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4 ohm amplifier with 2 or 8 ohm subwoofer???

Car Audio, Video, & GPS

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4 ohm amplifier with 2 or 8 ohm subwoofer???

  • I just bought a new subwoofer to put in my car as I blew out my old Kicker 12", but I'm realizing that I might have made a big mistake.  I have a Rockford Fosgate Punch 301S amp that is currently bridged out at 4 Ohms with 300 watts. It can also be wired at 2 ohms with 2 channels at 150 watts.  I purchased a RF Phase 3 D4 12" sub to go with it.  Then I realized that I could only run it at either 2 Ohms or 8 Ohms.  I have done some research and it seems like it would be a bad idea to run the amp at 4 ohms with the sub at either 2 ohms or 8 ohms.  Is this correct? 

    Yes, I know I'm an idiot and should have figured that out beforehand.  I just got overwhelmed with the choices of subs and read good reviews on this one and so I bought it. I'm not super car-audio-saavy, so I'm just trying to figure out what to do.

    I would prefer not to buy a new amp or new sub, but I want to make decent use out of what I have. 

    I am pretty sure that one option would be to just run the sub off one channel of my amp at 2 ohms (150 watts).  But the RF sub can take 500 watts RMS (1000 max), so that seems like I wouldn't be utilizing the subs full capability. 

    Thanks in advance for your help.  -Aaron

     

  • Can you return the subwoofer you bought?

    If you can't, then your best bet is to wire the sub in series, so the total impedance is 8 ohms. It is perfectly fine to hook a sub at 8 ohms up to a 4 ohm amplifier, you just don't want to go under 4 ohms, because the amp might not be stable.

    Unfortuanatly, your amp will probably only put out around 150 watts RMS at 8 ohms, which is way below what your subwoofer is rated at. But it should move the woofer, and hopefully get some reasonable bass out of it.

    You could try hooking the subwoofer up in parallel, which would make the total impedance 2 ohms. Keep in mind that the amp is not rated stable at that low of an impedance, so your amplifier could get very hot, go into protect mode, blow fuses, etc. However, I had a Rockford Fosgate Power 350S amplifier that I ran bridged at 2 ohms, and it ran great. Just keep in mind that it is NOT recommended by Rockford Fosgate or Crutchfield, and that just because mine ran great doesn't mean that yours will.

    I hope this helps, if you have anymore questions just ask!
     



    [edited by: nphilanthro at 1:21 AM (GMT -5) on Sun, Feb 17 2008]
    2005 Toyota Tacoma Access Cab Alpine CDA-9886 Head Unit Rockford Fosgate 550X amplifier Sundown Audio SAE-1000D Temp - Kicker Solo Baric L5 Temp - Kenwood Coaxials Soon - Fi Q 18, Hertz HSK 165, 120sq.ft. of Damplifier and Overkill
  • ATrain:
    I have done some research and it seems like it would be a bad idea to run the amp at 4 ohms with the sub at either 2 ohms or 8 ohms.  Is this correct?

    First off - the amp is rated as 4-ohm stable.  You can't run the amp at 4-ohms with the sub at either 2-ohms or 8-ohms.   The load determines what the amp runs at.  If you wire the sub at 8-ohms and connect it to the amp, the amp is at 8-ohms.  If you wire it at 2-ohms, you are running the amp at 2-ohms.  (You may not be running it for long at 2-ohms, but that is what you are doing.)

    I am pretty sure that one option would be to just run the sub off one channel of my amp at 2 ohms (150 watts).  But the RF sub can take 500 watts RMS (1000 max), so that seems like I wouldn't be utilizing the subs full capability.

    Correct, and that is probably the best you can do.  Keep in mind RF typically underrates their amps, so you might well get 200 or even 250 watts out of it this way - OTOH, you likely got more than 300W out of it before, and as you said, you won't be getting the capacity of the sub.

    Can you return the subwoofer you bought?

    Agreed, or if you can't, can you sell it to someone that wants it as basically new and get something that is a better match. 

    It is perfectly fine to hook a sub at 8 ohms up to a 4 ohm amplifier, you just don't want to go under 4 ohms, because the amp might not be stable.

    It's not quite that simple.  Think of the impedance (load) like rocks in the bed of a pickup truck.  If you hook up a lower impedance (2-ohm, less rocks) than the amp is designed for, the amp can draw too much current and overheat, going into protect mode (truck overspeeds and blows engine).  If you hook up a higher impedance (8-ohms, more rocks) than the amp is designed for, the amp has to work harder to push the load (can't go as fast) and can also overheat and go into protect mode.

    Generally, it's a LOT safer running at higher than lower recommended impedance, but neither is ideally recommended. 

    Unfortuanatly, your amp will probably only put out around 150 watts RMS at 8 ohms, which is way below what your subwoofer is rated at. But it should move the woofer, and hopefully get some reasonable bass out of it.

