You missed the important point Re: voltage! (archive)

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Which is that; THE VOLTAGES WITHIN THE AUDIO SIGNAL PATH HAVE ABSOLUTELY NOTHING TO DO WITH the power supply voltage!
Here's an example, using a theoretical power amp:
Lets say we want 50 watts of output power. Power(W) = Voltage (E) * Current(I)
(I'm ignoring the phase angle of the reactive loudspeaker loads for simplicity, OK?)
Ohm's Law states: E=I*R, by rearranging things, it becomes apparent the W = E^2. Therefore if we have a typical 4-Ohm load we need to swing an output voltage of slightly under 15V (I'm rounding). 15V^2/4Ohms=56.25W. (For an 8Ohm load we'd need 20V^2/8=50W)It doesn't matter if it is home audio or car audio, to get that power into that load you need that voltage. Period.
Now, where does the voltage that powers the amplifier circuit that powers the speaker come from? BZZZT! Wrong! Not the battery. Not the wall.
It comes from a separate power supply inside the amplifier, receiver, or whatever. The wall socket, or the battery powers the power supply. (I'm simplifying, but I think you know what I mean, right?) That power supply supplies the amplifier with bipolar voltages of a level appropriate for the power of the amplifier. In our 50W amplifier, the typical voltage might be anywhere from +/- 20V to +/- 30V depending on the efficiency of the particular output stage chosen and the load impedance that the unit was designed to drive. So, in a car audio amp, a switch-mode power supply takes the 12V input, switches it rapidly on and off under the control of a Pulse Width Modulator, sends this to a transformer, rectifies, filters, and regulates the output, and sends the resulting +/- DC voltage to the amplifier circuit. In a Home amp, the 120V AC comes in, goes to a transformer, the output is rectified, filtered, and (sometimes) regulated, and the resulting +/- voltage is sent to the amplifier circuit. The Important point is tht THERE IS NO DIFFERENCE in this +/- voltage at this point in the circuit between a home amp or a car amp. Anybody who tells you it is has held their fingers across their outlets too long.
Now, since the actual audio circuitry is operating off the same voltages with the same capabilities in home and car audio, where is the difference? It just ain't there!
: Let's stop and talk about MOSFET power designs.
Uh. Why? Its a non-sequitur. The point is not that "higher voltages create less distortion" (which I would also argue with, but not now), but that the voltages in the signal path (which is all that matters) are the SAME regardless of the power supply voltages that power that device. Since the laws of conservation of energy apply, to get a given power out, you must get that power in to the "box" somehow. Arbitrarily assuming a 50% efficiency (A reasonable assumption based on practical analog power amps), we must supply our theoretical 50Watt per channel stereo amp with 200 Watts to achieve rated power. Working backwards we find that the home amp requires 120V at 1.6Amps, and the car amp requires 12V at 16.6Amps. But it still comes back to the fact that, once you get through the power supply, the audio signal is exactly the same, home or car.

MOSFET, as you might know is an Electrical Engineering discovery in which diodes with doped ions in them gate or negate the flow of electrical current. In this study, the higher running voltage on the circuit will clearly get cleaner sound because of two things which you forget to mention: 1) Electrical signals in the circuit are created by the voltage drop.
However, it is imperative to know that all diodes, and MOSFET in particular react to higher voltage sources better and maintain a higher degree of excellence in quality in carrying the signal through. In digital media, this means that the digital input (CD) will probably most likely retain a higher level of quality because of the better matching of the digital signal. You might argue that all diodes create a 0.7 voltage drop no matter what the voltage source is, but it is particularly interesting to see that if there is a higher voltage source through the circuit, the standard in which the signals are traveling are maintained and hence bring better reliability and qualtiy in producing that signal because sometimes that 0.7 drop may not be maintained due to high resistance in copper wire. It is always a known fact that higher voltage source leads to better quality. And to the noise at home, maybe you haven't used a good surge protector which filters some of the hum.

There is too much uncorrelated nonsense in the above statement to respond to in detail. I recommend actually gaining some knowledge of the subject before continuing. Just a few points.
1.)Of course I know what a MOSFET is, but it is irrelevant.
2.) A MOSFET is not a Diode. Perhaps you are mixing this up with a Bi-polar transistor. Gate to Source voltage in the typical MOSFETS used in audio are more on the order of 4-6V, not .7V.
3.) It is not "always known that higher source voltage leads to better sound". Our head EE is rolling on the floor laughing. Take a look at THD vs output for a typical amp, you'll eat your words. It is true that a higher signal voltage level will lend itself to a better signal to noise ratio, and to greater immunity to outside interference, but this is not a guarantee based on higher voltage alone. It's all up to the individual circuit.
4.)A surge supressor will not have any effect on Hum. A surge supressor uses avalanch diodes to simply clamp voltage spikes which exceed (approx)140V and short them to ground, protecting the connected device. If you're talking about a "power line conditioner", that won't do anything either. It still sends 60Hz AC to the device. Hum comes from the nature of the 60HZ AC voltage coming into the device. The rectified voltage coming off the transformer is at double the line frequency at 120Hz, bith are well in the audible rance. It is up to the power supply filter capacitors inside the device to filter this out. Because of the low frequency of residential AC, these caps need to be large and expensive. With a switch-mode supply, like those in car audio and some of the newer home equipment, the operating frequency is much higher, often above 20hHz, which not only gets it above the audible range, but also allows the use of small value caps to filter the residiual ripple.

By the way, In all fairness I should mention that I do not completely disagree with your initial premise that the BMW DSP system is pretty good. This I would agree with. If I were not so entrenched in the audio business, and spoiled by how good a car audio system can sound I would probably be happy with it too. My only argument is that it could be even better for the money BMW must be spending on the hardware, and that there is a place for "better" depending on the individual's driving habits and degree of interest in music. I see it more as an issue of how big a part music plays in ones life as opposed to an automotive issue. Outside of the gross technical inaccuracies of your argument, the area I disagree with is the premise that one cannot get truly good sound in a car. Same as home? No. A rewarding experience measured with its own different set of guidelines? Absolutely.

Best regards,
Bob Hazelwood
Product Manager, Cambridge SoundWorks
'93 525i 5-sp. Sharked, BL/ss'd, H&R + Bilsteins
BMW CCA (Boston)

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