Wrong impedance!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by NoBuckers, Oct 23, 2005.


  1. NoBuckers

    NoBuckers Guest

    I have a nice 50W tube head that runs at 8 ohms, and I picked up an unloaded 2x12 cab for it not too long ago. It's been sitting around for a while, but I finally found two awesome G12H30s to slap in there. Only trouble is, one speaker's 16 ohm and one's 8 ohm! So, I was wondering...if I rigged an 8 ohm resistor in there with the 8 ohm speaker, would that solve the problem? Could it hurt the transformer? Could it hurt the speaker? Could it affect the output or tone of the amp? Might there be another solution besides looking for another speaker?
     
  2. bob-i

    bob-i Member

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    No you can't use a resistor. You simply can't use the 2 speakers together. If you do the 8 ohm speaker will receive 66% of the power and the 16 will receive the other 33%. The result may blow the 8 ohm speaker but more important it'll make the 16 ohm speaker very quiet, almost no sound at all.

    Sorry for the bad news.
     
  3. NoBuckers

    NoBuckers Guest

    Well, thanks for the reply...So, anybody wanna buy a brand new 8 ohm Celestion G12H30...?
     
  4. 908SSP

    908SSP Member

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    You can use an 8 ohm resistor in series with the 8 ohm speaker then wire that in parallel with the 16 ohm speaker to get a 8 ohm load for the cab. Both speakers will receive 50% of the power. The 8 ohm speaker will have a slightly different sound with the resistor then the 16 ohm speaker as the highest and lowest frequencies will be slightly rolled off. But it might sound great you will have to try it and see. Make sure you use a 8 ohm resistor rated at least the same wattage as the speaker. If your speaker is 30 watts get the 50 watt resistor. You can get load resistors from Weber for a couple bucks.
     
  5. John Phillips

    John Phillips Member

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    Actually neither of these things are true. The loss of volume from the speaker with the resistor will be audible but not 'very quiet'. It still isn't a great idea though because the speaker does not have a simple resistance, it has a frequency-dependent impedance, so if you wire a resistor in series, the power going to the speaker will vary depending on frequency - it will receive more treble than bass, since the speaker's impedance rises quite sharply at high frequencies.

    Both speakers will not receive 50% of the power. At the point where the 8-ohm speaker's impedance is equal to the resistor, the 16-ohm speaker will be taking 50% of the power, and the 8-ohm speaker and the resistor 25% each. But at higher frequencies the balance will shift away from the 16-ohm speaker and the resistor towards the 8-ohm speaker.

    You'd actually be better using a resistor higher than 8 ohms, to try to offset this a bit - the average impedance of an 8-ohm speaker is actually around 15-20 ohms. But there is no value that will give a consistent power and tone balance between both speakers, and you should really only consider it as a bodge IMO.

    You'd be much better simply replacing one of the speakers.
     
  6. bob-i

    bob-i Member

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  7. John Phillips

    John Phillips Member

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    No worries about disagreeing, but that's not been my experience at all. I have often run mismatched impedances (usually deliberately, to achieve a particular power distribution or sensitivity match) and never has the speaker taking less power sounded nearly inaudible. There's only a 3dB difference between the two, assuming the same sensitivity of speaker.

    Many people also run quite mismatched sensitivities together - eg a V30 and a G12M-25, which is also a 3dB difference.

    If you had an 8-ohm speaker and a 16 in parallel, and the 8 could in theory have taken 2/3 of the power of the amp, but in fact blew (and before that sounded far louder than the 16 to the point you could hardly hear the 16), could it have been already partially shorted and much lower impedance than 8 ohms? Just wondering...
     

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