Could someone please explain ohms to me?

Discussion in 'Amps and Cabs' started by tnrose, Apr 18, 2008.

  1. tnrose

    tnrose Member

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    Ohms, and how it all relates to speakers, wattage, amps, etc.

    I don't want to blow anything up, and I like having an understanding of things.

    Thanks!
     
  2. StompBoxBlues

    StompBoxBlues Member

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    Ohms are units used to measure resistance in a electrical circuit. If you use the classic water pipe/plumbing analogy, current (amperes) is like water flowrate, voltage is like water pressure, and resistance (ohms) is like restrictions in a water pipe. Picture a pipe run through a house, where a few feet the pipe is half the diameter.It sould restrict the flow, (resistance) and water pressure would build up on the upstream side of it...

    Wire coils, like speaker wire coils, because of magnetic fields built up in the coil have a sort of resistance, that is measured as "impedance" but measured in ohms. Impedance is like a resistor that changes its resistance depending on the frequency of the voltage applied to it.

    All this stuff, there are some very well done tutorials on the net already if you use Google and google for things like "ohms resistance theory" and "ohms primer" etc.
     
  3. RickC

    RickC Gold Supporting Member

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    The output of the amplifier has a certain output impedance (ohms), and for most efficient transfer of power to the speaker, you want the speaker to have the same impedance. The typical values you see are 4 ohms, 8 ohms, and 16 ohms, and occasionally 2 ohms. Also note that when matching, you have to consider the entire combined impedance of the speaker load. For example, in a blackface Super Reverb, there are 4 x 10" speakers of 8 ohms each, connected in parallel; the combined impedance of these is 2 ohms. Thus the output impedance of the Super Reverb amp is designed to be 2 ohms.

    Mismatching between output and speaker is usually not good. At best, you'll get slightly less efficiency to speaker and maybe a change in tone (which is sometimes desirable; Mesa for instance used to encourage experimenting with this). At worst, you'll damage the amplifier. In general, if the speaker impedance is a lower value than the amplifer impedance, the speaker will try to draw more current, which may lead to bad things in the amplifier. Connecting to a higher impedance speaker is safer, but in the case of tube amplifiers can still cause trouble.

    /rick
     
  4. RickC

    RickC Gold Supporting Member

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    one more -

    Rule of thumb is, with old Fenders it's pretty safe to use twice or half mismatch in speaker ohms. For example, a blackface Showman (not Dual) is designed for 8 ohms output impedance. You could safely connect either a 4 ohm speaker load or a 16 ohm speaker load to that amp.

    Old Marshalls are generally intolerant of any mismatch; you always want to use the correct speaker impedance.

    For anything else, ???? It's best to assume the worst and always match.

    /rick
     
  5. tnrose

    tnrose Member

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    awesome, thanks
     

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