Time to buy a class D amp?

Hello atmasphere,

     As I understand it from you and Bruno Putzeys as a layman, there is no such thing as too much feedback with class D amplification.  The generally accepted concept that feedback negatively effects the sound quality of traditional linear amplifiers may be true, but this aversion to feedback doesn't apply to class D amplifier design.  Very high levels of feedback, in an intelligently designed class D amp, are actually utilized to optimize the sound quality throughout the entire audible frequency spectrum.  
     Am I understanding this correctly?

Thanks,
  Tim

Very high levels of feedback, in an intelligently designed class D amp, are actually utilized to optimize the sound quality throughout the entire audible frequency spectrum.

This can also be said about linear amps, but they sound crap when done. The goal is to have the best engineered design with the lowest distortions without negative feedback, and then if it can be done just a little local feedback around the input/driver stage to clean things up, but no global feedback compassing the output stage as well.

It was Matti Otala back in 1970-80’s that was the guy behind it all in solid state amps, and is still used today by the very best, but hard to design for. Look for today's amps that say in their description/specs "no global negative feedback"


Cheers George

This can also be said about linear amps, but they sound crap when done. The goal is to have the best engineered design with the lowest distortions without negative feedback, and then if it can be done just a little local feedback around the input/driver stage to clean things up, but no global feedback compassing the output stage as well.

This is but one design methodology, and to say unequivocally it is the best is ... a leap not shared by all. It is more religion than science.

Can you define the time domain characteristics of feedback in a Class-D amp and linear amp?

The proof will be in the pudding, as there are Class-D’s coming out with little or no feedback, Merrill Elements being just one with it’s said to be a very good sound.
From EE Online

"If none of these issues are addressed, it is difficult to achieve PSRR better than 10 dB, or total harmonic distortion (THD) better than 0.1 percent. The standard solution for compensate for the poor performance of a Class-D system is to add negative feedback, but a large amount of negative feedback increases Transient Intermodulation Distortion (TIMD)."

Question is does Putzey’s designs with large amounts of feedback, ever state TIMD "Transient Intermodulation Distortion" I think not only ever seen THD specified
https://pdfs.semanticscholar.org/b3c0/a892a982ebde91f83f228905dac30186f827.pdf

Cheers George

Would you say there is inherent limitation in the phase shift that is a factor of the comparator speed, digital logic delays, and turn-off time of the FET? What do you think a practical limit is on that? That may put a practical limit of a few-10 degrees at 20Khz, but that would be inaudible.

The phase shift is indirectly related to the speed of parts like the comparitor (which typically has plenty of more speed than the output section). It is directly related to the filter at the output.

    As I understand it from you and Bruno Putzeys as a layman, there is no such thing as too much feedback with class D amplification. The generally accepted concept that feedback negatively effects the sound quality of traditional linear amplifiers may be true, but this aversion to feedback doesn't apply to class D amplifier design. Very high levels of feedback, in an intelligently designed class D amp, are actually utilized to optimize the sound quality throughout the entire audible frequency spectrum.  
    Am I understanding this correctly?

That's it in a nutshell.

The problem is phase shift in traditional designs prevents the application of the required amount of feedback in such amps; otherwise oscillation will occur. For this reason **every tube and solid state amplifier made has had insufficient feedback**. This has resulted in the applied feedback adding its own distortion- which is interpreted by the ear as brightness and harshness. This is why every amplifier with feedback up until now has sounded brighter and harsher than real life, and has fueled the tubes vs transistor debate all these decades. The industry has also been complicit in this problem, so when you see harmonic distortion measurements the fundamental frequency is usually fairly low (60Hz is common) so as to not get in trouble with the gain bandwidth product limitations of the amp under measurement! And this also explains why zero feedback amplifiers (like our OTLs) exist, as by designing a circuit that is linear enough to run without feedback the harshness and brightness of feedback is avoided.


Transient Intermodulation Distortion is a symptom of poor feedback loop design coupled with poor gain bandwidth product and insufficient feedback.

Can you define the time domain characteristics of feedback in a Class-D amp and linear amp?

Yes. In a class D amp its all about propagation delay. In a conventional amplifier its all about phase shift as capacitive strays roll off the response (introducing phase shift). Effectively both have the same effect- at some high frequency the feedback is no longer negative so oscillation can occur. But unlike a conventional design, in a class D you can take advantage of that oscillation by using it as the switching frequency.