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Ohms

Discussion in 'Amps and Cabs [BG]' started by Piezoman, Feb 10, 2003.


  1. Piezoman

    Piezoman

    Nov 29, 2002
    Bronx, New York
    What is an ohm. All you guys talk about is 700 watts at 8 ohms. I dont get it. What is an ohm and what is the system for them?
     
  2. JMX

    JMX Vorsprung durch Technik

    Sep 4, 2000
    Cologne, Germany
    Basically, all you need to know is this:

    A cab represents a certain load, measured in Ohm. This load is called impedance.

    The power an amp delivers depends on the load it is hooked on. The lower the Ohm rating of the cab, the more power the amp produces.
    E.g. your amp will give you theoretically 700W when hooked to a 8 Ohm cab. When hooked to a 4 Ohm cab you'll get roughly a third more power.

    Read this for more thorough basic info [very technical]

    http://whatis.techtarget.com/definition/0,,sid9_gci212333,00.html

    Or better, do a search for Joris' amp faq.
     
  3. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Here is a brief overview.
    Ohm's law describes basic electrical relationships
    in a circuit.

    To do this we need to define certain electrical events.

    Current {I} - The quantity of electrons passing a given point - {Unit: Ampere}

    Voltage {V} - Electrical Pressure or Force - {Unit:Volt }

    Resistance {R} - Resistance to the flow of a current -{Unit: OHM }
    and
    Power {P} - Work performed by a current -
    {Unit: Watt}

    Ohm's law defines the relationship between these things as follows:

    V = I x R or in English
    Voltage = Current x Resistance

    for example
    120V = 15 amps x 8 ohms

    one further concept

    remember Watts [ near and dear to our hearts ! ]

    P = I x V
    or
    P = I squared x R

    Using the above example

    1800 watts = 15 amps x 120 volts
    ---------
    Hope that helps.
    :confused:
     
  4. Schwinn

    Schwinn

    Dec 4, 2002
    Sarasota, FL
    If I have an amp with two channels, and I'm running in stereo or parallel mode, does each channel have separate impedence?

    For example, would one 8 ohm cab in each channel = 4 ohms total? Or am I getting 8 ohms with each channel?
     
  5. Thor

    Thor Moderator Staff Member Gold Supporting Member

    An amplifier with 2 separate channel outputs is actually rated at the output terminals for the impedance. So if you use one or both, it is the same output from the amplifier.

    HOWEVER, if you put a splitter and put two cabs on one output, you will now change the ohmage of [ impedance ] the speaker circuitry, risking some damage depending on the wiring.

    If you add 2 8 ohm wired in series, you will have a 16 ohm load.
    ------/\/\/\ r1/\/\/\ ---------/\/\/\r2/\/\/\------
    rtotal = r1 +r2 + ....
    If you wire them in parallel you will have a 4 ohm load

    -------------------------------------------
    | |
    > >
    < <
    r1 r2
    > >
    < <
    | |
    --------------------------------------------

    Rt = R1 x R2 4 = 64
    ------- --
    R1 + R2 16

    This is why, when building 4x cabs, they internally
    wire 2 sets of 2 speakers together in parallel and then hook the speaker sets together in series, bringing the whole array back to 8 ohms.

    8 ohm cabs with 2 speakers will have internal resistors often to accomplish the same thing.
    Most bass rigs have sufficient outputs or lines out to accomodate the users needs without worring about any of this.

    Matching your speakers to the amplifier outputs
    is sometimes referred to as impedance matching.

    Plug me in I'm ready to rock all night long!
    The Big O [ Not Big O from Ontario though ....
     
  6. Thor

    Thor Moderator Staff Member Gold Supporting Member

    I guess it got rearranged a little

    R Total =

    R1 X R2
    -------
    R1 + R2

    for the parallel circuit

    Big O
     
  7. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Guys this is all excellent but if I was a newbie to the whole ohms thing (which I presume Piezoman is)I'd be as confused as anything right now.

    Here's the simplified version.

    Don't get burried in relationship between watts and ohms. It not the most important part. Get the ohms wrong and you will blow your amplifier. All amps have a minimum ohms rating (eg 800w at 4 ohms). If I ran that amp at 4 ohms, it will produce 800 watts, 8 ohms = roughly 550 watts. However if I run a 2 ohm load, the amp will blow.

    When you hook a speaker up to an amp, the speakers create a certain amount of resistance to the amp. It's kinda like running up a hill. The slope of that hill is what we call ohms - 8 ohms is a steeper hill than 4 ohms. So to run at 8 ohms, the amp produces less watts because it's got more resistance to push against. At 2 ohms it's like running down a cliff face, the amp stumbles and will faulter.

    This stuff is really important, especially whan you use more than one cabinet.
     
