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Umm...Ohms? Ehh..help

Discussion in 'Amps and Cabs [BG]' started by Pawn, Aug 20, 2005.


  1. Pawn

    Pawn

    Aug 5, 2005
    Hello..im new to Guitar amps. Im going to get a used Peavey 115 amp. I understand that it is 200 watts in to 2 ohms, and 150 watts in to 4 ohms, what the hell does this mean? Does this mean i could hook up 2 4 ohm cabs and get an extra 200 watts? or 2 8 ohm cabs and get an extra 150 watts? Please help because i have no clue what this means. :eyebrow:

    Thanks :help:
     
  2. No. On all counts. Check out the Sticky, and/or the search function.
     
  3. Warwick player

    Warwick player

    Dec 31, 2002
    Bucks, UK
    Hiya,

    I taken it you're new to Talkbass.com as well as amplifiers.

    What Bassline Delux is trying to say is that this topic is covered quite a lot on here so check out the sticky at the top of the page about using the search function.

    Type in Ohms and cabs and some previous threads will probably come up with your answer. If you're still in trouble PM and i'll see if i can help

    Cheers and Beers

    Andy
     
  4. Pawn

    Pawn

    Aug 5, 2005
    emm..i did..the stickys didnt really cover it..and if i search for ohm ill get tons of threads
     
  5. its a combo amp, you wont usually use more than one external cab, if you do you would have to daisy chain them

    the 115 is 150 watts by itself, the speaker it has is 4 ohms, adding an external 4ohm speaker cabinet will lower the impedance to 2 ohms (4 ohms + 4 ohms in parallel = 2 ohms) and hence mean the amp will put out 200 watts
     
  6. Pawn

    Pawn

    Aug 5, 2005
    im sorry...im must of missed one of the FAQ's and now i see it. i have gone over them before. sorry for the thread.
     
  7. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA
    OK, I'll take a wag at it....

    Of course, in order to get sound from your amp, you must connect it to a speaker. All speakers have a certain impedance, which, oversimplified, is a sort of electrical "resistance". If the impedance is too low (below 2 ohms), your amp will likely burn up.

    If you hook up an 4-ohm speaker (or a connection of multiple speakers which combine to give an overall impedance of 4 ohms), your amp will put out 150 watts of power.

    Connect to a 2-ohm speaker (which are actually very rare; more likely, you'll connect to an array of multiple speakers which combine to give 2 ohms total impedance), and it'll pump out 200 watts.

    Does that help?
     
  8. Pawn

    Pawn

    Aug 5, 2005
    ok...after reading the sticky i was still puzzled...but that helped alot..thanks
     
  9. Interceptor

    Interceptor Supporting Member

    Mar 29, 2005
    Madison, WI
    I've seen far too much confusion over this. Perhaps this will help.

    Ohms is the measure of resistance in a circuit. Resistance calculations can be made for either series or parallel circuits using very simple math, as defined by Thevenin (physics really is fun!) In series circuits, the resistance of a circuit is the sum of the resistors in the circuit. As an example two 10 Ohm resistors in series = 10+10=20 Ohms.

    In a parallel circuit, which is what is almost always the case when adding speaker cabinets, the resistance is determined by 1/total resistance = 1/resistance 1 + 1/ resistance 2. As an example, lets consider two 16 Ohm cabinets: 1/16 + 1/16 = 2/16 or 1/8 as such the result is 8 Ohms.

    Applied to the resistances typically found in production cabinets, here we go...

    Two 4 Ohm cabs 1/4 + 1/4 = 2/4 which reduces to 1/2 as such the solution is 2 Ohms.

    Two 8 Ohm cabs 1/8 + 1/8 = 2/8 which reduces to 1/4 as such the solution is 4 Ohms.

    One 4 Ohm plus one 8 Ohm 2/8 + 1/8 = 3/8. Huh? To reduce this to 1/x, the solution is found by dividing by top number, which is three in this case, so, 8/3 = 2.67.

    Sorry, I'm convinced at some point, we just got to face the math and get over it.
     
  10. Interceptor

    Interceptor Supporting Member

    Mar 29, 2005
    Madison, WI
    Guess what, there is more of this math / physics garbage....

