So what do you think of Class D amp for subwoofers

Spatialking - As far as I know Nyquist and Shannon were describing sampled systems while class D is analog. Adding high frequency noise is often practiced (for instance in digital photography) to increase resolution. In image processing it is called dithering and is also used in audio processing.

I mentioned many times before, that noise is always produced - only high frequency is much easier to filter than 120Hz spikes (that carry a lot of high frequency energy) and that was main reason for Jeff Rowland Capri's SMPS.

I don't think I understand your statement "If it was conducted noise, moving the antenna would not have any effect" If you move antenna away and it stopped it was most likely conducted (capacitive) and not radiated (electromagnetic) noise.

At around 100MHz you're at such high harmonics of 0.5MHz carrier (Icepower) that there is almost no energy in harmonics to start with and even single capacitor would clean it (Icepower has second order filter).

I've read pretty much all posts on this forum (and others) regarding class D. 90% of people who dislike class D never heard one. Reading your original post I had impression that you don't even consider it for subwoofer much less for the main amp while according to all editorial reviews I've read bass is exceptional in class D.
(Just go back to my quote in previous post of John Atkinsons La Sphere system with 5200 watt Icepower system.)

At this point I'm not sure if you don't like it because of EMI, principle of operation (PWM) or the sound.

I don't know what Marantz screw-up in your amp (or what is wrong with your tuner/antenna combo)but mine is exactly under TV and has no detectable effect (speaker cables lying next to antenna cables).

If you believe that high frequency noise will affect audible band then go to Icepower 1000ASP datasheet and check FFT of its idle noise 0-20kHz - it is below -140dB (it should affect itself but doesn't). Also it has incredible IMD=0.002% and THD=0.007%. How many amps can output 100mW-1000W at 10Hz-20kHz with only 0.2% THD? For your subwoofer application DF=4000 should not be too bad as well as ability to output 40A for over 1 second.

As for disliking principle of operation (PWM) sigma-delta DACs, SACD and DSD studio recordings are doing exactly the same thing.

I can point you to review in Audioasylum where one guy bought Icepower amp to drive his rear spekers and after listening to it got rid of his main tube amp replacing it with Icepower. If you find Icepower sound not to your liking try Kharma or hundred other companies that produce class D audio amps.

It is entirely possible that you have better ears than mine or even more experience than John Atkinson (Stereophile) but if you just simply don't like the sound then don't speak poorly about whole class of amps that is not better or worse than other classes of amps and for the value are probably the best.

If you look into Icepower datasheet you'll find following statement:

EMI conforms to: EN55103-1
EN55103-2
FCC part 15b Class A

And don't worry about SMPS in your TV (about 200W) since it follows the same standards.

You said that class D noise is inherent and if it's not outside it means that designer had to take care of it. First of all - I pointed that switching noise in very high current spikes is inherent to any linear supply and second I could make similar statement of tube amps - "they have inherent dangerous high voltage and if it's not on the outside it is because of the designer (transformer). It always is - don't analyze and just find better one than Marantz.

Dont know if this is applicable or not,used on 2 way legacy speakers,sure the bass went deep,but it wasn't natural,and it wasn't any better then what comes out of my counterpoint,the counterpoint shows its bass entirely with whats on the recording,if its light on the recording that's what comes out if its good on the recording that's what comes out,class d seems to really bring out more then really whats on the recording,overemphasizes if anything.

Coffey - how do you know what is on the recording?

Bass of tube amps is in general different than one of transistor class AB amps - which one is "on the recording"?

Kijanki: Actually, a Class D amplifier uses a digital technique to reproduce the sound. The amp is actually a Pulse Width Modulator with a power output. Envision a one bit DAC reproducing the sound and you have an idea of what is going on in a Class D. Technically, it is incorrect to say it is a digital amplifier, the difference is described in the papers below. But do understand a Class D amplifier is certainly not a linear amplifier!

See John Guy's paper at http://www.national.com/vcm/NSC_Content/Files/en_US/Audio/ClassDAmplifierFAQ.pdf John is a very knowledgeable fellow, I worked with him at Maxim. Believe me when I say he knows Class D amps! The paper pretty well describes the Class D noise on the outputs.

John also published another paper in Audio Design Line which describes how a Class D amplifier works. I think you can get this magazine emailed to you directly if you are interested - I am not sure if you have to be an engineer or not to get it. Anyway, check out the paper at http://www.audiodesignline.com/howto/177102531;jsessionid=CP1YPZLBM2M5AQSNDLRSKH0CJUNN2JVN This is one of the best papers I have seen on how a Class D amplifier works.

It helps to know some algebra when you read these papers, as well as know a little bit about basic electronics. Otherwise, they are a good read.

It is incorrect to say I don't like my Class D amplifier; actually I have no plans to replace it. In fact, scroll up a ways and note what I said about listening to it with a CD as a source last week! That sound and bass was excellent! What I don't like is the radiated noise from the unit.

