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2 inputs to one cab, good idea?

Discussion in 'Amps and Cabs [BG]' started by Acepiloto, Sep 5, 2001.


  1. Acepiloto

    Acepiloto

    Aug 25, 2000
    I have a Rogue 300 watt, 4x10 cab w/no horn, it has two inputs and a switch for mono (4 ohm) and stereo (8 ohm). I realize that the main reason for two inputs are for the stereo operation of the cab. My Peavey Mark III head gives 210w at 4 ohms, and 300 watts at 2 ohms. I was wondering that if I hook up both outputs to both inputs, if it would lower the impedence to 2 ohms and give me the full 300 watts.:confused:
     
  2. JimM

    JimM

    Jan 13, 2000
    Northern California
    I wouldn't recommend it,at best you'd be accomplishing nothing,if the inputs are parralel that is,but if something isn't wired just right you could short the power amp.

    I think amps are rated two ohms for hype.I have a Sunn 1200s that supposedly will push 1200 watts through two ohms,but I'd need four 8 ohm cabs to do it.That's really making the amp work hard in my humble opinion.
     
  3. JimM

    JimM

    Jan 13, 2000
    Northern California
    Acepiloto,

    Sorry,upon rereading your post I see the switch is on the cabinet and it changes the impedence.

    The first thing I would do is get an ohm-meter and make sure the speaker outputs are in parallel,then make sure both cables are wired correctly,then check the cabinet impedence.better yet have a tech check it,or wait till someone like Joris who can do this stuff in his sleep reads the post.

    I can sympathize with wanting to get as many watts as possible without having to buy new gear.
     
  4. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    getting warmer
    If i'm reading your post correctly then the answer is no, you'll still be pushing 4 ohms.

    In stereo each side is 8 ohms, think of it as being two 8 ohm 2x10 cabs. Since the load will be paralleled (sp?) at the amp it will end up being 4 ohms again.

    Whatever you do make sure the switch is in the stereo position if you try this or else VERY BAD THINGS will happen to your amp.
     
  5. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania
    As far as I've known, your amp (unless it has a switch or specific speaker outputs) chooses how many ohms it will produce, meaning, if you put a 4 ohm load to it, it will produce 4 ohms.

    I don't see what you'd be accomplishing other then lowering your ohms, and producing a humble amount of power more. Check with the more experienced guys though... But I don't think you can "trick" your amp into producing a 2 ohm load when it's making itself produce a 4 ohm load.

    And, you'll be accomplishing nothing by hooking it up in stereo. If it only runs in stereo at 8 ohms, you'll be losing wattage.

    Check with the tech guys though, that's just as far as I've ever understood though... I'm more of the kinda person who hears stuff, and just goes off that, but is more concerned about playing then equipment (especially since I have all my equipment I'll need). :)



    To the person who said 2 ohm cabs are just hype? I think that's bull. I run my Peavey Firebass II 700 at 2 ohms, I've been running it at 2 ohms for about 2 years now, no problems, nothing. It's actually supposed to be run at 2 ohms, as far as I gather.

    ie: Firebass II 700
    I doubt they just like the way "700" sounds, and it just so happens to be 700 watts @ 2 ohms. :)


    Edit: You wouldn't need four 8 ohm cabs, They DO make 4 ohm cabs. ;)
     
  6. JimM

    JimM

    Jan 13, 2000
    Northern California
    It may be fine to run two ohms under ideal or close to ideal conditions,but I seem to remember reading on this board that going for several hours in a hot environment that the amp would be in greater danger of overheating than with four ohms,especially if the line voltage is going through a long extension cord etc.

    I work on appliances and I've seen microwave oven circuit boards that have lasted for years burn up when the customer decided to cook something for thirty minutes instead of the usual two minute cup of coffee.on the other hand,if your amp has already been through that and survived,it probably doesn't have the weak solder joints that lead to these problems.


    Edit: You wouldn't need four 8 ohm cabs, They DO make 4 ohm cabs. ;)
    [/QUOTE]

    I would want eight ohm cabs to go with the two eight ohm cabs I'm using now.otherwise,sure the amp will put 1200 watts into two eight ohm cabs and one four ohm cab,but why do that?The four ohm cab would be getting 600 watts and my two eight ohm cabs would get 300 each.It could be done,but would only be worthwhile if the four ohm cab had double the speaker area of one of the eight ohm cabs have.and I don't want to carry an 8x10 cab.it was a hard enough desicion to get the 4x10 instead of a pair of 2x10s.I also have an old 1x15 at eight ohms.

    that outta teach you not to make me try to 'splain stuff in the future. ;)
     
  7. CrawlingEye

    CrawlingEye Member

    Mar 20, 2001
    Easton, Pennsylvania


    I would want eight ohm cabs to go with the two eight ohm cabs I'm using now.otherwise,sure the amp will put 1200 watts into two eight ohm cabs and one four ohm cab,but why do that?The four ohm cab would be getting 600 watts and my two eight ohm cabs would get 300 each.It could be done,but would only be worthwhile if the four ohm cab had double the speaker area of one of the eight ohm cabs have.and I don't want to carry an 8x10 cab.it was a hard enough desicion to get the 4x10 instead of a pair of 2x10s.I also have an old 1x15 at eight ohms.

    that outta teach you not to make me try to 'splain stuff in the future. ;)
    [/QUOTE]


    No such luck with teaching me! :D

    I was speaking in terms of running just 2 cabs, like I do. I run two 4 ohm cabs.
    Each cab receives 350 watts, which sets my rig up perfectly, considering both my cab's RMS is 350 watts.

