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Secrets Q&A with John Siau

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A recent interview with John Siau was published in the Secrets of HT and High Fidelity.


I enjoyed getting further insight into the AHB2 and was particularly intrigued by one point made by John Siau with regards to distortion at low volume settings:

"Class AB amplifiers often produce a small transient every time the output voltage crosses 0 volts. These crossover distortion transients are visible on test equipment, and they raise distortion to a high percentage of the signal when the amplifier is delivering a fraction of a watt. This crossover distortion does not generally show up in THD+N measurements that are made at high power levels, but it becomes the dominant cause of distortion when playing at low power levels. It turns out that most of the sounds we hear in a recording are reproduced using a fraction of a watt. For this reason, I suspected that we should be able to create a simple listening test that would prove that crossover distortion is audible."

He refers to an ABX study (how refreshing!) that showed this distortion was audible.


He also refers to distortion in D class amps:

"The AHB2 is a linear amplifier, but its efficiency is close to that of a class-D amplifier. The difference is that the AHB2 does not produce the distortion and high levels of ultrasonic noise that are always produced by class-D amplifiers. Class-D performance measurements can be deceiving because the measurements are made with a 20 kHz brick-wall lowpass filter that removes the ultrasonic noise. That test-equipment brick-wall filter is missing when you connect your speakers to a class-D amplifier. The ultrasonic noise and distortion are sent directly to your tweeters where they can be demodulated back into audible frequencies. For this reason, class-D amplifiers do not sound as good as the measurements would suggest."

If these topics have been discussed elsewhere, please advise. I own the AHB2 as well as several D class and A/B class amplifiers and haven't noticed audible differences between them; I will focus more on low volume listening going forward.
I post this to see if this is something others have noticed and comments from the experts on the accuracy of these statements. I understand Mr. Siau is a contributor to this form and perhaps he will comment as well. I could not find his member name however.
 

PierreV

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There are many crooked (at worst) or deluded (at best) characters in high-end audio. John Siau is clearly not one of them. That being said, extremely competent and honest designers are always biased in favor of what they design. While I don't own a AHB, I do own other competently designed amplifiers and have never been able to tell them apart either.

I am also extremely cautious when I am told to listen for something specific, as it is a very generic salesman trick that biases us into suddenly hearing that specific thing.
 

Larry B. Larabee

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"Class AB amplifiers often produce a small transient every time the output voltage crosses 0 volts. These crossover distortion transients are visible on test equipment, and they raise distortion to a high percentage of the signal when the amplifier is delivering a fraction of a watt. This crossover distortion does not generally show up in THD+N measurements that are made at high power levels, but it becomes the dominant cause of distortion when playing at low power levels. It turns out that most of the sounds we hear in a recording are reproduced using a fraction of a watt. For this reason, I suspected that we should be able to create a simple listening test that would prove that crossover distortion is audible."
This is a deliberate misrepresentation of a class AB amplifier's operation.
 

McFly

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I suspect a large reason the AHB2 is so quiet is due to the SMPS, as opposed to linear power supply that AB manufacturers have persisted with since the dawn of solid state.
 

BoredErica

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"The AHB2 is a linear amplifier, but its efficiency is close to that of a class-D amplifier. The difference is that the AHB2 does not produce the distortion and high levels of ultrasonic noise that are always produced by class-D amplifiers. Class-D performance measurements can be deceiving because the measurements are made with a 20 kHz brick-wall lowpass filter that removes the ultrasonic noise. That test-equipment brick-wall filter is missing when you connect your speakers to a class-D amplifier. The ultrasonic noise and distortion are sent directly to your tweeters where they can be demodulated back into audible frequencies. For this reason, class-D amplifiers do not sound as good as the measurements would suggest."

If these topics have been discussed elsewhere, please advise. I own the AHB2 as well as several D class and A/B class amplifiers and haven't noticed audible differences between them; I will focus more on low volume listening going forward.
I post this to see if this is something others have noticed and comments from the experts on the accuracy of these statements. I understand Mr. Siau is a contributor to this form and perhaps he will comment as well. I could not find his member name however.
In Pa5 thread alone pma and Matias have "discussed" this on 2 seperate occasions. Ended so far, both times with Matias saying music is -40db down at 20khz or so and thus the problem isn't audible.

For this reason, class-D amplifiers do not sound as good as the measurements would suggest.
As far as I'm concerned we're comparing 1 transparent amp to another. And so I take issue with this statement as it implies the flaws are audible.
 
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restorer-john

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This is a deliberate misrepresentation of a class AB amplifier's operation.

You are correct.

