Amplifier basics

Discussion in 'Amps and Cabs [BG]' started by checkmate, Sep 20, 2003.

  1. checkmate


    Sep 20, 2003
    Hello everyone! I'm new to the forum and just wanted to ask a question about amplifier ratings. I see ratings such as 500 watts @ 4 ohms, 400 watts @ 8 ohms, etc... and I was wondering what exactly this means? And how does different speaker configurations determine this rating? I'm new to these things too, so nothing too technical please. Thanks for your help.
  2. kirbywrx

    kirbywrx formerly James Hetfield

    Jul 27, 2000
    Melbourne, Australia.
    ohms means resistance.

    Thats all i know. I know nothing when it comes to resistance of amps ect. But stay tuned, someone will help us soon :D
  3. miccheck1516

    miccheck1516 Guest

    Feb 15, 2003
    two 8ohm cabs give a resistance of 4 ohms
    two 4 ohm cabs give a resistance of 2 ohms
    There is a little more to it than that, but that gets me by
  4. Mcrelly


    Jun 16, 2003
    Minnesota, USA
    check out this link from a local electronics wizard...

    The lower the ohms the less "resistance" the amp has in front of it when trying to make 'sound'. less resistance means the 'watts' of the amp will increase.

    your example I assume is from one amp? I don't know if the same 400w into 8 is actually quieter than the 500w into 4ohms, but if there are more speakers involved to lower the ohms to 4ohms then it probably will sound louder with same amp. also some will say that 4ohms infront of the amp will sound more 'punchy' and quick I suppose this is true, but I've not experimented.

    500w into 4ohms is adequate for most situations and so is 400w into 8ohms IMHO.
  5. wneff


    May 27, 2003
    Woburn, MA

    The ratings and the Ohms are relatively simple.

    All this I am describing here is for Transistor amps. Tube amps have a switch in the back so you can match the amp to the cabinet your'e using.

    Every power amp is designed to be used with certain speakers. The Ohm number answers two questions:
    1) Is it possible to use a particular amp with a particular speaker cabinet
    2) How much power can I expect from this combination.

    How much power an amp delivers depends on what speaker you connect to it. Most common for bass are speaker cabinets with 8 Ohm and with 4 Ohm.
    Typically (but not always) a cabinet with 1 or 4 speakers have 8 Ohm, 2 or 8 speakers in a cabinet have 4 Ohm.

    If you hook up more than one speaker cabinet to the same amp the Ohm value the amp "sees" is lower than that of only one.

    Here is a list of common values:
    8 Ohm + 8 Ohm: amp sees 4 Ohm
    4 Ohm + 4 ohm: amp sees 2 Ohm
    8 Ohm + 4 Ohm: amp sees 2.7 Ohm

    Amplifier are designed either for 2 Ohm or 4 Ohm MINIMUM load. This means that all speaker hooked up to the amp can not go below 2 Ohm or 4 Ohms.

    But any vlue higher than that is OK, but you'll not get the full power out of the amp. So, you can hook up a cab with 8 Ohm to an amp that is dsigned for 4 Ohm.

    Most amps have about 70% at twice the load and 50 % at four times the load.

    Example 1: You have a power amp and a cab with one 15" speaker.
    1) Look at back of cabinet: Label says 8 Ohm
    2) Look in manual of power amp: Minimum load is 2 Ohm.

    -> You can hook up the 15" speaker
    -> Amp will delivers about 50% of the power it is designed for.

    Example 2:
    You have an amp that is made for 4 Ohm minimum load (very common) and you want to connect a 2x12" (4 Ohm) and a 4x10" (8 Ohm) cabinet:
    -> Total load is 2.7 Ohm, below the min. rating of the amp. You can not ust the cabinets with this amp.

    Example 2b:
    BUT: You can have the 2x12" re-wired (VERY simple if you don't have a tweeter in there) so it has 16 Ohm. Its not on the list but the 8 Ohm and the 16 Ohm is going to be somewhat higher than the two 8 Ohm. So, the total load is higher than 4 Ohm, you're fine, and you'll get almost all the power.

    The symbol for Ohm is the greek Omega
    To see one, type a "W" in a word processor like Openoffice ( and change the font to symbol. That is to avoid confusion with the zero.

    For all practical purposes the power you lose by not perfectly matching cabinets and amps is not that great, Even if you lose 50%, don't sweat it, in terms of loudness its not that severe.

    Let me know if you want to know more.
  6. checkmate


    Sep 20, 2003
    Thanks for the explanations. I'm a lot clearer now on what this means. One more question. You said, for example, that if the if the amp is rated for 4 ohms minimum, you can't use a speaker configuration that would give you 2.7 ohm. Is this because the amp would overpower the speakers and damage them?
  7. No, it's because the speakers would draw more current than the amp is capable of delivering, and thus could burn up the amp.

    Think of it this way. You have a barrel of water, with a hose adding water in the top of the barrel. Now you put a hole in the bottom of the barrel. The hole lets water out...that would be like adding one speaker. You put another hole in the bottom--adding another speaker--and even more water flows out. But if you put enough holes in the barrel, eventually there's going to be more water flowing out than the hose is putting in. The barrel soon runs dry.

    Tiny holes would be like speakers with large resistance, say a 16 ohm speaker. You could put several tiny holes in the barrel, or one bigger hole--say a 4 ohm speaker.

    The current (amperage, abreviated to amp) that an amp can deliver is limited by its power transistors. They can only provide a certain amount of current (the hose in the barrel). When you plug in more speakers than permissible, the speakers are "asking" the transistors to provide more current than they are capable of providing, and they can overheat and literally fry. Higher quality amps provide more current capacity, in general, and they can run speakers with low impedances, down to 2 ohms. Poor quality amps may only run an 8 ohm speaker, trying to plug a 4 ohm speaker into an amp that's only rated for an 8 ohm speaker will draw too much current and possibly harm the amp.
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