So what do you think of Class D amp for subwoofers

Kijanki: What you wrote is not exactly correct nor complete. The conduction current spikes you mention do exist in the manner you describe, however your analysis is not complete.

First, the conduction spikes caused by diode action you describe are well known and are published in every engineering textbook that discusses converting AC Mains power to unregulated DC voltage. I am pretty certain the first engineer to build a circuit which did this was in the mid or early 1800's, long before DeForrest invented the triode vacuum tube in 1906, I think it was.

Don't think for an instant that switching noise spikes don't exist in a SMPS, they do and are larger in magnitude than in a linear PS. They are, however, quite different in nature in a SMPS than in a linear PS. In addition to this, you also have noise from the clock frequency to deal with. There are SMPS out there with clock frequencies at 1MHz and higher, although I believe most SMPS run in the 100KHz-ish range.

The method to eliminate radiated noise caused by the current spikes in a transformer, whether SMP or linear power, are the same and is not difficult nor impossible to do. It does add cost however, and that is the key.

In addition, the frequency of operation also comes into play: the higher the frequency the smaller the size of transformer for a given power output. However, higher frequency does not mean lower radiated noise. What it does mean is that the transformer is smaller and thus the shielding can be smaller, meaning lower cost. SMPS's are also significantly less expensive than linear power supplies for a given power. They are also much more complex. Note that the stability requirements for a SMPS are the same as for a linear, regulated supply as both involve loop feedback.

The first approach to remove radiated transformer noise, which is probably the most common, is to design in a Faraday shield in the transformer. A transformer using a torrid design is a better and more expensive approach.

Second, a Faraday can could be placed around the transformer and depending on how much money you want to throw at it, you can make it out of iron, copper, iron and copper, or mu-metal.

Third, you can remove the transformer from the chassis and place it in another chassis several feet away from the sensitive electronics. Certainly this has been done numerous times in high end audio equipment. Fourth, you can use a combination of these approaches, not to mention this is not an exhaustive list. For example, I did not mention the quality of the transformer design.

If you want to remove the current spikes caused by the full wave diode bridge circuit in front of a huge capacitive input filter for a large power amplifier, the best way is to eliminate the capacitor based filter which causes the current pulses.

Instead, design a PI based filter where the series inductor is designed to handle the worst case current the amplifier is expected to deliver. In this case, the current spikes which occur become nothing more than 60 Hz sine waves where the magnitude of the current is about what the power supply is delivering.

Don't misunderstand me, this is still an unregulated raw supply. To complete regulation, you still have to add a linear regulator on the output, which is effectively adding another amplifier in series with the signal amplifier. A SMPS does exactly this, as the SMPS is effectively another amplifier whose output is DC.

I am sure you can see that a SMPS power supply is less expensive than the standard regulation circuit just described. Just as there is no question a SMPS is less expensive, there is also no question a SMPS is more noisy and that noise has to be dealt with.

Note one can use all the noise killing techniques described above, including putting the PS in another chassis, and then using a regulated SMPS as well to reduces the cost over a linear supply.

This is indeed a less expensive approach, but it is still more expensive than using a full wave, capacitive based, raw supply which is why this is still the most common approach in linear power amplifiers. Since the amplifier has inherent power supply noise rejection, and the 120 Hz ripple left over in the output voltage is well within the feedback loop bandwidth, I am sure you can see why this is such a popular approach.

There is no question, at least in my mind, that there are better sounding approaches to power supply design in power amplifiers than an unregulated raw supply, it doesn't change the fact it is still a popular approach.

Kijanki, I forgot to mention that I agree with your statement that Class AB is not expensive to operate, I disagree with your statement that distortion mechanisms is the reason for Class D in audio today.

The reason Class D is in audio today is the lower cost to produce a given power output and efficiency of operation. Let me discuss these separately, since they apply to different markets.