  8. I could be wrong here, but I remember reading somewhere that with tube bass heads it was the other way around. You could safely run cabs with a lower rating than the head, but it would have problems with cabs of higher impedence. Anyone?
     
  9. There are far more qualified TB's than me, but here goes. For instance, My Mesa Boogie 400+ is an ALL tube amp. The power section has a different output section than SS or Hybrid heads. The output transformer which I believe converts ac to dc power or the other way around, is the big difference when it come to resistance/impedence/ohms (all the same thing as stated above) You still must have speakers with the correct load/ohms when you hook up a tube amp, ANY amp. It's just that all tube amps see the resistance a little differently. My 400+ has two 8ohm speaker outputs, two 4ohm, and two 2ohm. Boogie's hookups are a little confusing at first at least to me. You can mismatch cabs, but they recommend using equivalent cabs. If I were to use two 8ohm cabs I would plug them into the two 4ohms outputs (see, that is were Boogie is confusing) if I had two 4ohm cabs I would plug them into the two 2ohm. If I used one 8ohm I would plug into a single 8ohm. The reason they have to 8ohm is if you had a couple of 16ohm cabs which I've never used.

    Bottom Line. Read your owners manuel, and if you don't understand it, ask the OEM (original equipment manufacturer) That's what I did and cleared it all up for me.

    It's like Petebass said before, you'll blow up your stuff if you don't hook it up right. It's frustrating, but it will all make sense if you take the time.
     
  10. Piezoman

    Piezoman

    Nov 29, 2002
    Bronx, New York
    what happens if im using 2 cabinets, Do I have to do something. Or Can you control the ohm "load" or something like that.

    Thanks for the two replies I could understand
     
  11. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yep the rules for tube are different... I'll let a tube expert tackle this one.

    When you use more than one cab, it changes the overall ohm value. For example, run 2 cabs that have an 8 ohm resistance each, the total load becomes 4 ohms if theyre linked in "parallel", or 16 ohms if it linked in "Series". It's very rare that cabs will be linked in Series. 9 times out of ten the extra cab will be linked in Parallel. For example, if you link the cabs via the input/outputs on the back panel, that's parallel. Or of your amp has 2 speaker outs and you plug a speaker into each one, that's also a parallel link.

    There is a formula for working all this out and I won't bore you with it here. Just be aware of these things when adding cabs to your rig, especially cabs with ohms rating different to each other.
     
  12. Piezoman

    Piezoman

    Nov 29, 2002
    Bronx, New York
    What is the difference betwen series and parralel. How do I control that
     
  13. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Let's say you have a simple amplifier and 2 loads,
    such as 2 plain speakers.

    From the amp if you connected the positive +
    lead of the amp to speaker 1 + terminal, and then
    connected a wire from speaker 1 - neg terminal to the + terminal of speaker 2 and then connected a wire from neg - terminal of speaker 2 to the negative terminal of the amp, they would be wired in series. One after the other, like batteries in flashlights.

    For parallel, connect + terminal on amp to speaker 1 + positive terminal, and then connect to speaker 2 + positive terminal. Similarly connect amp negative to speaker 1 negative and speaker 2 negative.

    At this point, a trip to Radio Shack or the library would be most beneficial. Find a book on basic electricity, and circuits and read it over. This will
    greatly improve your understanding of electrical energy and how basic amps work. You don't really need to be able to calculate all this stuff on your own, but basic knowledge of amperage, voltage,
    impedance, power and voltage drop can only be helpful to understanding your equipment.

    --------
    I got friends in low places
    ----- Big O
     
  14. Piezoman

    Piezoman

    Nov 29, 2002
    Bronx, New York
    Thnx for all your replies but would like to commet Pete on the only understandable one. I did find the scientific ones rather funny.
     
  15. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Hey, I like the scientific ones :)
     
  16. Thor

    Thor Moderator Staff Member Gold Supporting Member

    Piezoman,

    I'm sorry you felt the responses were not
    understandable. You expressed a desire to
    know what this was about and how it works.
    I think I tried to address that need.

    The formulas and concepts are based on simple
    arithmetic. Just like practicing the bass, these
    concepts will become simple and comfortable
    for you if you do your homework on them.
    That means studying your amps, speakers, the
    owner manuals, and relating this all together
    using the concepts above.

    Electronics has its own language, you have to
    learn the basics to be able to utilize it. A really
    basic book on electronics would help you get
    started.

    I find it interesting you chose the name 'Piezoman'.
    'Piezo ' in electronics has to do with the changes
    in electric charges in a crystal such as quartz when
    mechanically deformed, or the vibrations produced in the solid when an electric charge is applied to it.
    Such as a buzzer or cell phone speaker. Piezo speakers such as tweeters have very different properties from regular speakers. This part of the
    electronics field is about as technical as it gets,
    so as I was curious why you chose that.

    Big O
    ----------
    I'm really ready for summer now.