Reactive / Resistive dummy load

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jhana, Mar 24, 2008.

  1. jhana

    jhana Member

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    I’m planning to build a switch box to mount into the back of a 1x12 8ohm 65w solid state combo. The idea is to have a switchable configuration of extension cabinets with the following 4 options:

    1. Internal speaker only
    2. Internal speaker + dummy load + 2x12 extension cab
    3. Internal speaker + 1x12 extension cab + 2x12 extension cab
    4. Internal speaker disabled + 2x12 cab + 2x12 cab

    To enable the use of a 2x12 cab (option 2), I originally planned to use a couple of aluminium clad 100watt resistors (4R7 and 3R3 in series) to act as a resistive load in series with the 8ohm internal speaker.

    However now I’m wondering whether it would be better to design a reactive load instead of a resistive load? What would be the difference to the sound overall? Any risk to the amp etc?

    Any advice appreciated. Thanks!!
     
  2. donnyjaguar

    donnyjaguar Member

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    I wouldn't bother with a reactive load myself, but if a customer wanted one I'd plot the impedance characteristic of the speaker(s) and design one that would smooth out the bumps. If nothing else it would allow you to get more power from the amplifier. I can explain the procedure if interested.
     
  3. jhana

    jhana Member

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    Thanks, I expect a resistive load would effect the overall impedance curve of the other speakers although I don't fully understand the rules of interaction here.

    I think I'll experiment with a resistive load initially to see what happens.

    Would it be better to double the resistive load to about 15ohms as a more representative average for an 8ohm speaker? Or is this no concern for a solid state amp and probably better (power wise) to keep it 8ohms?

    Yes I would be interested to see how you go about plotting the impedance characteristics of a speaker.

    Again Donny and anyone else thanks for your advice.

    JC
     
  4. donnyjaguar

    donnyjaguar Member

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    I personally don't worry about driving a 16Ω load with a solid state power amplifier, its almost certainly perfectly safe.

    As for measuring the impedance curve I use a pair of AC voltmeters (or my 'scope) and a solid state amplifier. Place a resistor of equal value in series with the speaker and sweep the speaker using your audio signal generator. You can determine the load the speaker is presenting by measuring the voltage drop across the resistor. If you need the formulae let me know, but basic Ohm's law here. I generally give the speaker a fair pounding so its a rather noisy exercise. This is generally of more value in hi-fi speaker design. If you get a wild peak at a given frequency you need to engineer a band-pass filter to ground to negate the effect of the impedance peak. Interestingly enough, many so-called hi-end loudspeakers don't bother with this but it makes a difference if you're driving with a tube amplifier.
     
  5. EarlTX

    EarlTX Member

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    JMO, but since a SS output section has, basically, no output 'impedance' - aside from some minimum value load for current overload - I'd just wire a switch that connects the various loads in series, and ditch the dummy loads. The amp won't care what you hang off it, as long as you don't get below the ratings of the output section.

    The effective output resistance of the amp is probably in the 10th's of an ohm (or less), so the speaker wire is gonna be the biggest factor, as far as damping is concerned.
     

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