We are sometimes asked our opinion of exotic power cords, and have met more than a few people who have laid out some serious investment to have what they see as the very best sort of cord running from the wall socket to the IEC socket on the back of a power amp or CD player. At our company, Blue Jeans Cable, we don’t sell power cords, and apart from the possibility that one of these days we may sell some short-length IEC cords for people who like to have tidy rack installations, it’s not likely that we ever will. Why? Well, read on to find out.
The Signal Chain
All of the audio and video cables we sell have one thing in common, whether they are made for audio or for video, digital or analog: they’re all in the signal chain, carrying actual audio and video signals from device to device. Too much is often made of the impact of cable attributes upon video and audio quality, but it’s fair to say that any cable in the signal chain, carrying signals, has at least the potential to have an impact upon the quality of your system’s audio or video output.
Power cables are a different matter. At no point in use do they carry signals–they simply convey power from your home’s main supply to your devices’ power supplies. This is important, to be sure, and without a power cable your system isn’t going to do very much at all, but the question of course is not whether the cable is essential but whether there are differences in performance between reasonably-well-constructed power cords, and on that question we have to say that the answer is simply “NO.”
What Matters in a Power Cable?
You’ll note that we said “reasonably well-constructed” power cords, and there is a reason behind that. It is possible, under some circumstances, to cause a system to behave suboptimally by using a poorly-designed power cable, but the main consideration at work here is simply wire gauge, and in such a circumstance all one really needs to do is increase the gauge of the cable. So, while a power cord upgrade in such a situation may indeed be advisable and helpful, it’s likely to be a five or ten dollar solution, not a five-hundred-dollar or higher solution.
To see how gauge can be important, let’s talk about what a power cable does, and about what the power supply it feeds does.
Most circuits in use in consumer electronics run on Direct Current (DC) voltages, which can run anywhere from a couple of volts up to, in the case of tube equipment, hundreds of volts. Ideally, these DC circuits should be fed a constant, clean, level voltage. What comes down the street, through your house’s service entrance, and to your wall socket, however, is not DC but AC, Alternating Current. AC power supplied to your home changes its voltage dramatically from moment to moment, and ideally, presents a sine wave which cycles from zero, to negative, through zero to positive, and then back to zero, sixty times a second in the USA and 50 times a second in most other countries. Incidentally, although household current in the US is fed at what we call “110 volts,” that’s the RMS voltage; the voltage actually cycles from about 170 volts positive to 170 volts negative.
Quite obviously, DC circuits, with their demand for clean, stable voltage, cannot be powered just by plugging them in to the AC power that comes to your wall socket. Accordingly, devices which need DC get it through a built-in power supply which accepts the AC from the wall and converts it to the desired voltage or voltages. The classic design for a power supply, and still the dominant one, consists of a transformer, a rectifier, and a filter; many modern supplies add to this a voltage regulator.
Power comes in through the socket and enters the transformer, which is typically an iron core with coils of copper wire wound tightly around it. One rather odd thing about a transformer is that, to look at how it’s made, one would think that the power couldn’t get out of the wall and into the circuits, because the wire that runs the power from the wall into the transformer simply makes a series of turns around the transformer core and then leaves for the wall, with no interconnection to the other wires on the transformer core. Thanks to the property of inductance, however, what is actually happening is that energy running through this winding of the transformer induces a field around the other windings, and causes current to flow in them. The result is that the AC from the wall is turned into AC of whatever voltage or voltages are required.
Now, this is still AC, and unsuitable for powering DC circuits, but we’ve stepped the voltage up or down to the level needed. The next step is to run it through a rectifier, which typically is a set of diodes, which act as one-way valves for electrical flow. By the right configuration of diodes, e.g., what is commonly known as a “bridge rectifier,” one can switch the connections around in such a way as to “flip” the negative voltages in each cycle and turn them into a positive voltage, or vice versa, as required. Instead of a voltage which fluctuates from negative through zero to positive and so on, we now have a voltage which fluctuates from positive to zero, then zero to positive, and so on.
