The Future of Audio Amplification

Wrong!!! look at it and think again. It’s Stereophiles, there’s your hint, they’ve NEVER EVER given a Class-D 20khz square wave, as it would look unrecognizable as a square wave.

https://www.stereophile.com/images/1212AM1fig03.jpg

You are correct and I was in error- that is a 10KHz squarewave.

So that makes the scan frequecy in this example only 200KHz(!), which means the class D in question had to have been built in the **1990s**. By the early 2000s, everyone was doing well over 300KHz. That means that the bump on the leading edge is the phase shift caused by a filter that is likely at only 40KHz.
So using this as an example of **all** class D amps is still a Hasty Generalization and like any logical fallacy, is false.

20kHz is hard for any amp to do, much less well???

http://www.firstwatt.com/pdf/prod_f5_man.pdf

Look at page 17 where the FW F5 absolutely nails a 200KHz square wave at 1 watt. A lot of amps are designed to roll off at or past 20KHz more for purposes of self preservation than technical limitations. You’re always going to have that guy who wants to try Litz wire ICs and create a high power oscillator. 

I agree- but the Pass amps IMO are some of the best solid state amps made :)

Our OTLs are quite good at 1 watt 20KHz squarewaves too. And they don't care about the Litz wire; I'd be shocked if the Pass amps do!

Regarding distortion- I agree wholeheartedly that small amounts of particular kinds of distortion are easily heard. This is why I've avoided feedback in our amps; while feedback suppresses distortion in doing so it adds some of its own, and its all IMD and higher ordered harmonics which are easily heard.
But it is also true that you can build a class D with zero feedback and there are a number of them around. At that point, the limit of the distortion becomes the precision of the encoding scheme. For example if you use Pulse Width Modulation using a triangle wave to set the switching frequency, the distortion is all about how precise the triangle wave is and the speed and offsets of the comparitor circuit used. It happens that with that sort of circuit, as the amp approaches full output, it can have a form of soft clipping as the encoding scheme starts to fall apart. So the resulting amp can have very low distortion and a lack of higher ordered harmonics over most of its range.

So that makes the scan frequency in this example only 200KHz(!),

Wrong again when have you seen an amp that can do 200khz, have a slewing audio frequency square wave like this, the sides are collapsing in like the leaning tower of pizza

Our OTLs are quite good at 1 watt 20KHz squarewaves too.

And that is where you should stay linear and with tubes, otl, even add trany coupled, so your not hamstrung to certain speakers. You built your rep on them and now your going to destroy that?. Not a good move, it will come back and bite you on the arse, your just another respected amp manufacturer saying "give me some of that class-D action"

So that makes the scan frequency in this example only 200KHz(!),

Wrong again when have you seen an amp that can do 200khz, have a slewing audio frequency square wave like this, the sides are collapsing in like the leaning tower of pizza

I think you misinterpreted this. The amp at the link you provided was an older amp that was only switching at 200KHz. Nobody has been building amps with switching speeds that low in a long time, unless its for subwoofers only. Also, we don't know why the risetime is as slow as it appears since the data on the amp itself is lacking. That can be caused by measurement errors, but it can also be caused by analog signal processing before the encoding scheme.

So its erroneous to apply the results of this nearly 20-year old circuit to newer amps.

I think you misinterpreted this.
Nobody has been building amps with switching speeds that low in a long time

Geez Ralph other way around, get with it, in this case I wasn’t talking about 200k switching speeds, I was talking band-width and the slewing of this square wave I posted up, with and without switching noise embedded in it.

Ralph, you are more capable than anyone on this thread to explain why class D is the future from a technical standpoint.  Lay it on us.  Also, have you (as George is implying) had a religious conversion to the world of class D to the point of implementing it in your products?