# Advantages to running at higher/lower ohm loads?



## Mazda3SQ (Nov 11, 2006)

I'm in the process of planning out the next system for my next vehicle and had a question in regards to amp choice in relation to ohm rating. Right now I intend to buy a dual 4 ohm sub, question is what advantages/disadvantages would I have in purchasing a pdx 1000.1 and running it at 8 ohms (about 500 watts i'd imagine) verses buying a 600.1 and running it at 2 ohms? Subwoofer choice is not relevant to this discussion other than the fact that the few I have in mind all have an rms right around 500 watts and will be run IB in the rear deck. I'm aware of the power limitations inherent in running IB but as always a little bit of headroom is never a bad thing.


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## Oliver (Jun 25, 2007)

Higher ohms = easier on electrical system


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## Mazda3SQ (Nov 11, 2006)

What about in terms of signal clarity? Is a 2 ohm load as "clean" as an 8 ohm load? I understand that for all intensive purposes it'll be negligable with a sub anyway but just for my own knowledge if I ever run some dvc mids or something.


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## Oliver (Jun 25, 2007)

jury never came back in on that one :blush:


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## MadMaxSE-L (Oct 19, 2006)

Mazda3SQ said:


> What about in terms of signal clarity? Is a 2 ohm load as "clean" as an 8 ohm load? I understand that for all intensive purposes it'll be negligable with a sub anyway but just for my own knowledge if I ever run some dvc mids or something.


Seeing that most amplifiers are rated (for whatever that's worth ) to have higher distortion numbers at numerically lower ohm loads...

-Matt


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## phatredpt (Feb 22, 2006)

Running a dual 4 at 8 ohms (Alchemy RX) on a Blau Velocity 2200 (should be about 400 @8 bridged) and I am loving it! 
Amp stays cool... "seems" pretty darn clean... 
I had the same concerns before I did it


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## Oliver (Jun 25, 2007)

The vast majority of my subs are dual 4 ohm [ wired in series for an 8 ohm load as seen by the amplifier ]


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## drake78 (May 27, 2007)

a$$hole said:


> Higher ohms = easier on electrical system


X2, You amp will will run cooler with higher impedances(=longer amp life). In relevance to power maximization should be more the majority of the concern. On paper amps have lower thd in higher impedance loads. The difference in thd is not audible to the human ear. Unless the thd is increased to around 10%.


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## thylantyr (Jan 21, 2008)

Assuming amplifier efficiency is the same.


Which one is better.

1. You draw 500w from the car's electrical system to drive
an 8 ohm speaker.

2. You draw 500w from the car's electrical system to drive
an 2 ohm speaker.


Second question.

What weighs more.

1. one pound of feathers
2. one pound of gold


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## Mazda3SQ (Nov 11, 2006)

thylantyr said:


> Assuming amplifier efficiency is the same.
> 
> 
> Which one is better.
> ...


Wouldn't you be drawing amperes off the car's load rather than watts?...And if I'm not mistaken amp draw at different ohm loads is different...correct me if I'm wrong.


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## thylantyr (Jan 21, 2008)

Mazda3SQ said:


> Wouldn't you be drawing amperes off the car's load rather than watts?...And if I'm not mistaken amp draw at different ohm loads is different...correct me if I'm wrong.


If the car battery is 12 volts and you draw 41.66 amperes,
that is 500 watts. [12v * 41.66A]

What if the amp draws less than 41.66 amperes ?
What happens?

Lets say it draws 20A.

12v * 20A = 240 watts. <<----

Lets say you draw 100A.

12v * 100A = 1200 watts.


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## Mazda3SQ (Nov 11, 2006)

thylantyr said:


> If the car battery is 12 volts and you draw 41.66 amperes,
> that is 500 watts. [12v * 41.66A]
> 
> What if the amp draws less than 41.66 amperes ?
> ...


That is all assuming a constant load...for example take the tutorial on JL's website relating to gain setting. For any given amp, lets use for example the 1000/1, the power that is put out by the amp at different ohm loads is different (goes from 38.7 [email protected] ohms to 63.2 [email protected] ohms) Now these are all at a constant 1000 watts. Maybe I'm missing something in the translation from amps to volts...


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## phatredpt (Feb 22, 2006)

thylantyr said:


> Assuming amplifier efficiency is the same.
> 
> 
> Which one is better.
> ...