The problem with rectified AC is that our DC circuits still won’t work well on it, because they are designed to take an input of a constant voltage, not a continually fluctuating one, and audio circuits in particular will sound awful if we feed them a power supply that carries a loud, low buzz like this. Somehow we’ve got to smooth these fluctuations out, and this is where filtration comes in. A power supply will typically have a filter capacitor, which allows power to store up during peak voltages and be drawn down when the supply voltage drops, and may also have chokes, which are large, heavy coils whose inductance discourages the rate of current flow from fluctuating. To this may be added a voltage regulator, which typically is a solid-state device designed to prevent the voltage from exceeding a certain value. Assuming that the design of the power supply has been done right, by this time we have taken the nasty up-and-down, humming wall current and turned it into nice, usable, clean, quiet DC at whatever voltage, positive or negative, our circuits require.
When Power Cords Go Bad
There’s not a lot that can go wrong with a power cord, but there’s one thing in particular which can be of importance. An undersized power cord (that is, a power cord of inadequate gauge for the current running through it) can be dangerous, because it can overheat and start a fire. Apart from being dangerous to life and limb, interestingly, an undersized power cord can, under just the right sort of conditions, have an adverse impact upon audio quality as well.
We ordinarily think of the voltage which appears at the input of a power supply as being the same as the voltage supplied to the home, but this is not quite actually the case. Unless your home is wired with superconductors, every foot of wire along the way presents a bit of resistance, which results in some of the current which flows in that wire being turned into heat. The circuit really begins with this wire “resistor” on the live line, then the load of the device’s power supply, and then another wire resistor on the neutral line back to your service panel. For most loads, these resistances in the wire are insubstantial and can be ignored. However, in accordance with Ohm’s law, the energy traveling in the circuit is consumed by the circuit’s elements in proportion to their resistance; a high-impedance, low-power load such as a cell-phone charger consumes almost all of the small amount of power flowing in the circuit, but a low-impedance, high-power load like a space heater, by virtue of its low internal resistance, causes a higher proportion of the power in that circuit to burn up in the wires that feed it. When that happens, another property of the circuit varies: the voltage across those power supply input terminals. If the supply voltage is 110 volts, and the device presents a resistance of, say, ten ohms while the wire presents a resistance of one ohm, one eleventh of the power supplied is burning up in those supply wires and the voltage across the power supply terminals will be not 110 volts, but 100 volts. As that voltage drops, the voltages coming out of the other end of the power supply transformer drop proportionally, and this sort of thing can cause some trouble.
The power supply of an amplifier relies upon the filter capacitor to hold enough charge to provide the desired voltage at the power supply output when the incoming rectified voltage is at a low point, and so the capacitor is drawn down when the rectified voltage is low and charged up when it is high. Some things can draw that power faster than others, and heavy bass is one of those things. Let’s say that we have a badly undersized power cord, and are playing bass-heavy music on a power amp. The bass draws down the filter capacitor, and the power supply draws current to try to replenish that charge; however, the more current the supply demands, the lower its impedance, and with an undersized power cord, this means that the supply voltage drops. If the voltage drop is strong enough, the filter capacitor will not fully charge up on the next cycle of high voltage, and this means a reduced supply voltage to the amplifier circuits. This phenomenon will occur in direct relationship to how much of this loud bass is being produced, and what does that mean? The bass note is now modulating the entire output of the amplifier which can in turn increase intermodulation distortion.
Now, this is an unusual scenario. There are any number of reasons why, in any particular system, it may not happen. The filter capacitor may be amply sized; the power cord may be of perfectly adequate gauge; the power supply may be delivering a slightly high voltage into a regulator anyhow and so the voltage fluctuations are stopped at the regulator. But if the factors are all right, it is indeed possible for a power cord to have a negative impact upon sound quality.
But what would fix this? The answer, of course, is simply that a power cord of adequately large gauge would fix it, if anything will. If that won’t fix it, the problem is in the power supply design and no power cord, of any sort, is going to help. No exotic design, full of silver conductors, braided wires, shielding, Teflon, or what-have-you, is going to help resolve this sort of problem any better than any other cord of similar gauge because the one and only problem is that the resistance of the cord is too high.