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Class D principle, audible "higher frequency noise"

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#1
Hello,

I am new here in this forum and joined now, because I have a dedicated question regarding class D amplifying technic:

The situation:
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.
I do not know the exact frequency, but this noise is audible at the tweeter.
This noise is very low in volume, I can hear it up to a distance of appr. 20 cm from the speakers, I cannot hear it from my usual listening position.
The noise is independently from other connections, devices, rooms or chosen wall-sockets.

In another forum a guy from "Nuprime" stated with one sentence that is is "normal" and - as far as I understood - has to to with the "switching frequency". I asked for more information, but it seems that the guy noticed that it is not the best kind of marketing to declare an audible noise as normal ;-)... and did not respond any more to this issue in a helpful way.

So I wonder now, why this noise is "normal", because I already used two different class D implementations in my rooms (Lyngdorf TDAI 2170 and Dynaudio Focus XD active speaker), both of them are absolutely silent, even if you put your ear close to the speaker.

My questions:
Can a noise like this really be considered as "normal" in some class D implementations?
If yes, what are the differences in comparison to "silent" class D implementations, and are their any advantages in the "noisy" one?

Note:
Please don't get me wrong: I do not want to blame Nuprime (I still like their devices, and both devices are doing their "main job" fine,) and I do not want to judge this noise, I just want to understand the main reason (at least a little bit), understand the differences to other implementations, and learn a bit more what is going on... and of course any link to existing information is also very much appreciated... I guess I would anyhow need some time to read and understand ;-).
 

kaka89

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#2
I am also a happy Nuprime owner. I used to have STA-9 and now using ST-10.

I think it is normal for tweeter to have (very) low noise, and this is not limited to class D. Class A and AB also have the same problem, and tube amp would be worst.

What speaker are you using?
 

March Audio

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#3
T
Hello,

I am new here in this forum and joined now, because I have a dedicated question regarding class D amplifying technic:

The situation:
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.
I do not know the exact frequency, but this noise is audible at the tweeter.
This noise is very low in volume, I can hear it up to a distance of appr. 20 cm from the speakers, I cannot hear it from my usual listening position.
The noise is independently from other connections, devices, rooms or chosen wall-sockets.

In another forum a guy from "Nuprime" stated with one sentence that is is "normal" and - as far as I understood - has to to with the "switching frequency". I asked for more information, but it seems that the guy noticed that it is not the best kind of marketing to declare an audible noise as normal ;-)... and did not respond any more to this issue in a helpful way.

So I wonder now, why this noise is "normal", because I already used two different class D implementations in my rooms (Lyngdorf TDAI 2170 and Dynaudio Focus XD active speaker), both of them are absolutely silent, even if you put your ear close to the speaker.

My questions:
Can a noise like this really be considered as "normal" in some class D implementations?
If yes, what are the differences in comparison to "silent" class D implementations, and are their any advantages in the "noisy" one?

Note:
Please don't get me wrong: I do not want to blame Nuprime (I still like their devices, and both devices are doing their "main job" fine,) and I do not want to judge this noise, I just want to understand the main reason (at least a little bit), understand the differences to other implementations, and learn a bit more what is going on... and of course any link to existing information is also very much appreciated... I guess I would anyhow need some time to read and understand ;-).
This is not "normal". It should be silent. This is not an inherent class d issue.

It could be specific to the nuprime, but there is however a situation where noise can be created. Before I go into detail can you check one thing.

Disconnect the source, dac or whatever, and see if the noise is still there.

Is it just a hiss or more like a "digital" sort of noise?
 

kaka89

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#4
Well, by normal I meant it is not defective.
Nuprime noise floor is higher than Hypex product that is for sure. If you want absolutely silent then try Hypex, Purifi or other measure well modules .

I agree with @March Audio that you should try disconnect with all sources, to check if the noise actually comes from other source or ground loop.
 

DonH56

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#6
What everyone else said. All amps have noise; it is more audible with some than with others. It is not specific to any one class of amplifier. As for switching frequency noise, well, even young folks' hearing rolls off just a little over 20 kHz and class D amplifiers typically switch at 400 kHz or more -- about twenty times the highest frequency we can hear, and way beyond what your tweeter can produce.
 
