# ohms/resistance

noob
Copper - Posts: 92
Joined: June 13, 2003
Location: United States
Posted: June 14, 2003 at 11:40 PM / IP Logged
can someone explain resistance to me......in its entirety......or at least link me to a nice site that can explain it well......any help is appreciated
Sound Pressure
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Posted: June 15, 2003 at 1:11 AM / IP Logged
https://www.the12volt.com/ohm/ohmslaw.asp
Sound Pressure
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wvsquirrel
Gold - Posts: 1,237
Joined: July 27, 2002
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Posted: June 15, 2003 at 2:24 PM / IP Logged
Put simply, less ohms = less resistance = more power and increased efficiency.
Here's an example,
2 amplifiers, one rated at 400 x 1 @ 2ohms and the other at 400 x 1 @ 4ohms. Both amplifiers will produce 400 watts (given the proper conditions), but the amplifier running at 2ohms will be running more efficiently because the resistance is less then the amp running at 4ohms.
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the12volt
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Posted: June 15, 2003 at 3:42 PM / IP Logged

wvsquirrel wrote:
 Put simply, less ohms = less resistance = more power and increased efficiency. Here's an example, 2 amplifiers, one rated at 400 x 1 @ 2ohms and the other at 400 x 1 @ 4ohms. Both amplifiers will produce 400 watts (given the proper conditions), but the amplifier running at 2ohms will be running more efficiently because the resistance is less then the amp running at 4ohms.

If two amplifiers have a maximum sustained current draw of, for example, 30 amps at maximum output and the first amplifier puts out 400 watts per channel and the second puts out 200 watts per channel, both x 2 @ 4 ohms, the first amplifier would be more efficient than the second. Now if it was the same amplifier with, for example, ratings of 400W x 1 @ 2 ohms and 250W x 1 @ 4 ohms, the second rating would be more effiecient, but you are comparing two different amplifiers and those factors alone (wattage & nominal speaker impedance) can not determine which amplifier is more efficient, so less resistance does not necessarily equate to increased efficienty, however it can equate to more power. So if the two amplifiers in your example had the same maximum sustained current draw, which one would be more efficient?

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wvsquirrel
Gold - Posts: 1,237
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Posted: June 16, 2003 at 10:21 AM / IP Logged
Ok, I can see how you said with two different amps, both using the same current draw and the same speaker impedance, the one putting out 400 watts would be more efficient then the one putting out 200 watts. That part I understand
But I'm a little confused about what you're saying when comparing the same amp. I guess I thought an amp running at 2ohms would be running more efficiently than the same amp running at 4ohms with less output wattage (given the same factors: power, current draw, input sensitivity, etc).
I'm even more confused because when dealing with the same amp, all the threads I've read have always leaned towards an amp running at 2ohms with more output would be running more efficiently then the same amp running at 4ohms with less output (or maybe I'm just confusing the way the circuitry works between a class AB and a class D?).
I know this is going to sound wierd so I'll try to word it as correctly as possible...
I always thought that an amp was able to produce more power at 2ohms (say 400 x 1 @ 2ohms) than at 4ohms (say 200 x 1 @ 4ohms) because at 4ohms there is more resistance and the amp has to work twice as hard to produce the output (which would explain why most manufacturer's specs have the 4ohm rating at about half of the 2ohm rating for a mono amp). So that said to me that if an amp only has to work half as hard at 2ohms to produce more power then at 4ohms, then it would be operating more efficiently at 2ohms then at 4ohms. I'm sure there's something very simple that I'm missing that has my thinking in reverse of yours when dealing with the same amp, I just don't know what it is!
After all that, I think I just figured part of it out (which also leads me to your last question).
An amp running at 400 x 1 @ 4ohms would probably have a 2ohm rating of about 800 x 1 @ 2ohms. And based on your first example of comparing 2 different amps at the same impedance, the amp running at 800 x 1 @ 2ohms (or 400 x 1 @ 4ohms) would be more efficient then the amp running at 400 x 1 @ 2ohms. Or am I still thinking backwards?
Squirrel
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the12volt
Administrator - Posts: 3,955
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Posted: June 16, 2003 at 10:53 AM / IP Logged

My point is that efficiency of an amplifier and power output of an amplifier are not the same. Yes, an amplifier will produce more power at a lower resistance, however it doesn't mean that the amplifier is more efficient at a lower resistance. In some cases, current draw may be more than twice as much at 1/2 the resistance and the loss to heat may also be more than twice as much, making it less efficient while producing more power. Efficiency with an amplifier would relate more to the ratio of power output to current draw, and power output to power lost to heat.

With regards to my question, both would be equally efficient, but yes the second amplifier, in theory, would be capable of producing twice as much power if it were stable at 2 ohms mono :)

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wvsquirrel
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Posted: June 16, 2003 at 3:06 PM / IP Logged
Thanks for the clarifications, it helps alot!
Squirrel
"No more Cpt. Kirk chit chat"
If its too loud, then you're too old
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the12volt
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Posted: June 16, 2003 at 3:53 PM / IP Logged
You're welcome ;)
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