Class D = Trash?

atmasphere,

      Thank you for replying to my questions.

    Your confirmation that current class D amp switching frequencies are not causing 'inharmonic distortion', along with class D Ice and Hypex NCore power module inventor Bruno Putzeys' statement that current class D switching frequencies in the mid-500kHz range are "completely reasonable", refutes the theory that they need to be raised to the 3-5mHz range (to not affect frequencies in the audible range) as far as I'm concerned.
     These confirmations make sense since they mirror the more subjective perceptions many class D users have, including myself, of the total lack of sonic anomalies and very high quality performance levels of class D amps..

    I'm looking forward to hearing the results of your first class D amp design effort.
   I suspect your class D amp will be transparent, detailed, with a very neutral overall quality while also having the extreme low noise and distortion levels that all seem to be common characteristics of good class D amp designs.  
   I believe you'll also find that an amp with the above traits will be a great match with high quality tube preamps since the amp will operate as the audio ideal of 'a straight wire with gain' which  will allow the sound qualities of your tube preamps to be amplified faithfully without any alterations. 

Tim
    

This is a statement from Optoma USA customer help, a world wide company who I believe handled NuForce, maybe still do.

Digital Switching Amplifiers (commonly known as Class-D) have been around for years. Nevertheless, it is nearly impossible to engineer a conventional Class-D amplifier that handles the full requirement, 20-20,000Hz, for full-bandwidth music reproduction. A Class-D amplifier works by utilising a high-frequency saw-tooth waveform to modulate the music signal (to learn more about how Class-D amplifier works, click here).

The constant presence of the saw-tooth waveform, which is very high in frequency spectrum and its inevitable frequency jittering, can mask or corrupt low-level music signal. The output filter designed to filter out noise and overtones caused by the saw-tooth waveform adds a 180 degree phase shift to Class-D output stage, causing possible instability and adding distortion due to its own inherent non-linearities.

Additionally, the output filter presents frequency-variant output impedance that can interact with a speaker’s complex impedance. Variants of Class-D amplifiers with the addition of Digital Signal Processor claim to improve music reproductions.

However, because of their lack of close-loop design, especially from the speaker’s terminals, spurious interaction between the speaker’s complex impedance and back-EMF with the amplifier’s resonant output filter can result in harsh sound reproduction.

The fundamental flaws of conventional Class-D amplifiers remain unresolved.

Cheers George

It's an interesting statement you posted George, and that’s from a large company that promotes/distributes Class-D amplification.
Kind of gels with what I hear with Class-D. I must admit I’m still waiting for it to come of age, 'till then I’ll also stick with my linear amps. I'd say the writings on the wall though, with the advancement of Class-D’s higher switching frequency, one that Technics have used with their SE-R1 (which I have not heard yet)
Great discussion nonetheless & hope to learn something from the various luminaries (and others) chiming in .... 

Regards

S

tradeontheweb
  with the advancement of Class-D’s higher switching frequency, one that  Technics have used with their SE-R1 (which I have not heard yet)

Doubt you will S, I contacted Technics and it is still special order only ex Japan, and none yet have been allocated for Australia, even if you have the deep $40kaud pockets.

Cheers George 

Hi George,

From my technically limited layman’ s perspective, the quote you posted from Optoma USA actually seemed logical and made some sense to me. If I didn’t know better from years of personal experience, I’d probably just accept what they stated as the truth, assuming they knew what they were talking about.

However, I find myself still resisting this theory that current class D switching frequencies cause sonic artifacts/anomalies in the audible range for several main reasons:

1.Two well credentialed and well respected amp designers whom I also respect and trust, atmasphere and Bruno Putzeys, have made statements that there’s no need to raise the switching frequencies for class D amps from the current mid-500kHz to the 3-5 mHz range.

2. If these proposed sonic artifacts/anomalies and supposed phase shifts actually do negatively affect the sound quality as theorized, I would think that a certain percentage of the currently thousands (millions?) of class D amp users, including myself, would be capable of detecting any peculiarities in the sound quality in their systems. I would also expect a certain portion of these users would be alarmed enough to post their observations on this or other audio forums seeking explanations and advice.
Oddly, the only comments I’ve read questioning the sound fidelity of class D amps were made by you, George.
To be fair, I have read comments by others claiming that they thought class D amps sounded cold and sterile but I’m not sure if these perceptions are related to sonic artifacts. I always made sense of these comments by questioning the quality of the class D amp they heard. After using 3 different class D amps from 3 different companies for soon to be the past 3 years, I’ve never perceived any of them to be cold and sterile and have never heard anything I would classify as a lack of fidelity.
I’ve asked you numerous times whether you’ve ever personally heard these elusive,quiet and apparently very difficult to notice sonic artifacts from a class D amp and if you could describe what they actually sound like but you never responded.
I wanted to keep an open mind and know what to listen for when I actually spent a couple hours listening to some very familiar music trying to be hyper-critical and detect anything that didn’t sound quite right. I didn’t hear anything amiss.

3. There seems to be a serious lack of any scientific studies or research concerning the affects that particular switching frequency ranges utilized in class D may have on the the actual audible musical signal frequencies being amplified. I’m referring to carefully controlled tests and experiments, with the results required to be consistently and reliably achieved when conducted by independent scientists before results can be verified, conclusions reached and a knowledge base on the subject can begin to be established.

The absence of a solid knowledge base allows individuals such as yourself, George, to put forth any theory that serves their purposes without being constrained by anything as silly as needing to support their theory with actual evidence such as facts or at least agreed upon truths.

But enough of this scientific mumbo jumbo, the simple truth is that I would have sent back my first class D amp for a ’no questions asked’ full refund, and bought a more expensive traditional linear amp, if any of these theorized sonic artifacts or phase shifts caused an audible lack of fidelity in any part of the audio spectrum.
The only thing I heard was a small, lightweight, efficient/electricity sipping, cool running, $630 Class D Audio SDS-440-CS class D amp significantly outperforming my large, heavy, inefficient/electricity gulping, hot running, $2,400 former Aragon 4004 MKII class A/B amp in almost every amp criteria that I care about: much better bass response, a much lower noise floor, better dynamic range and a much more neutral and detailed presentation. Both amps had a roughly equal ability to present a solid, stable and realistic 3D sound stage illusion while both had similarly very good mid-ranges and trebles that were well extended but not fatiguing.

Tim