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Why do home theater speakers have such high resistance?


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4 replies to this topic

#1 of 5 OFFLINE   swob111

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Posted July 07 2014 - 05:45 PM

I used this forum many years ago, and I was more than pleased with the help i received. I am now starting another project.

 

I plan to use an old car amp that I have to create a speaker system for my college apartment. So far I have taken an old computer power supply to power the amp, and it works great! The amp does about 175 watts per channel and it is a dual channel. It can be stable down to two ohms.

 

My question is why are all home audio speakers only stable down to 4 or 8 ohms? What am I missing? Any other advice would be appreciated.



#2 of 5 OFFLINE   schan1269

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Posted July 07 2014 - 10:28 PM

Car audio uses 4 and 2 ohm to juice the system. Home audio doesn't have that problem when your wall socket is 15 amps on 120.

And it isn't "home audio speakers only stable to 4 or 8".

Speakers present a load. Loads are always stable. It is the other end driving it (ie, the amp) That is stable...or not.

#3 of 5 OFFLINE   Robert_J

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Posted July 08 2014 - 05:54 AM

To produce more power, you need to draw more voltage or amperage.  Like Sam mentioned, wall sockets are normally limited to 15 amps and you are stuck with 110v AC.  My sub amp is on a 20amp dedicated circuit with larger wire running to the outlet.  Guys on other forums have installed 220v outlets for amps in the 4,000w range.

 

The current trend for car amps is basically no limiters on their design.  Look at the sub amps especially. 

@ Max Power 13.8 VDC – 1 OHM: 9595W RMS

@ Max Power 13.8 VDC – 2 ohms: 6305W RMS

@ Max Power 13.8 VDC – 4 ohms: 3646W RMS

@ Max Power 12.6 VDC – 1 OHM: 8000W RMS

@ Max Power 12.6 VDC – 2 ohms: 5256W RMS

@ Max Power 12.6 VDC – 4 ohms: 3040W RMS

 

See the jump in power from a 1 volt increase in the charging system.  If that amp is stable to 16v then it would really push the power.  But one spec that is very, very important - Consumption Maximum Music @ 12.6 VDC: 431 Amp.  That's right 431 amps of power.  On a compact car, the alternator puts out about 120amps with about 75 to 85 used to power the car and accessories like A/C.  That leaves 40 to 50 amps to charge the battery and power the audio system.  Pull more than that and you have headlights dimming and soon a dead battery.  In the home environment, you can't pull more than the rated amperage or you throw a breaker.

 

So why don't home systems use low impedance speakers?  There's no use in doing so.  And with most manufacturers sticking with 8 ohms, it makes compatibility easier.



#4 of 5 OFFLINE   swob111

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Posted July 08 2014 - 03:47 PM

Thank you very much for the detailed response. I wish I understood more about electricity, but I think I followed everything that you said. I was hoping that I could give you the specific set up that I am planning and then you could maybe make some recommendations.

 

The amplifier is a rockford fosgate punch 360 a^2. Here is the manual. Hopefully you can find the specs for it in there somewhere. I am not really sure what to look for. I know that the amp is fairly old, but it worked great in my car until I upgraded, which is why I was hoping to utilize it.

 

Originally I was powering it with a very old computer power supply. It was working great, but I don't think the amplifier was getting the power that it needed. Here are the specs: 20140708_183937.jpg

 

Then I saw a sale on tiger direct for a different power supply, so I purchased that. The specs are: 20140708_184032.jpg However I have yet to get that one to work with the amp, but I haven't been trying for very long.

 

Please let me know what you think of this set up so far. Thanks! 



#5 of 5 OFFLINE   Robert_J

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Posted July 09 2014 - 10:55 AM

Please let me know what you think of this set up so far. Thanks!

You asked.  I think it is a waste of time.  Why?  Even though that power supply is rated for 12v @ 25amps, it is designed to provide a steady stream of power.  Car amps pull in bursts and many of those bursts are well above the rated amperage listed.  For example, my old PPI 2025AM has a 10 amp fuse.  Does it always pull 10 amps?  No.  Will it ever pull more than 10 amps?  Yes.  Fuses are not hard safety net.  They have tolerances built it.  Yes, it's rated for 10amps but the amp can easily pull 20 or 30 amps of power for short bursts without blowing the fuse.

 

For a moderate system, you need a different type of power supply and Pyramid is usually the cheapest.  Google Pyramid PS26KX and you will see some prices and other options depending on how much current you actually need.

 

Wait.  I just looked at your link to the RF owner's manual.  That 360 amp pulls 40 amps of current.  Yes it worked great because it pulled a LOT of power to make power.  It's old so it is not efficient.  You need a power supply like PYRAMID PS52KX.  By the time you buy everything you need, you could have purchased the proper equipment.

 

That RF amp is 140w by 2 channels.  A Behringer A-500 is 230w by 2 channels and costs $50 less than the power supply that you need.






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