Class D Technology

I do not have "ice modules" . i have stated repeatedly I have BelCanto Ref1000m amps. These use ice modules but also custom input and power circuitry and the price and sound reflects that accordingly.

Again generalizing and glazing over facts and details to help an argument helps nobody and is a disservice.

I would likely not have been satisfied using that older revision of vanilla Ice modules to drive my speakers which is why I opted for a better design. The original Bel Canto ref1000 was vanilla icepower (and much cheaper) and the differences between the two are well documented.


I don’t know what KEF uses or why. Its their choice so better to ask them. Probably just easier or cheaper or better for them to biamp in general which opens up options to use different amps for different purposes rather than try to get either one to do it all best at their price point. I’ve heard those those KEFs in my home and they are small power and current hungry speakers.   I'd definitely use Class D for teh bass with them but if biamped then many decent options for the less demanding  (power and current-wise) high end.

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