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On Class D Amplifiers Measurements

What's the tweeter inductance?That goes a long way toward reducing the current.

Of course I was waiting for this reasoning ;). I measured tweeter current as a voltage drop on 1 ohm low inductance resistor, "audio" signal was a 5kHz sine. The result is attached. As the plot is calibrated, dBV reading directly reflects in dBA reading (1 ohm resistor). See the peak at >400kHz.

1574009152773.png
 
The only source of such a high frequency I can think of just now would be the noise shaped stuff coming out of an SACD player or a DSD DAC.

Are you sure? Did you make some research and measurements on this?

1574010606795.png
 
Of course, there's a lot of room for unintended pickup of HF signals...

I put a 1R in series with a tweeter, then fed it with the amp I showed upthread, while getting the spectrum across the 1R. Current at the switching frequency is about 1 mA (and that may or may not be 100% actual VC current, but does set an upper bound). 750mV*1mA= 0.75mW. That is not terribly worrying to me.

Class D idle into tweeter.png
 
Are you sure?
No - as I wrote I remember having seen plots somewhere at ASR. :oops:
Did you make some research and measurements on this?
Since I don't have an SACD player I cannot measure its noise. I'm sure though that someone else with more knowledge than me will correct me.;)
 
Does that mean there is a market for $400 “line conditioners” to remove spurious high frequency signals for Class D amps and this time show there is audible difference with and without them? :D
 
What Im saying is can you demonstrate this is a real world problem?

Can you, on the other hand demonstrate it is not? The spurious/out of band output content of digital source components connected to the various Class D amplifiers is an unknown.

There will be 'audiophiles' with NOS filterless D/A converters putting out goodness knows what into their Class D amplifiers, just as there will be guys with SACD/DSD players with unknown HF content.

In real terms, PMA has done (unintentionally) what the high end companies do (and you heard it from here first). He has created, or discovered, a problem that really doesn't cause a serious issue on the whole, but is rooted in science, and all he has to do now, is manufacture and market a grossly overpriced solution in the form of nice wooden or extruded aluminium cased special filter with gold jacks and a nice velvet presentation box.

All it needs is a catchy name. Audiophiles will lap them up by the thousand, others will deconstruct and make their own, but it will become an essential add-on. "Do you want a filter with that Class D amplifier, sir?" Even if Purifi and others put perfectly adequate LPFs into their gear, the standalone filter will be a "belts and braces" approach no audiophile will be able to resist. Without one in the system, their friends won't take them seriously. In fact, each digital source component (an FM tuners) needs one too. I sense a new cash cow!
 
PURIFI 1ET400A has a PSRR of 90 dB, very good data. Forced 1Vrms f≤1kHz ripple, either rail.

Hypex NC500 has a PSRR: 75 dB (min) and 85 dB (typ).

The Hypex module needs an aditional RF / EMI filter (if mains is noisy, usually in urban concentrations). And two stages filter work without problems. They are economical.
 
At first thought it's interesting. Then you have to think about it.
A better approach is to use pink spectrum NID multitone input signals to test this. First limit the bandwidth to 20khz then increase to 35khz, 45khz,80khz to see at what point it starts to affect the spectrum under 20khz.
One thing to point out, instead of using sampling as analogy, it's actually more like AM signal. That's how you get to see the signal being "demodulated" into audio band.
Rod Elliott talked about this before, directing the actual rf interference present in the environment. http://sound-au.com/amp_design.htm#s12
this seems like a very reasonable approach to see whether this affects a given cd design, and if i am not wrong, something we have people around can do, to know whether there is any fire correspomding to this smoke :p
 
At first thought it's interesting. Then you have to think about it.
A better approach is to use pink spectrum NID multitone input signals to test this. First limit the bandwidth to 20khz then increase to 35khz, 45khz,80khz to see at what point it starts to affect the spectrum under 20khz.

Thinking about this, I am not sure pink noise at some reference level across the band would necessarily prove or disprove anything about the real world. If the out of listening band signal is not necessarily controlled by the gain setting in the real world and hence uncorrelated with the reference level used within the audible region, the level of any perturbation induced by it might be significant or not depending on the gain setting.

What I am trying to get at is that there are 3 possibilities to eliminate to relate to real world

(1) eliminate any possibility that any induced or leaking out of band signal does not create audible noise when there is no signal in the audible range (like audible hiss or hum when no content is playing), which is what I think OP covers. So don’t have any signal in the audible range but keep increasing the band above it until you see any effect in the auditory range.
(2) eliminate any possibility that even if there is any audible effect in (1) above that it is not masked by audible band signals at reference level.
(3) eliminate any possibility that any audible effect is not masked at low levels of signal as may happen when listening at low levels. This would require a sweep across gain levels.

In other words, do the real world device gain controls affect the out of auditory range signals proportionally relative to the auditory range as measuring instruments would? I don’t know the technology enough to answer this.
 
In other words, do the real world device gain controls affect the out of auditory range signals proportionally relative to the auditory range as measuring instruments would? I don’t know the technology enough to answer this.

Oh, this is the perfect audiophile silver bullet solution! Introduce doubt as to whether instruments can measure something the golden-ears claim to be able to hear, and then present the solution.

