Any benefit (Pros/Cons) to 4 Ohms v. 8 Ohms?

Discussion in 'Amps [BG]' started by tastybasslines, Jul 21, 2010.


  1. tastybasslines

    tastybasslines Supporting Member

    Joined:
    May 9, 2010
    Location:
    Los Angeles, CA
    I get it, the higher the impedance, the more power you need to drive the speaker/s...thus,

    4 ohm at 200 watts

    (all power measurement are exactly equal for this post)

    should be equal to 400 watts going into 8 ohms.

    Is there really any difference? I imagine if your amp is powering a 4 ohm speaker there could be slightly more chance to blow your speaker because it is "weaker"...is this true?

    Why would there be any benefit to using 8 ohm speakers...won't it just push the amp harder/use more power?

    I am planning on getting the GK MB115 which is 200 into a 4 ohm speaker. I'm not a "power user" so it won't affect my buying decision, but I am curious...

    What are the pros and cons of each?
  2. brendanbassist

    brendanbassist Supporting Member

    Joined:
    Jul 9, 2010
    Location:
    Albuquerque NM
    Actually, you've got it backwards. Amplifiers produce more power at lower impedance. If the impedance goes to low, you can damage the amplifier. The advantage of 8 ohm cabs is you can run 2 cabs in parallel to get a 4 ohm load.
  3. Travis_Bass

    Travis_Bass

    Joined:
    Jul 8, 2010
    Brendan is right. You get more power from an amp the lower the independance, or Ohms. I run 450w @ 4ohm because my cab is 4ohm. But I could also run 275w @ 8ohm, or run 450w @ 4 ohm, running 2 cabinets rated at 8ohms each. If I ran 2 cabinets rated at 4ohms each, it can stress out the amp significantly, unless your amp can do a 2 ohm load (SVT classic head can). Just remeber when you add 8+8, you get 4. 4+4 = 2. 16+16= 8. It can get confusing, so keep it simple if you can.
  4. bassybill

    bassybill The smooth moderator... Supporting Member

    Joined:
    Mar 12, 2005
    Media:
    1
    Location:
    West Midlands UK
    Tehcnical bit first... the power going to your speaker is determined by it's impedance and the output capability of your amp. So if, your amp can deliver 200W into 4 ohms and 120W into 8 ohms (harder load to drive), that's the most it's rated by the manufacturer to deliver with cabs of those impedances. The power rating of the speaker you use is independent of this. So using a 4 ohm cab rated as capable of handling 500W won't mean that the amp I just mentioned will deliver that much with that cab - you'll still only get 200W out of the amp before you're likely to be asking too much of it.

    Anyway, here's a fairly typical process you might go through when looking for cabs - not the only way, and lots of variables involved. But this is just an example.

    Very generally speaking, when looking at what impedance is best for you to use, you should consider the total load presented to the amp. So for example, with my amp a 4 ohm load is a good idea for me as I get more clean power that way (500W at 4 ohms compared to 300W at 8 ohms) and I don't want to overdrive my amp. Other players might like the overdriven sound and decide that 300W is a better idea and choose an 8 ohm load instead (to get the overdrive they're after easier and without having to crank quite so much power and blast their band into oblivion).

    Having decided what load you want to use, then you could decide what cab configuration you want. Say I want a 4ohm load - do I use one 4 ohm cab or two 8 ohm cabs to give 4 ohm total? With what size drivers in each? Is this likely to work the way I want? A lot of personal preference and circumstance here, really.

    Then having settled on a cab configuration that gives me the desired load, I could look at power handling of the cab or cabs and ask whether they'll safely handle the power each wil get, and whether they'll get enough power to work efficiently / be loud enough / sound they way I want in terms of how they're being driven.

    Now we know the load, the configuration and rough power handling requirement for each cab I want to use - time to draw up the shopping list and go try some.

    In my case, I want to use a 4 ohm load (to maximise the clean power) and I want two cabs, so they need to be 8 ohms each. Each will handle 400W so will be in no danger from half each of the 500W hopefully clean watts I could be sending them, but will still be getting plenty of power to drive them properly (in most cases 250W should easily drive a cab rated at handling up to 400W). See how that fits?

    Like I said, just an example for illustration of a few key points.
  5. billfitzmaurice

    billfitzmaurice

    Joined:
    Sep 15, 2004
    Location:
    New Hampshire
    Disclosures:
    Owner, Bill Fitzmaurice Loudspeaker Design
    Halved impedance doesn't give twice the power, it's closer to 1.4 times as much. That equates to about 2dB, which is not much. The only time a 4 ohm load is of any benefit is if your amp is seriously lower powered compared to the speakers, as in less than half. Otherwise stick with 8 ohms and leave room for future expansion.
  6. bassybill

    bassybill The smooth moderator... Supporting Member

    Joined:
    Mar 12, 2005
    Media:
    1
    Location:
    West Midlands UK
    Putting it like that, Bill, I guess I like the 4 ohm load as it means I get to use my extra cab for more speaker area and that = louder.

    But wouldn't it be the case that the amp would still have some headroom if I were on a gig and putting out, say, 250W into a single 115 4 ohm cab, whereas with a single 115 8 ohm cab 250W would be getting close to the max (if the amp is rated 300W at 8 ohms / 500W at 4 ohms)?
  7. billfitzmaurice

    billfitzmaurice

    Joined:
    Sep 15, 2004
    Location:
    New Hampshire
    Disclosures:
    Owner, Bill Fitzmaurice Loudspeaker Design
    The main reason why you're significantly louder with two cabs versus one is the increased sensitivity, not the lower impedance. If you had a pair of 16 ohm cabs versus a pair of 8 ohm cabs you'd still have at best about a 2dB difference between the two. If your amp had enough power to drive them to full output the difference would be 0dB, as maximum output is determined by displacement, and displacement isn't affected by impedance.
    The 'extra' headroom of the lower impedance load applies mainly to voltage swing, and it's obtained at the expense of current draw, so there's no real benefit there either unless as I stated previously, when the amp is so anemic that it lacks the voltage swing capacity to drive the speakers to full output.
  8. bassybill

    bassybill The smooth moderator... Supporting Member

    Joined:
    Mar 12, 2005
    Media:
    1
    Location:
    West Midlands UK
    Very clear answer - thank you. I could add that by saying "I like the 4 ohm load", I only meant that for loud gigs I usually take both cabs!