OP
D
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Thread Starter #7
T

This is not "normal". It should be silent. This is not an inherent class d issue.

It could be specific to the nuprime, but there is however a situation where noise can be created. Before I go into detail can you check one thing.

Disconnect the source, dac or whatever, and see if the noise is still there.

Is it just a hiss or more like a "digital" sort of noise?
Hi and thanks for the answer.
I already tried quite a lot, including: Connect only the ST-10 or or the STA-9 to speakers. No other connection (means: also no pre-amp), all other devices in the room completely disconnected from any power-socket. I also tried this test with different power and speaker cables, also in a different room. There was no influence to this noise (means: This "higher frequency" noise was not influenced by any other change).
Of course I first thought it would be defect, but the same defect in two devices... hm... and according to some opinions or Nuprime users this noise seems to be "normal" or at least not unusual, which was also surprising to me...
 
OP
D
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Thread Starter #8
What everyone else said. All amps have noise; it is more audible with some than with others. It is not specific to any one class of amplifier. As for switching frequency noise, well, even young folks' hearing rolls off just a little over 20 kHz and class D amplifiers typically switch at 400 kHz or more -- about twenty times the highest frequency we can hear, and way beyond what your tweeter can produce.
OK... I just thought that it is a class D issue, so maybe this is wrong...
And yes... the switching frequency is much higher, and if I still can hear it, it mus be less than about 13kHz (the noise, I mean)...;)
 

MediumRare

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#9
Personally, I do not find any audible noise at the listening position to be acceptable. Use your smartphone and a free app to measure and you’ll see if it’s true noise (like white/pink static) or some combo of harmonics and at what point in your room it’s measurable. In any case, why would you buy anything new with noise when the SNR should be 85+ ?

[edited]
 
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DonH56

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#10
Stick your ear in the speaker and most every system will have some noise at some combination of gain and such. Given the dynamic range of most recordings (audio or video) I don't care unless it is clearly audible at the MLP with the volume at or a little above my normal max setting.
 

MediumRare

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#11
Stick your ear in the speaker and most every system will have some noise at some combination of gain and such. Given the dynamic range of most recordings (audio or video) I don't care unless it is clearly audible at the MLP with the volume at or a little above my normal max setting.
I will concede "at MLP", but I like blackness where the recording calls for it.
 

Eetu

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#12
The Nuprimes you tested have SNR specs of 91 & 95db, could be better. The Hypex NC252MP (class D) has a noise floor of ~-120db (power/load dependent). The hiss is also influenced by whatever DAC & preamp you're using with the Nuprimes vs Lyngdorf/Dynaudio. Also, if you used RCA inputs, maybe try balanced to see if the hiss is reduced.
 
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#13
I’m using two Nuprime STA-9 as monoblocks to my Canton SLS 790 DC and I also have noise (hiss) from the tweeter. I’ve had more or less noice with all my speakers and amps. I don’t worry about it since it can only be heard 5 cm from the tweeter.

I have amplifiers from Arcam and Emotiva and they too produce noise at very close distance with other speakers.

From my experience the higher sensitivity of the speaker, the higher noise.
 
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tmtomh

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#14
Stick your ear in the speaker and most every system will have some noise at some combination of gain and such. Given the dynamic range of most recordings (audio or video) I don't care unless it is clearly audible at the MLP with the volume at or a little above my normal max setting.
Agree - I've never not heard noise when I stick my ear right in front of a tweeter. However, the volume has varied dramatically. With my prior amp (a Class AB Adcom), I heard a clear hiss from the tweeters that was audible from about 2 feet away if I kept my ear directly facing the tweeter (e.g. head turned to the side, ear aligned on-axis). With my new amp (Audiophonics Purifi-based), I have to get within about 8 inches of the tweeter (again, ear on-axis directly facing it), and the hiss that I do hear at that point is lower in volume than with the Adcom. I've tried putting my ear directly on the tweeter and the hiss is still lower than it was with the Adcom when I was a foot or more away.

The Audiophonics is so quiet that it revealed the contribution of additional noise when my source component - an Oppo UDP-205 - is turned on. In fact, I have a four-outlet wall plate - so two 2-outlet modules behind that wall plate - and I even detected a slight difference in the noise level depending on whether I have the Oppo plugged in below my amp (in other words into the same physical outlet module) or next to the amp (in other words in the other outlet module). The noise difference through the tweeters in those two scenarios is subtle - I would guess perhaps 1-2dB - but clearly detectable.

