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Yet another impedance question...

Discussion in 'Amps and Cabs [BG]' started by Chillihead, Jun 13, 2004.


  1. Chillihead

    Chillihead

    Mar 31, 2004
    New Zealand
    Hi low-enders,

    I realise that is probably an often asked question, but a search didn't produce an answer that satisfied me:

    I bought a Rockit 120 Bass amp, and the seller threw in a cab with a 15" JBL speaker, which was blown. I've sent the speaker off for repair. In the mean-time I found out the speaker is rated at 8 ohms, and the amp puts out 4 ohms.

    My question is then - what are the ramifications, if any of using these two together? Would I get better performance if I put another 8 ohm resistor in parallel with the speaker to lower the resistance to 4 ohms?

    Thanks for your time.
     
  2. You'd be wasting half your amp's output, in the form of heat--the resistor would just heat up. The speaker would still be getting the same amount of power, only the amp would be working twice as hard. Half of the amp's output would now go to the resistor, the other half to your speaker. Not better performance, just wasted energy.
     
  3. Chillihead

    Chillihead

    Mar 31, 2004
    New Zealand
    So, it's best just to use as is then? Or if not, what would you recommend?
     
  4. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Yep just leave it. You may be getting less watts but the over all volume difference is usually barley noticable due to the loagarithmic nature of decibels. Doubling your watts only gives you an extra 3dB. To double your volume, you need to multiply your watts by 10.

    I'd leave it as an 8 ohm speaker. Then maybe get another 8 ohm speaker later on and use 'em together for a 4 ohm load.
     
  5. Chillihead

    Chillihead

    Mar 31, 2004
    New Zealand
    Aha! Makes sense. Actually, I thought that might be where this was headed, so I've been eyeing up another JBL 15" speaker (also 8 ohm). I'm not sure the cab is big enough to fit two 15"ers though...

    Thanks for the heads up people.
     
  6. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Probably not. ou might consider running 2 seperate 15 cabs. That way you're not going to break your back carrying both at once, and you end up with a modular rig. Just take on to the smaller/quieter gigs, take both if you think you'll need em.

    Also, speaker cabinet design is a bit more complicated than whacking a speaker into a wooden rectangle. The size of the cab is very important, as it the design of the porting that lets air escape.

    If you need help designing cabs that get those speakers working th their potential, start a thread here on TB. There's quite a few guys on here who can help you. Just mention the exact model of JBL 15" speaker.
     
  7. Chillihead

    Chillihead

    Mar 31, 2004
    New Zealand
    Cool, thanks Petebass.
     
  8. Hooking up another 8 ohm cab, in parallel with the first 8 ohm cab, will be a 4 ohm load, hence the amp will produce more power, and this power will go to the speakers, and therefore there will be more volume. (Though as Petebass pointed out, not twice as much volume, but a preceptible difference nontheless).

    However, from the original post, I inferred that he was inquiring about merely putting an 8 ohm resistor (NOT a speaker, just a resistor) in parallel with the 8 ohm speaker. I would like to to reiterate that this will indeed make the amp put out more power....but the extra power will just disappear into heat being generated by the resistor. The speaker would still be seeing the same amount of power as before.
     
  9. Chillihead

    Chillihead

    Mar 31, 2004
    New Zealand
    Hi Nashvillebill

    I read and understood your first post, many thanks for that. I now better understand how things work, as a result of both your and PeteBass's posts.