    On the subject of sensitivity, is there a ballpark way of combining figures for two cabs to give an indication for overall volume potential? I'm guessing that could be tricky as many sensitivity figures I see seem pretty undetailed, e.g. "100 dB SPL" with no further specifics provided.
  9. tastybasslines

    tastybasslines Supporting Member

    Joined:
    May 9, 2010
    Location:
    Los Angeles, CA
    Thanks man! That helped alot.

  10. bassybill

    bassybill The smooth moderator... Supporting Member

    Joined:
    Mar 12, 2005
    Media:
    1
    Location:
    West Midlands UK
    Okay, good - but make sure you read Bill FM's take on the impedance thing in the posts after mine, he knows way more stuff about this stuff than I do.
  11. billfitzmaurice

    billfitzmaurice

    Joined:
    Sep 15, 2004
    Location:
    New Hampshire
    Disclosures:
    Owner, Bill Fitzmaurice Loudspeaker Design
    Only when they're identical, otherwise you'd need detailed specs to calculate the result, specs that no manufacturer will give out, assuming they have them to give. With identical cabs two will give 6dB more than one.
  12. FortessOne

    FortessOne

    Joined:
    Nov 8, 2004
    An old bassteacher of mine only used 8 ohm cabs because he thought they sounded tighter.
    And, if you use a head with a lot of power it can work out good to use an 8 ohm cab in stead of a 4 ohm because you can drive your amp a little further, often resulting in better tone.
  13. Rick Auricchio

    Rick Auricchio Registered Bass Offender

    Joined:
    Nov 27, 2007
    Location:
    Cambria, CA (Central Coast)
    But all this is purely subjective, based on what he liked to hear. As for "tighter sound," I doubt he compared the exact cab in both four and eight ohm versions.
  14. FortessOne

    FortessOne

    Joined:
    Nov 8, 2004
    I don't know, but he was pretty sure of this. This guy was playing for years and years and went through a lot of amps and cabs, he really knew what he liked and what he was talking about...
  15. StuartV

    StuartV Out of GAS!! Supporting Member

    Joined:
    Jul 27, 2006
    Location:
    Bristow, VA
    OTOH, if you are buying a cab and think you might get an SVT tube amp (CL, VR, 2 Pro) at some point, you might want to get a 4 Ohm, as those amps won't run an 8 Ohm load. But they will run 2 Ohms.
  16. jastacey

    jastacey Supporting Member

    Joined:
    Feb 8, 2009
    Location:
    Houston,Tx
    I might add that using a 4 ohm load applies mostly to amps that have a solid state power section ...most tube amps have a impedance selector switch, but having a 4 ohm cabinet allows you to get more power to your speakers, if your running a solid state amp
  17. 1n3

    1n3

    Joined:
    Sep 13, 2007
    Location:
    USA
    This might touch on something I've wondered about - the effect of load on damping for solid state amps. I'll start with what little I know about this, then ask my question, and hope that someone more knowledgeable can run with it.

    The damping factor determines how tightly the amplifier controls the motion of the speaker. It's largely determined by the ratio of impedance between the source and the load. Typically, the output impedance of a SS amp is very low (a small fraction of an ohm), so the damping factor is high. But the damping factor will be higher for an 8 ohm load than a 4 ohm load.

    So my question: Given a modern SS amp, identical speakers, and equalizing for loudness, can there be an audible difference between an 8 ohm load and a 4 or 2 ohm load?

    And if there is an audible difference, how much is it dependent on the resonant characteristics of the speaker/cab combo?
  18. billfitzmaurice

    billfitzmaurice

    Joined:
    Sep 15, 2004
    Location:
    New Hampshire
    Disclosures:
    Owner, Bill Fitzmaurice Loudspeaker Design
    Not when the only difference is the impedance. But different impedance versions of the same driver don't have identical specs, so there will be differences. The effect of damping factor though is nil.
  19. 1n3

    1n3

    Joined:
    Sep 13, 2007
    Location:
    USA
    Thanks, Bill. I didn't word my question very well. I was thinking of this scenario: SS amp driving an 8 ohm speaker. Add identical speakers in parallel to create a 4 or 2 ohm load. Reduce power to compensate for the increase sensitivity. Compare.

    Of course, there would be greater extension; not sure how you'd compensate for that. But would there be an audible difference attributable to the decrease in damping? You say no, I believe.

    Some guitar players I know go for things that reduce damping - tubes for starters, then run a 4 ohm load off the 16 ohm tap, and use cheap speakers. The guys here who like running 2 ohm loads make me think of them.
  20. billfitzmaurice

    billfitzmaurice

    Joined:
    Sep 15, 2004
    Location:
    New Hampshire
    Disclosures:
    Owner, Bill Fitzmaurice Loudspeaker Design
    All for naught. Tubes have low damping factor, SS have high damping factor, but there's no particular advantage to either, it's just how they are. As for the technical prowess of guitar'd players, these are guys who think a 4x12 with 10 degree dispersion in the mids makes sense. :rollno:

Share This Page