    My amp can only peak at 1400 watts, if I hit something very hard, and have everything cranked fairly loud. And, that's EXACTLY how much my cabs
    can take.

    I've never played my amp under what you'd call "hot" conditions, although, after about 5 hours in a room, in the summer, it tends to get hot in my house, the guitarists' head/amp usually heats up the room quite quickly. Although, even after a 2-3 hour practice session of me by myself, my head isn't even remotely hot. I've TRIED to get my head to heat up, but it won't.


    About the 2 ohms vs 4 ohms being more dangerous... That's just a rule of the thumb, less ohms = more danger your heads in, because of the more power that's being used. :)

    If you'd want to run your amp at 2 ohms, you could always cheat, and go two 4x10's and two 1x15's or something to that effect.

    Although, if you're producing enough power right now, it's really pointless.

    My point in saying that my rig works at 2 ohms and runs fine, is A) it does... and B) it annoyed me that you said it's just hype

    :)
     
  8. JimM

    JimM

    Jan 13, 2000
    Northern California
    Mark,

    Your point is,of course,well taken.Hype was the wrong word to use.
    In this age of consumer litigation,if a manufacturer says it'll do two ohms it will.I'm just a little over protective of gear.Especially after blowing out an Eden VT-300 by running it outdoors for two days in a row (it wasn't on the whole time) using long extension cords.In my youth I was more careless with gear,even tube gear,and it never gave me any trouble.

    At least I got someone stirred up enough to help answer the original question.:D
     
  9. MikeyD

    MikeyD

    Sep 9, 2000
    Amps do not "produce" ohms, nor do speakers "hold" watts. If anyone refuses to believe this, please study some electrical engineering.
    - Mike
     
  10. I'm gonna go along with JIMM on this one. Its not really hype, more of not reccomended by the manufacturer even though the amp can run at that level. Your amp may be an exception, and i'm not arguing that. I specificly remember somone talking about how some swr heads can go down to 2 ohms. Then some people had problems with it buringing out the amp. So when they asked the SWR tech support about it, they said "yeah, it CAN run at 2 ohms, but that should only be in certain situations.

    The problem with running an amp at 2 ohms is the heat. Over heating causes the amp to burn out, and with that, you have some problems.
    The lower the impedence goes, the harder the amp has to work to push out that strong of a flow, therefore causing the amp to get hotter faster.

    If you play an amp in one situation, run it at 8ohms, then set up the same situation, but running the amp at 4 ohms, you'll notice the fan will go on much faster. Why? the amp is working harder.

    I run my amp at 8ohms mono-bridged (800w). It heats up pretty quickly with the master at 3 o'clcok and master at 12. I'll be able to play about a half hour before the fan goes on. I've never had the "thermal shutdown" and i hope i never do. This happens only when the room is around 90-100 degreese. If i ran my amp on one side, 8 ohms, i'd get considerably less wattage, but i'd also have the fan turn on less often. why did i type this part up? Simply to show, that 2ohms isnt some magic number, and that some amps can handle a lower impedince, and sometimes, the maximum wattage of the amp can be very high, at a high impedance. There is a reason why eden says the minimum impedance for the amp mono bridged is 8ohms.
     
  11. Hategear

    Hategear Workin' hard at hardly workin'.

    Apr 6, 2001
    Appleton, Swissconsin
    Actually, the lower the resistance (ohms), the less your amp has to "work," but the "hotter" it will run. That's why the output goes up when running at a lower resistance.

    If you think of a straw -- the smaller the diameter of the straw, the less you have to work to suck or blow something through it. Imagine a coffee stir-stick straw as being 8 ohms and a McDonald's shake straw as being 2 ohms, with an in-betweener as being 4 ohms. It's easiest to suck through the shake straw than it is to suck through the in-betweener, which in turn is easier to suck through than the stir-stick. See? Less work, greater gain.

    Man, there are a lot of "sucks" in this post. :D
     
  12. MikeyD

    MikeyD

    Sep 9, 2000
    While we're grasping at straws... Work is force times distance. Force blowing through a straw is the pressure times the cross-sectional area. The work done is the pressure multiplied by the area multiplied by the distance the air travels. Power is the rate of work, or the force times the distance divided by the time it takes to move that distance. Alternately, power is the flow rate (volume of air flow per unit time) times the pressure.

    So - in the case of the amplifier analogy, an amplifier delivers more power at lower impedance because it is delivering more energy (work) per unit of time: this means it IS working harder. The straw analogy is okay, except what you're missing is that, from the amp's perpective, the pressure is the same whether the "straw" is big or small: the flow increases with bigger straw, however, and therefore the work done increases (per unit time).

    For these same reasons, your vacuum cleaner whines at a higher pitch when you block the hose: you are stopping the air flow (think of impedance going to infinity), thereby unloading the machine so the blower is unable to do useful work.

    - Mike
     
  13. JimM

    JimM

    Jan 13, 2000
    Northern California
    So by that logic,if your amp sees a zero ohm load,it will do no work at all.I agree with that!

    Sorry Hategear,I couldn't resist.
    Seriously,I think heat is the real enemy of amps.