Let me explain the difference between conducted and radiated. Conducted noise is conducted along the wires, say the line cord, RCA cables, or speaker wires. Radiated means the noise is traveling through the air, much like a radio broadcast. If the noise is conducted, it will disappear when you unplug the offending wire or at least diminish when you put some ferrite suppression on it. If the noise is radiated, then simply moving the affected gear farther away from the noise source will reduce or eliminate it since radiated energy decreases as a square of the distance.

Note that sometimes it is difficult to determine if it is radiated or conducted. For example, if the noise was conducted from the AC power cord, unplugging it would also kill the unit. The only way to find out if it is conducted from the power cord in this case is to swap out power cords. It is further complicated by the fact the noise can be conducted out of the chassis by a cable and then radiated from there.

The FCC Class A you mention above is required for everything that is sold which has a clock in it above 10 KHz, I believe. The Class A or Class B spec dictates how much noise is allowed to pass, per FCC. Class A is allows ~10dB more noise in selected frequencies and is normally used for industrial equipment. Class B is normally used for consumer goods, such as radios, TVs, or stereos with computer chips in them. Class B is significantly more strict with respect to radiated and conducted noise. Actually, I am a bit surprised to see audiophile grade gear sold with a Class A level, but the intended application is what normally dictates the A or B designation, not who buys it.

Also, just because the frequency goes up doesn't mean the harmonics diminish. Once you are above 1 to 10 MHz or so, the phase-coherent relationship outside the chassis dictate how significant the emissions are. I have worked in EMI suppression before and it is a common mistake to assume a 2 MHz clock won't have significant or fail Class B emissions at 800 MHz, but don't count on it. I have seen units fail at those frequencies with a clock in the 2 MHz region. FCC requires testing up to 1GHz and sometimes higher just for this reason.

Also note when I say bandwidth, I don't mean the audible bandwidth at 20 KHz, I mean the bandwidth of the amplifier. If a given amp has a bandwidth of 60 KHz, then any injected noise in the region up to 90 KHz or so will have an effect on the sound, depending on how fast the response rolls off. But, lets keep this in perspective - one can inject a tiny amount of 60 KHz noise and the damage is causes may not be audible to our ears depending on our ability to hear it and our systems to resolve it, even though the sound did change a bit in the audible octaves.

I can give you a splendid example of this in one of my own designs. About two decades ago I was consulting for a successful audio amplifier company. They had a real winner of a power amp, it was listed in all the recommended components. Their preamp left a lot to be desired though, and that is why I was hired. I sketched out a phono preamp over dinner one night with the CEO of the company and their primary engineer.

I built it a few days later and plugged it into my system. The line stage was about perfect but the phono stage was a bit on the hard sounding side. Try as I might over the next week, I could not find the reason for the hardness. Since we were having a design review on Saturday, I had to fix it and fix it fast. Finally, around midnight on Friday, I found the problem. There was a 6 dB bump in the frequency response at 14 MHz and that is what was causing the hardness. Once I found the source for that, the hardness was gone completely. That bump at 14 MHz caused audible effects in the 1 to 3 KHz range even though at 14 MHz the gain had already rolled off quite a bit, beyond the -6 dB point. The gain was still significantly higher than unity though and that was the problem. Like I said, if it is in the bandwidth of the amp and the noise is bad enough, you will hear it!

Spatialking - Class D is absolutely analog, at least in Icepower or Hypex implementation. I think you're very confused here. We deal with digital when when resolution has certain limits. There is no resolution limit here. Analog voltage signal comes from the input to analog modulator that changes voltage (in linear fashion)to duty cycle WITH UNLIMITED RESOLUTION. Output parameter until filtered is time (duty cycle). Type of parameter does not have any bearing on being digital or analog. Voltage can be analog or digital as well as any other parameter.

Digital implementation wouldn't be even possible since to get 16 bit resolution and 20kHz bandwidth carrier frequency would have to be 1.3GHz (Nobody makes N-channel Mosfets this fast). You can read, Karsten Nielsen (founder of Icepower) doctorate work on principles of Icepower (in English on Icepower website). B&O was so impressed with his work that created subsidiary company for him and gave him stock (private company) first time in their 70 years history. This, and the fact that Jeff Rowland put his name on changing whole production to class D only, makes me believe that it is not so bad (at least for me) and even for very discriminating audiophiles should be satisfactory for subwoofer.

You stated that class D produces other kinds of distortion - could you comment on this? What other kinds of distortion? Do they have name?

Harmonics produced by class D are mostly even (like in tube amp) - just opposite to class AB producing higher order odd harmonics (especially at low levels).

I never said that there is no noise on the output of class D amp. I stated that any amp produces a lot of switching noise and that the one created by class D does not affect the sound. In order to affect the sound something would have to show in FFT of idle noise in audible band (0-20kHz).
It doesn't and Icepower itself is the best proof because it doesn't contaminate itself having idle noise 0-20kHz below -140dB.

No matter what can be measured or proved - the best proof is listening session. Listen to one of top Rowland amps that Guido recommended and prepare to be amazed. Mine is the cheapest one and I think it's great.