2x12 wiring question

Discussion in 'Ampage Area' started by KaHOnas, Jan 14, 2022 at 8:01 AM.

  1. KaHOnas

    KaHOnas Electromatic

    I got a hold of an old busted AC30 cab. No speakers and no amplifier. Just the wood box. But I did have a little 50W Vox MV50 microamp laying around doing nothing so I figured I'd get the two together. I picked up a pair of 8Ω Peavey Blue Marvels relatively cheap, mounted them with minimal modification to the cabinet, and wired it all up.




    Now to my question: the output switch on the amp has selections for 4, 8, and 16Ω so I can wire these however I like. So, what's the advantage of series over parallel or vice versa? Right now I'm running them in series and have the amp set for 16Ω. Unless my math is wrong, if I were to run them in parallel, the total impedance would be 4Ω, right?

    MV50 output label:

    50W @ 4Ω
    25W @ 8Ω
    12.5W @ 16Ω


    Any input?

    Thanks everyone.
    Last edited: Jan 14, 2022 at 8:30 AM
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  2. thunder58

    thunder58 Super Moderator Staff Member

    Dec 23, 2010
    tappan ny
    Admin Post
    Attn: @hcsterg ( thanks buddy ) ;)
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  3. Ricochet

    Ricochet Senior Gretsch-Talker

    Nov 13, 2009
    Monkey Island
    Correct, running 2x 8 Ohm speakers in parallel would yield 4 Ohms, so you would be able to run your amp at the full unadulterated 50W.
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  4. loudnlousy

    loudnlousy Gretschified

    Oct 18, 2015
    I would run them parallel. So if one of them blows up you`ll still have the other running.

    Wired in series the whole box would be dead if one speaker is down.
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  5. wabash slim

    wabash slim I Bleed Orange

    Feb 10, 2010
    lafayette in
    It depends on how loud you want to be. With two speakers, you can do 4 Ohms in parallel, or 16 Ohms in series. One speaker alone will give you 8 Ohms. What also matters is what wattage your speakers are rated for.
  6. KaHOnas

    KaHOnas Electromatic

    As far as I know, they're rated for 75W each.
  7. wabash slim

    wabash slim I Bleed Orange

    Feb 10, 2010
    lafayette in
    If so, you'll be safe at 4 Ohms in parallel.
  8. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    Sure ! If I can help, it's always with pleasure... ;)

    You are right, @KaHOnas. :cool:

    In the case of your 2x12" with two 8R speakers, if you want maximum efficiency in dB/watt, you should wire them in parallel, and hook them to the 4R output of the amp.

    Doing so, the global output level in dB will be increased by +6dB versus the single speaker :
    * +3dB due to the doubling of the cone surface
    and :
    * +3dB due to halving the impedance.

    If you choose instead to wire the two speakers in serie, and hook them in the 16R output of the amp, then the efficiency will not be changed versus the single speaker. The global output level in dB will be :
    * increased by +3dB due to the doubling of the cone surface
    but :
    * minored by -3dB due to doubling the impedance.

    That said, as long as the output impedance selection of the amp matches the resulting impedance of the speaker association, there will be no difference (and no issue) than the resulting dB loudness per watt.

    I mean, in you case, if you play at say 5W output on your amp, the loudest solution will be to wire the 2x12 in parallel, and hook it on the matching Z output of your amp. Doing this, you will have a gain in loudness of +6dB versus the single speaker arrangement.

    Many years ago, for my pal the boss of our local music shop (who was a bit lost on the subject), I wrote some tables about speaker association concerning the resulting impedance, and resulting efficiency - sorry Guys, it's in French :oops:, but I think that's understandable nevertheless... If not, of course : feel free to ask ! :)

    The usual cases :

    The math rules :

    The resulting efficiency in dB :

    I hope I'm clear... o_O:D

    Last edited: Jan 14, 2022 at 6:46 PM
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  9. twangmanster

    twangmanster Electromatic

    Dec 29, 2021
    Long Island
  10. KaHOnas

    KaHOnas Electromatic

    Brilliant explanations! Thank you so much for sharing this with all of us. I leaned a lot more than I expected to. And I also got to brush up on my French, too!
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  11. new6659

    new6659 Country Gent

    I need to print this out!
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  12. GlenP

    GlenP Country Gent

    Jul 23, 2019
    How do you know which poles on the speaker should connect to the tip and ring/shield of the speaker cable? Is there a standard convention for that? If the speaker has + and - poles, which one goes to the jack tip and which one goes to the “shield” on a TS two conductor speaker jack & cable?

    planning a cabinet build myself, one of these days and just wondering.
  13. j.s.c

    j.s.c Country Gent

    Aug 19, 2008
    I love that school... docs in french is even better :)
    Tx @hcsterg
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  14. j.s.c

    j.s.c Country Gent

    Aug 19, 2008
    Ha! I think I could help here with the old 9V battery trick, just contact the speaker with both + & - of the battery if the bumper cone goes in your face (move ahead) then the polarity of the speaker in the same as the battery. In reverse, if it goes to the back side, than you could inverse the result. Am I right ?
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  15. GlenP

    GlenP Country Gent

    Jul 23, 2019
    Right, but I was not sure which one goes to the jack connectors. I found a diagram here, looks like the speaker + or red goes to the tip of the jack, the - or black wire goes to the ring. Of course, my old Peavey amp has blue and yellow wires on the speaker.


    Screen Shot 2022-01-14 at 9.16.09 PM.png
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  16. hcsterg

    hcsterg Friend of Fred

    Feb 13, 2012
    Yes. Still simple and reliable.

    Yes. That's usually what is done - but the battery trick as mentioned by j.s.c will confirm or not the speaker polarity.

    Assuming a 2x12" cab where the speakers are wired in reverse polarity (= reverse phase). Usually :
    - the output level in dB will be neatly decreased.
    - the bass level will be strongly reduced.
    - the tone will be rather nasal.

    There's no danger for the amp, as long as the impedance matches : it's only a matter of loudness ans tone - or taste !

    I remember a Punk-Rock player that much preferred the reverse phase tone than the correct phase tone on his 1964 VOX AC30 Twin Top Boost that I restored for him.

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