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An Impedance-reducing Resistor?

Discussion in 'Amps [BG]' started by domjohnson, Feb 17, 2013.

  1. domjohnson

    domjohnson

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    My understanding is that when you plug two cabs into an amp head, the impedance is reduced because you get the same effect of resistors in parallel - In other words, the speaker itself is acting as a resistor, with the added (and desired) effect of outputting sound - and so the overall volume is increased not only because of the extra speakers, but also the reduction in impedance. However, I can't really afford a second cab, nor would I be able to get a second one up to a gig.

    So, my question is: is there a product that is basically a great big high power resistor that gets hooked up to the other output on the head that will therefore reduce the impedance of the stack?

    Cheers,
    Dom.
  2. MAMMOTHvolume

    MAMMOTHvolume

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    Yep you can buy big resistors if you like!
  3. B-string

    B-string Supporting Member

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    But you will split the total power between the resistor and the speakers. Thereby reducing the actual power the speaker receive. Have fun!

    Speaker are not just a resistive component, the resistance (actually impedance) varies with frequency. They also have inductance and capacitance.
  4. Jazzdogg

    Jazzdogg Less barking, more wagging! Supporting Member

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    Last time I checked, there was still no such thing as a free lunch. ;)
  5. B-string

    B-string Supporting Member

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    Sure there is! But it comes with a pink slip. :eek:
  6. bassgod0dmw

    bassgod0dmw Supporting Member

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    If your goal is to get more volume, there is no replacement for displacement!
  7. beans-on-toast

    beans-on-toast

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    Well, there are speaker attenuators that allow you to reduce the power coming out of your amp. They do this by using a circuit to emulate a speaker and reduce the output. Here is an example of one. There are many similar products out there. These tend to work with low power amps so it depends on what amp you are talking about. They can also get expensive and cost more than a cab.

    Check this out to see what a speaker model can look like. You don't have to understand the math (which includes something called real and imaginary numbers, I kid you not!) just look at the pictures to get an idea of what is involved.

    This will give you less volume out. If you are trying to get more volume out, you might be able to rewire your cab to give you less impedance. If you have a tube amp you will get a little more volume at the expense of running your output transformer and tubes harder. The gain is marginal. If you have a solid state amp, more power will be delivered to the speakers. You might gain more with a more efficient cab but a second cab really is the answer.
  8. Mr. Foxen

    Mr. Foxen

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    Disclosures:
    Amp tinkerer at Ampstack
    Volume control is a much easier way to make your rig quieter.
  9. 1958Bassman

    1958Bassman

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    All that would do is waste the power it took to create heat produced by the current flowing through the resistor. The results aren't worth the expense and even if the amp's power doubled, only about half would go to the speakers, which is what you already have. Actually, cutting the impedance in half only adds about 40% more, on a practical level. Save your money for another cabinet- the results are worth the price. Besides- a non-inductive resistor that would handle the power costs almost as much as a cabinet.
  10. SGD Lutherie

    SGD Lutherie Supporting Member

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    Resistance and impedance are two different things. A resistor will reduce the volume, but the impedance of the cab will stay the same.
  11. nashvillebill

    nashvillebill

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    They are different, but impedance (Z) is a product of R, C, and L. Change any of the 3, and Z will change.
  12. Codger

    Codger Bradley Baker Supporting Member

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  13. SGD Lutherie

    SGD Lutherie Supporting Member

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    But putting a resistor in series with the speaker won't do what the OP wants. If anything, it will raise the impedance, not lower it.
  14. Passinwind

    Passinwind Charlie Escher Supporting Member

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  15. nashvillebill

    nashvillebill

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    Correct, putting it in series won't help a bit.

    Nor will putting it in parallel (which would halve the impedance) help.

    I think everyone is in agreement: if the OP wants more volume, add another cab. Want less volume? Turn the amp down....this isn't a guitar amp.
  16. SGD Lutherie

    SGD Lutherie Supporting Member

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    Yes, just wanted that to be clear. :D
  17. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Not to mention (as pretty much everyone so far has hit the nail on the head) you wouldn't notice a humongous difference anyway. Twice the watts does NOT translate into twice the volume. I won't even go down that road because 1) it's complicated and 2) it would probably start a debate that would be WAY off topic. It just suffices to say that half the ohms doesn't mean twice the volume, even though in most cases it doubles (or greatly increases) the watts. There are sticky posts that cover all of this. And there are web sites by the thousands that do as well. Just google "watts and decibels" or something like that.

    The short version is you have three choices.
    1) Get another cab that will get louder than yours by itself (for various reasons not just that it would be 4ohms) to replace your current cab.
    2) Get another (identical) cab and be louder with them stacked but have more to lug.
    3) Leave it as is.
  18. SGD Lutherie

    SGD Lutherie Supporting Member

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    Right, you need a 10 times the power (=10dB), to get roughly double the volume. So, a 100 Watt amp is only twice as loud as a 10 Watt amp! That's not even getting into speaker efficiency, etc.
  19. two fingers

    two fingers Loud Mouth Know It All Blowhard Gold Supporting Member

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    Nice and concise way of putting it. I just figured any further down the road would get into the weeds. Hopefully the OP will do some reading. I must admit, I came here years ago with many of the same ways of thinking. I have learned a ton, and still do almost daily.
  20. Mehve

    Mehve

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    Bassman/B-string already aluded to this, but this won't work, period. Yes, putting a resistor in parallel with the speaker would allow the head to output more power, BUT none of the extra power would actually make it to the speaker and create volume.

    At best, the speaker would receive the same power it already did, with all of the extra power all being dissipated by the resistor. In a less-ideal case, the amp would hit its current limits and the speaker would actually see a little less power. Lay it out in a diagram and do the math, it becomes pretty easy to see.

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