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ohm problems......any way to make conversions?

Discussion in 'Amps and Cabs [BG]' started by BassWizard55, Aug 31, 2003.


  1. BassWizard55

    BassWizard55 Guest

    Dec 21, 2002
    Rome, Ga
    I have a head with a 4 ohm out, and a cab that is rated at 8 ohm. I need to use them together....what are my options??
     
  2. I'll go out on a limb here-

    Take a speaker cable and plug it into the "out" on the back of the amp then plug the other end into you cabinet!

    ;)

    Seriously though, you can run one 8ohm cabinet off your head. It won't hurt anything. The 4ohm rating on the back of your amp just means that (4ohms) is the lowest impedence your amp is rated for.
     
  3. BassWizard55

    BassWizard55 Guest

    Dec 21, 2002
    Rome, Ga
    but it will only be half as powerful right
     
  4. Unfortunately, you are correct.

    Sorry!:(
     
  5. goran

    goran

    Dec 17, 2002
    Croatia
    Endorsing Artist: Bartolini
    Is it a tube or solid state amp?

    My tube driven Bassman 100 wants to see exactly 4 ohm load. You could damage amp by not matching impedance of amp and cabinet.
    Does it say "total load 4 ohms" or "minimum load 4 ohms"?
     
  6. Hey guys

    As far as trying to get as much power into the cab as possible can you run the cab and an 8 ohm resistor with jack together and get the same sound as just using a 4 ohm cab? If you can do this I'd go for it (and possibly will:D ). C-YA

    Ben
     
  7. goran

    goran

    Dec 17, 2002
    Croatia
    Endorsing Artist: Bartolini
    I had similar question in one thread last week. Answer is NO!!! Due to high current (my guess would be more than 20A) that would flow, resistor would get very hot and eventually would fry.

    Better is to hook another 8ohm cabinet.
     
  8. Duh! With all due respect, that's plain stupid! Exactly half of the amp power will be very effectively converted to heat inside the resistor!!!!! You'll get even LESS power that way, because at 8 ohms, an amp will usually still provide about 2/3 of its rated 4-ohms-power.
     
  9. If you are using a solid state amplifier, there is no problem running an 8 ohm cabinet from a 4 ohm output. Tube amps however like to have cabinet impedances matched to the amplifier's output transformer.

    In theory you get less (1/2 to 3/4) power running an 8 ohm speaker from an amp rated at 4 ohms, but in practice the difference in volume will be barely noticable.

    On the other hand, you don't want to run a 4 ohm speaker from a solid state amplifier rated for 8 ohms. This will cause excessive current draw from the amplifier and could lead to its demise.
     
  10. Hey Joris

    It's a good think you prefixed your statement with "with all due respect" cause with all due respect that's just plain rude. I may be no electronics major but I'not entirely sure I understand the difference between adding 2 8ohm cabs together to produce an overall load of 4 ohms and wiring a resistor in line as opposed to the cab. Presumably there is a way that this could be done (cheaply) so that you could use an 8 ohm cab by itself but with an overall load of 4 ohms.

    When you wire the 2 cabs together does one of them get hot? If not then do the speakers have some kind of heat sink (like could be attached to the stupid resistor:D ). I think it would be really useful to be able to run an 8 ohm cab as a 4 ohm cab by just adding a small piece of equipment. Cheers

    Ben
     
  11. Johnny BoomBoom

    Johnny BoomBoom Supporting Member

    Jun 8, 2001
    Glasgow, Scotland


    When you have wired the 2 speaker cabs in parallel, both get the same amount of power distributed to them. They both disspiate this power by producing sound, in theory neither will get hotter than the other. Some energy will turn to heat in the voice coils.

    Whilst your ida might be a good one, practically the best bit of kit you can add is another 8 ohm cab, or change the speakers and build yourself a 4 ohm cab.

    As the song says 'You canny change the laws of physics......':)
     
  12. Hey Johnny

    Thanks for helping with my somewhat naive(sp?)question. I'm actually considering getting a new acme cab at the moment and am trying to decide between the 8 and 4 ohm varieties. So this is pretty helpful stuff. I think I'll look into it further but I'll probably end up buyinn the 4 ohm option. Cheers

    Ben
     
  13. goran

    goran

    Dec 17, 2002
    Croatia
    Endorsing Artist: Bartolini
    Actually, majority of energy will turn to heat. Only in SF movies you can find cabinet with 112dB sensivity at @1W @1m where all energy goes into sound. For example, if your cabinet has sensivity of 100dB, only about 6% of electrical energy will convert into cone movement, while rest goes into heating voice coil
     
  14. Johnny BoomBoom

    Johnny BoomBoom Supporting Member

    Jun 8, 2001
    Glasgow, Scotland
    Speaker cab choice is all about personal preference. Tone, volume (well, the efficiency part) and what your gonna be runing it with. Also, how big a rig you're gonna be prepared to humph around (well, that's how I see it anyway!)


    I have a rig that has 2 8 ohm cabinets - 'cos I like the sound you get from mixing speaker sizes. I have always liked the way a 4 x 10 along with a 1 x 15 sounds. I also think it works well for 5 string basses(I've got a couple of them!)

    Thing is I actually traded my 4 x 10 and 1 x 15 for my new rig Ashdown Mini 4 x 8 and Mini 1 x 15. Biggest difference is the 4 x 8 speakers as opposed to the 4 x 10! To me tonally there wasn't anything in it, but cab size (portability) and weight difference for my back (about 12 kgs or 28 lbs) helped me choose.


    I've found that there is a lot more to choosing a cab than just it's impedance!
     
  15. BenHack said...

    ...I think it would be really useful to be able to run an 8 ohm cab as a 4 ohm cab by just adding a small piece of equipment.

    Joris doesn't need anyone to speak for him but with all due respect just because you think something should work doesn't mean it will work, Ben.

    First, an 8 ohm resistor big enough to handle the kind of power we're speaking of would be huge - and expensive.

    Second, resistors work by converting electrical energy to thermal energy - so although your huge resistor would absorb half the power it wouldn't make any sound. Speakers convert electrical energy to mechanical (and to a lesser extent, thermal) energy. Half the amplifier's power would go toward heating the resistor - not toward making sound. As Joris said, a single 8 ohm cabinet with no resistor would be louder.

    But - thanks for playing. We have some lovely parting gifts for you ;)
     
  16. Thanks Allan

    It's much nicer to be told your stupid by reading between the lines with a bit of tongue in cheek. It was a lovely game and I may even sign up again :D I guess it would be fair to say that with all the intellectual gifts I've been blessed with and understanding of electronics is not one of them. oh well you can't have everything;) . Cheers

    Ben
     
  17. I'm not inferring anyone is stupid, Ben - as a matter of fact I had this same idea several years ago and it made perfect sense to me.

    Remember, discoveries are made by not following instructions. Every invention we use today was called a stupid idea by someone :)
     
  18. Hey Allan

    Sorry if I offended (or gave the idea that I was) I was trying some of that sarcasm stuff. But to coin a saying I learnt while living in Virginia "no harm, no foul". :p

    Ben
     
  19. No worries here, Ben - you didn't offend me at all :)

    Keep thinking this stuff up, though. A lot of the stuff you come up with might be kinda silly but who knows - you might have an idea someday that completely changes the way we do something. I'll bet most inventors have some harebrained ideas from time to time - and the worst thing a person can do is stop imagining.