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4Ohms Volume into a single 8Ohm cab

Discussion in 'Amps and Cabs [BG]' started by TaySte_2000, Aug 23, 2002.


  1. TaySte_2000

    TaySte_2000

    Jun 23, 2001
    Manchester, UK
    Endorsing Artist: Mojohand, Subdecay, Overwater, Matamp
    Right I'm interested in a ashdown ABM 500 and the ABM 4x8 mini cab well this cab is only 8Ohms but I want to be able to get the full 575w out of the head. The speaker is capable of handling 600w but is there something you can buy to put in between the head and cab so the head thinks theres 2 cabs attached when there is only one. I really like to keep the 4x8 on it's own so theres less to take to gigs.

    Thanx for any ideas you can provide.
     
  2. awesome

    awesome

    Aug 14, 2002
    Belgium
    I don't think that's possible .. sorry.
    The thing is that if you add another 8 Ohm cab, the 575w will be divided over the 2 cabs, resulting in less power per cab (only 287.5w for 600w cab)
    (I would like some backup on this one guys)

    greetz
     
  3. Nope, sorry. If you want a 4ohm load you'll either have to buy a 4ohm cab or add another 8ohm cab.
     
  4. geshel

    geshel

    Oct 2, 2001
    Seattle
    A transformer. :D
     
  5. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Glockenklang used to offer a special transformer for that purpose - can't remember the specs though (power rating etc.) and I've never seen anybody use it.
    Better get a 4 Ohm cab...
     
  6. embellisher

    embellisher Holy Ghost filled Bass Player Staff Member Supporting Member

    A robot in disguise?:confused:?























    :p
     
  7. Short of a transformer, there isn't really any way to do this. If you used, say, a dummy load resistor or a power brake, you would get extra power out of the amp but you'd probably get less going to the cabinet! If you used a dummy load of 8 Ohms to present a 4 Ohm load to the head. The head would produce 575W but only half of that (about 285W) would go to the cab, the other half would go to the dummy load. The amp will probably produce somewhere around 400W at 8 Ohms (this is just a guess, Ashdown hasn't provided this info). So you're better off just going with either the 8 Ohm cab as is or with a 4 Ohm cab.
     
  8. steve-o

    steve-o Guest

    Apr 17, 2002
    marshall makes a power brake for this reason.
    will it work on bass?

    steve
     
  9. geshel

    geshel

    Oct 2, 2001
    Seattle
    The power brake is so you can push your amp into power-section overdrive without blowing your eardrums or killing your neighbors.

    So what it does is *reduce* the amount of power coming from the amp that goes to the speaker (turns it into heat, basically).

    (BTW, this is exactly what Spacegoat describes above! :) )

    To actually utilize more of your amp's power (IOW, higher volume) into an 8-ohm speaker you'd have to use a transformer (but there are downsides to that, hence it's very uncommon).
     
  10. geshel

    geshel

    Oct 2, 2001
    Seattle
    I'd like to see an amp advertised as having "high-current, toroidal Go-Bots" someday. :D
     
  11. Hategear

    Hategear Workin' hard at hardly workin'.

    Apr 6, 2001
    Appleton, Swissconsin
    Does the amp really divide that power between the two cabs? If I'm using a Peavey FireBass II (which I am) and I plug one 4-ohm cab into it, the amp puts out 475 watts, which only goes to that one cab, because that's all I have plugged into it, right? Now, if I plug another 4-ohm cab into that head, I reduce the total load to 2-ohms, which in turn boosts the output of my FireBass to 700 watts. Doesn't that 700 watts go into both cabs equally? Amps don't divide that in half and send half to one cab and half to another, do they? Maybe another way to think about it would be like this: If one cab is rated for 300 watts and another is rated for 400, the amp doesn't only send 300 to one and 400 to the other, they would both get the same amount of "juice" -- whatever the maximum output for that load is, right?

    Right?

    :confused:

    With that being said/asked, I was once told to add a dummy load to my amp, to simulate another cabinet, if I didn't want to add another cab, but wanted to increase the output of my amp.
     
  12. Hategear

    Hategear Workin' hard at hardly workin'.

    Apr 6, 2001
    Appleton, Swissconsin
    [​IMG]
     
  13. Hategear:

    When you have two cabinets of equal impedence connected to a single amplifier channel, each cabinet will receive half the output power of the amp. If what you were say was correct, each driver in a cabinet would also recieve the full power of the amp (say in a 410). So with the 2 4Ohm boxes attached, the amp will produce 700W. Each cabinet will receive 350W (this is a bit simplistic, but we'll keep it there for the sake of clarity). This is because the cabs are in parallel. When any two components with equal resistances are in parallel (speakers, resistors, whatever) the voltage across them will be equal but the current through each will be half the source current. Power (Watts) = Voltage*Current, which gives half the power for each component. It's called the Current Divider rule. So the power brake/dummy load thing is not really effective. :D
    Hope that makes sense!
     
  14. Jazz Ad

    Jazz Ad Mi la ré sol Gold Supporting Member Supporting Member

    There is a way, but it's pretty stupid.
    I've done it once with an oversized cab (600 W pushed by a 300 W head)

    Let's assume your 4x8 must me made with 4x8ohm speakers, seried and then paralleled 2 by 2 like this :
    Code:
     __8__8__
     !       !
    _!_8__8__!_ = 8 ohms
    
    (which is probably the case)

    THen you can short circuit one of the speakers and go like this :
    Code:
     __8__8__
     !       !
    _!____8__!_ = 5.33... ohms
    
    This way, you have a 1/(1/(8+8)+1/8)=5.33... ohm cab.
    You'll push more power than with your 8 ohm cab, but only thru 3 speakers instead of 4.

    I know it's pretty stupid, but it's the only way I know to achieve your thingy.
     
  15. Hey Jazz, now that is creative thinking. :)
     
  16. awesome

    awesome

    Aug 14, 2002
    Belgium
    Jazz Ad got me thinking ... what would happen if you put to 4 speakers parallel to get 2 ohm and put a 2 Ohm dummy load in series to get 4 Ohm.
    Is this right what I'm saying? :confused:
    If so, I have a few questions.
    How much power would each speaker get? Is the dummy load going to change you sound?
     
  17. geshel

    geshel

    Oct 2, 2001
    Seattle
    Half the power would still be wasted by the dummy load.

    Using a resistor of about the same impedance as the cab, whether in series or in parallel, will burn a lot of power.
     
  18. awesome

    awesome

    Aug 14, 2002
    Belgium
    I knew something had to be wrong.

    Thanks geshel
     
  19. Jazz Ad

    Jazz Ad Mi la ré sol Gold Supporting Member Supporting Member

    As usual, I look for cheapo inventive solutions :D

    geshel is right. We're talking physics here. Not voodoo.
    You won't square a circle. Whatever you do with those 4 speakers, if you go 4 ohms half of the power will be lost.
     
  20. just buy an ashdown mini 110 or 112, small, but will boost your sound loads, and your getting the 4 ohms u want.

    *Si*