A Class AB, two channel, 200 or 300 Watt power amplifier is probably affordable by most audiophiles unless one considers the more exotic and expensive designs. Certainly, I don't need to mention the more expensive brand names here. However, given the cost of the more mainstream 200W stereo amplifiers, it should be obvious that a 200W Class D amplifier is less expensive for a consumer to buy than the same power in a Class AB amplifier.

Class D has really come into its own due to the proliferation of home theater systems where one needs 6 or more channels of operation, the cost of these systems are much lower using a Class D approach. In addition, the proliferation of portable sound systems demand the efficiency of Class D operation since they have to operate by battery power and conserving battery power is everything in a battery based design. Battery powered personal sound players is obviously a big market as are laptops computers which also have to produce sound as well.

As pointed out elsewhere in this thread by knowledgeable folks, implementation of a technology is more significant than the technology itself. Granted, a Class AB design has some inherent distortion mechanisms eliminated by a Class A design, that doesn't mean a Class A design sounds better than a Class AB design. Whether it does or does not is dependent on the trade offs chosen by the design engineer and the manufacturing techniques and quality.

The same applies to a Class D design. I disagree with your statement that a Class D amplifier has inherently less TIM distortion than Class AB design; whether it does or does not is irrelevant. Frankly it adds new distortion mechanisms that do not exist in a Class AB design. Given the increase in complexity of a Class D design and circuit, implementation is substantially more significant to the quality of the output signal.

This reminds me of CD player manufacturers which still specify wow and flutter as a distortion mechanism. It is obviously designed out in a CD player but it is rare to see a jitter specification in inexpensive CD players. Jitter is a new distortion mechanism which did not exist in a LPs.

Spatialking - It is not so bad with transformer radiated noise - toroid should solve it. It has 10x less radiated noise than standard transformer and my Benchmark DAC1 has measured 140dB S/N with power supply toroid next to circuitry and without any shielding. I was talking about current spikes coming thru power cord of linear supply.

Just opposite to good SMPS that switches at zero voltage - zero current, linear power supply switches at maximum voltage.

As I said before linear power supply is in reality 120Hz SMPS where width of current spikes from mains depends on the load.

Yes, linear power supply can be filtered out with pi filters as well as regulation can be added but they don't do it. Pi filter costs and inductor would have to be huge while regulation will add enormous amount of the heat (and therefore heatsinks) since it needs to cover typical 90-132V range. I don't know of any audio amps that have regulated linear power supply - but also I don't have a lot of experience in audio gear.

Jeff Rowland used SMPS in his CAPRI preamp not because of lower cost but because of lower noise. Linear power supply - even regulated would be very inexpensive for the power needed by preamp - SMPS would not give him any savings.

Noise from SMPS is, as you stated, usually around 100kHz (but can be even 1MHz for lower efficiency/lower power demand). This frequency is non-audible and its harmonics would be perfectly filtered-out before we get to your FM radio frequency range. I agree that SMPS is often used to save cost with undersized transformer and poor filtering (computers) but it doesn't have to be. Everything is in the hands of designer.

My computer, monitor, TV, amplifier, DVD player, electric shaver, cordless phone, Cell phone, cell phone charger and who knows what else in my home - ALL have SMPS power supplies. I'm not worrying about noise produced by my class D amp because it is well below FCC requirements.

"There is no question, at least in my mind, that there are better sounding approaches to power supply design in power amplifiers than an unregulated raw supply, it doesn't change the fact it is still a popular approach."

Spatialking, Higher end class D amplifiers use a high degree of SMPS regulation. E.g. JRDG 300 series amps: 301, 302, 304, 312, as well as Bel Canto Ref Mk.2 series. JRDG also makes use of active PFC in its pre power supply circuits. Switching frequency of JRDG 302 SMPS is in the 1Mhz range. In my experience, a discussion of unregulated SMPS implementation is not applicable to newer design amps ranging in cost from $5K to $50K.