My overall conclusion is that some very low level of electrical noise is either unavoidable, or at best a matter of luck.

But at the same time, psychologically I find it much more difficult to live with a louder hiss that I can hear from a couple of feet away, even though I still cannot hear such a hiss from my listening position.

I feel better and relax more when I know the noise is truly minimal (and when I don't hear it as I, for example, pass close to a speaker as I walk up to the rack to change a CD). While that is a psychological issue rather than a strictly performance-related issue, listening is about relaxation, enjoyment, and engagement, and so to me it's worth chasing lower noise because it indirectly but very clearly enables me to enjoy listening more.
 

Vasr

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#15
The only known cure to idle background noise (that isn't related to ground loop) is inefficient speakers. ;)
 
OP
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Thread Starter #16
T
Is it just a hiss or more like a "digital" sort of noise?
I forgot to answer the second part of your question: It is mor a "hiss"... somehow like you would take "pink noise" and remove all ferquencies lower than x, means: sounds to me like a mixture of several higher frequencies.

Regarding/Summarizing the other posts:

- Up to now it seems that other members here have same or at least similar experiences (I never had anything similar... and I am quite sensitive to noise, maybe I just was lucky up to now...). So it seems to be a comon thing... not nice maybe, but at least common...

- The noise is most probably not related to the implementation type (class D in this case)

- Regarding the suggestion testing XLR-Input: The noise is also there if NOTHING but the speakers are connected to the ST-10 or STA-9 (no Pre-amp, no RCA connection, no XLR connection, I even tested with no other electrical device in the room, and also in an additional room). I do not have the possibility to test XLR, but I guess the test would not change anything, because even with no connection at all the noise is there. Within my limited possibilities I can connect whatever wherever I want: The noise is there and does not change.

But... does somebody know where this noise is coming from (I mean the technical root cause)?
Or is it just hard or impossible to say because in every system/amp there may be different root causes?
 

kaka89

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#17
I forgot to answer the second part of your question: It is mor a "hiss"... somehow like you would take "pink noise" and remove all ferquencies lower than x, means: sounds to me like a mixture of several higher frequencies.

Regarding/Summarizing the other posts:

- Up to now it seems that other members here have same or at least similar experiences (I never had anything similar... and I am quite sensitive to noise, maybe I just was lucky up to now...). So it seems to be a comon thing... not nice maybe, but at least common...

- The noise is most probably not related to the implementation type (class D in this case)

- Regarding the suggestion testing XLR-Input: The noise is also there if NOTHING but the speakers are connected to the ST-10 or STA-9 (no Pre-amp, no RCA connection, no XLR connection, I even tested with no other electrical device in the room, and also in an additional room). I do not have the possibility to test XLR, but I guess the test would not change anything, because even with no connection at all the noise is there. Within my limited possibilities I can connect whatever wherever I want: The noise is there and does not change.

But... does somebody know where this noise is coming from (I mean the technical root cause)?
Or is it just hard or impossible to say because in every system/amp there may be different root causes?
Normally the noise should not bother you, if it does it could be a defected product, I recommend send back to check.
 

SIY

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#18
But... does somebody know where this noise is coming from (I mean the technical root cause)?
Or is it just hard or impossible to say because in every system/amp there may be different root causes?
Indeed, many possible causes, but it could be easily determined with the amp on hand.
 

SIY

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#19
Agree - I've never not heard noise when I stick my ear right in front of a tweeter.
Ditto IME, until a few months ago. I have nCore and Purifi amps on hand where there is absolutely no audible noise from the tweeter (or any of the other drivers). I thought I heard some, then kicked myself after figuring out by putting my ear next to the baffle that it was the seashell effect.
 

MediumRare

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#20
I just measured mine: with the gain turned all the way up about 5 dB in the area from 4 kHz to 20 kHz, measured 1 cm from the tweeter, about 3 dB at half gain. Completely inaudible given a quite silent (of course, not actually silent) room. When I change the source to an empty preamp input the noise goes to 1 to 2 dB. Background noise is almost all below 500 Hz.
 
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