Somebody please explain speaker impedance...

Discussion in 'Amps and Cabs' started by Nuclearfishin, Feb 15, 2009.

  1. Nuclearfishin

    Nuclearfishin Supporting Member

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    Boy, I still don't get it. Could somebody please explain speaker/amp impedance in layman terms? I have a Peavey 5150 combo and it says on the back, no less than 16 ohms. The speakers each have a sticker that say 8 ohms. I have a '64 Bassman head which is supposedly at 4 ohms, matched with an avatar cabinet built for this head, and it says 4 ohms on the cab. I want to buy an attenuator, but I'm not buying anything until I understand what the heck I need. When the impedance of a cab is listed, is it per speaker, or for the entire cab? Same for the head--when it says 16 ohms minimum, how do I interpret that?

    Any help explaining all this would be greatly appreciated. Thanks!
     
  2. mc5nrg

    mc5nrg Supporting Member

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    When the impedance of a cab is listed it's for the whole cab. Depending on how its wired different conbinations of speakers of certain impedances will combine to create a cab of X impedance. There is a math formula to figure this out. Its easier to do this with a couple speakers of the same impedance, Two 8 ohm speakers wired in parallel create a 4 ohm load, wired in series its a 16 ohm load. It gets more complicated when wiring a 4 speaker cab which generally are wired series/parallel (but not always) or mixing speakers of different impedances. In the example of a typical 4-12 Marshall cab w/individual 16 ohm speakars, series/parallel* wiring gets you a 16 ohm load. There are better explanations on various sites including the full math formula such as the weber.vst.com site and the alternate.guitar.amps google site FAQ though I forget exactly how to get there at this hour. One thing to keep in mind that actual resistance varies with frequency and if you measure a cab you usually get a figure a bit lower than the rated impedance.

    Knowing exactly what the figure listed near the amp output jacks means, can vary depending on just what the manufacturer has in mind. Is it a head, an extension jack for a combo with an already connectred speaker etc...In your case, they are strongly suggesting to only use a 16 ohm cab with that jack. Is that the only output jack and does it go to the speakers in the combo or is it an extension jack for another cab with the combo speakers remaining powered? No impedance selecter? Peavey itself would be a good sorce of info but the intent seems pretty clear.

    A typical tube head would have different output jacks or a selecter to allow for different speaker loads which the amp "wants to see" aka drive comfortably. In general ,tube amps can tolerate an impedance mismatch of a factor of 2, with lower mismatch being harder on the tubes and upward harder on the output transformer. Some amps don't much like this however. SS is different and a load below x ohm minimum generally means blown amps, upward usually not a problem. SS amps give there full rated power at a certain load, upward speaker loads mean less watts output.

    *or parallel/series

    As for the attenuater question it either should be selected for the speaker load the amp will drive (entire cab) or be switchable...I guess.
     
  3. Nuclearfishin

    Nuclearfishin Supporting Member

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    Thanks, that actually helps a lot. The Peavey is a combo with no selector switch, and the 16 ohm minimum sticker is on the extension jack with the combo speakers remaining powered. So it sounds like the Peavey is a 16 ohm cab, and the Avatar is two 8-ohm speakers wired in parallel to create the 4-ohm cab for the bassman. When you add an extension cab, I assume it is wired so the load on the combo speakers doesn't change? Does the jack for extension cabs typically put out the same load as the main speaker jack?

    Thanks again!
     
  4. rockon1

    rockon1 Member

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    If the internal speakers remain powered when the 16 ohm ext. jack is used the amp most probably switches to 8 ohms when anything is plugged into the extention jack (just like the Classic 30). Thats why it calls for a 16 ohm ext. cab since 16 and 16 in parallel is an 8 ohm load. Bob
     
  5. michael.e

    michael.e Supporting Member

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    There is SO much info here about this subject. Use the "search" button.

    I just typed in "Speaker Impedance" and got 250 hits.
     
  6. jeak

    jeak Member

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  7. riffmeister

    riffmeister Gold Supporting Member

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    1) The two 8 Ohm speakers are wired in series* which produces a total load of 16 Ohms.

    2) I am going to guess that the Avatar cab also has two 8 Ohm speakers in it. But this time, they are wired in parallel* for a total load of 4 Ohms

    3) The cab impedance is the total load of all the speakers in the cab. Your Peavey cab is a 16 Ohm cab, your Avatar cab is a 4 Ohm cab.

    4) Sounds like your amp's output transformer wants to see a 16 Ohm load. The Bassman's output transformer wants to see a 4 Ohm load. Many modern amps have an impedance switch on the back panel so you can switch the output transformer to 4 Ohms, 8 Ohms, or 16 Ohms, depending on the impedance of the cab(s) you are using.

    *There are two ways to wire a pair of speakers, in series or in parallel. I could explain the difference, but it would probably be confusing. Pictures of each would be the best way to convey the differences.

    Once you know whether it is series or parallel wiring, here is how you calculate the total load:

    Series: Speaker A + Speaker B

    Parallel: (Speaker A x Speaker B)/(Speaker A + Speaker B)
     
  8. Lespaulsignature 74

    Lespaulsignature 74 Silver Supporting Member

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    "I want to buy an attenuator, but I'm not buying anything until I understand what the heck I need"

    There are several attenuators that can be used for varying speaker loads. I have the Dr.Z Air brake which can be used for most speaker loads (4,8,16-ohms) for any amplifier up to 100 watts maximum.
     
  9. Nuclearfishin

    Nuclearfishin Supporting Member

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    Great, did you read most of those? Try searching "Speaker Impedance explanation." You only get 7 hits and none of them explain it as well as this thread.
     
  10. Nuclearfishin

    Nuclearfishin Supporting Member

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  11. michael.e

    michael.e Supporting Member

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    As a matter of fact, over the years, yes.


    Mucho info if one cares to delve. Tons of info on the information superhighway as well....
     

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