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Heads and Cabs

Discussion in 'Amps and Cabs [BG]' started by Angry_Monkey, Feb 17, 2003.

  1. Ok
    First of all, Hi my name is Nick, I know basically nothing about amps and ohms and stuff so in my current search for a better setup, I am seeking knowlege from the ones who know best.
    ( Everyone except me )
    I was given the impression that if you have a 400 watt head, and two 4x10's pushing 400 watts each. I kinda thought that you would have to have an 800 watt head to push 400 watts from the two cabs.
    So, can a 400 watt head give enough power to two 4x10's.
    Sorry I'm trying to learn. lol. If you have any other comments about amps and stuff, feel free to let me know.
    Thanks a bunch.
  2. xyllion

    xyllion Commercial User

    Jan 14, 2003
    San Jose, CA, USA
    Owner, Looperlative Audio Products
    Where do we start on this one?

    Let's start with cabinet impedance. Most cabs are either 4 ohm or 8 ohm cabinets. Other values are possible but much less common. If you have two cabinets that are both 8 ohms and drive them in parallel (the most common way to wire them), then you will have a combined impedance of 4 ohms. With two 4 ohm cabinets, your combined impedance will be 2 ohms.

    On the amp side of things, your amp will be rated as a power at an impedance. For example, it might be rated as 400W @ 4 ohms which means that it will deliver 400W into a 4 ohm load. If that 400 ohm load is 2 cabinets, then it will split the power equally putting 200W into each cabinet.

    This being said, the numbers tell you absolutely NOTHING about how loud your rig will be. Those numbers aren't even guaranteed to mean anything when you compare to different brands. Everybody rates their equipment in a different way. In fact you can bet that some equipment can deliver higher than specified where as other equipment barely makes it to the rated power. On top of that, different speaker cabinets require different amounts of power to acheive a similar volume.

    The bottom line is that you will need to listen to different rigs to determine if they are loud enough for you. BTW, you'd be surprised how far a 100W amp can take you if you get the right amp.
  3. Actually, speaker cabs don't push watts or anything else except air. It's the amp that does the pushing and the cab that does the resisting. The less the cab resists (ohms), the more the amp can push (watts). Up to a point that is. If the cab(s) put up too little resistance then the amp tries to put out more watts than it was designed for, it gets hot and turns into a steaming puddle of goo.

    With some exceptions, solid state bass amps can handle a total speaker load of not less than 4ohms. If the resistance is greater than 4ohms, solid state amps generally don't care except that the more resistance the cab puts up (ohms), the less power (watts) the amp can put out. An amp that delivers 200 watts into a 4ohm load, for example, typically only delivers 130 watts into an 8ohm load. Running an amp with a resistance load less than the manufacturer's reccomendation is risky business and could put somebody's eye out.

    The ratings on speakers cabs are nuts. You'll hear all kinds of alien terms and most of them actually do mean something, but the power rating we concern ourselves with most is RMS. Now there are various arguments and volumes of confusing facts written about what RMS (root mean square) actually means, but for our purposes as bass players RMS means how much clean, undistorted working power a speaker can handle. If you feed a speaker a distorted signal then all bets are off about how much it can take. There is also much debate, and much of it can be found on this board, about what is and what is not distortion and how much of a role it actually plays in destroying speakers, but everybody seems to agree (for whatever reason is in style this week) that sending a clipped signal to speakers is a bad thing. Clipping is what happens when you turn an amp, particularly a solid state amp, too loud and it starts distorting. The tell-tail sign of a amp that's clipping is when the speakers sound like they're farting.

    To avoid farting you want to be sure the signal you feed to your speakers is free from clipping which means that you want to have enough amplifier power on hand to crank it loud without distorting. If you have a cab that can handle 400 watts RMS of clean power the best way to be sure it gets 400 watts of clean power is to use an 800 watt amp. If you have 2 speaker cabs that combined can handle 800 watts, then a good way to ensure that they're getting 800 watts of clean power is to use a 1600 watt amp which brings us finally to your original question... can a 400 watt head give enough power to two 4x10's.

    The answer...yes, but watch for farting.

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