Class D Technology

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.

@mapman 

I will not agree to any such prejudices. While I can say that class A lacks notch distortion, it is by no means the only type of distortion or non-linearity!

We can't on the one hand talk tech. theory on a subject like notch distortion while rejecting THD figures, which is (oddly) what a lot of audiophiles like to do.

The distortion and noise measurements between high end class A, A/B and D heavily overlap.

I'm also not sure that what audiophiles like to hear is always "better" if "better" = "more accurate."

I think audiophiles like sweetening (which is fine!) in which case trying to match an argument with notch distortion and sound quality blows up. The "sound quality" isn't found in the technology per se. If I had all the time, money and space in the world I would not mind having a pair of Conrad Johnson Premier 8 for instance, knowing full well they lie lie lie. :)

Best,

Erik

Erik I’m just trying to frame up the comparison between the various amp technologies in purely technical terms as an extension of georgelofis argument against Class D based purely on switching frequency.

I am of the mindset that understanding the technology helps to make decisions where needed but alone does not dictate what sounds best in the end. Technology alone cannot predict what sounds best in specific cases. A system together makes the sound not just one component. Its a team sport as they say. Plus individual preferences vary for many reasons though some technical people have a greater tendency to associate these with specific aspects of technology that matter to them whereas others (I’d like to think myself included) are not so fast to jump to conclusions.

So I think we are on the same page in practice.

Guess what? My Class D amps were preceded by two different Class A amps that were fine on their own terms but far from perfect in my application. The Class A amps were preceeded by many a Class A/B amp and even a 70’s vintage Class G that was nice overall in its day especially if space is limited but sound quality could not hold a bone compared to any good quality newer amp I compared it to.

I heard the Benchmark amp at Capital Audiofest last summer (along with every other kind of amp imaginable) and thought it to be a top performer easily in the same league with the best I heard there.  I would own it in a heartbeat.   I am strongly considering giving Benchmark preamp/DAC units I heard there a try which if if successful (as I expect it would be)  would finally happily eliminate the last tubed  gear in my house.  I'm all about the sound.   The technology used is just a means to the end.

Here is another argument for Class D. It can be argued that once you get the bass right the rest is not so hard to sort out. I have found a lot of truth to that in recent years. Efficient Class D amps are both on paper and in practice hands down best at handling bass. That seems to be common knowledge based on application and sure enough I have found it to be true in practice. In a nutshell that’s mainly because of Class D high efficiency and the fact that doing extended bass well is exponentially more work for an amp than doing higher frequencies and efficient amps do it best. Just like efficient speakers make it easier for an amp to do bass well. The end result when done right (ie sufficient efficiency or efficiencies in play to facilitate doing the needed work) tends to sound similarly good in the end I find.