He's been called on it before. Crossover distortion is a non-issue in any competently designed amplifier for the last 50 years. The Benchmark 'scope' shots are blatantly deceptive and in no way representative of any Class AB amplifier residual distortion I have ever seen. It's just marketing BS and disappointing.
 

pjug

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"The AHB2 is a linear amplifier, but its efficiency is close to that of a class-D amplifier. The difference is that the AHB2 does not produce the distortion and high levels of ultrasonic noise that are always produced by class-D amplifiers. Class-D performance measurements can be deceiving because the measurements are made with a 20 kHz brick-wall lowpass filter that removes the ultrasonic noise. That test-equipment brick-wall filter is missing when you connect your speakers to a class-D amplifier. The ultrasonic noise and distortion are sent directly to your tweeters where they can be demodulated back into audible frequencies. For this reason, class-D amplifiers do not sound as good as the measurements would suggest."
I think it is annoying that benchmark keep making this claim about speakers demodulating the switching residual, but won't demonstrate that it is true. I asked about this on ASR to Bruno Putzeys and John Siau when both were pretty active on here. BP gave a thoughtful reply (I thought convincing) that this can't be so. JS did not reply to me or rebut what BP said about it.
 
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SIY

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You are correct.

He's been called on it before. Crossover distortion is a non-issue in any competently designed amplifier for the last 50 years. The Benchmark 'scope' shots are blatantly deceptive and in no way representative of any Class AB amplifier residual distortion I have ever seen. It's just marketing BS and disappointing.
IIRC, I posted actual measurements illustrating this (i.e., it's a non-issue in any engineered amp) the last time it was brought up.
 

Matias

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John Siau himself spreading Class D FUD? Huh.

The FAQ comes to rescue.

 

pjug

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Benchmark can't make these amps fast enough. Why make yourself look bad with BS trashing the competition when the performance of their product has them running at max capacity and that doesn't look to change any time soon.
 

Pdxwayne

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In Pa5 thread alone pma and Matias have "discussed" this on 2 seperate occasions. Ended so far, both times with Matias saying music is -40db down at 20khz or so and thus the problem isn't audible.


.....
I thought the issue is:
"...The ultrasonic noise and distortion are sent directly to your tweeters where they can be demodulated back into audible frequencies....."

May I know if Amir tests for this ultrasonic noise and distortion? For example, any of his measurements here can tell me more about those ultrasonic noise over 20khz?

I wonder if anyone else here in ASR, other than Benchmark, tested this theory using speakers that can go as high as 45khz by performing double blind listening tests?
 
OP
E

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There are many crooked (at worst) or deluded (at best) characters in high-end audio. John Siau is clearly not one of them. That being said, extremely competent and honest designers are always biased in favor of what they design. While I don't own a AHB, I do own other competently designed amplifiers and have never been able to tell them apart either.

I am also extremely cautious when I am told to listen for something specific, as it is a very generic salesman trick that biases us into suddenly hearing that specific thing.
I agree, and if these comments had not come from a highly qualified engineer I would have dismissed them as generic audioBS. And the ABX test was another factor supporting the comments. However, like clinical studies funded by the manufacturer of other products we have to remain highly skeptical until there is independent corroboration.
 

DDF

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I thought the issue is:
"...The ultrasonic noise and distortion are sent directly to your tweeters where they can be demodulated back into audible frequencies....."

May I know if Amir tests for this ultrasonic noise and distortion? For example, any of his measurements here can tell me more about those ultrasonic noise over 20khz?

I wonder if anyone else here in ASR, other than Benchmark, tested this theory using speakers that can go as high as 45khz by performing double blind listening tests?

A tweeter's sensitivity is extremely low at class D switching frequencies. This means the tweeter isnt really moving in response to class d switching ultrasonic noise, so no dynamic non linearities (whether motor or suspension) can be excited.
The impedance of a tweeter at class d switching frequencies is also extremely high due to voice coil inductance. The current draw is very low in response to class d switching, and so static heating is a non issue.
I believe it was Alan (March) that tested the demodulation theory and objective testing turned up no issues due to class d switching noise.
 

Pdxwayne

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A tweeter's sensitivity is extremely low at class D switching frequencies. This means the tweeter isnt really moving in response to class d switching ultrasonic noise, so no dynamic non linearities (whether motor or suspension) can be excited.
The impedance of a tweeter at class d switching frequencies is also extremely high due to voice coil inductance. The current draw is very low in response to class d switching, and so static heating is a non issue.
I believe it was Alan (March) that tested the demodulation theory and objective testing turned up no issues due to class d switching noise.
May I know what freq range are those switching freq?

I know certain speakers can go up to 45khz. Are those not high enough to be affected?

Thanks!
 

pjug

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May I know what freq range are those switching freq?

I know certain speakers can go up to 45khz. Are those not high enough to be affected?

Thanks!
Amir shows this in a lot of the Class D reviews. Typically mid to high hundreds of kHz. Here is Purifi:
index.php
 

Pdxwayne

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Amir shows this in a lot of the Class D reviews. Typically mid to high hundreds of kHz. Here is Purifi:
index.php
Thanks! Should I assume ICEPower basically the same? My Pioneer SC95 uses ICEPowee, I think.
 
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