So what do you think of Class D amp for subwoofers

First, let me state I "fixed" the radiated noise from the Class D amplifier interfering with the FM radio. I bought a new antenna and positioned it 20 feet away from the Class D amplifier. Now, there is no audible noise in the FM. I also found that the antenna I used had more of an effect on sound quality than the FM tuner!

Using the standard, cheap, 300 Ohm lead dipole that comes with every FM tuner you buy could produce either good sound or poor sound, depending on how you orient and where place it. Switching to a directional, high gain, wide bandwidth FM antenna on a rotor could make the sound quality better or worse than the cheap dipole, depending on where it is placed and its orientation. I think the next step is to buy a better directional antenna and install it in the attic with a rotor.

There is no question that both of my Class D amps radiate like the dickens and the pickup is through the FM front end. Adding massive EMI cable ferrite assemblies on the coax, line cord, and speaker wires did little to solve the noise problem.

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