Resistance vs. wattage output; Manufacturers' specs

Discussion in 'Amps and Cabs [BG]' started by Deman669, Aug 19, 2001.

  1. Deman669

    Deman669 Guest

    May 11, 2001
    Caldwell, ID, USA
    Alright - beng the know it all that I am, not knowing the corret answer to this is beginning to get to me.

    Here's the question:

    If an amp is rated at 300w @8 ohms, and 600w @ 4ohms, why is a very simlilar amp by another manufacturer rated 300w @ 8ohms and 450w @ 4ohms?

    This is purely theoretical, but I have seen such examples far too many times to count. I have a few guesses such as efficiancy and cap/transformer size, but I am looking to the more tehnically inclined in this forum (i.e. MikeyD, BGavin) for a technical, mathematically supported answer (if possible). Thanks for any help!
  2. MikeyD

    MikeyD Guest

    Sep 9, 2000
    Hi - Bob Lee from QSC would probably give a much better answer, but it is a good question. If the amplifier's output stages could maintain full voltage at all rated impedances, then you'd be correct - a halving of impedance would exactly double the power. However, for various reasons, as the current increases to a lower impedance, the output stages see increasing I*R losses and/or the power supply rails start to sag under heavy load. When the available voltage starts to deteriorate, the power is reduced. From what I have seen in amplifier specs, it is quite common. I have not studied amp topology in a long time, so if Bob Lee replies, you'll get a more competent answer. Paging Bob!! :)
    - Mike
  3. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio!

    Jul 3, 2001
    Chester, Connecticut
    Former Technical Communications Developer, QSC Audio
    What Mike said. ;)

    This applies to solid-state amps, not to tube amps:
    If an amp could have perfect regulation of its power supply (the supply would effectively have zero series impedance), its maximum power specs would be inversely proportional to the load impedance:

    X watts @ 8 ohms
    2X watts @ 4 ohms
    4X watts @ 2 ohms

    But nearly all power amps use unregulated supplies, because regulation wastes power. So their supplies have some series impedance. The more current the output circuitry draws, the more voltage (I*R, as Mike pointed out) drops in the supplies internal impedances, causing the supply rail voltages to sag more. Consequently, the maximum voltage the amp can produce sags correspondingly (meaning it clips at a lower voltage), so it never quite gets to a precisely inverse relationship between maximum output power and load impedance.

    The load impedance doesn't affect gain, though. So below clipping, you will get an actual doubling of power by halving the load impedance, but the amp's maximum power capability will not quite double.

    You can tell something about an amp's power supply by looking at how its power specs compare with a perfect theoretical amp. In an amp with a solid supply, the power spec won't necessarily double going from an 8-ohm load to a 4-ohm load (or from 4 ohms to 2 ohms) but it'll be respectable, like maybe 160% to 175%, while with an amp with a wimpy supply it might be only around 125%.