So what do you think of Class D amp for subwoofers

"i will listen to em again" - to Glen Back and Karl Rove?

Muralman - I agree. It is more revealing. With class D I discovered how cheap $25 aluminum dome tweeter with Mylar cap in series sounds like. I ended up with much better $250 tweeter and $15 capacitor but still with some harshness and sibilants. I changed caps to $100 kind and it becomes sweet and extended plus soundstaging improved. My previous amp was covering it or maybe I did not expect much. Other way around - putting great speakers on cheap electronics and cables will reveal it ruthlessly.

Kijanki:

I did go to the Icepower site and I downloaded his thesis. I skimmed through it yesterday and today at lunch and I focused on some of his major points. There is a lot there and he is certainly the first to put phased pulse code modulation into a Class D amp. I can see the advantages of that since it allows one to effectively have a higher sampling frequency while using a lower clock frequency.

But he did not invent phased pulse code modulation! I am pretty certain I worked on a PPCM circuit when I was in the Air Force working on some old DEW radar equipment; I know it has been used in various circuits since then as well.

You asked me to explain distortion mechanisms in Class D amplifiers. Either you did not read the links I posted above or you did not understand them as the biggest distortion mechanisms for Class D are described there. However, given that those papers were written for electrical engineers and not the layman, I think I will describe those in a separate post later. I will also include Class A and Class AB distortion mechanisms since they also apply to a Class D amplifier.

You are correct that Class D is analog, however it is most certainly not linear. Class D amps have much more in common with digital amplifiers or switching power supplies than linear Class AB amps. If I used the word “analog” in place of “linear” somewhere I apologize as that would be the wrong term.

However you are absolutely wrong when you think that Class D amplifiers have no resolution limits. Indeed they have at least as much or more than Class AB. The reason for this is that in a Class D amplifier there is a quantization error that exists because the modulator is trying to approximate the area under the analog input waveform in a given time sample using a varying pulse width approach, that is a PWM Rectangular waveform. As soon as you use that approach you develop a quantization error which does not exist in linear Class A or AB amps unless the units are severely bandwidth limited and even then it would not be a quantization error, it would simply be rolled off. I don’t know who told you or why you would think a Class D amplifier has infinite resolution but it most certainly does not. There is nothing available at any price, using any form of amplification, such as tube or solid state, in an amplifier which has infinite resolution.

Using the IcePower approach with phased PWM one can obtain a reduction of a factor of four in quantization error, but that does not mean some number X divided by 4 is zero. Hardly! Note that if the sampling frequency was unscaled by four, you would have exactly the same thing as a phased PWM at 1/4 the sampling rate. The big advantage using the IcePower approach is for any given sampling frequency, the effects are equivalent to increasing it by a factor of four, or some integer multiple of that given the number of phased arrays you have.

But there is a trade off here: you can’t increase the number of phased array output devices without dealing with increased problems in dead zone, turn on, turn off, rise time, and fall time problems. The more devices you put in parallel the greater problem you have with dead zone and the resultant harmonic distortion that results from it. The paper does mention this.

You also mention that 16 bit resolution with a 20KHz bandwidth is not possible unless you are sampling at 1.4 GHZ? Where did you get these numbers? I have news for you: we have an honest 16 bit systems at 196KHz and 20 KHz bandwidth working in the world today and in consumer gear, too. The big limitation in achieving 16+ bits of resolution is NOT in the digital domain but in the analog. There are few linear or analog ICs available that produce an honest analog equivalent in Signal to Noise ratio of more than 18 bits. Maxim has an audio IC which is guaranteed to produce 16 bits SNR, bu the price of that is so expensive hardly anyone uses it.

Discreet 16 bit SNR circuits are possible but add other issues – In short, we are a long ways from true 24 bit SNR on the analog side. The clock speed is just not a problem and neither are the digital chips to run at that speed. Also, if you are speaking strictly in the domain of Class D, there is a LOT more to deal with than just the clock speed. Besides as stated in the thesis, it is clear that an effective 1.6GHz sampling rate is reasonable in production.

I disagree with your statement that only even harmonics are possible in Class D amplification. This is nuts really; if you look at the papers I posted above you can see that a dead zone in the region of 20 MHz to 500 MHz bandwidth causes as much as 2% harmonic distortion and this is not just even harmonics. And we did not even discuss clock jitter, whose effects are amplified by phase shifted pulse code modulation. Besides, the claim that odd harmonics are more audible than even harmonics was dispelled at least a decade ago. Music itself produces both even and odd harmonics as do loudspeakers. The Class D amplifier is very much like a push pull output stage in a linear amplifier, both even and odd harmonics will be produced. Even the thesis published distortion curves which are virtually identical to Class AB amplifiers.

Lastly, you continue to quote to me reviews by noted authors in various stereo review magazines as if that is supposed to prove to me that Model X amplifier is obviously a fantastic product. Frankly, if someone publishes something good about some product, I consider it is worth a listen but nothing beyond that. I hardly think for an instant that just because someone published an article stating that Model X amplifier it as close to perfect as possible and it is indeed so worthy of my cash that I will plunk down money for it. It might be great for their ears but I wear mine, not theirs. Consequently, I listen critically before I spend money.

I quit listening to that sort of review blabber at least 20 years ago when I walked into an expensive stereo salon in Beverly Hills, CA, and the sound there was substantially worse than what I had in my living room. And, the salespeople there were trying to tell me what they had was as perfect as I could possibly buy!

I walked out and didn’t bother to spend a dime there, considering when I walked in I was ready to dump, in 1981 dollars, around 600 dollars on a phono cartridge.

