Ohms Question

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  • 02-12-2004, 06:20 AM
    poneal
    Ohms Question
    I'm not really that technical but I am curious about how something works. If you have an amp that is rated at say 200wpc into 8ohm and 300wpc into 4ohm and you have a speaker that is rated as 4ohm, how does the amplifier know which voltage to send to the speaker? I have read that 8ohm is a higher voltage output than 4ohm. So how does the amp know which to send? Or does it really matter. Thanks, Paul.
  • 02-12-2004, 06:56 AM
    markw
    Oversimplified response
    The voltage delivered is the same in both cases. Watts are basically derived by dividing the voltage by the impedance. Since the voltage is the same in both cases, more watts is available at the lower impedance (4 ohms) than is available at 8 ohms.

    Assume 100 volts. at 4 ohms it would be 25. At 8 ohms is would be 12.5

    These are not to be considered fact, merely mathmatical examples. You might want to do a quick study on Ohm's Law (and it's derivitives) for a more accurate rendering of this oversimplified example. It's got to do with how voltage, current and resistance all interact to produce "watts", as well as other goodies.
  • 02-12-2004, 08:08 AM
    poneal
    What happens if
    I have a 25 year old integrated amp rated from 8 to 16 ohms. Its rated for 100wpc. If I use this to drive my 4ohm subwoofer will this be bad? I think I know the answer (yes) but from a technical point I guess I do not. Is there anything I can do to make it safe to use until I can afford to buy another amp? Thanks.
  • 02-12-2004, 08:21 AM
    markw
    well, maybe if you play it real low you might get away with it.
    In any case, you are playing with fire, possibly literally.

    I can't think of a reasonable workaround to make it "safe". In any case, 25 watts isn't really a drop in the bucket for a subwoofer.

    Poneal, do you know anybody from Yorktown Baptist?
  • 02-12-2004, 09:22 AM
    poneal
    Sorry, but I do not know anyone from there, however, I do know where its located :-). Did you live here previously?
  • 02-12-2004, 12:24 PM
    This Guy
    you should be fine for a temporary amp, just don't turn it up too high or you could ruin the amp.

    -Joey
  • 02-13-2004, 07:28 AM
    F1
    Quote:

    Originally Posted by poneal
    I have a 25 year old integrated amp rated from 8 to 16 ohms. Its rated for 100wpc. If I use this to drive my 4ohm subwoofer will this be bad? I think I know the answer (yes) but from a technical point I guess I do not. Is there anything I can do to make it safe to use until I can afford to buy another amp? Thanks.

    Well, I believe that adding a resistor in series with your speaker will make it safer. Make sure the resistor can take a few watts like this:
    http://www.partsexpress.com/pe/pshow...er=004-2&DID=7
    Some receivers have a switch to select 4ohm or 8ohm speaker. My guess is the 4ohm position is to engage a dummy resistor to increase speaker impedance.
  • 02-13-2004, 08:42 PM
    Mash
    Basic Amplifier EE
    Receivers/amps used to have impedance switches such as 4 ohm/8 ohm to select the proper output taps on their coupling transformer. Changing the switch connected your speakers to different transformer turns-ratios so the amp's internals saw the same load for, say, an 8 ohm speaker as for a 4 ohm speaker.

    Later 'direct-coupled' amp designs did away with the coupling transformer and so the amp's internals would be designed to drive different load impedances safely. But driving, say, 4 ohm or worse yet 2 ohm loads with such an amp rated for use with no lower than 8 ohm loads usually leads to an overheated amp. Overheated amps will often be shut down by internal protective circuits, but sooner or later amps persistently run overheated will die.

    Amps tend to operate in one of two basic output-circuit flavors: constant voltage and constant current.

    The majority of amps will "tend'' to operate as constant output-voltage, and you will note that a TRUE constant output-voltage amp will deliver twice the power at 4 ohms that they will deliver at 8 ohms. Then you will see something like "100 watt/ch at 8 ohms and 200 watts/ch at 4 ohms". The Musical Fidelity A2 and A220 will, for example, deliver exactly twice the power at 4 ohms that they will deliver at 8 ohms. Most amps "tending'' to operate as constant output-voltage will have specifications something like "100 watt/ch at 8 ohms and 160 watts/ch at 4 ohms".

    Constant output-current amps, on the other hand, will tend to deliver, say, twice the power at 16 ohms that they will deliver at 8 ohms. The Futterman OTL tube amps fall into this category.
  • 02-23-2004, 01:52 PM
    poneal
    Mash
    Can you recommend an amp for driving my 350watt rms into 4ohm subwoofer? I've been loooking at the outlaw audio m200 and crown ce-1000aes. Thanks, Paul.
  • 02-23-2004, 06:24 PM
    bturk667
    4 ohm speakers demands more current from an amp than 8 ohm speakers do. Here is where the differences comes in. Not all amps are up to that task!