    Agreed - in theory, the amp should produce half as much power at 8-ohms as 4-ohms, giving you the same power as the single channel setup (but that is a known stable condition).  In reality, it sometimes gives a bit more power in the bridged 8-ohm mode. 

    You could try hooking the subwoofer up in parallel, which would make the total impedance 2 ohms. Keep in mind that the amp is not rated stable at that low of an impedance, so your amplifier could get very hot, go into protect mode, blow fuses, etc. However, I had a Rockford Fosgate Power 350S amplifier that I ran bridged at 2 ohms, and it ran great. Just keep in mind that it is NOT recommended by Rockford Fosgate or Crutchfield, and that just because mine ran great doesn't mean that yours will.

    Agreed - might work, might burn up the amp - could take the sub with it as well when the amp goes, I think.  Not recommended, but might work. 

    2002 Ford Focus JVC KD-A815 Sony CDX-GT410u Sony XT-100HD HD Tuner Stock speakers, no amp, no subs

  • That's what I figured.  Thanks for the help!  I really appreciate it!!!
  • i have a quick question. I have two rockford fosgate 12 inch subs. one is 8 ohms and one is 4 ohms. i went to a local audio place and they said that all i needed was a mono channel amp and that would give sufficient power to both my subs and it would be fine. So my question to you is, can i just get a mono amp or should i try to sell one and get one of the same ohm power? im not sure wat to do.

  • You really need both subs to match when running them off a single mono amp.

    GLH Geeked

  • Try to sell one and find a different audio shop.

    See here.

    Short answer - assuming they are both Single-Voice-Coil (SVC), you could wire them in parallel at 2.667-ohms, but the 4-ohm sub would see twice the power - so if the 4-ohm sub was 500W RMS, and the 8-ohm sub was 250W RMS, and you bought an amp that was around 900W RMS @ 2-ohms, you would be okay power-wise, but it probably wouldn't sound great anyway.

    Now if by chance, they are the same RMS and the 8-ohm sub is DVC (or really vice-versa also), you can do much better, but for ideal sound you really want both subs to be identical brand, make, model (and ideally year of manufacture).

    Hope This Helps!!!

    2002 Ford Focus JVC KD-A815 Sony CDX-GT410u Sony XT-100HD HD Tuner Stock speakers, no amp, no subs

  • go online and check out the owners manual for that amp. I'm almost positive that amp is stable at 2 ohms. most punch and power amps are stable down to 1 ohm. you certainly won't hurt the amp by running it at 8 ohms either. you should be quite alright running it either 2 or 8 ohms for your amp. another thing you can do is call your local rf dealer they will tell you.

  • http://www.crutchfield.com/learn/how-to-match-subwoofers-and-amplifiers.html?g=500&tp=13

     

    All the info you could possibly need

  • Amp isn't 2-ohm stable bridged.

    (And yes, the thread is almost 3 years old and the post I am replying to is almost 10 months old.Devil

    2002 Ford Focus JVC KD-A815 Sony CDX-GT410u Sony XT-100HD HD Tuner Stock speakers, no amp, no subs

  • I dont really have an answer, but you question made me wonder, can you hook up a 4-ohm subwoofer to an 8-ohm home audio reciever? 

    Im wondering this because I have two Kicker Comp 12in in the kicker box at 4-ohms, and the only home reciever I have is 8-ohm.

     

    If you wondering why I don't connect them to my car, is because I don't have a car yet, I'm 15.  

     

    I wanna be able to use these in my house and I just wanna know if I can with what I have...

  • Home amps generally are not rated as stable under 8 ohms.  If your receiver has a subwoofer channel you could wire the 2 subs in series for 8 ohms.  My guess though is that your home amp probably isn't much more than 100w @ 8ohms.  That is woefully insufficient to run 2 Kicker Comp 12's.

    • Pioneer DEH-80PRS
    • Kicker ZX700.5 5ch amp 4x90w + 1x570w
    • 12" JBL P1220 Factory Ported Enclosure
    • 2-12" Polk Audio DXI-124 Custom Ported (soon)
    • Polk Audio dB651 6.5" & dB 691 6"x9"
  • Some home amps will work at 4-ohms - most won't.

    If I read the post correctly, though, I think you likely have two Kicker Comp SVC-8-ohm subs in a box at 4-ohms final.  You can't wire those in series for 8-ohm at the amp, but they are 150W RMS subs and you could connect ONE of them to the amp and get decent results (assuming the box is divided so the sub isn't seeing double the airspace it wants.

    Hope This Helps!!!

    Anyone else have suggestions?

    2002 Ford Focus JVC KD-A815 Sony CDX-GT410u Sony XT-100HD HD Tuner Stock speakers, no amp, no subs