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Class D principle, audible "higher frequency noise"

restorer-john

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#41
Well, a SNR at 80 dB is terrible. Most dacs are way over a 100 dB, so that points to the amp. As far as resolution less than 14 bit
Don't conflate SNR (signal to noise ratio) and resolution in bits- they are unrelated.

An amplifier with audible hum can still achieve a S/N well over 120dB with enough power on tap.
 

March Audio

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#42
Don't conflate SNR (signal to noise ratio) and resolution in bits- they are unrelated.

An amplifier with audible hum can still achieve a S/N well over 120dB with enough power on tap.
This is why I have always wanted to see a risidual noise measurement in uV in @amirm reviews. It provides an absolute indication of how noisy an amp is. Can be very relevant especially with more sensitive speakers.
 

tmtomh

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#43
This is why I have always wanted to see a risidual noise measurement in uV in @amirm reviews. It provides an absolute indication of how noisy an amp is. Can be very relevant especially with more sensitive speakers.
Yes , that would be great. So many otherwise quiet amps have it at audible levels.
 
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Thread Starter #44
I have received/read in another forum following statement:
"If you listen close to the tweeter (within a feet), all variants of Class-D amps have the so called switching noise. That is normal and does not affect the listening experience. The noise level is dependent on the speaker's efficiency and tweeter."
The statement was posted by someone who is not really independent (strongly connected to a company selling good amps, but producing this kind of noise).

According to what I have experienced and learned so far, these sentences are not correct, they would be correct if they were modified e.g. as follows:
"If you listen close to the tweeter (within a feet), several variants of Class-D amps have the so called switching noise. That is normal for some manufacturers and if it does affect the listening experience, everybody has to judge for themselves. The noise level is dependent on the speaker's efficiency and tweeter. But: There are also absolutely quiet class D implementations out there..."

My Lyngdorf e.g. is absolutely quiet, so I can confirm that there are quiet class D implementations out there (quiet means in this case: no audible noise, I am not talking about noise-measurements), and several members confirmed and reported this here also: sure there are some class D implementations producing this noise, but others don't!
I just like to correct things, because there are several people out there searching for useful information in forums... and I don't like if someone with strong relation to a manufacturer tells them "fairy tales", so at least these people can find correct information here.
But of course someone may correct me, in the case that I am wrong.
 

SIY

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#45
Some amps produce audible noise. Some don’t. Class D has nothing to do wit it.

Switching frequencies are generally HUGELY higher than even bats can hear.
 

Killingbeans

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#47
According to what I have experienced and learned so far, these sentences are not correct, they would be correct if they were modified e.g. as follows:
"If you listen close to the tweeter (within a feet), several variants of Class-D amps have the so called switching noise. That is normal for some manufacturers and if it does affect the listening experience, everybody has to judge for themselves. The noise level is dependent on the speaker's efficiency and tweeter. But: There are also absolutely quiet class D implementations out there..."
It has nothing to do with the switching noise. It usually sits around 500KHz or higher. Even if a tweeter could output that, there's no way your ears would be able to pick it up. All amps, no matter class, has residual noise on the output, and sometimes it's audible. It's not a Class-D specific phenomena.
 
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Thread Starter #48
It has nothing to do with the switching noise. It usually sits around 500KHz or higher. Even if a tweeter could output that, there's no way your ears would be able to pick it up. All amps, no matter class, has residual noise on the output, and sometimes it's audible. It's not a Class-D specific phenomena.
Yes, you are right. And the word "switching" within the phrase "... the so called switching noise...", is an important detail within this context.
And as far as I know (at least up to now, I am learning right now every day a bit more about audio-technique ;-) ), the word "switching noise" is common within the context of "switching regulators" (and not when talking about noise audible at the tweeter).
There can be found quite good explanations in the internet explaining the "switching noise", explaining the fact that you mentioned:
The "switching noise" introduced at the output would have the same frequency as the switcher (and some harmonics of that frequency, means even higher frequencies).
So you are right, it's not correct to call an audible noise at the tweeter "switching noise" in the context of class D amplification, because the switching-frequencies are much higher.
 

restorer-john

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#49
it's not correct to call an audible noise at the tweeter "switching noise"
Just call it residual noise. Specify a bandwidth, weighting and other conditions (shorted input/volume max etc) and specify/measure it in micro Volts (uV). That's the way it was done forever.
 

Panelhead

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#50
Just call it residual noise. Specify a bandwidth, weighting and other conditions (shorted input/volume max etc) and specify/measure it in micro Volts (uV). That's the way it was done forever.

