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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: This might explain why. posted by Granholm on October 4, 2001 at 01:04:43:
"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."Actually the two are related. Temperature rise is due to resistive heating, and resistance determines voltage drop. Same thing in the end.
"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 Has Ohm's Law been repealed?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, but when compared to normal variations on an AC power supply rated to be within some nominal variation (10& or so), even drops due to the other effects mentioned are of low consequence.
"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. "
He is talking about DC on caps in the unregulated part of the power supply. Any decent design will allow enough voltage leeway that the regulator can still achieve the required output. Also, if you put a resistance between a voltage source and a capacitor, it will still charge to the same voltage. All you effect is the time to charge.
"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. "
The external effects are much less than the transformer in the equipment itself, and all may be isolated by appropriate filtering and regulation in a decent power supply.
"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."
Yes, but these 'distortions' and 'hash' are always present and are generated by much worse things than mentioned above. You are running all sorts of things in your house that are much worse at causing it.
"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. "
Not when you take it all into account. Plus if the voltage drop is considered as a percentage (say 5% for the sake of argument) you reduce it from 5 to 2%, which doesn't sound all the impressive, especially when he already acknowledged that the cable is only a small part of the story.
"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,"
Reduce interference? Maybe... but how does it raise power due to this shielding?
Enough for now, the rest does little more to convince me. In fact, some of things mentioned, such as conductive fluid jacketed cables and the claims for them make me shake my head in wonder.
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- thoughts on this - preamp 16:09:06 10/04/01 (0)
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