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Convert 4 Ohm Speaker to 2 Ohm


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juicejug 
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Joined: May 15, 2013
Location: Illinois, United States
Posted: May 15, 2013 at 10:23 PM / IP Logged  
I have an 07 Acura TL. I've had to replace the rear stock speakers once in this vehicle - due to some typical jackassery on my part. When I replaced them I found that the originals were 2 ohm, but 4 ohm speaker won't cut it with the stock nav head unit. I bought some replacement 2 ohm speakers, but one of those took a dump on me - just through regular usage - no jackassery. I'm not willing to screw around with replacing the head unit, adding amps, buying new 2 ohm speakers (as the options are limited and don't trust the replacement brand), etc. I was talking with someone else and they suggested just jumpering a set of 4 ohm speakers, down to 2 ohm.
I want to take this route, but want to jack around with this setup with an old head unit and some old 4 ohm speakers I have.
My questions, since my electricity math sucks-
To make these 4 ohm speakers I have, would it be best to place a 2 ohm resistor across the + and -, or in series on either the + or -?
Is a 2 ohm resistor proper for this reduction or does the max watt rating of a speaker effect this resistance?
I am going to be trying this out, regardless if it's best practice. I won't personally hold anyone responsible for any thoughts they have and want to discount any replies that say I shouldn't do this because I'll wreck something.
Experiments always come with risk but are fun.
Thanks all!
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i am an idiot 
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Joined: September 21, 2006
Location: Louisiana, United States
Posted: May 15, 2013 at 10:39 PM / IP Logged  
A 4 ohm speaker can not become a 2 ohm. There are several manufacturers that make 2 ohm drivers. Infinity is one that comes to mind.
KPierson 
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Joined: April 14, 2005
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Posted: May 15, 2013 at 10:49 PM / IP Logged  
Just in case you didn't catch that, a 4 ohm speaker can never become a 2 ohm speaker.
The issue at hand is that at 4 ohms you don't have enough power to get adequate results from a system that was designed to run at 2 ohms. If you take 4 ohm speaker and wire it in parallel with a 4 ohm resistor you WILL get 2 ohms of total resistance. However, your speaker will still get the same amount of power and all the additional power created by dropping the ohm load will go to the resistor and be turned to heat. If you were bright enough to properly size the resistor for the amount of power it will be dissipating and took care mounting the resistor away from anything that could catch on fire you may not start a fire in the car. If you undersized it or keep it near carpet / insulation you may catch the car on fire. Those resistors can get HOT!
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haemphyst 
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Joined: January 19, 2003
Location: Michigan, Bouvet Island
Posted: May 16, 2013 at 7:53 PM / IP Logged  
here...
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: May 18, 2013 at 4:15 PM / IP Logged  
The only way to convert a 4-ohm speaker into a 2-ohm speaker is to replace the voice coil.
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oldspark 
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Posted: May 18, 2013 at 7:13 PM / IP Logged  
Or use a transformer.
PS - I'm not saying that's practical in this case, but it is - or was - a common technique.
But to apply to a big or LF sub etc - forget it! A new sub or amp would be far lighter. And impedance matchers - a new sub or amp would be cheaper.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: May 19, 2013 at 9:43 AM / IP Logged  
oldspark wrote:
Or use a transformer.
PS - I'm not saying that's practical in this case, but it is - or was - a common technique.
But to apply to a big or LF sub etc - forget it! A new sub or amp would be far lighter. And impedance matchers - a new sub or amp would be cheaper.
But that still does not change the impedance of the loudspeaker, it only changes the effective impedance at the amplifier and it is a power-waster. :)
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oldspark 
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Posted: May 19, 2013 at 9:53 AM / IP Logged  
Partly true. Obviously the speaker impedance does not change, but the amp sees 2 Ohms and thus delivers its full power.
Paraphrased, the 4 Ohm speaker receives the same power as a 2 Ohm speaker would (less a few% inefficiency).
PS - apart from the slight transformer inefficiency, no power is wasted - it is almost fully transferred.
DYohn 
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Joined: April 22, 2003
Location: Arizona, United States
Posted: May 19, 2013 at 11:22 AM / IP Logged  
100% true. You cannot change the impedance of the speaker using any sort of external device. Period. So, if your transformer has a 2-ohm primary causing the amp to produce, say, 100 watts at full power, and it is then coupling that into a 4-ohm load, how much power will be utilized by the 4-ohm load? The voltage ratio will still be 1:2, yes?
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oldspark 
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Posted: May 19, 2013 at 5:57 PM / IP Logged  
100W.
No - not 1:2, ~1:1.4.
PS - I didn't specify a 2R primary. If you had a 2R primary, then it's only 50% transfer.
I assumed a close to a 0R primary RESISTANCE.
If the amp can't handle low ohmage DC then insert a resistance or use a 2R primary. Same thing - then you lose 50% of the power so there is no reason to do it - ie, the 4R gains no power (unless the amp pushes more than twice the power into a 2R than 4R).
But IMO it doesn't matter. As I said, impedance matching transformers simply are not practical - not for the power we are discussing, nor to match impedances of the same magnitude.
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