So what do you think of Class D amp for subwoofers

However, when not listening to the FM radio, the amp sounds perfectly wonderful! I had some classical music on last night - piano, horns, and violins - and the sound was truly amazing!

Spatialking - Noise in linear power supply comes from the fact that capacitors are recharged only in narrow spikes. Width of the current spikes, often called "conduction angle" can be found by analyzing output impedance of transformer and ESR of capacitors. Guy named Schaffer made diagram (it was long time ago - I'm not sure of the spelling). These spikes can be very narrow and heat-up the core of transformer by high frequency content and windings by big rms value (and you'll see them in power cord). Transformers in linear supply are therefore designed for over 150% of needed power. Unfortunately nothing will remove noise they produce. People call it linear but it is in fact SMPS operating at 120Hz. Traditional transformer is also very ineffective since 2" toroidal transformer at 60kHz can carry the same power as huge 8" toroid at 60Hz.

Switching power supplies are wonderful since they work at non-audible frequencies (easy to filter out), don't radiate since frequency is too low and the only questionable might be capacitive coupling that is easily handled by shielding. Also, they can be designed extremely quiet (that's why Jeff Rowland used it in Capri preamp), and are line and load regulated.

Linear power supply is a dinosaur because to provide any temporary regulation you have to put a lot of capacitance and this brings inductance into the picture (unless you use expensive caps like slit-foil). Inductance slows response and putting film or ceramic caps in parallel creates parallel resonant circuit that rings (and this circuit is in series with the speaker). Response of SMPS is very good and you can find in reviews of Bel Canto s300 or REF1000 that they keep composure and dynamics even at very high levels.

Class D is not so difficult to filter out. Zobel network filter according to my estimates leaves only about 1% of the switching frequency (about 0.5MHz) on the speaker cables and almost nothing of higher harmonics. At this frequency speaker cable has to be few hundred feet long to be 1/4 wave antenna and going below 1/8 wave will produce extremely low radiation (antenna becomes very poor). I cannot detect on my TV in any mode and even with very weak signal any difference in noise while switching my 2x200W class D that is directly (less than foot) under TV. TV itself has most likely switching power supply as well.

I cannot say how good is class D in comparison to great tube or AB amp but I know that is very good for the money.
I have one of the cheapest class D amps and it is way better than class AB I had before.

What class D amps did you audition?

Spatialking - One more thing. Class AB even at 2x300W is not expensive to operate since average music power is only in order of few percent of maximum power (Class A is a real problem though). Operating cost was not a reason for class D in audio. Class D doesn't produce TIM distortions and in effect higher order odd harmonics. Time used insted of voltage doesn't have big nonlinearities typical for class AB output transistors. Class AB amp has gain before feedback of many thousands to make low THD and IMD (class A has at least 10x less gain) but tends to produce TIM ("transistorish sound").

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.