The output noise is always there. The lack of an established standard for comparison prevents comparing these numbers. I see numbers specified for many recent amplifiers. Usually in double digit uV.
This is very quiet.
If the analyzer has enough resolution, there is measurable noise at every point in the system, thermal noise level.
 

DonH56

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#51

PierreV

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#52

DonH56

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#53
By linear extrapolation, we can deduce that in a few years we'll have a new class of integrated devices that combine a wifi router, a micro-wave oven and an amplifier. :cool:
Pretty sure that is non-linear extrapolation... :)
 

restorer-john

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#54
The output noise is always there. The lack of an established standard for comparison prevents comparing these numbers.
You don't need an "established standard" if the conditions are stated. That said, shorted input- gain wide open over a 20KHz bandwidth either weighted or not, is pretty much the defacto standard.

I measure noise with a shorted input (or 5.1K if DC coupled on the input), unweighted, over a 200KHz bandwidth on analog amplifiers, with or without a 400Hz filter (for hum and harmonics).
 

dfuller

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#55
So this does beg the question: Why are a lot of class D amps exceptionally noisy? What about the nature of the architecture makes them prone to noise?
 

DonH56

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#56
So this does beg the question: Why are a lot of class D amps exceptionally noisy? What about the nature of the architecture makes them prone to noise?
What amps do you have in mind when you say "lot"? IIRC, most of the ones Amir has measured exhibit very low noise.

The chart has a lot of class D amplifiers at eh far left (best SINAD) side... https://www.audiosciencereview.com/...iver-review.14946/#lg=attachment75333&slide=0

What we don't have AFAIK is a chart showing the actual residual noise at the outputs with inputs shorted, but you can infer reasonably low noise if SINAD is high.
 
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VintageFlanker

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#57
During the last weeks, I could test two different amplifiers from "Nuprime" (STA-9 and ST-10). Somehow I like them, but I had all in all three different noise issues.
To make things not to confusing, I want to focus here only on one of the three noises:
If one of the amplifiers is connected to the speakers and powered (means: in state "ON"), there is an audible noise in the higher frequency range.
Had the exact same issue with my previous STA-9s bridged. NOT with all my amps since (Class AB, Ncore, IcePower). Class D has nothing to do with that.
 

dfuller

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#58
What amps do you have in mind when you say "lot"? IIRC, most of the ones Amir has measured exhibit very low noise.

The chart has a lot of class D amplifiers at eh far left (best SINAD) side... https://www.audiosciencereview.com/...iver-review.14946/#lg=attachment75333&slide=0

What we don't have AFAIK is a chart showing the actual residual noise at the outputs with inputs shorted, but you can infer reasonably low noise if SINAD is high.
Sorry, I should've specified - most of my experience is with class D chip amps in active speakers, almost all of which have much higher residual noise (no signal applied, balanced connections) than class AB or G amps in the same situation.
 

Killingbeans

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#59
So this does beg the question: Why are a lot of class D amps exceptionally noisy?
Sorry, I should've specified - most of my experience is with class D chip amps in active speakers, almost all of which have much higher residual noise (no signal applied, balanced connections) than class AB or G amps in the same situation.
My bet would be cost/profit margin. I can imagine that most (low cost?) active speakers use whatever of-the-shelf chips the designer can find that fulfill some completely different criteria than low noise. These chips are probably made for other uses, like cheap portable comsumer electronics, and while proper care for the implementation could most likely lower the noise, it simply wasn't deemed necessary.

What about the nature of the architecture makes them prone to noise?
Nothing really. It's all about implementation and cutting corners.
 
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Thread Starter #60
So this does beg the question: Why are a lot of class D amps exceptionally noisy? What about the nature of the architecture makes them prone to noise?
I don' think that class D amps are generally exceptionally noisy (like explained by some other members here).
The only "special" noise I know caused by the class D principle is the "switching noise", but this noise has the same frequency as the switcher or its harmonics (e.g. for the Nuprime STA-9 this should be e.g. 550kHz, 1100kHZ, ...), means: far outside the audible range. And anyhow, (at least theoretically) this noise should be filtered off before the signal is sent to the speaker drivers (at least I hope so ;)).
If we are talking about audible noise: I am using Lyngdorf TDAI 2170, I also had for some days a TDAI 3400, and for some month I used Dynaudio Focus XD active speakers. All of them class D, all of them absolutely quiet (quiet means "no audible noise" in this case), so there are Class D implementations out there which do not produce audible noise. But of course noise has many different types and possible root causes...
 

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