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Question about ohms
#342521 03/18/11 07:28 PM
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Hello,

I am currently taking an introductory course into principles of electronics and speaker ohms has me a little confused.

I have learned from class that the greater the ohm rating, the greater the resistance. So it seems a little counter intuitive to me that the M80s, with 4 ohms, would be harder to drive than say an M60 with an 8 ohm rating. I tried to ask my instructor about it and he really did not have an answer to that, either.

Thanks


Speakers: Energy RC-70s, RC-LCR, RC-Rs and eD A2-300 x 2.
Receiver: Marantz 6006
Re: Question about ohms
Seekinganswers #342522 03/18/11 07:30 PM
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It's not harder to drive, it's more demanding on the amplifier, which will have more current flowing through it, because of the lower impedance. More current equals more heat and more demand on the power supply, which equals more chance of clipping or shut down.

Last edited by ClubNeon; 03/18/11 07:31 PM.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris
Re: Question about ohms
ClubNeon #342523 03/18/11 07:34 PM
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Ahh, that makes so much sense! Haha, I wish I would have thought of that before.

So basically, a lot more current will be running through the speaker, thus requiring more power from an amp in order to avoid overheating and clipping or shutting off?


Speakers: Energy RC-70s, RC-LCR, RC-Rs and eD A2-300 x 2.
Receiver: Marantz 6006
Re: Question about ohms
Seekinganswers #342525 03/18/11 07:37 PM
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The speaker allows current to flow through it more easily.

It just requires an amp that is designed to be able to handle that extra power moving through it. A larger power supply is the start, and a more robust output stage that can remain stable, along with better cooling to remove the heat.


Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris
Re: Question about ohms
Seekinganswers #342527 03/18/11 07:44 PM
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Originally Posted By: Seekinganswers
Hello,

I am currently taking an introductory course into principles of electronics and speaker ohms has me a little confused.

I have learned from class that the greater the ohm rating, the greater the resistance. So it seems a little counter intuitive to me that the M80s, with 4 ohms, would be harder to drive than say an M60 with an 8 ohm rating. I tried to ask my instructor about it and he really did not have an answer to that, either.

Thanks


Yeah, your way of thinking is correct as the advantage of a 4ohm rated speaker would be that it presents less resistance to the current flow which will allow the speaker to pull more power from the amp/receiver. However, because of this the amplifier must be designed to handle this greater flow of current. Therefore, with low impedance speakers (impedance varies and it is the dips/phase angle interaction to worry about) can sometimes cause the amplifier or receiver to overheat and/or go into current limiting when pushed hard. In either case will cause the electronics to shutdown or in the worst possible case, permanent damage.


I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne
Re: Question about ohms
BlueJays1 #342532 03/18/11 08:00 PM
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in many ways, it's like an electric light bulb;
one that has a filament with high resistance in it will not be as hot and bright and will not consume as much electricity as one that has a low resistance filament in it.

lower resistance in a wire (or a voice coil) will let pass more current.
low resistance/impedance = low Ohms
if the resistance is low, then current going through is not impeded as much as when the resistance is high.

Re: Question about ohms
BlueJays1 #342533 03/18/11 08:01 PM
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Since this is for a class lets get the terms right.

When talking about alternating current (like audio signals) Ohms measures impedance, not resistance. Resistance only applies with the phase angle is constant at 0 degrees, i.e. DC.

And speakers don't "pull" anything. They are simply there to impede the flow of current from one pole of the output to the other. If the speaker wasn't there, and speaker wire was made into a dead short (milliohms) the amp would self destruct (or more likely go into protect) because the current flow would be so high.


Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris
Re: Question about ohms
ClubNeon #342545 03/18/11 10:09 PM
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You got it.

And you got the resistance vs impedance. Although we use ohms to quantify both, resistance implies that the voltage and current are always in phase (0 degrees, like for a resistor) while impedance implies that they are out of phase. When you plot impedance then on an XY graph, it'll have a "real" part and an "imaginary" part (implying phase). The magnitude of that vector is measured in ohms.

Since you're in class, another way to think of the lower speaker impedance being more difficult to drive is to consider power. P=V^2/R. So if we assume that the voltage is the same (volume knob in the same spot), when the "R" gets smaller, the power goes up.

Cary

Re: Question about ohms
Cary #342555 03/19/11 12:12 AM
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and P=RI2 so as the power goes up so does the current. Back to school...


Bruno
M80s/VP180/QS8s/EP600/AVR-890
------------------------------------
"The problem is choice..."
Re: Question about ohms
Seekinganswers #342568 03/19/11 02:07 AM
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Seek, this is yet another of the things widely known which happen not to be true. Believe it was Mark Twain who said "It ain't the things that we don't know that get us into trouble, it's the things we know for sure that ain't true".

As Chris and others have pointed out, the lower impedance hinders the input voltage less and the result is that for a given voltage the current is higher. This is because of Ohm's Law, which I'm sure that your instructor is familiar with, even he's not much into audio technology. One form is I=E/R(current equals voltage divided by resistance[impedance for AC]), which shows the effect of the lower impedance. Since another iteration of Ohm's Law is that power equals voltage times current, the effect of the higher current is also higher power for a given input voltage from the amplifier.

So, in that sense lower impedance makes it easier to realize power in the speaker, but the higher current possibly could cause overheating in the amplifier and its protective circuits would shut it down. However, there's no brick wall suddenly created by impedance lower than some magic number, as some seem to imagine. It's entirely possible for some speakers rated at 4 ohms(e.g. the M80)to be more sensitive than some speakers rated at 8 ohms, to require less power for a given sound level, and to be less likely to cause amplifier shutdown.

Let us know what your instructor thinks about this.


-----------------------------------

Enjoy the music, not the equipment.


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