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70v amp, 8 ohm speakers?

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  • 70v amp, 8 ohm speakers?

    What would happen if I connected an 8 ohm speaker to a 70v amp output?

  • #2
    Re: 70v amp, 8 ohm speakers?

    Originally posted by emilime75 View Post
    What would happen if I connected an 8 ohm speaker to a 70v amp output?
    Bad things, unless you get a transformer for the 8ohm speaker
    “Never ask people about your work.”
    ― Ayn Rand, The Fountainhead

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    • #3
      Re: 70v amp, 8 ohm speakers?

      Originally posted by DE Focht View Post
      Bad things
      Can you be a bit more specific?

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      • #4
        Re: 70v amp, 8 ohm speakers?

        Short answer: If you hook up an 8 ohm speaker directly to a 70V output, you would overload the amplifier at anything above volume level 0.5/10.

        Detailed answer (This answer assumes a lot and simplifies everything such as ideal transformers with no loss. Even with real world parts, the principles remain the same): The calculations below all operate on the P=V^2/R formula in one fashion or another. In a 70V system, the transformer at the speaker end transforms the impedance of the speaker up to (hopefully) not exceed the load rating of the amplifier. For example, if you have a 100W 70V amplifier, the amplifier is expecting a load impedance no lower than 49 ohms (R=V^2/P : 70*70/100). If you connect a 100W 70V transformer with an 8 ohm tap to the amplifier, it should drop the voltage to ~28V (V=sqrt(P/R) : sqrt(100*8)). The voltage ratio of the transformer is ~2.5 (70V/28V). The impedance ratio is a square of the voltage ratio, which means the impedance ratio is ~6.25 (2.5^2). So, if you put a speaker that has an impedance of 8 ohms on the transformer, it looks like a ~50 ohm load to the amplifier (6.25*8).

        Amplifiers are limited by current output, so if you attach an 8 ohm speaker directly to the 70V output of an amplifier, you could probably play the speaker at low volumes with no problems, but you will nowhere achieve close to the full output potential of the amplifier, and you risk damaging it. Another example: If you have a 100W 70V amplifier, your output current will be limited to ~1.43 amps (I=P/V : 100/70). A current of 1.43 amps into 8 ohms yields 16.3 Watts (P=I^2*R : 1.43^2*8). This will be well below the maximum voltage output of the amplifier, but driving it any harder will begin to draw excess current through the power transistors and will damage them (or send the amplifier into some sort of protection if it has this feature). If you were to supply the proper 50 ohm load, the amplifier would deliver the full 100 watts (P=I^2*R : 1.43^2*50).

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        • #5
          Re: 70v amp, 8 ohm speakers?

          Originally posted by emilime75 View Post
          Can you be a bit more specific?
          get a transformer for the 8ohm speaker. Transformers are going to step down the voltage and keep proper impedance for the 70v side of things.

          If you were to take a typical in ceiling speaker down from your Dr. Office you would find a little transformer on the speaker. 70v into it and whatever the speaker needs out of it.

          Get speakers meant for 70v distributed audio or get the appropriate transformer.

          http://www.parts-express.com/cat/com...r-systems/3431

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          • #6
            This can harmful to your speaker because the voltage is higher. Be careful with electronic gadgets.

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            • djg
              djg commented
              Editing a comment
              Wise words indeed.
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