That one is easy... choice #1
While your arguement looks good on paper the reality of the situation is that MOST of the time you will not use that 500 watts... the answer lies in your first statement.. assuming amplifier efficiency is the same.
An amp loaded down with a 2 ohm load is going to run hotter than one breezing along into an 8 ohm load. (higher rail voltage, lower current on the outputs)
Since we know that heat is wasted energy...  
Aside from the other advantages already listed of longer life, stable, damping factor (I know it doesn't mean anything)
So In real world application I still feel the higher impedance is the better choice...

Oh and the Gold weighs the same, worth ALOT more, but isn't nearly as comfortable if you have to sleep on it!


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## phatredpt (Feb 22, 2006)

Mazda3SQ said:


> That is all assuming a constant load...for example take the tutorial on JL's website relating to gain setting. For any given amp, lets use for example the 1000/1, the power that is put out by the amp at different ohm loads is different (goes from 38.7 [email protected] ohms to 63.2 [email protected] ohms) Now these are all at a constant 1000 watts. Maybe I'm missing something in the translation from amps to volts...


The voltage listed is on the outputs....
The current required from the 12 volt supply is the same
except for the amount wasted as heat!


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## Dillyyo (Feb 15, 2008)

Mazda3SQ said:


> I'm in the process of planning out the next system for my next vehicle and had a question in regards to amp choice in relation to ohm rating. Right now I intend to buy a dual 4 ohm sub, question is what advantages/disadvantages would I have in purchasing a pdx 1000.1 and running it at 8 ohms (about 500 watts i'd imagine) verses buying a 600.1 and running it at 2 ohms? Subwoofer choice is not relevant to this discussion other than the fact that the few I have in mind all have an rms right around 500 watts and will be run IB in the rear deck. I'm aware of the power limitations inherent in running IB but as always a little bit of headroom is never a bad thing.


The disadvantage for the higher impedance load is a waste of power. With subs, you can spend less money and optimally push your subs, without to much worry in harmonic distortion. If you think you are going to hear the difference between .05% distortion and .1% distortion, in your sub frequencies, I have a bridge to sell you. Only advantage that I see, with subs, is cooler running amps, but any well designed 2 ohm amp, should have no problem. Especially if its a class d.


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## Mazda3SQ (Nov 11, 2006)

phatredpt said:


> The voltage listed is on the outputs....
> The current required from the 12 volt supply is the same
> except for the amount wasted as heat!


Gotcha, thanks for clearing that up.


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## thylantyr (Jan 21, 2008)

Mazda3SQ said:


> That is all assuming a constant load...for example take the tutorial on JL's website relating to gain setting. For any given amp, lets use for example the 1000/1, the power that is put out by the amp at different ohm loads is different (goes from 38.7 [email protected] ohms to 63.2 [email protected] ohms) Now these are all at a constant 1000 watts. Maybe I'm missing something in the translation from amps to volts...


The JL outputs 1000 watts regardless of the impedance
is 1.5 ohm up to 4 ohms.

The stress on the electrical is the same regardless of 1.5 ohm or 4 ohm assuming the same amplifier efficiency.


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## MadMaxSE-L (Oct 19, 2006)

thylantyr said:


> Assuming amplifier efficiency is the same.
> 
> 
> Which one is better.
> ...


Except that doesn't aplifier efficiency will decrease as you impose more load onto it, correct? 

The whole point here is that you won't need to drive the 8ohm speaker with 500w (if you had to drive the 2 ohm'er at 500w) to get as loud as the 2 ohm speaker, right?

-Matt


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## thylantyr (Jan 21, 2008)

phatredpt said:


> An amp loaded down with a 2 ohm load is going to run hotter than one breezing along into an 8 ohm load. (higher rail voltage, lower current on the outputs)


For fun, lets go with what you said.
//////////////////////////////////////////////////////////////
simplified;

Generic 100w @ 2 ohm single channel amplifier.
* 15 volt rms [21.2v rail]
* 7.5 amperes rms

5v drop across the transistor = 25w

Generic 100w @ 8 ohm single channel amplifier.
* 30 volt rms [42.5v rail]
* 3.75 amperes rms
//////////////////////////////////////////////////////////////

This is a simplified examples of one amp 100w @ 2 ohm
vs. another amp 100w @ 8 ohm. Rail voltage is different
and current draw is different.

