Class D Technology

Thanks mapman. That is consistent with the new KEF LS50 active monitor. It uses Class D for the bass and Class AB for the highs. 

Non-linear & lots of distortion that needs to be cleaned up with an output filter.

I don't understand what you mean by that.  Class D is extremely linear.  Class D modulator, converting voltage to duty cycle, resembles Delta-Sigma D/A converters (principle of operation is the same) that are extremely linear.  Output filter, Zobel network,  is only used to obtain average value and reduce amount of noise on the speaker wires (that is inaudible).  Like with any amp - don't guess and just listen.  I like sound of my small class D Rowland amp.  I like imediacy, transients, liquid midrange, low noise floor and composure under loud passages (regulated power supply).  The only problem I had initially was lower midrange.  It was a little thin sounding.  I replaced speaker cables with Acoustic Zen Satoris and lower midrange came back.  Cello sounds like cello and I can hear "chestiness" in male voices.  Perhaps the question is not whether class D is worse or better than class A (Jeff Rowland makes some wonderfull class D amps), but rather if it is better for you, for the money you can spend.  Switching technology is very promising - just look at SACD - pretty much the same as class D.  I agree with George that switching frequency should be increased, since 50-60kHz bandwidth limitation introduces around 20deg phase shift at 20kHz (causing wrong harmonics summing), but I'm not even sure I can hear such nuances.  There are many highly praised amps that have similar bandwidth, I'm sure.  

georgelofi you are correct about switching frequency being teh key to even better Class D amps in the future.

Have you listened to any good quality class D amps or are you judging solely on technical interpretation?

Yes the new Belcanto 600 monoblocks, they were the best of the Class D's I've heard so far, but still didn't convince me to go to the dark side yet.

In the future when the switching frequency is up around 3-5mhz then the filtering can do it's job properly without it's effects influencing the audio band.
The Belcanto 600's did a series of output filters to counter this, but it has it's own set of problems.

The Technics are up to 1.5mhz but you pay for this $$$$$

http://www.technics.com/us/products/r1/se-r1.html#overview

Cheers George

I agree with George that switching frequency should be increased, since 50-60kHz bandwidth limitation introduces around 20deg phase shift at 20kHz (causing wrong harmonics summing), but I’m not even sure I can hear such nuances. There are many highly praised amps that have similar bandwidth, I’m sure.

Switching frequencies are up around 600-800khz at the moment, except for that Technics above.
The problem is the switching noise output filter has to handle the full power of the amp and can only be low order, otherwise it will burn out. And being low order it has an effect down into the audio band. If the switching frequency is 3-5MHz then it can do it’s job well away from the audio band without having phase, fr, ect limitations effects .

Just look at the ringing artifacts in Stereophiles 1kzH square waves of Class D, they now test with a inline heavy filter to make this (ringing) disappear, that Audio Precission’s filter they use makes the test wave look clean, but it’s a con job as it’s done at very low power so it doesn’t burn out, in real use it would last a microsecond.
I have asked JA to include both unfiltered (normal) and filtered square waves, just so readers can see the real deal.
Read comments at the bottom of the page, no answer from him.
http://www.stereophile.com/content/bel-canto-eone-ref600m-power-amplifier#G7miwIhm5pj5EDlK.97

Cheers George

I don’t buy the "gas guzzler" A/B vs. "green" class D arguments for the latter. Kilowatt hours are cheap and relatively clean until you pile up a WHOLE lot of ’em. With typical audiophile usage patterns, and the fact that we’re a VERY small segment of the population, there’s no significant ecological impact here.

The extreme case would be a 100+ Watts pure Class A amp user, who leaves amps on 24/7, switching to a class D amp. Then yes, you’re going from a $1,000+ yearly power bill to like 100 bucks. But that’s an outlier. With more sane usage patterns -- e.g. a class A/B tube amp that you turn on/off as needed -- the difference is close to negligible. And as far as ecological impact, class D probably makes things worse overall, due to the fact that audiophiles ship ’em over, decide they don’t like the sound, and then ship ’em right off again.