Which is worse: lower or higher impedance ...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by iggs, Sep 13, 2006.

  1. iggs

    iggs Member

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    I know neither is good but ...

    Which is worse:

    1) running 16 ohm cab plugged into a 8 ohm output on the amp

    or

    2) running 8 ohm cab plugged into a 16 ohm output on the amp

    :confused:
     
  2. Gordon

    Gordon Silver Supporting Member

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    Running an 8 ohm cab with your amp set on 16 is the one that you definetely don't want to do.
     
  3. John Phillips

    John Phillips Member

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    This is a common misunderstanding... that's exactly what you can do.

    (Probably because it's the opposite way round for tube and solid-state amps.)

    That way round - 16-ohm amp into 8-ohm cabinet - is safe for almost any tube amp. In fact, both are usually safe, but a too-high impedance load is more risky than too low.

    The danger to a tube amp comes from excess flyback voltages generated in the OT by having too little load (ie too high impedance) on the output - the worst case is no load (ie infinitely high impedance), which most people are aware is a bad thing... so why the idea that higher impedances are safer? :confused: In fact, usually up to a factor of two mismatch (ie running into double the intended impedance) doesn't cause enough excess voltage to damage things - but be careful, especially with older amps with more fragile OTs with weak insulation.

    A too-low load, or even a short circuit, is bad for the power tubes and in extremes might actually blow them, but in most cases won't damage the amp directly. The current in the OT does rise, but not by that much because the output power is reduced by the mismatch, not increased as it would be in a solid-state amp. It's still not a good idea to go outside a factor of two mismatch though (ie running into less than half the intended impedance), and again with amps with more fragile transformers you might not want to run at full power for extended periods into a too-low load.

    (BTW, Fender BF and SF amps not only have the Extension speaker jack in parallel with the Main - guaranteeing a too-low mismatch when it's used - but also have a shorting switch in the Main jack, to protect the OT in the event of no speakers being connected. These amps are very reliable.)

    This is all because a tube amp has an OT, which requires a match for maximum power transfer and loses efficiency the further away from that you go. A solid-state amp usually does not have an OT, so the load is directly coupled to the power transistors; the lower the impedance the more power is generated, which increases the current, so you must not run a solid-state amp below its minimum impedance (although they're usually completely safe with any impedance above that, including no load).

    The two types are totally different, and you can't apply one rule to both.


    Generally -

    Tube amp: safe between half and double the matching impedance, power is reduced in either direction (typically by about 20%).

    Solid-state amp: safe above the minimum impedance, power is reduced proportionately the higher you go (typically by about 30% for each doubling, as a rough rule of thumb).


    There are a few examples of both types which don't follow the rules though - either because of under-rated components running very close to their limits, or design quirks that make them unstable into the wrong loads - this includes a very few solid-state amps too, so if in doubt it's always best to match the load correctly.
     
  4. Antero

    Antero Member

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    That's the first explanation of that I've seen that made any sense. Rock.
     
  5. iggs

    iggs Member

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    Thanks for the info!
     

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