Class D Technology

I am not sure where obsession with the switching frequency comes from.  It is inaudible

This statement needs clarification! It is very easy to demonstrate that switching frequency is audible. Once it gets above a certain minimum it becomes progressively harder however.

Ok, let me clarify then: I cannot hear 500kHz, not only because of my hearing apparatus (at my age I have difficulty hearing 15kHz), but also because tweeter’s output drops like a rock with combined effect of membrane inertia and increasing tweeter’s impedance (easy enough to check tweeter’s frequency response chart). There might be intermodulation products, but only if tweeter’s membrane can move at 500kHz - no chance . In addition output filter, set to about 1/10 of the switching frequency, is at least two pole resulting in >40dB/decade suppression. 500kHz frequency that I cannot hear and tweeter cannot response to is already suppressed by >40dB from the peak loudness. I challenge anybody to detect, form the seating position, when my amplifier is on.

Can someone tell me what can be observed when measuring a class a/b amp that indicates its inferiority to pure class A?

I am off the opinion that Class A is the bomb when possible. However  class A amps to power less efficient speakers are large heavy expensive and power hungry.   So enter Class D as a practical option to inherently inferior (to class a amps) class ab amps. 

Inferiority... well, I don’t believe this is true either... but! :)

In Class A the transistors (or tube) never switches "off". It’s at least minimally on through the entire voltage swing. This is what causes all the heat. At 0 output volts, the + and - transistors are maximally conducting!!

To swing in the positive direction the negative transistor has to start reducing current, and vice-versa.

The result of all this is that a class A biased device never has "notch" distortion. This is caused by the transistors shutting off during part of the swing. To turn them back on the voltage has to exceed the diode voltage, when it suddenly "snaps" on. It’s not perfectly linear from 0 to 1 volts After the transistor is conducting it gets linear quickly. Whether this is audible.... well, that’s a whole other discussion. :) There's also a lot of debate and misinformation about feedback. This notch distortion is measurable, but reducible by increasing feedback, to the point where the distortion figures can become incredibly tiny, regardless of bias type.

There are also a number of interesting single-ended Class A designs of very low power, which use a single device. Look up some of the Pass Labs First Watt designs for lots of info on this.

Best,


Erik

Erik it seems reasonable to me to say Class A/B is technically inherently inferior to Class A for the reasons you cited. IS not good Class A always preferred over Class A/B for sound for the technical reasons you cited? Class A/B is used to keep cost power demands and size and weight down for the masses mainly (sound familiar?), not for better sound or performance. Whereas Class D is totally different and can actually challenge Class A especially as the technology continues to improve even further. Of course I prefer my Class D amps over any Class A/B amp I’ve tried already and even a lesser Class A I once owned.

The most unique value of Class D is when much power and current is needed (mainly for bass) as in case of many smaller more extended less efficient speakers that many prefer. As speakers get bigger, loads easier, and efficiency higher, then most any good quality amp can perform well without being too large, heavy and expensive for most, although each will still probably sound different and individual considerations including personal preferences come into play.

So practically, it really pays to decide what one wants or needs the physical profile of their gear to be to meet their needs then choose speakers and amps together accordingly to meet those needs. This practically is the most important thing to consider together I would say way more than any particular technical achiilles heel of any specific amp technology one might obsess on.

No amp including most Class A amps I would say are perfect so choose your poison.

In general., I find Class D amps are the best of all at taking total complete control of speaker which I find tends to yield the best results possible. The result is greater articulation and detail in the bass and a cleaner more dimensional sound overall. Cleaning up/controlling teh bass prevents masking detail at higher frequencies often buried by poor quality or muddy bass when amp is not up to the task of totally controlling the speakers, especially at the low end which is the most demanding and requires an amp to work hardest. Class D amps are MOST efficient and that helps enable achieve this more often than not compared to the competition I would say.

Bass reproduction requires the most work by far and is the biggest challenge for most amps. Class D does this best. For higher frequencies, other amps perform well as do the better Class D amps around these days so not as much to differentiate the technology there although skeptics will attack class D high end as its relative weakness which is a fair statement at least historically. But that "weakness" is one that seems to not matter at all practically as best I can tell these days. Even my modest newer BelCanto C5i integrated that I picked up for a mere $1000 used seems to have gotten everything just about as right as possible and that is a complete integrated amp with phono and DAC 60 watt amp and headphone amp. Its easily as good sounding as anything I have heard within its modest 60w/ch power limitation which will come into play in some cases. OF course no single sound will appease everyone so YM will always vary.