# 8 ohms speaker to 4ohm amplifier ???



## duckymcse (Mar 15, 2009)

I had a 8ohm midrange speaker rated at 100W @ 8ohm and use it with an amplifier rated 100W at 4ohm. 
How much wattage is the speaker getting?
Will running this setup have potential harm to the speaker?


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## basicxj (Jan 1, 2008)

duckymcse said:


> I had a 8ohm midrange speaker rated at 100W @ 8ohm and use it with an amplifier rated 100W at 4ohm.
> How much wattage is the speaker getting?
> Will running this setup have potential harm to the speaker?


It'll see about half that power @ 8 ohms, and no, it won't harm the speaker.


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## t3sn4f2 (Jan 3, 2007)

A speaker's wattage rating is not @ an ohm load. The speaker is the load. The amp is rated @ the load presented to it (ie the speaker).


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## m R g S r (Oct 1, 2009)

don't worry about overpowering the speaker with that amp.


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## Vermithrax (Aug 21, 2007)

basicxj said:


> It'll see about half that power @ 8 ohms,


That's a general assumption. Not all amplifier designs operate the same way. What is the brand and model of the amplifier you have?

Irregardless, if the amp outputs 100W @ 4 ohms and the speaker is 8 ohms and will handle 100Wrms you won't damage it assuming quality non-faulty equipment and proper installation techniques are used.


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## t3sn4f2 (Jan 3, 2007)

Always half or more if the amp is not bridged. Never less.


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## basicxj (Jan 1, 2008)

Vermithrax said:


> That's a general assumption. Not all amplifier designs operate the same way. What is the brand and model of the amplifier you have?
> 
> Irregardless, if the amp outputs 100W @ 4 ohms and the speaker is 8 ohms and will handle 100Wrms you won't damage it assuming quality non-faulty equipment and proper installation techniques are used.


I offered that comment as nothing more (the OP didn't state the amplifier or speakers involved) .


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## duckymcse (Mar 15, 2009)

The amp is McIntosh MCC404 and the speaker is Dynaudio MW142 (3" midrange). I have been playing around with it for a week, so far so good.



Vermithrax said:


> That's a general assumption. Not all amplifier designs operate the same way. What is the brand and model of the amplifier you have?
> 
> Irregardless, if the amp outputs 100W @ 4 ohms and the speaker is 8 ohms and will handle 100Wrms you won't damage it assuming quality non-faulty equipment and proper installation techniques are used.


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## duckymcse (Mar 15, 2009)

So I am getting about 50W min to the speaker, right? 
If a speaker rated 100W at 8ohm, is it true this speaker is rated 50W at 4ohm? I kinda confused now 



t3sn4f2 said:


> Always half or more if the amp is not bridged. Never less.


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## t3sn4f2 (Jan 3, 2007)

duckymcse said:


> So I am getting about 50W min to the speaker, right?
> If a speaker rated 100W at 8ohm, is it true this speaker is rated 50W at 4ohm? I kinda confused now


Yes 50. Likely not more since that it such a good amp.

Again a speaker is rated at 100 watts period. Not into an ohm. A speaker of the same make and model but with a different ohm rating (ie 4 or 8 ohms) has the same power rating. The speaker determines the ohm not the amp.

4 ohm
8 ohm


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## circa40 (Jan 20, 2008)

OP this may be helpful...

http://www.diymobileaudio.com/forum...d-our-members/31-real-deal-8-ohm-drivers.html


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## duckymcse (Mar 15, 2009)

Thanks for clarifying. It does make sense now.
Wish the speaker is 4ohm. It does sound pretty good already. But I can felt it can take more power into it.



t3sn4f2 said:


> Yes 50. Likely not more since that it such a good amp.
> 
> Again a speaker is rated at 100 watts period. Not into an ohm. A speaker of the same make and model but with a different ohm rating (ie 4 or 8 ohms) has the same power rating. The speaker determines the ohm not the amp.
> 
> ...


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## duckymcse (Mar 15, 2009)

Thanks. Good read. Now I know why my amp is much cooler than before.
The MAC amp does get pretty HOT.



circa40 said:


> OP this may be helpful...
> 
> http://www.diymobileaudio.com/forum...d-our-members/31-real-deal-8-ohm-drivers.html


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## Sarthos (Oct 29, 2010)

I explained this recently in a thread where someone asked about running a PDX amp into 8 ohms. Basically there are 3 things that can destroy an amplifier: Voltage in the output section, current in the output section, heat in the output section. 

Now, at 4 ohms and 100 watts W=V*V/R or V=sqrt(W*R)=sqrt(400)=20 volts
I=V/R=20/4=5 amps
So you need to have less than or equal to 20 volts, and less than or equal to 5 amps when you go into 8 ohms. If you gave other values for 2 ohm or 1 ohm power, figure out those currents and those voltages. The highest ones are your limiting factors for any resistance you run in. Luckily when you run into higher resistances, voltage is the limiting factor, so don't run it past 20 volts.
Into 8 ohms, W=V*V/8 so W=20*20/8=400/8=50 watts. That's 20 volts, and only 2.5 amps, both are safe for your amplifier.
For heat in the output section, heat is proportional to the watts lost in the output section, which is equal to watts*resistance of output section divided by (load resistance+output resistance)
So at 4 ohms, heat is proportional to 100*(r)/(4+r)
at 8 ohms, it's proportional to 50*(r)/(8+r)
now, as a ratio of 8 ohms over 4 ohms
(50*r*(4+r))/(100*r*(8+r)= 0.5*(4+r)/(8+r). If you assume r is extremely small, it reduces to 0.5*4/8=0.25. If you assume r is extremely large, it reduces to 0.5*1=0.5. So your amp should generate between 25% at 50% to the heat at 8 ohms and 50 watts as it would at 4 ohms and 100 watts. Generally closer to 30% I believe.


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