It is true, if the output stage isn't designed to pass the
appropriate current we have no amplifier. But lets assume
we chose the correct transistor for the job. You can estimate the heat produced by the transistor in both
scenarios. This is an important consideration when designing the amplifier. The goal is to have a transistor
that doesn't heat up, if it does, we need to heatsink it
properly.



> Generic 100w @ 2 ohm single channel amplifier.
> * 15 volt rms [21.2v rail]
> * 7.5 amperes rms


The voltage across the single transistor could be approx
21.2v - 15v rms = 6.2v

6.2v drop across the transistor generates this much heat:
6.2v * 7.5 amperes = ~ *46 watts*




> Generic 100w @ 8 ohm single channel amplifier.
> * 30 volt rms [42.5v rail]
> * 3.75 amperes rms


The voltage across the single transistor could be approx
42.5v - 30v rms = 12.5v

12.5v drop across the transistor generates this much heat:
12.5v * 3.75 amperes = ~ *46 watts*

The transistor seems to dissipate the same heat
in both scenarios, 46 watts.

This is just a simple exercise.


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## MadMaxSE-L (Oct 19, 2006)

Autiophile said:


> That is entirely dependent upon sensitivity of the drivers in question.


The same exact driver; won't the 2 ohm version be less efficient than the 8 ohm - therefore not getting as loud on the same input power as the 8? 

Meaning; you don't need as much input power with the 8 ohm speaker...


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## BEAVER (May 26, 2007)

Don't the sensitivity ratings offset, more often than not?

Meaning... When looking at the same exact driver if the 4ohm version is rated at 90dB, the 8ohm is 87dB... in theory the same volume with the same power.


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## MadMaxSE-L (Oct 19, 2006)

Autiophile said:


> The different versions would have different sensitivities. If there is any difference in efficiency is captured in the sensitivity specification of the drivers.
> 
> In practice it is true that 8ohm drivers tend to be have higher sensitivity, but not just because they have a higher resistance. The higher resistance is a result of the differences in the coil which impact the sensitivity. If you tossed a resistor onto the terminal of a 2 ohm speaker (without any coil changes) to make it an 8 ohm load, you wouldn't change the sensitivity.


Correct, at least I'm thinking straight 

-Matt


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## thylantyr (Jan 21, 2008)

*Except that doesn't aplifier efficiency will decrease as you impose more load onto it, correct? *

I think you have to blame the specific amplifier design
for this and not blame the concept of low impedance.

For example. I won't blame the idea that 2 ohms is worse
than 8 ohms.

First, I buy a true class A 100w amplifier to drive the woofer. It's consuming 1000 watts of power to give me
100 watts @ 8 ohms.

Later I realize this amplifier design sucks and I found
a class D. I can get 100w to my 2 ohm speaker and 
consume far less.

The idea of using 2 ohms wasn't a bad idea. I just didn't
have a good amplifier to do this job well.


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## BEAVER (May 26, 2007)

I've got a related question, but if it's too off topic, just ignore me...

Amp X puts out 55WX2 @ 4ohms / 75WX2 @ 2ohms. 

Why does it take 14.8V to do this at 4ohms and only 12.25V at 2 ohms? Would the amplifier clip if set at 14.8V and 2ohm speakers were connected? Why?


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## mitchyz250f (May 14, 2005)

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

I think the better question is;

1. You draw 250w from the car's electrical system to drive
an 8 ohm speaker.

2. You draw 500w from the car's electrical system to drive
an 4 ohm speaker.


Second question.

What weighs more.

1. one pound of feathers
2. one pound of gold

They weigh the same, but have completely different uses.


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## 89grand (Nov 23, 2006)

BEAVER said:


> I've got a related question, but if it's too off topic, just ignore me...
> 
> Amp X puts out 55WX2 @ 4ohms / 75WX2 @ 2ohms.
> 
> Why does it take 14.8V to do this at 4ohms and only 12.25V at 2 ohms? Would the amplifier clip if set at 14.8V and 2ohm speakers were connected? Why?


Because the 2 ohm load draws more current. Current times voltage equals power output.

Besides, we don't set our gains by measured voltage from a DMM around these parts anyway.


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## 89grand (Nov 23, 2006)

mitchyz250f said:


> --------------------------------------------------------------------------------
> 
> I think the better question is;
> 
> ...