Before you expend any more energy attempting to prove the supposed inherent musical inferiority of class D designs to more traditional ones on theoretical generalizations, you may want to let your ears do the walking. Consider granting some in depth listening to the recent statement level creations of leading class D amplifier designers. If after that you still did not like them, you would at least anchor your preference on applicable personal experience. Guido

Kijanki, I don't believe any SMPS is quieter than a linear regulated supply regardless of what the diodes are doing in a linear PS. Sure you can design SMPS to switch on and off at zero crossings but you still have to deal with clock noise. If a SMPS is low noise, it is only because some design engineer somewhere worked pretty hard at getting the noise out.

There are cases where linear regulators are used after SMPS regulation simply to eliminate the SM noise. I think Maxim has a patent on this and they do use this approach in many of their sensitive chips. I know, I worked there as an apps engineer for a while.

Also, please note that any noise, at any frequency, injected in the bandwidth of the amplifier has a negative impact on the sound quality. This is especially true at higher frequencies where PSRR of the amplifier drops off.

The reason for this is that any noise injects additional energy into the bandwidth of the amplifier. Sure, you may not hear it as "noise" but any increase in energy within the bandwidth of the amplifier will sound different. This is the basis Nyquist theory and all digital audio: a given bandwidth and given energy level sounds the same. Change either one and the sound changes.

Again, as I have stated at least three times now, just using a SMPS, or a Class D amplifier for that matter, doesn't mean the sound is worse than an Class AB design, it could be substantially better. It all depends on the trade offs made by the design engineer and how it was manufactured.

Guido: I stated above that I believe the sound of my bass is "a bit dead and not overly lively". I based this on listening to it, not from some datasheet or preconceived made up opinions about Class D amplifiers. I did not state that all Class D amplifiers produce bass that is dead or not overly lively. I did state that this is what I hear in my system and it is caused by my Class D amplifier. I was curious to see if anyone else had experienced the same sonic effects which I have.

Let me put it another way, approximately 1000 folks have looked at this thread. If say 30 to 40% of them stated something like "Oh, heck I have the same problem with my Class D" I would be inclined to find an engineering reason for it. This has not happened. Whatever I am experiencing here seems to be more related to my own Class D amplifier.

If I felt that Class D amplifiers were bad from bias, I wouldn't have spent a grand on Class D amplifier in the first place! I could have continued using the two Amber 70's I had. Actually, the only complaint I had about the 70's is the fact they only produce 70 watts. This is not a lot of power and I can clip them if I was not careful on the volume control. Otherwise, they sounded perfectly fine as bass amps. My subs are nominally 6 Ohms and the Amber's have plenty of current to drive 6 Ohms. The problem is they were running out of voltage, not current. A well designed amplifier should run out of voltage when they clip, well designed amps should not run out of current.

These aren't theoretical generalizations I am making up. If any Class D amplifier is going to have less radiated or conducted noise than a Class AB, it is because the design engineer made it happen because Class D designs have inherent noise that Class AB amps don't have. That is a fact of life, just as tubes have filaments, mosfets are voltage based devices, and transistors are current based devices.

I will admit that I am conservative when it comes to making changes in my stereo - This is the reason I have a Class D amp for the subs and I haven't sold my Premier Five CJs on the upper octaves. I could have sold the CJ's and ran the entire system off this monster Class D amplifier. I didn't do it, I wanted to see how it worked on the subs.

Please scroll up and read what I wrote about radiated noise from my Class D amplifier causing FM interference. If it was conducted noise, moving the antenna would not have any effect. But the fact that I moved the antenna and eliminated the interference proves the noise is radiated. Also, simply turning the Class D amplifier off eliminated the noise also proves my Class D amplifier is the source of the noise. If anyone still believes this noise is not from the Class D amplifier, they will simply have to stop by for a visit.

Again, let me state one more time simply using a Class D amplifier does not mean your system will sound worse than a Class AB or Class A or even a tube amplifier. It does mean that if you don't have a noise problem then the design engineer who designed it dealt with the noise problem. Whoever was the design engineer on my Class D amplifier did not do enough homework or I would not have had the interference problem.