Why do designers/manufacturers...?

Tweaks huh? A long time ago we started using toroidal power transformers, and we noticed some odd things. One was that if the amps were too close together, they sounded different. Another was that the bolts supplied by the transformer manufacturers were heating up- in fact were hotter than the transformers!

Turns out that the magnetic field of a toroid is not as compact as they say. The bolts were heating up because they were ferro-magnetic. We switched to non-magnetic stainless mounting hardware and the heat problem went away and the amps sounded better! We moved the transformers in such a way that the amps could sit side by side with no ill effects.

Now we noticed that about 25 years ago yet to this day all transformer manufacturers will supply magnetic bolts that heat up. I don't see anyone in the industry talking about this either. This effect is sonically more pronounced than many tweaks I have seen audiophiles do.

BTW, you can see the links inside a fuse 'dance' when you put power through them. If you think that the fuse does not make a difference you are simply not paying attention. We figured out they were a problem in 1990 when we were designing the MA-2 amplifier. It uses a different kind of fuse that is much larger (the fuse rating is an FNM8 in the MA-2)- we figured out that they sound better that long ago.

I can easily measure differences between power cords. I have seen them rob an amplifier of as much as 40% of its total output power. With an effect that profound, you think it won't be audible? When I have people tell me that power cords don't make a difference, I always ask them to show me their measurements, because I have measurements I have done with a simple 3 1/2 digit multimeter that says they are wrong! But so, far, none of the skotics saying that power cords don't make a difference has ever had any numbers to support their position.

Jea48 is correct, power cords are not just power delivery. The measurable differences don't seem to be profound, but they can affect power amplifiers in particular in a number of ways.

First is the voltage drop across the cable. When I saw 40 watts lost in one of our power amps, the voltage drop across the cable was about 2.4 volts. But a cable has to have high frequency capability as well- anyone that knows power supplies knows why- the rectifiers in the power supply only turn on (commutate being the technical term) when the voltage from the power transformer is higher than that of the filter caps.

Now the amp only drains the filter caps a small amount between AC power cycles. So the result is that the rectifiers only commutate on peaks of the AC waveform, perhaps only for a very brief time! So if the cord lacks high frequency capability, the power supply won't charge up properly and you can hear the result, plus you can measure the power supply voltage as well.

ROMEX works surprisingly well, so the real problem is in the power cord, not the wall wiring unless the building is quite old. However it is illegal to use solid core wire in an AC power cord. Thus, the cable has to flex, can't have any voltage drops (the problem areas are often at the terminations, but the cord itself can heat up from voltage drops as well) and has to have good bandwidth.

If the cord satisfies these requirements it will work fine and won't matter what brand it is.

We use an IEC connector for the simple reason that no matter what cord we put on the amp, the likelihood is high that it will be discarded anyway in favor of the audiophile's preference.

We do however make sure we have the best resistors and capacitors in the actual circuit, or at least have them optionally available. I have seen too many times where someone thought they could improve the amp or preamp, only to turn it into a rat's nest in the process. Believe it or not, neat wiring often sounds better since stray capacitance is better controlled!

Scvan, sure. How a power supply works is you have the transformer, rectifiers and filter caps. When the amp is running, it discharges the filter caps slightly between each peak of the AC power waveform. The rectifiers only conduct when the filter cap voltage is lower than that of the power transformer. What this means is that current will only flow at the peak of the incoming AC waveform.

That current spike can have some pretty steep risetimes, in order to charge the filter cap properly. A steep risetime corresponds to a high frequency. If the power cord limits the risetime at that frequency, the power supply will not charge up properly.

You can see this in an amplifier by running it at power and then substituting power cords. You can often see a difference in the power supply voltage, even though at the input of the amp the line voltage is unchanged. This is why some power cords can have an audible effect- and its also measurable. Anytime you can hear it and measure it you can also regard it as real.

Now this says nothing about the cost of the cord. It does not have to be expensive; it simply has to work.

Romex is solid core. If you flex it enough, it will break.

A legal power cord will be multi-stranded which means it can flex without breaking.