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Adding resistance to Increase Power Output

Discussion in 'Amps and Cabs [BG]' started by Redman, Oct 1, 2005.


  1. Redman

    Redman

    Oct 21, 2002
    Nashville TN
    Greetings fellow Bassists

    I have a question. First a little info. I recently got a killer deal on a new SWR Golliath III (4x10) 8ohm cab. I'm running a SWR Bass 350 (blackface with chrome) head. 240 watts at 8 ohms 350 at 4 ohms. Is there a device (short of buying another 8 ohm cab) that can increase the power to the 4 ohm load with only an 8 ohm cab? The 240 watts drive the cab kinda ok, but an increase to 350 without adding additional speakers would be great. I know I could re wire the speakers in the cab to get 4ohms, but I don't want to screw up my warranty. Any ideas and suggestions would be greatly appreciated...

    Redman
     
  2. No and no you can't rewire the 4x10 to equal 4ohms either. Sorry.
     
  3. To elaborate, suppose you put an 8 ohm resistor in parallel with your speaker cab. Now the amp will see a 4 ohm load, so it will put out 350 watts. This will be divided equally, so 175 watts will go into the speaker cab, and 175 watts will go into the resistor. Sure the amp will be working harder...to produce power which is going into the resistor (as heat!!).

    So the amp will work harder, but the speaker cab won't be getting more power-- it actually will get less. (If the amp was theoretically perfect, it would put out double the power if the load impedance dropped in half, but in reality amps don't operate at theoretical perfection.) Nevertheless, even if the amp did put out its theoretically perfect 500 watts at 4 ohms instead of 250 at 8 ohms...the power would still be divided equally, so the cab would still only get its original 250 watts!! The other 250 watts would go into the resistor, and dissipated as heat.

    And exactly as Mudbass said, you cannot rewire an 8 ohm cab to get 4 ohms. To get more info, look for the article on speaker wiring posted on this site:

    http://www.colomar.com/Shavano/construction.html
     
  4. Lowtonejoe

    Lowtonejoe Supporting Member

    Jul 3, 2004
    Richland, WA
    Yo, Red,

    Your amp is rated down to 2 ohms.

    I believe the Goliath III is loaded with 8 ohm speakers. Hook up all your speakers in parallel and enjoy your 450w.

    :D

    Joe.
     
  5. Redman

    Redman

    Oct 21, 2002
    Nashville TN
    OK, that makes sense. I see your point, and accept it and I'll just use it the way it is, but SWR offers this cab in either a 4 or 8 ohm configuration, isn't the difference accounted for by speaker wiring? ie series or parallel? or is there more to it? Now I'm just curious....

    Thanks Again for the Info
     
  6. Hi,

    There are only 3 ways to make 4 speakers turn out 8 ohm total (assuming they're identical speakers):
    1. Four 2 ohm speakers in series.
    2. Four 32 ohm speakers in parallel
    3. two 8 ohm speakers in series, and in parallel with another two 8 ohm sepakers in series.

    To make it 4 ohm total there are also only 3 ways:
    1. Four 1 ohm speakers in series.
    2. Four 16 ohm speakers in parallel
    3. two 4 ohm speakers in series, and in parallel with another two 8 ohm speakers in series.

    As You can see the ohmage of the speakers never match, so either way it's wired (and I'd guess number 3 is the winner) there's no way to just rewire an 8 ohm, 4-speaker into 4 ohms.

    But... buy another one, and put it in parallel with the first one, and You'll be rockin your full 375 watts with double the speaker area... :bassist:

    Edit:
    Forgot to mention: SWR probably uses 4 ohm speakers to make the identical, but 4 ohm, speaker

    /Ff
     
  7. A typo there I believe:
    You mean two 4 ohm speakers in series and in parallel with another two 4 ohm speakers in series...

    Regardless, everybody is trying to say the same thing: You can't go from an 8 ohm to a 4 ohm cab, or vice versa, with a mere wiring change. Read the article I linked to earlier and try doing the math yourself.
     
  8. They use different drivers for the 4 and the 8 ohm version. There's no way to cut the impedance in half by rewiring. You can only change the impedance by a factor of 4. So yes, you may be able to rewire your 8 ohm cab to 2 ohms. If you're currently wired all in series, go to series-parallel. If you're currently series-parallel, go to all parallel.

    Randy
     
  9. Yup, classic copy-paste typo... :rolleyes:
    Sorry about that.

    /Ff
     
  10. Nightbass

    Nightbass

    May 1, 2001
    Seattle, WA
    If the SWR fairy were to descend on your humble abode and change your cab from 8 to 4 ohms, I suggest that you would hardly notice it. On paper, it may represent a 3dB increase in SPL, but in practice it will be less, and you will more than likely not notice it.

    I've compared many 4 and 8 ohm cabs and the difference is subtle. My philosophy is simple: get the cab you like and make sure you have enough power to drive it. If you need more volume, add more cabs.

    So really, you're not really going to be missing much, imo.

    Nighbass
     
  11. Redman

    Redman

    Oct 21, 2002
    Nashville TN
    Thanks Everybody, I appreciate all the comments and info. I try to run my equipment "resistantly sound" and knew I was on uncertain ground here, but was trying to get just a little bit more from my rehearsal setup without dragging around more stuff. I appreciate all the comments and info. Funny, when I was a kid you just plugged a speaker cab in every hole you could find, then wired in a few more and didn't think much about it. But then it was more about quantity of sound (actually noise) than quality.
     
  12. Blues Cat

    Blues Cat Supporting Member

    May 28, 2005
    Katy, Tx
    You could wire your cab for 2 ohms but if someone adds another cab to your head & they don't know, it could blow your head. I wouldn't do it.
    I have an SWR 350 redface pre fender w/the same ohm-power #s. W/ my goliath 2 & it being rear ported, I could barely keep up w/ a 60watt tube guitar amp on 3. I sold the cab & a friend of mine let me use his Bergantino 310 & @ 4 ohms I noticed a nice difference from the 8 ohm. I didn't have to dig in as hard & fewer speakers were pushing more air. It gave me the minimun headroom I needed to play in a pretty loud blues/rock band. But it wasn't a huge jump in volume. You ought to think about getting a four ohm cab for med to small gigs & add the other 8 ohm cab for a total of 2.66 ohms getting you almost up to max wattage.