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Surround sound speakers & amp impedance mismatch

Discussion in 'Electronic Design' started by Bob Engelhardt, Mar 25, 2012.

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  1. My step son & family have bought a second home and are furnishing it
    with 2nd hand stuff. My neighbor donated a Sony receiver/surround sound
    without speakers. I found speakers at our dump's "swap shop". Trouble
    is the amp uses 3 ohm speakers and the ones that I found are 8 ohm.

    The speakers will need 1.4 times the voltage to output the same power,
    but levels are non-linear between the dial & the output, so I'm not sure
    if I should expect the amp to have enough power (voltage). In other
    words, if a level of "5" would be normal for 3 ohm speakers, would a
    level of "10" be enough to drive 8 ohm ones?

    Fidelity is not a big concern - this is for a TV system. But if
    distortion is going to be major, I could match impedance by adding a 5
    ohm resistor in parallel with each speaker. But then the power demand
    on the amp would be that much greater.

    I could use some advice.

    Thanks,
    Bob
     
  2. Guest

    Most likely, "3 ohm" refers to the minimum allowed speaker impedance,
    going any lower could permanently damage the amplifier. Thus any 4-8
    ohm speaker would be suitable for this amplifier.
    A surround sound amplifier might be specified as 5x100 W, while the
    power supply might be able to deliver perhaps 150 W constantly :).
    The 100 W output power might have been measured with one channel
    driven at a time and even then only for a short time (and then the
    number is multiplied by 5).

    In practice, the power supply voltage will sag, when multiple
    channels are driven at the same time, thus reducing the output
    available from each channel.

    By using 8 ohm speakers instead of 4 ohm speakers, the speaker will
    draw less current and the power supply voltage will not sag as much,
    delivering a slightly higher output voltage to the speaker, before
    distortion starts.

    With a stabilized power supply, the audio output level would drop by 3
    dB when going from 4 to 8 ohm (assuming same speaker efficiency),
    however, with a badly regulated power supply, the loss in SPL might be
    just 1-2 dB.
     
  3. Not loud. You got the "cheap" part right <G>. Thanks, Bob
     
  4. That's something that I didn't even think about, but it makes sense.
    Good, again. The concerns that I had were enough power & distortion.
    'Looks like we're OK on both counts.
    Thanks. This is the kind of "thumbnail" knowledge that I like to have
    (just enough to make me sound like an expert to my step son <G>).

    ....
    Great, I was doubtful that I'd find a set of 3 ohm speakers at the dump
    & now I can just scratch "speakers" off of their "needed" list.

    Thanks a bunch,
    Bob
     
  5. ....

    Thanks to all reply-ers. We're going to go ahead and use them.

    Bob
     
  6. Sjouke Burry

    Sjouke Burry Guest

    8 ohm is fine , you just get barely detectable less maximum power.
    Most amplifiers have to much output power anyway.
     
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