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The usual scientific explanation

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AC Power Cord Effects

So, how in the world can a power cord make any difference, if it is properly rated for the current draw, and is UL listed?
Like many questions about audio systems, this one seems very logical and reasonable. Just use Ohm's Law to calculate the current draw, and viola, we have the voltage drop, and we have the cold hard facts, Yes?

No.

Like many overly simplistic answers, this one does not take into account all the facts. Let's look at a hypothetical power amp to start. Lets say it is a moderately high power design, and is rated to draw about 8 amps from the wall at full rated power. That's about 960 watts from the wall. We won't even get into side issues of switching power supplies, which play total havoc with the simple Ohm's Law approach, we will stick to linear power supplies.

So if the AC cord is UL listed for 10 amps, it is likely an 18 gauge cord. Many 18 gauge power cords are rated for 10 amps of current. How is this rating determined? By how hot the cord gets while carrying the rated current, NOT how much voltage drop there is.

In our hypothetical amplifier, the RMS voltage drop in the 6 foot cord would be approx. 1/3 of a volt according to Ohm's Law. This does not take into account the wall outlet contact, or the IEC connectors if present, nor does it take into account any other factors, just the resistance of the 18 gauge wires in the AC cord.

On the face of it, this seems quite harmless. How could a third of a volt make any difference? Well, because the voltage drop is NOT 1/3 of a volt! Has Ohm's Law been repealed? Are engineers all insane?

No, just an overly simplified analysis that failed to take into account ALL the factors. So what could possibly make that much difference. Well, for one, it would help to know that linear power supplies refresh their DC reservior from the AC line in bursts of current, current peaks that are in time with the peaks of the AC line. That is when the filter capacitors are refilled, when the AC line voltage its it's peak value, and the output from the secondary reaches a level above that which the PS capacitors have been drained to. Instead of a nice steady drain of 8 amps, we have current peaks that
last only for a brief moment, and the 8 amps is an RMS amount.

What this means is that because the current is being drawn only for a fraction of the AC voltage cycle, or for about 1 thousandth of a second (one half cycle lasts for 0.0083 seconds). The peak currents can easily reach ten times the RMS value. What is the voltage drop for 80 amps? At this point, the resistance of the AC plug contacts, and even the bond inside the cord between the plugs and the internal wires become a significant factor. It is not hard for the voltage drop to reach 5 volts or more. The AC line peaks never reach the full value, because the line cord has lost some of it.
Power amps depend on receiving the full measure of AC line for their full rated power, so such a drop will reduce the actual output power in a seemingly disproportionate amount. A 200 W amp may be reduced to 170 watts before clipping.

The above (still simplified) analysis assumes a steady signal, and a steady current draw. Musaical dynamics make it a much less consistent thing, and the dynamic demands will cause dynamic perturbations.

This is not the only effect on the power amp. These current peaks can easily cause AC line distortion, and the heavy current draw can generate harmonics on the line, the hash from the rectifier diodes can increase, a whole series of events occurs that are not immediately obvious just by thinking of the amp as a simple resistor and using Ohm's law.

Most house wiring is either 12 gauge or 14 gauge, while many OEM cords are 18 or at best, 16 gauge. Most OEM cords do not have shielding or any provision for reducing radiated EM fields, do not have premium AC outlet plugs or premium IEC plugs for better electrical contact at these junctions.

So what happens with a bigger power cord? Replace that 18 gauge cord with a 14 gauge cord, and the voltage drop will go down by a factor of about 2 and a half.

Include shielding in that cord, and the possiblity of radiated EMI/RFI goes down compared to an unshielded cord.
Shielding and radiated EM fields might not seem relevant since the house wiring has neither advantage. However, the house wiring is not laying right next to the other component's AC line cords, or right next to the line level or speaker interconnects. Reduced levels of induced RFI and radiated EMI/hum fields would not hurt SOTA sound reproduction.

Since the AC power cord is usually laying right in there with all the other AC cords, and probably the speaker cables, and the interconnects (some people even bundle them all together for neatness, OUCH!), it is quite possible that a premium AC cord will help reduce interference in the system, and raise the amount of power available before clipping,
and smooth any AC line distortions, etc.

This is all without even going into secondary effects, or other more esoteric aspects. Just a more nearly correct way of applying Ohm's Law to the real situation. Add in ferrite filters, built-in filter components, shielding effects, and the esoteric aspects, and it should be obvious that AC cords are not at all simple, nor are they a no-brainer.

There is the issue of resonant situations. Certain power cords and power supply transformer primaries might tend to resonate at RF or high frequencies. This resonance might make RFI/EMI problems worse, changing to a different cord will change the resonant frequency, and change the RFI/EMI effects.

The plugs are not a trivial issue either, and may be more responsible for sonic improvements than the other factors.
Hubbel and other premium plugs and sockets will increase contact area and pressure, reducing contact resistance and other contact related problems. It has been claimed that poor AC plug contact can cause micro-arcing, with it's attendant hash being injected directly into the audio component.

It is not as simple as just simple wire resistance. The connection at each end of the cord adds resistance, the wall outlet socket adds resistance, etc.
For the raw wire, round trip:
12 gauge, approx. 6 feet = 0.0206 ohms
14 gauge, approx. 6 feet = 0.0328 ohms
standard AC power cord
18 gauge, approx. 6 feet = 0.0830 ohms

Measuring real AC power cords, I get around 0.128 ohms for an IEC 18 gauge power cord, and about 0.022 ohms for a 12 gauge IEC cord, not including the AC wall socket connection.

Why is the 18 gauge resistance so much higher than just the wire resistance? Ever tear one of those cheap cords apart? Poorly crimped or barely soldered connections are responsible for the bulk of the extra resistance.

Preamps and CD players all have their special requirements: CD players require shielding to help keep the digital hash that back-feeds from the circuitry out of the rest of the equipment, preamps need a nice steady voltage for minimum noise, and freedom from RFI, etc.

Fancy AC power cord geometries might also reduce the inductance of the 6-8 feet of line cord, raising the available voltages. but this would be limited to the ratio between the length of the power cord vs. the wall run. Such geometries often reduce the radiated energy, and aid shielding of the cord.

Some power cords might have a built-in filtering action, like the water jacketed ones, that have the conductors and insulation surrounded by a conductive fluid. This fluid might short out and reduce/damp any EM fields the cord would conduct to the component besides the 60 Hz AC power signal.

It also helps to keep in mind that we are not supplying a Sears rack system, that any system which aspires toward the SOTA is going to be more sensitive to minute effects and minute improvements. How much does a big fat shielded power cord help things? Probably about as much as upgrading from an OEM interconnect or zip cord speaker cable to
some decent aftermarket cables, some systems are more sensitive to AC cords, some are less sensitive to cords.

As always, the bottom line is: you have to listen for yourself, and see if their is any benefit for you, on your system, with your listening habits.

Do AC cords have the potential to influence high end sound? Yes. Does anyone who believes this also believe that high end cord costs are justified? No.

In my DIY AC Cord note, I recommend an $11 shielded heavy duty Belden (now Volex) cord, not a thousand dollar high end cord.

See my DIY AC cord note at:
http://members.nbci.com/Jon_Risch/ac-cords.htm

Some of the cord/cable manufacturers get carried away with using only the very finest materials and assembly techniques, carrying over the technology and costs from their high end audio interconnects and speaker cables.

Is this necessary? I don't know, I will not discount it out of hand. Does it make the cords cost a lot? Yes.

Does this make them dishonest or imply that they are deliberately trying bilk the customer? Not at all.

Jon Risch


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