You stated that listening is proof of concept, I do agree with that statement but did you read above where I did listen to my amplifier and I came to the conclusion after much listening over a period of time that my bass is somewhat dead and lifeless? I didn’t measure this, I didn’t read it, I determined that by listening to it. Actually, the Class D amplifier is only amplifier I have yet to put on a bench and test.

Perhaps your bass is fine, perhaps not, I don’t know but I can assure you, my bass is not. Frankly, the bass was somewhat better when I had the Amber 70s in the system; at least until they ran out of voltage. It is hard to compare the sound of a 70 Watt amplifier to a 450 Watt amplifier when the 70 Watt amp clips occasionally. To my knowledge, my Marantz Class D amp has not clipped once. But that doesn’t change the fact the bass now has a problem.

Kijanki, if you really believe that your Class D amplifier has 140 dB SNR, then do this experiment: take a battery powered AM radio and tune it to a spot on the dial which has no station. Then move it close to your Class D amplifier and see if it picks up any radiated EMI. An amplifier that has 140 dB SNR will not make a sound on the radio.

Frankly, I believe if you got that 140 dB SNR figure from some literature, they sure aren’t measuring it with respect to 1 Volt! Not to mention the fact that to measure the output of a Class D amplifier, one has to use a very steep filter to squash the noise on the output. Gosh, if I put a 5th order filter cut to 20 KHz on the output of a Class AB amplifier, I would get fantastic SNR too!

My advice to you is to start listening to your ears, accept the fact you can't afford the most expensive gear, and quite accepting reviews as gospel. In the long run, you will have a better sounding stereo and more money in your pocket.

"My advice to you is to start listening to your ears, accept the fact you can't afford the most expensive gear, and quite accepting reviews as gospel. In the long run, you will have a better sounding stereo and more money in your pocket."

Tell the truth Spatialking, I was tempted to forge similar language for your own benefit. . . but then, naaah. . . I would not do that, would I. On the other hand, you seem fixated on the issues you verified on a Marantz amp, and you are performing a somewhat bold yet unorthodox induction step, which is leading to potential fallacy. Remember the old bad syllogism:

Socrates is a man.
Socrates has bad breath
All men have bad breath.

Well. . . 'tis probably almost true, but.. . you know what I mean (grins!). Thus, instead of expending furter energy in the futile attempt of associating the scurge of audio halitosis to all class D amps, why not take a moderate pinch of your own free advice. . . get out of the house, and listen to some true high end switching amps. . . with and without switching power supplies. Chances are, you may find some pearls. G.

PS. The only part I am not sure of. . . is the one about the very un-audiophile solace of finding more money in your pocket.

Spatialking - what quantization error you talking about? Quantization happens only in sampled system - this is purely analog. Imagine perfectly linear ramp generator that converts voltage to duty cycle and filter that converts it to average value. That way you can get absolutely any value - therefore unlimited resolution. Oh yes - it is also very linear. Nonlinearities come from errors in switching times and feedback corrects it. Other classes of amps also use feedback to correct nonlinearities - even in class A. Icepower of course is more complicated than that. Modulator uses sine wave and not the ramp etc. but principle is the same.

When I mentioned -140 dB idle noise shown on FFT it is the noise of amplifier in 20Hz-20kHz bandwidth and not the wider bandwidth noise. My point was that if other audio gear is affected by switching noise (since you stated that it must be audible)in audible band then why Icepower doesn't affect itself? Maybe simply because carrier is way outside of the audible band.

Please read carefully what I wrote. I didn't say that digital implementation of 16 bit and 20kHz is not possible. I said that digital class D is not possible with 20kHz and 16bit. Each period of 20khz would have to have 65536 individual codes - so the width of the smallest pulse in PWM would correspond to 20kHz*65536.

Same about careful reading applies to even harmonics. I did't say that Icepower plays only even harmonics - I said exactly "Harmonics produced by class D are mostly even" - completely different thing. Produced simply means that amplifier exaggerates mostly even (like tube amp) and not odd harmonics. Class AB amp tends to exaggerate odd harmonics because of TIM (overshoots pulses).

As for the radio - I did exactly what you asked. I set portable AM radio less than a foot from the amp and set to different frequencies where there is no station. Then I plugged and unplugged my amp - no difference in noise. I even tried lowest frequency 540kHz assuming that it will be closest to the carrier - still no difference. As I mentioned before my TV that is foot above and has antenna cable running next to power cables shows no difference on any of the analog channels - even very weak ones. I just cannot detect if my amp is ON.

I'm very happy that you agreed that class D is analog because I was about to say that if everything that switches is digital then FM radio is digital radio. Perhaps in future I will be lucky to convince you that it is as linear as class A and doesn't suffer from TIM. Just perhaps.

Guido - I like your Socratese example of bad logic. Let me offer you one from "Alice in Wonderland"
"I see what I eat" is not the same as "I eat what I see"

I'd like to keep open mind about audio since I've learned that not everything can be explained by specifications and electrical engineering (and that's the beauty of it).
Person with negative attitude toward particular construction of an amp has no chance to fully appreciate it because of it (placebo effect). Trying to convince, as I found, makes it worse since it creates avalanche of counterarguments and fortifies hate toward the subject.

I have never said that class D is perfect - no class is but just in my humble opinion it is pretty good (and good for the money). I like neutral and open sound but I also really enjoy sound of good tube gear and cannot understand why people have to be so polarized about it. Our discussion here took turn toward technical side and I think I should perhaps sign off this thread.

Cheers,