sure that it was protected against it. If there is, then it would be worth establishing the outcome in the same way OP did. And if that is done, then they can be tested as well. I don’t really see a problem with doing such a test. But if I was worried about the optics of such a test on a class of amp I was selling, I would be. It could lead to people with vested opposing interests making it a talking point in their narrative.

Adcom amps always had a bad rap for not having speaker protection although getting a reliable report of an Adcom amp failure actually taking out a speaker were hard to come by and it wasn’t because the amps never failed, just didn’t fail in a mode that would fry the speakers while theoretically possible. They just didn’t realize how insecure people would feel with even a suggestion that their expensive speakers could be taken out by an amp failure.

So, I always thought Adcom messed up by not putting some protection as a precautionary measure rather than letting that narrative grow from a marketing point of view while trying to dismiss such concerns. And making that protection circuitry a matter-of-fact bullet in their brochures.

Seems to me, from a marketing point of view, Class D amp manufacturers would be better off just owning this possibility and simply saying it is a non-issue given the RF filters as you did (and making the effectiveness of the filter a talking point) but not protest so much about whether it is chasing ghosts because it cannot happen. Don’t need the latter. It does feel like it is over-defensive even if technically correct and that can create the kernel of doubt that lets opposing narratives grow.
Yes. All classes can be affected by RF, demodulating, dc offsets etc.

I am not in the slightest bit concerned about doing some tests. I have suggested Amir does exactly this.

Did you not notice?

Spreading FUD is rife in audio. You won't counter it with a few tests on ASR. Non technical audiophiles will continue beleive what they want.
 
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DonH56

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@audimus : RFI has been an issue for decades and was long before class D audio amplifiers became common. See earlier posts from @restorer-john and others. You need a path for RF to reach a junction (which can be most any transistor) and those are all too common. Good design practice is to include a filter (often just a capacitor, sometimes an LC filter) to reject out-of-band noise. <Edit: @March Audio beat me to it.>

@RayDunzl : A line of tall towers, sometimes with a couple at one end offset in sort of a "V", is usually a directional AM station antenna. The wavelengths are long and the entire tower is the antenna. The line of towers comprise an antenna with one driven (powered) element (tower) and the rest are to direct the signal to avoid interference with other nearby (or far away) stations or other sensitive locations (airports, Area 51, whatever).
 
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Xulonn

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the faster amplifier
Back about 1967, I moved an Eico ST-40 integrated amp in my 1965 maroon Corvette, just like the one below, and hit 100mph on the freeway. That Eico became the "fastest" amplifier I ever owned.

1965-corvette-4.jpg
 
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Matias

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Spreading FUD is rife in audio. You won't counter it with a few tests on ASR. Non technical audiophiles will continue beleive what they want.
Would you mind creating a new topic "Class D Mythbusting" or something? It should have following:
1. It generates high frequency noise I can hear.
2. High frequency noise can blow the tweeters.
3. Power is not continuous, as per their datasheets.
4. Phase shift on the high frequencies.
5. They sound cold and sterile.
6. No way a small and light amplifier can supply those power ratings.
7. I heard a class D some years ago, it sounded bad, so all class D must sound bad too.
8. Power does not double when halving the load impedance.

These are the top of mind, sure there may be other myths worth answering. :)
 
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audimus

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Yes. All classes can be affected by RF, demodulating, dc offsets etc.

I am not in the slightest bit concerned about doing some tests. I have suggested Amir does exactly this.

Did you not notice?
Why include what can be construed as snark at the end? This is what makes people think you are less than objective. :)

I was replying to your very post that had the caveat in response to my earlier post “ONLY if you first measure DACs.... etc” and because of that caveat. My post was to specifically suggest you don’t need to have that caveat with the risk of being seen as in denial.

In terms of RF interference, are there any measurement tests published anywhere of what happens in a recent vintage non class D amp in the audible region? Is it similar to Class D or a different artifact? The explanation in the OP seemed like there was a particular interaction because of the inherent design. Hence the need for testing. Just saying they all can be affected seems like defensive deflection more than specifics.
 
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