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Class A vs AB vs D amplifiers


This isn't the write up I had in mind, but mentions some of the same things regarding heating tweeters and IMD.

Ran across someone with the idea to feed tube filaments with high frequency class D amps to avoid hum. Seems DC would be better.
DC could be thought of as zero or as infinite frequency, I suppose. :)
 
Ran across someone with the idea to feed tube filaments with high frequency class D amps to avoid hum. Seems DC would be better.

His name wouldn't have been David Berning, would it?

Jim
 
Regarding electrostatics and Class D amplifiers, my Acoustat Spectra 1100s are rather inefficient and dip to slightly less than 2 ohms at very high frequencies. I have had no trouble driving them at "normal" levels (i.e. Dolby Atmos reference levels with real-world material) with Class D Aiyma A07s. I've run them full range and bi-amped with external DSP for crossovers, EQ, etc. I currently bi-amp them with Class D Fosi V3s running on 48VDC/10A power supplies with zero issues. (Note that in the Spectra 1100, a sealed 8" woofer operates between 35Hz and 250Hz. The electrostatic panel is crossed over at 250Hz electrically at 6dB/octave, when operated in stock, full range mode. In biamp operation I use 24dB/octave slopes at 250Hz.)
 
Regarding electrostatics and Class D amplifiers, my Acoustat Spectra 1100s are rather inefficient and dip to slightly less than 2 ohms at very high frequencies. I have had no trouble driving them at "normal" levels (i.e. Dolby Atmos reference levels with real-world material) with Class D Aiyma A07s. I've run them full range and bi-amped with external DSP for crossovers, EQ, etc. I currently bi-amp them with Class D Fosi V3s running on 48VDC/10A power supplies with zero issues. (Note that in the Spectra 1100, a sealed 8" woofer operates between 35Hz and 250Hz. The electrostatic panel is crossed over at 250Hz electrically at 6dB/octave, when operated in stock, full range mode. In biamp operation I use 24dB/octave slopes at 250Hz.)
I've run Soundlabs which dip to below 1 ohm in the treble using a Wyred4Sound amp for years. Best amp I've ever had on them.
 
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If 5W is 90dB, then 600kHz is 60dB

Audible frequency or not, that's still quite a bit of power that reached the APx555
 
Was this EVER proven or is it a mere hypothesis? Have you measured how much of the 400kHz switching residuals REALLY finds their path to the voice coil over the system inductances? I did the homework.
Can you include the homework sir?
 
View attachment 373026

If 5W is 90dB, then 600kHz is 60dB

Audible frequency or not, that's still quite a bit of power that reached the APx555
Not really. Is roughly .005 watts if the impedance at that frequency is still 4 ohms. For any tweeter impedance will not be that low at that frequency. Wattage will be even lower.

I think you misread the chart. 5 watts is labeled as 0 db, and it is -30 db from there. I am assuming it is a voltage scale.
 
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Not really. Is roughly .005 watts if the impedance at that frequency is still 4 ohms. For any tweeter impedance will not be that low at that frequency. Wattage will be even lower.

I think you misread the chart. 5 watts is labeled as 0 db, and it is -30 db from there. I am assuming it is a voltage scale.
5W is about 90dB SPL on many speakers
 
Any tests results you have would be very relevant to this discussion.
Not test results, but this was put up on ASR by Bruno Putzeys some years ago and I wonder if you would argue against his points. I admit a lot goes over my head, but what about the point "But hey, should anybody really want to make a point of this claim, the onus is of course on them"


That’s a curiously shaky argument to put forward in favour of a fine amplifier like the AHB2. It doesn’t need to make any excuses. I imagine that it was just a throwaway comment, otherwise it’d qualify as Spreading Uncertainty and Doubt.

I am fully aware that literature exists testing tweeters at blistering levels with signals close to the audio band, but it’s quite a stretch to extrapolate from there to a small 500kHz residual and side bands. Keep in mind that the electrical impedance of a speaker goes up with frequency so the residual (heavily attenuated by the LC filter) amounts to an absolutely tiny current. But hey, should anybody really want to make a point of this claim, the onus is of course on them to provide data showing that the switching residual of a good class D amp adds distortion to the acoustical output that isn’t there when the switching residual isn’t there. If such data were around, it’d probably be online already.

That’s not a very informative answer, so let’s see if we can take the argument anywhere useful. This is about demodulating the sidebands of the switching residual downward. The word “noise” is potentially misleading here, it’s not noise as in “hiss” but as in “signal I didn’t pay for”. The switching residual is actually a highly predictable signal.

Anyhow. Demodulating requires an even order distortion mechanism. One which, as several of you noted, would have to be present even when the voice coil isn’t moving anymore. For the sake of an argument, let’s just imagine that there is something in the tweeter that is able to turn not just the current but also the square of the current into a force. If you designed a tweeter specifically to be that bad, you could actually do that, but it wouldn’t do much else besides: the exact same nonlinearity would also produce absolutely massive amounts of distortion in the baseband in the regular way.

The hypothesised additional distortion would be the intermodulation between the carrier and its sidebands, and between the sidebands amongst themselves. Spectrally they would therefore appear exactly where regular distortion components are already present, except much lower. These regular distortion products are caused by the same nonlinearity but with a much larger input signal (the actual audio signal instead of the filtered residual). The hypothesised fold-down distortion would be much smaller than regular distortion and indistinguishable from it because it’s in the exact same place.

If we put in a better tweeter, regular and fold down distortion (if any) would fall in tandem. There’s simply no scenario in which fold-down distortion could appear separate from its associated regular base-band distortion and/or with remotely comparable magnitude.