    Power amplifiers output levels in solid state amps are primarily determined by the voltage of the amplifiers power supply and the load resistance. A "perfect" amplifier would exactly follow the model of Volts (Squared)/resistance = Watts. It isn't really that simple in practice, due to internal resistances, power supply design, etc. When we break it down once again in pretty simple math, the amplifier's voltage remains a constant, with resistance being the variable. As an example, an amplifier with a 50 Volt power suppy into an 8 Ohm load:

    (50*50)/8 = 2500/8 = 312.5 Watts.

    Now, let's try that amp (in a perfect world) at 4 Ohms

    (50*50)/4 = 2500/4 = 625 Watts

    I hope this helps
     
  11. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA
    When you're dealing with two cabs (and it's the rare bass rig that uses more than two cabs), I've always found the multiply/sum calculation method much easier:

    Two 4-ohm cabs: 4X4/4+4 = 16/8 = 2 ohms
    Two 8-ohm cabs: 8X8/8+8 = 64/16 = 4 ohms
    One 8-ohm, one 4-ohm: 8X4/8+4 = 32/12 = 2.67 ohms

    Nothing wrong with your method at all; this is just a little more intuitive, and easier to remember (at least for me)....
     
  12. Pawn

    Pawn

    Aug 5, 2005
    so if i wanted with the peavey i could add

    1) 2 4 ohm cabs @ 2 ohms to get a total of 200 watts
    2) 4 8 ohm cabs @ 2 ohms to get 200 a total of 200 watts
    (if i was crazy)
    3) 1 4 ohm cab @ 4 ohms to get a total of 150 watts
    4) 2 8 ohm cabs @ 4 ohms to get a total of 150 watts
    5) 8!!! 16 ohm cabs @ 2 ohms to get a total of 150 watts
    (if i was evean more crazy!)
    6) 4 16 ohm cabs @ 4 ohms to get a total of 150 watts

    right? and its not really # of cabs but number of speakers right?

    Thanks for all the help bigbeefdog,Interceptor,i_got_a_mohawk :hyper:
     
  13. the thing is, its a combo amp (i take its a TNT if those are the wattages you were told)

    so you more than likely can only add ONE cab in parallel (because the speaker in the combo is always there, so think of it as an amp that always has a 4ohm cab attatched), as there will only be one spare output

    so, you could only really add another 4ohm cab (to give 200watt) or an 8ohm cab (2.67 ohms so say about 175 watts)


    And the speakers have thier own impedance (ohm rating), but because of the way they are wired its the cab's impedance you look at, a 4ohm cab coould have 4x10" speakers, each speaker at 16 ohms, but because they are wired in parallel it means the cab (all the speakers in the box together) are 4ohms, because thats what the amp would "see"
     
  14. of course, you could daisy chain cabs, but thats another story, thats putting the cabs in series instead of parallel, so two 4 ohm cabs in series would give 8ohms, but, i dont want to confuse you :p
     
  15. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA
    Not always.

    In many cases, daisy-chaining cabs actually connects them in parallel.... depends on the particular cab, and it's internal wiring......
     
  16. bigbeefdog

    bigbeefdog Who let the dogs in?

    Jul 7, 2003
    Mandeville, LA
    You have it now. :)

    But no, it's not the number of speakers - it is the number of cabs, or "loads". For example, you might have an 8x10 cab that has an overall (cab) load of 4 ohms. Or you might have a 1x12 that is also 4 ohms.

    They both factor in the same in the calculation - as a 4-ohm cab, and as a single 4-ohm load when you calculate total connected load.
     
  17. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    It's a great amp to start with. I wouldn't get too worried about connecting more cabs- I think you'll find that you spend a lot to only get a little louder; 200w just doesn't go that far. It's a good enough amp for practice, and if it isn't loud enough for shows, I'd look at another amp completely and keep this one for practice (saves toting junk around).
     
  18. ah right, i thought it would always be, but i guess, i can kinda picture the wiring making it in parallel
     
  19. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    Actually, daisy-chaining not just sometimes but *nearly always* results in a parallel connection, unless you have special wiring somewhere.
     
  20. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    No, you can't "add" a 2 ohm load *if you're already driving the 4 ohm load represented by your internal speaker*. If you drive a 2 ohm load in addition to a 4 ohm load, you get a total lower than 2 ohms, which is bad. All the speakers hooked up to the amp, *including the internal one* (if you're using that one too)* must add up to a load no lower than 2 ohms.

    Thus, if you use the 4 ohm internal speaker, any other cabs you hook up, whether a single cab or more than one, must add up to a total of *no less than 4 ohms*. This means, for example, one 4 ohm cab, two 8s, an 8 plus two 16s, or four 16s.