Ok, I'll stop now. :)
 
Thinking about this, I am not sure pink noise at some reference level across the band would necessarily prove or disprove anything about the real world. If the out of listening band signal is not necessarily controlled by the gain setting in the real world and hence uncorrelated with the reference level used within the audible region, the level of any perturbation induced by it might be significant or not depending on the gain setting.

What I am trying to get at is that there are 3 possibilities to eliminate to relate to real world

(1) eliminate any possibility that any induced or leaking out of band signal does not create audible noise when there is no signal in the audible range (like audible hiss or hum when no content is playing), which is what I think OP covers. So don’t have any signal in the audible range but keep increasing the band above it until you see any effect in the auditory range.
(2) eliminate any possibility that even if there is any audible effect in (1) above that it is not masked by audible band signals at reference level.
(3) eliminate any possibility that any audible effect is not masked at low levels of signal as may happen when listening at low levels. This would require a sweep across gain levels.

In other words, do the real world device gain controls affect the out of auditory range signals proportionally relative to the auditory range as measuring instruments would? I don’t know the technology enough to answer this.
Pink spectrum is to mimic the spectrum of music. You cannot have a white spectrum in any way. So pink is a good option. As pink spectrum decreases along the frequency increase, it shows how much effect there will be with music playing.
In design we think about worst possible case. So varying gain is not helping.

If you are talking about the rf interference not coming from the source, it can be filtered out and mostly they are not strong enough to push enough current in a low impedance input terminal.

The most possible case is when playing hires audio music. Otherwise there shouldn't be any issue.
 
Oh, this is the perfect audiophile silver bullet solution! Introduce doubt as to whether instruments can measure something the golden-ears claim to be able to hear, and then present the solution.

Ok, I'll stop now. :)

Don’t. Perfect cynicism for lazy Sunday. :)

The irony in this particular case is the opposite of the usual claim. That the measuring instrument gain controls are too good relative to the real world and hence control the gain across any bandwidth and thereby prevent audible artifacts in the audible range while the real world gain controls are not and so allow audible artifacts... mind boggles.
 
Pink spectrum is to mimic the spectrum of music. You cannot have a white spectrum in any way. So pink is a good option. As pink spectrum decreases along the frequency increase, it shows how much effect there will be with music playing.
In design we think about worst possible case. So varying gain is not helping.

If you are talking about the rf interference not coming from the source, it can be filtered out and mostly they are not strong enough to push enough current in a low impedance input terminal.

The most possible case is when playing hires audio music. Otherwise there shouldn't be any issue.

My point wasn’t about pink noise.
 
I do not understand why usual measurements are almost always restricted to audio band measurements, 20Hz-20kHz, maybe 50kHz. SINAD, THD vs. amplitude, THD vs. frequency. Is that enough, really? Isn't it pointless? Above some level of parameters, like SINAD = 80dB, do we find differences by listening? Why do we concentrate our efforts at those standard measurements only? Because they are easy to make? Why do not we try to find broader consequences? In the age where we are surrounded by EMI pollution, digital sources having MHz and tens of MHz rubbish at the output? Isn't it the key to perceived differences, these out of audio band interferences transformed as intermodulations and aliases into audio band? Why do we resist to investigate something else than a conventional, decades old audio band measurements?
 
I do not understand why usual measurements are almost always restricted to audio band measurements, 20Hz-20kHz, maybe 50kHz. SINAD, THD vs. amplitude, THD vs. frequency. Is that enough, really? Isn't it pointless? Above some level of parameters, like SINAD = 80dB, do we find differences by listening? Why do we concentrate our efforts at those standard measurements only? Because they are easy to make? Why do not we try to find broader consequences? In the age where we are surrounded by EMI pollution, digital sources having MHz and tens of MHz rubbish at the output? Isn't it the key to perceived differences, these out of audio band interferences transformed as intermodulations and aliases into audio band? Why do we resist to investigate something else than a conventional, decades old audio band measurements?

I'd ask the question the other way- if there's nothing significant being introduced into the audio band, why would I be interested in RF stuff?
 
The only source of such a high frequency I can think of just now would be the noise shaped stuff coming out of an SACD player or a DSD DAC. I seem to remember having seen worst case levels up to -30dBFS ( I may be wrong though) which could lead to spurious signals within the audible range.

Yep, that should be done always and fixes many potential problems.

Yes dacs can output RF but it will more typically be around 350kHz and I have never seen it as high as - 30dB.
 
Another thing - is it OK if the power amp sends 460kHz rubbish like this into the tweeter? Are there any possible nonlinear effects in the tweeter driver? Or we just keep saying the only issue is negligible amount of added heat? Any serious explorations on this?

View attachment 39112

No there aren't. A tweeter simply does not respond at 460kHz. You can't generate other spurious signals from a position of no response.
 
I'd ask the question the other way- if there's nothing significant being introduced into the audio band, why would I be interested in RF stuff?

To answer the question whether something is being introduced into the audio due to RF stuff being present in signal?

I understood OP’s point to be introduce out of band signals to measure any audible artifacts and if there is proof that such artifacts exist then measure out of band to see if and which upstream devices send such signals.
 
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