From that thought experiment we can conclude it’s rather unlikely that any measurable or audible mechanism exists that can be traced back to the carrier residual.

As an aside I find the “demodulation” argument also a bit of an own goal when proposed in favour of high-res audio. What they’re saying there is that maybe a bad tweeter might manage to produce distortion components inside the audio band which wouldn’t be there if either the tweeter were better or the bandwidth of the audio signal were limited to 20kHz. That sounds more like an argument against mindlessly doubling the sampling rate every 3 years and inventing a new format every decade.
 
Not test results, but this was put up on ASR by Bruno Putzeys some years ago and I wonder if you would argue against his points. I admit a lot goes over my head, but what about the point "But hey, should anybody really want to make a point of this claim, the onus is of course on them"


That’s a curiously shaky argument to put forward in favour of a fine amplifier like the AHB2. It doesn’t need to make any excuses. I imagine that it was just a throwaway comment, otherwise it’d qualify as Spreading Uncertainty and Doubt.

I am fully aware that literature exists testing tweeters at blistering levels with signals close to the audio band, but it’s quite a stretch to extrapolate from there to a small 500kHz residual and side bands. Keep in mind that the electrical impedance of a speaker goes up with frequency so the residual (heavily attenuated by the LC filter) amounts to an absolutely tiny current. But hey, should anybody really want to make a point of this claim, the onus is of course on them to provide data showing that the switching residual of a good class D amp adds distortion to the acoustical output that isn’t there when the switching residual isn’t there. If such data were around, it’d probably be online already.

That’s not a very informative answer, so let’s see if we can take the argument anywhere useful. This is about demodulating the sidebands of the switching residual downward. The word “noise” is potentially misleading here, it’s not noise as in “hiss” but as in “signal I didn’t pay for”. The switching residual is actually a highly predictable signal.

Anyhow. Demodulating requires an even order distortion mechanism. One which, as several of you noted, would have to be present even when the voice coil isn’t moving anymore. For the sake of an argument, let’s just imagine that there is something in the tweeter that is able to turn not just the current but also the square of the current into a force. If you designed a tweeter specifically to be that bad, you could actually do that, but it wouldn’t do much else besides: the exact same nonlinearity would also produce absolutely massive amounts of distortion in the baseband in the regular way.

The hypothesised additional distortion would be the intermodulation between the carrier and its sidebands, and between the sidebands amongst themselves. Spectrally they would therefore appear exactly where regular distortion components are already present, except much lower. These regular distortion products are caused by the same nonlinearity but with a much larger input signal (the actual audio signal instead of the filtered residual). The hypothesised fold-down distortion would be much smaller than regular distortion and indistinguishable from it because it’s in the exact same place.

If we put in a better tweeter, regular and fold down distortion (if any) would fall in tandem. There’s simply no scenario in which fold-down distortion could appear separate from its associated regular base-band distortion and/or with remotely comparable magnitude.

From that thought experiment we can conclude it’s rather unlikely that any measurable or audible mechanism exists that can be traced back to the carrier residual.

As an aside I find the “demodulation” argument also a bit of an own goal when proposed in favour of high-res audio. What they’re saying there is that maybe a bad tweeter might manage to produce distortion components inside the audio band which wouldn’t be there if either the tweeter were better or the bandwidth of the audio signal were limited to 20kHz. That sounds more like an argument against mindlessly doubling the sampling rate every 3 years and inventing a new format every decade.

I have always had questions regarding this "fold-down" intermodulation distortion. This is the first counter-argument that is logical and makes sense.

Thank you, @pjug and @Bruno Putzeys

Jim
 
5W is about 90dB SPL on many speakers
Do you know of any tweeters that are stressed by .005 watts even if it results in 60 db SPL? Plus, your original post was implying a considerable amount of power into the APx unit, but no it wasn't. Plus, plus, the tweeter won't be putting out 60 db SPL at 500 or 600 khz switching frequency. Anything that somehow modulated down to the tweeter would be less still. Plus, plus, plus, the tweeter won't even see .005 watts at the switching frequency as it will have a far higher impedance at those frequencies. So a big nothing-burger.
 
TF-10 preamp, mid 1970s. Can't remember if he did the same for the BA-150 Audionics.
One of Berning's other amp designs. Not a low distortion device. They mentioned the smooth treble with Soundlabs, I guess so with a 3 ohm output impedance which would have really softened the bite into those Soundlabs.

Some info on the BA-150 on page 38, but not the detail you would really like to know.

1717471681122.png
 
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Measured tweeter current from Hypex NC400, with 5kHz signal frequency and carrier frequency residuals. HF current was 16mA peak. Tweeter heating? :D

View attachment 373082
Don't think so. The audio current is much higher than the RF residudal. Therefore the audio current would make the heat.
 
It's not the tweeter I'm worried about. But rather if we had a tweeter that has flat frequency response up to 1MHz, it's my ears that I'm worried about, even if this is ultrasonic.

I usually listen around 60dB and I assume class D switching doesn't change with signal volume.
 
It's not the tweeter I'm worried about. But rather if we had a tweeter that has flat frequency response up to 1MHz, it's my ears that I'm worried about, even if this is ultrasonic.

I usually listen around 60dB and I assume class D switching doesn't change with signal volume.
Show us a tweeter with 1 mghz bandwidth. Ha ha ha!. Oh and should I show the attenuation in air of a 1 mghz signal vs distance? Ha ha ha!

And beyond all that ridiculousness should we consider your ear at 1 mhz? Please the irony of that is killing me.
 
Some info on the BA-150 on page 38, but not the detail you would really like to know.
I found my copy of the schematic (actually the EA version, sold under his name rather than Audionics)- conventional linear supply.
 
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