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!
