Negative Feedback a deal killer?

Unsound - As I understand it class D doesn't need a lot of negative feedback since duty cycle is much more linear than transistor voltage in class AB output stage*. In addition it has inherent low output impedance since speaker is always connected to zero source impedance (GND, VCC) with very low resistance Mosfets. I would just guess that it is less than class A amps (gain before feedback is few hundred) and much less than most class AB amps (gain in order of few thousands). It is also worth mentioning that conversion of voltage to duty cycle is done in one stage oscillator to drive output Mosfets while traditional amplifier requires multiple output stages (hence delays) to get signal from input to output. Negative feedback causes, as Atmasphere mentioned, late summing with input signal and overshoots (time domain) or enhanced odd harmonics (frequency domain). It is called Transient Intermodulation TIM and was unknown until 70's. Some class AB amps even have saturation of output transistors that causes momentary gaps (charge trapped at the junction)with fast changing input signals (our brain compensates but makes us feel tired) - not even possible with class D since output transistors are in saturation anyway (time is analog quantity and not the voltage). For the same reason class d amp cannot become unstable.

Icepower has multiple feedbacks. One feedback, I believe, is for output voltage while another might be controlling time.

* Class AB has additional problem of different (changing) trans-conductance (gain) crossing from one transistor conducting to both transistors conducting (around zero). It is voltage dependent gain that needs negative feedback.

"50us/0.2us = 500" ?

"better dynamic range then all others based on tiny switching power supplies (again - huge exception is Jeff Rowland)"

Not only that most (if not all) of Rowland Icepower amps use SMPS supplies but he uses them as well in Capri preamp and new generation of class AB amps. Rowland amps like 102, 201, 501 use standard B&O modules with built in SMPS.

Dob,

Mentioned Rowland 102, 201, 501 have standard SMPS without anything coming from Rowland. You can buy amplifiers with built in PFC like model 312 or buy additional power supply but most of Rowland Icepower amps have only standard B&O modules with integrated SMPS.

Please check power amp listed in my system before you make another silly personal remark involving elephants.

Magfan - local feedbacks are always better than global one but their application is limited.

Class AB amp design has to start with excellent linearity and wide bandwidth since one of conditions to eliminate TIM is to limit bandwidth at the input to one amp had before feedback was applied. Feedback can fix inherent class AB problems like "gm doubling" (different gains for small and large signals) but cannot fix poor design. AFAIK there is always some form of feedback (local one) even in zero feedback amp.

Magfan - ASC series has also SMPS. I think that it was AP series but now Icepower makes different modules.

B&O SMPS are pretty good - very quiet (zero voltage / zero current switching) and very strong (1000ASP can deliver 40A output for 0.5s). What people often don't realize is that SMPS supplies are line and load regulated while linear supplies aren't. Linear supplies require a lot of capacitors to keep voltage steady and filter out 120Hz. They are also noisy since switching is done in audible frequency at max voltage. Linear supplies are also huge with big transformers. Small 1"-2" dia. toroidal transformer at 100kHz can deliver same power as huge toroidal transformer at 60Hz.

Best Icepower amps use 1000ASP with addition of extra supply that feeds over 400VDC. From what I read it has positive effect on the sound but module still uses its own supply. It is perhaps bunch of extra capacitors at high voltage (low losses).

Smaller Icepowers like 200ASC used in my Rowland 102 have higher carrier frequency and therefore wider bandwidth.

If you plan to use one, you might want to investigate input circuit that increases input impedance from 10k to 40k based on THAT1200 instrumentation amp (tiny board). That's the only Rowland contribution (other than beautiful case) to my amp.
I have to admit that he deserves credit for recognizing genius of Karsten Nielsen and B&O company. B&O was so fascinated with the project that they sponsored Nielsen's doctorate and gave him shares of company - first time in 70 years of B&O (being private company).

Michael_moskowich,

Rowland uses SMPS in all class D models (including integrated). In higher models like 312 or integrated Continuum he uses additional PFC unit. For the rest like 102,201,501 you can buy external PFC unit. Such unit supplies about 450VDC and you don't need any DC/DC converter (term tossed often to impress people) because B&O module can be supplied directly with 450VDC.

There is more than 100 companies manufacturing class D amps any many of the use SMPS. Example of company that uses only SMPS is Bel Canto, NuForce, Red Dragon.

New Rowland high end class AB amps costing $48k also use SMPS. Read this thread:

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1263307524&read&keyw&zzcriterion

For class D amplifier SMPS is obvious. If you think that class D amp is good enough then SMPS, that is the same thing (class D), should be as well.

I know, you know all that and am just restating but as for the rest of the world using only linear power supply - I don't know. I wouldn't risk such statements without checking. Others with more experience can perhaps help but I remember that Linn amplifiers have SMPS and are not a class D amps. Even some preamps like Rowland Capri use SMPS. I can tell you that linear power supply is easy to design in comparison to SMPS. In addition there is natural resistance as it was at the beginning to class D amps. It is very difficult to change opinion of people (like you) so designers design for current demand (I don't blame them).

One more thing - PFC is not conversion but correction. It is beneficial for power company but not so much for the end user (lower overall efficiency since PFC unit has its own efficiency <1). What counts is just whole bunch of electrolytic caps at high voltage fortifying SMPS but the same thing (extra capacitors) could be included in SMPS to start with. Again "PFC correction" is more impressive than just "extra caps". Bel Canto has similar thing inside of cabinet but they call it "increased energy storage". Their amp Ref1000M made Stereophile class A rating. There is not so much sound difference from the original Ref1000 but it allows some critics who made previously stupid reviews to save face and reevaluate (Martin Colloms gave this amp score of less than one of 10).

Shadorne,

Absolutely true, but linear power supplies also create HF noise in the AC mains by drawing current in short narrow spikes of huge amplitude. They are practically switchers at 120Hz. Modern SMPS with zero current / zero voltage switching can be extremely quiet. I don't see other reason for Jeff Rowland using them in preamp (Capri) where regulated linear power supply could be used.