Is Power Additive in Series And Parallel?

Discussion in 'Amp Tech Center' started by HD28, May 16, 2021.

  1. HD28

    HD28 Tele-Meister

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    It seems like the interweb answer is yes and no.
    In guitar and amp circuits, regarding resistors, is power (watts) additive in both series and parallel?
    In guitar and amp circuits, does Ohms law E=IR equally apply as does P=I^2R?
    If you use E=IR the answer appears to be no if voltage remains constant.
    If you use P=I^2R, the answer appears to be yes if amperage remains constant.
    The question stems from trying to build a 10% power switch using https://robrobinette.com/AB763_Modifications.htm#10%_Power_In_External_Box
    I wish to use two 3ohm 100W and two 4ohm 100W resistors in series to make a 6ohm and 8ohm 200W 10% power switch, suitable for a 4ohm output 10% power switch for a dual showman, twin reverb, or similar 4 power tube fender amp.
     
  2. RLee77

    RLee77 Friend of Leo's

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    Yes. In both cases, your 3 + 3 = 6 ohm resistor and 4 + 4 = 8 ohm resistor are the equivalent of a single 6 ohm and 8 ohm resistor, with a power rating of 200w each.
    Used as shown in Rob's circuit, they will give you the 10% power attenuation, with a total load resistance of 4 ohms. (8 + 4 = 12 ohms, in parallel with 6 ohms, is 4 ohms.)
     
    Last edited: May 16, 2021
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  3. beanluc

    beanluc Tele-Holic

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    Power is voltage divided by current.

    Voltage is additive in series, not in parallel.

    Current is additive in parallel, not in series.

    So a power switch logically (to me, I haven't looked at this type of circuit) could be additive in series if it affects voltage, or additive in parallel if it affects current.

    In other words, there's no single answer, and whatever answer makes sense for your application is probably not even related to my over simplification.
     
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  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You would not question this when adding two speakers together.
    Two 25W speakers in series = 50W of power handling.
    Two 25W speakers in parallel = 50W of power handling.
     
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  5. HD28

    HD28 Tele-Meister

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    Power is additive in both series and parallel.
     
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  6. HD28

    HD28 Tele-Meister

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    Thank you.
     
  7. RLee77

    RLee77 Friend of Leo's

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    Just to show how the power splits up in your attenuator, here's a rough sketch, showing watts for each resistor and speaker with 100w total going in; the total load on the amp is 4 ohms. Since the two 4 ohm R's in series with the speaker won't see as much power, you could scale those back to two 50w R's if you wanted. I'd leave the two 3 ohm R's at 100w each for safe power dissipation.

    atten.JPG
     
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  8. blowtorch

    blowtorch Telefied Ad Free Member

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    power is addictive
    and power corrupts
    absolute power corrupts absolutely
     
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  9. RLee77

    RLee77 Friend of Leo's

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    Bass players are therefore more corrupt than guitar players.
     
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  10. SRHmusic

    SRHmusic Tele-Afflicted Silver Supporter

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    Power = voltage x current = V x I

    Also P = I^2 x R = V^2 / R, from substituting V=IR or I=V/R.

    Power always adds; when looking at a circuit the total power dissipated is the sum of the power dissipated in each element.

    One method: first sort out how much current flows in each branch, then use P=I^2 R to find the power in each resistor.
     
  11. blowtorch

    blowtorch Telefied Ad Free Member

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    and drummers are pure evil
     
  12. Dacious

    Dacious Poster Extraordinaire

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    Singers are the worst - they get thousands of watts in dedicated foh and foldback,and still complain....
     
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  13. HD28

    HD28 Tele-Meister

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    Thank you.
     
  14. beanluc

    beanluc Tele-Holic

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    yeah, I blew it.
     
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  15. kbold

    kbold Friend of Leo's

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    Well ..... maybe not.
    Bassist requires 10 times more power than the guitarist.
    So, it only takes one tenth of the power to corrupt a guitarist (compared to a bassist).
    Therefore the guitarist is 10 times more corrupt. Allow a guitarist to play through a bass amp ...............
     
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  16. kbold

    kbold Friend of Leo's

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    . duplicate
     
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  17. radiocaster

    radiocaster Poster Extraordinaire

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    The problem is how you calculate the perceived power rating of the amp and resistors.

    First of all, a 100W amp is rated at that clean, so if you're cranking it you'll get something like 160-200W.

    Second, the ratings of the resistors are understood to be with heavy heat sinks, and a fan might help as well. For example, here's one rated at 100W:
    https://www.tube-town.net/ttstore/high-power-resistor-3r9-4-ohm-100-watt.html
    as you can see, the rating without a heat sink is 40W. So if you just use two of those in the circuit with a Showman without heat sinking, you'll likely fry them and possibly damage your amp.
     
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  18. the fatch

    the fatch Tele-Meister

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    Actually, power is voltage multiplied by current. Voltage is shared between resistances in series, and is the same across both when in parallel. Current is shared by resistances in parallel and is the same in both when in series.
     
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  19. dsutton24

    dsutton24 Doctor of Teleocity Ad Free Member

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    In DC circuits, and AC circuits through the audio range, power is pretty well behaved. Once you get into RF, though, things can get weird.
     
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