Why do AC cords make so much difference?

The better ones do a good job of filtering out common mode noise from electrical and magnetic fields and also filter noise from equipment (digital) to the outside. But not surprising, some of the cheaper ones can do as good of a job, if not better, than some of the more obscenely expensive ones.

...and I absolutely agree that the power supply design is everything, where a well shielded power cord compliments it rather than trying to wrestle it into submission.

Although I have not experimented with AC cords (for lack of time .. young daughter, and daipers being a key reason) my theory is that cords can make a difference for two reasons :

1) they can act as low pass filters. I believe that for digital components this can be important to prevent the digital component from polluting the power supply of your analogue components (preamp, amp etc). It also helps that the digital component doesn't receive noise.
2) They must allow free flow of potentially large amounts of current to the power amp.

I can quite understand why power cords might be good in different applications : a CD transport might benefit from a cord with tons of ferrite to prevent it from modulating noise back onto the power supply. This cord might strangle the dynamics of a power amp that requires large bursts of power.

Ultimately I agree with Sean that if a power supply is built correctly the device should have enough reserve current and should not be either susceptible to incoming noise, nor radiating its own noise back onto the power supply.

Interesting theories ... I hope that potty training will soon give me some more time to experiment.

I've come to believe that the quality of dielectric used can have a bearing on performance, in addition to shielding and current capacity. Hence I'm a fan of well-shielded all-Teflon designs. Write me for details if you wish. Cheers.

Cables are mysterious ~! But I think I am getting close to figuring it out. There is a great deal more to why power cords effect the sound of a component than simply "cleaning" the power or filtering out all the bad stuff.

Applying power to a conductor causes the conductor to resonate. The resonant frequency of the conductor establishes a base line for the cables sonic signature. But this is only the beginning.

As I understand it (I am not an EE) with 60Hz power there are 120 pulses per second. A pulse is the time when power can flow through the power supply transformer - when the rectified secondary voltage is higher than the stored voltage in the filter capacitor.

During the pulse, a heavy magnetic field is created and collapsed quickly within the power cord. Conductive material within reach of the magnetic field will dissipate energy through circulating currents, however some materials will attempt to store some of the magnetic field energy until the next pulse (or dissipate it as heat).

By adding material to a power cord one can change the resonant frequency of the wire, dielectric properties, and the pulse itself.

For example Taking a 10' long twisted pair of copper wire and applying more force - to twist it down to 6' in length will change the resonant frequency of the wire. Shielding that wire with tight braided copper, sticking that wire inside a tube & then filling with iron powder, fiberglass, sand, crushed aluminum, sawdust, crystals, or whatever, will again change the resonant frequency, dielectric properties and the pulse.

Each combination of materials will react differently to the magnetic field created by the pulse and presto... you have a new sonic signature in your system.

hmmm, thanks bwhite, that sort of makes sense to me.

so far though, swapping PCs in my system hasn't made much difference. other cables, both interconnects and speaker cables, i hear marked differences. but not PCs.