So what do you think of Class D amp for subwoofers

Spatialking,
I have a Mcintosh pre in front of a Mc power amp (300x2)driving the front and a Cerwin Vega (2800x1)driving 2-Electro Voice EVX 180's. In my apinion,you do'nt need a reel good amp for bass. The bass is tight hard hitting and the Mc run very cool because it's not driving any bass.By the way,I use a E.V. 2 way active X-over that has a sub out.I bought the class H amp new for around $450.
It costs a lot more to have the Mc serviced and I bet I won't have to have the Mc serviced for a long time because it's not driving the bass. If I blow the C.V. it's no big deal. Just buy a new one. Take care,Bill

Kijanki: Sometimes I write in the manner I speak rather than write in a more coherent manner. As an Electrical Engineer, I know the difference between Watts and Kilowatts. I apologize for not being more clear. Frankly, I wish the station broadcast in the MegaWatt range, at least the signal would be a lot stronger here. My favorite Jazz station is privately funded - their broadcast is so limited I can only listen to it clearly in Silicon Valley. East of the hills in the East Bay, their signal is very limited. Sigh...

High Frequency noise is "easy" to filter out within a band of frequencies. However, as the bandwidth increases, filters become more difficult to design and thus noise become more difficult to eliminate. When you consider you also have to filter electromagnetic noise as well as electrostatic and conducted, and the fact our noise spectrum now extends into the GHz range, it is no longer a trivial task.

Do understand that any noise spikes created by the diodes in a linear power supply is insignificant in comparison to the noise created by diodes and components in a switching power supply or class D design. This doesn't mean anything with a switching power supply is bad, but it does mean more engineering work has to be done to eliminate the resultant noise that comes along with it. Also note that this diode "switching noise" in a linear amplifier is actually the transformer radiating, not the diode. But, again, this noise is insignificant in comparison to the noise created by a switching power supply of comparable quality.

Class D amps grossly violate the KISS principal, but it is being widely embraced because most of us who have a multichannel 300W+ per channel Class D amps couldn't afford it if it was Class AB and certainly couldn't afford to operate it, much less buy it, if it was pure Class A. This is in addition to the fact that Class D makes higher power, battery operated personal stereos possible.

Given a fixed amount of money to spend, choosing a Class D amp for the midrange and upper frequencies would be my last choice over a good tube or Class AB solid state design. I do believe it is much more difficult to design an equivalent sounding Class D amp to a Class AB design.

Tgrisham: I agree with you on implementation. This Marantz amp I have had a sticker price in the 2000+ dollar range. I don't remember exactly, I would have to look at the receipt if I still have it, I am thinking it was around 2400. I bought it at 60% off due to store closure and the fact it was only four channels and not six made it hard to sell. Clearly, it isn't a cheap design - even though it is Class D it still weighs over 60 pounds. It is possible that I have a bad amplifier but I am more inclined to believe I have an implementation problem of some sort here at the house. A Class D amplifier design engineer I know now works at National Semiconductor, I think I will look him up tomorrow and discuss this noise problem with him.

Bill: I agree, the use of an amplifier as a subwoofer only makes the choice of amps much more easy. For one thing, it has only a very limited range of frequencies to reproduce and any nonlinearites created by producing 80% of the power in music is not fed to the upper octaves where your ear is most sensitive.

In addition, designing a subwoofer only amplifier is easier in that more feedback can be used without the resultant hardness through the midrange. The additional feedback increases the damping factor which usually improves the bass and the limited frequency range of the bass frequencies means upper bandwidth can be sacrificed for the increase in feedback to maintain stability.

Unfortunately, history has shown it is more difficult to sell a standalone, subwoofer only amplifier. Although once installed in the subwoofer cabinet, they seem to be selling quite well!

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").