If you're drawing more power from the cars electrical system by using lower impedances, then you must be getting more power output out of the amp as well. If not, it would be the same.


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## BEAVER (May 26, 2007)

> Besides, we don't set our gains by measured voltage from a DMM around these parts anyway.


I know. I'm just trying to work the numbers in my head. 

So the current is equal, even though the voltage is different? Interesting.


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## 89grand (Nov 23, 2006)

BEAVER said:


> I know. I'm just trying to work the numbers in my head.
> 
> So the current is equal, even though the voltage is different? Interesting.


I was thinking along the lines of input power. I'm not sure how the output voltage translates to output power. I never pay any attention to it.


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## Fixtion (Aug 25, 2006)

*link*
http://www.diymobileaudio.com/forum/showthread.php?t=31

*-fixtion*


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## MadMaxSE-L (Oct 19, 2006)

Fixtion said:


> *link*
> http://www.diymobileaudio.com/forum/showthread.php?t=31
> 
> *-fixtion*


THANKYOU! - That is the link I've been searching for for the past hour:blush: 

-Matt


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## Oliver (Jun 25, 2007)

Fixtion said:


> *link*
> http://www.diymobileaudio.com/forum/showthread.php?t=31
> 
> *-fixtion*


quote from link ^^^


Remember, most speakers are rated at 2.83V. A 4 ohm speaker rated for 90db spl at 2.83v is really being rated at 2 watts! Whereas an 8 ohm speaker rated for 90db spl at 2.83v is only being rated at 1 watt. You can do the math for yourself, Power = Voltage^2 / resistance. At 2 watts, we can assume that same 8 ohm speaker is actually rated at 93db spl (remember, every doubling of power gives you a theoretical 3db gain in spl). So using that 8 ohm speaker will give you the same amount of output, at half the power required as a 4 ohm speaker. Your amps run cooler and draw less power from your vehicle's charging system, your speakers run cooler, and everyone is happy!


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## 89grand (Nov 23, 2006)

Yeah, see an 8 ohm speaker that happens to be 3db more efficient will give you the same output at half the power, but I think some in this thread think an amp draws more from the cars electrical system if you run a lower impedance and that is not true. It's only true if the amp is also putting out more power due to the lower impedance.


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## phatredpt (Feb 22, 2006)

Just to recap what I mentioned earlier...

Totally subjective...

My experience...

dual 4 in series...

Totally happy!


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## 89grand (Nov 23, 2006)

In the OP's case, I see no real benefit to buying a more expensive amplifier, to end up get less power from it than the cheaper amp when the PDX-1.600 will run a 2 ohm load no problem.

I'd buy the 1.600 and run it at 2 ohms over buying the 1.1000 and running it at 8 ohms.


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## thylantyr (Jan 21, 2008)

BEAVER said:


> I've got a related question, but if it's too off topic, just ignore me...
> 
> Amp X puts out 55WX2 @ 4ohms / 75WX2 @ 2ohms.
> 
> Why does it take 14.8V to do this at 4ohms and only 12.25V at 2 ohms? Would the amplifier clip if set at 14.8V and 2ohm speakers were connected? Why?



Amplifier X was designed to give you 75w x 2 @ 2 ohms @ 12.25V.

If you increase the load from 2 ohms to 4 ohms, that amplifier will only
do 37.5w x 2 @ 4 ohms @ 12.25V.

If you raise the voltage from 12.25V to 14.8V, the power rises from
37.5w to 55w per channel.

The reason the power increase is because the amplifier rail voltage increased
when it was connected to 14.8v from 12.25v.

What you described is how a typical car amplifier works.


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## thylantyr (Jan 21, 2008)

mitchyz250f said:


> 1. You draw 250w from the car's electrical system to drive
> an 8 ohm speaker.
> 
> 2. You draw 500w from the car's electrical system to drive
> an 4 ohm speaker.


1. You draw 1000w from the car's electrical system to drive
an 8 ohm speaker.

2. You draw 10w from the car's electrical system to drive
an 4 ohm speaker.

3. You draw 1w from the car's electrical system to drive
an 0.1 ohm speaker.


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## Oliver (Jun 25, 2007)

mitchyz250f said:


> --------------------------------------------------------------------------------
> 
> Second question.
> 
> ...


Different to me  

http://en.wikipedia.org/wiki/United_States_customary_units


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