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Cable Asylum Interconnects, speaker wire, power cords. Ask the Cable Guys. |
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Cable Asylum Interconnects, speaker wire, power cords. Ask the Cable Guys. |
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In Reply to: Is There A Reasoned Explanation Of Why Power Cords Work? posted by Mister Pig on August 2, 2018 at 10:21:38:
I have to admit, the first time I heard how much difference a power cord could make, it really bothered me. It wasn't like it was subtle.
But its not hard to measure the effects of a power cord and with minimal equipment, you can sort out what's going on. I've already done that. So:
There are two aspects, AC voltage drop and high frequency current limiting.
AC voltage drop is the voltage dropped from the wall to the input of the equipment in use. I've measured a loss of 40 watts on an amp that makes 140 watts, so no-one should be surprised that that might be audible as well. I used a 3 1/2 digit DVM to measure the voltage drop and it showed around 3 volts. This was a pretty standard but inexpensive Belden cord. A more expensive Belden cord with heavier gauge showed a lessor drop and more power out of the amp. So no mystery here.
The second issue is the high frequency current limiting. This is a bit trickier to understand, but its not quite rocket science. Almost any power supply consists of a power transformer, rectifiers and filter capacitors. When the the transformer voltage is higher than the capacitor voltage, the rectifier commutates (a fancy word for turns on and conducts). At that point the filter capacitors can charge up and will do so until the power transformer voltage falls low enough that the rectifiers cut off.
At that point the circuit using the power supply drains the filter caps. Since this happens 60 times a second, the drain is usually not very much at all, so its only at the very peaks of the AC waveform that the caps are be replenished. There might be only a few microseconds or milliseconds that this can happen, and quite a bit of current might have to flow during that time, essentially a high frequency event.
If the power cord limits current during this period, the performance of the circuit using the power supply might suffer, possibly due to increased IMD since the DC might have a bit more of a sawtooth on it than if the current was not limited.
There are some take-aways; if the circuit is heavily regulated, the power cord will make less difference. If the connections at either end of the cord gets warm during operation, you can count on a voltage drop. How much the voltage drop in a power cord affects the audio performance depends on the AC wall voltage and the equipment itself.
A lot of people point out that there is wiring in the walls and from the power company and so on. Of course! But Romex is pretty high performance; if you could legally sell power cords made of Romex they would have excellent performance. But that would pose a fire and shock hazard so power cords are all about how to work with stranded wire.
One way to measure a power cord's performance is to measure the effect it has on the equipment in use. This is how I discovered that 40 watt loss I mentioned. If you have enough time and toys, you can set up a microphone in the room and measure frequency response and distortion rather than just testing the gear on the bench. I have a customer that used this technique to test filter capacitors in the power supplies of his amps.
Anyone who tells you power cords can't make a difference hasn't measured them.
BTW, you can measure fuses too- they too have a voltage drop. There is a persistent tale about how fuses are directional, which is nonsense, but by measuring voltage drops you can see how this tall tale came about since fuses rarely sit perfectly in their holders. But you don't have to reverse them (that produces a random result) you can just rotate them in the holder for lowest voltage drop. A simple DVM is all you need for that...
BTW, the amp in question was a tube amp with a considerable filament circuit.
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