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The Ohm / Output / Volume Question

Discussion in 'Amps and Cabs [BG]' started by Soulbanana, Sep 26, 2005.

  1. Soulbanana


    May 7, 2004
    I have an SWR Stereo 800 power amp, and there are some questions I would like to ask about it, and power amps in general.

    When an amp puts out 800watts @ 4ohms for example, why is it that the phrase "would normally put out xxx watts @ 8ohms" is used? Do different amps react differently to 8ohm impedance?

    How much output does the SWR Stereo 800 put out, bridged into a single 8ohm cabinet?

    Is there a difference in output / response were I to run 2 8ohm cabs in parallel out of one side, versus running one 8ohm cab out of each side of the power amp in stereo?

  2. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    1. Unless we have a power rating from the manufacturer, we guess from it's 4 ohm power. OR Assuming the power rating to be a part of marketing, and as such, overrated or overblown, we give a more realistic number.

    2. How much does the manual say? If I had to guess, I'd say about 2/3*800, around 533.

    3. Theoretically no, but practically yes: if you run a SS amp at 4 ohms, it runs a bit hotter than at 8 ohms. So there's a bit more strain. Also with the power drawing, you can reach the limits of headroom faster. However, I'm an economist, not a tech, so I don't have too much real knowledge behind this.
  3. Solid state amps put out different amounts of power based on the load (cabs) connected to them. Example: the fender 800 Pro puts out 800 watts into a 4ohm load and 1200 watts into a 2 ohm load - ie. the lower the ohms, the more power the amp puts out (tube amps put out the same power into different loads). two 8ohm cabs together makes a 4ohm load. 2 4ohm cabs together makes 2ohms - note: some amps can't run a 2ohm load

    biamps/stereo amps apply the same principal. each side can run down to a specific load. Example: the Ampeg SVT4-PRO can run down to 2ohms on either side and puts out 900watts each at that load, but when you bridge the two together, it can only go down to 4ohms and 1600watts

    check out the specs on your amp and see what it says...
  4. Soulbanana


    May 7, 2004
    Be sure that the speakers you connect to the Bridge output can handle the wattage delivered by the Stereo 800. Under clipping, the Stereo 800 delivers a MINIMUM of 700 Watts RMS @ 8 ohms and 800 watts RMS @ 4 ohms. We recommend consulting the manufacturer of your speaker system if you are unsure BEFORE using them in this mode.

    From the SWR discontinued products instruction manual. Is this a trustworthy account? It doesn't seem to give a 2/3rds difference between 8 and 4ohm output. Why is this so?

  5. Don't forget to consider cabinet/speaker sensitivity. It can make a huge difference in volume levels, all-else the same.

    - Andrew
  6. IME more sensitive speakers/cabs do not compensate for more speakers when it comes to volume and neither does more power.

    IMO if you want volume get more speakers.
  7. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    That's why.
  8. Frank Martin

    Frank Martin Bitten by the luthiery bug...

    Oct 8, 2001
    Budapest, Hungary, EU
    Hey, have you seen that 36*10 that Ampeg made back then? You have gas for one of those, right? :p :rolleyes: :D

    How much volume are you looking for, anyway?
    It's not just the quantity of speakers that matters, but also the sensitivity of the cab and the power it's fed.
    IMO it's easier to haul a lighter, but high-power head with, like a 410, then brign an 810 and a low-power head.
  9. I can't say for sure, but I still don't think that a high powered head with a sensitive 410 can match up to a decent head with an 810

    Unless it is a behringer - jokes*
  10. SWR amps in general have a 8-ohm output that's closer to the 4-ohm output than many other amps. My Workingman's 2004 (baby baby brother of the 800) does 200 at 4 ohms and 160 at 8 ohms. 2/3 would put it at 133 watts.

    Looking up the manuals for several current models, they do tend to follow the 2/3 estimate.
  11. Soulbanana


    May 7, 2004
    I think I've solved the amp/cab query. It's not about impedance. It's about ideology. The SWR is made in the YOO ESS AAYEE and stands up for life, love and liberty. My Behringer is made in China by less attractive Chinese people (just like myself). They are at loggerheads!

    Thankfully all will be solved when my Lab Systems 4x10 comes in tomorrow. Made in Australia by like-minded Australians who share similar values. Maybe then the 2 of them will deport the poor Behringer on a boat back to wherever it was from. :smug:
  12. tadawson


    Aug 24, 2005
    Lewisville, TX
    In an ideal world, the 2 ohm power would be twice the 4 ohm power which would be twice the 8 ohm power. Power is voltage squared divided by resistance, so mathematically, you can see where this relationship comes from. Now, having said that, in the real world, this does not always happen. In PA type amps, 4 is typically twice 8, but 2 is less than double. Why? Power supply limitations, and the inherent resistance of the output devices. What that means is, that at the higher current levels of lower resistances, a lot of amp power supplies can't hold their voltage like they could into 8 ohms, and so the max output is less (reference equation above). The second, output device impedance, simply means that losses across the output devices become more significant at lower load resistance. IE, .2 ohms in the output stage on an 8 ohm load is only 2.4 percent, but with a 2 ohm load it becomes 9 percent. That percentage is dumped as heat in the amp, and never makes it to the speakers. (Note: The ".2 ohm internal resistance" I have used is for example only, and does not relate to any actual amplifier!) When you run an amp bridged, each channel effectively "sees" half the load - so an 8 ohm cab on a bridged amp will look like 4 ohms to each channel of the bridged amp, and as such, the bridged power output should be very close (if not exactly) twice the 4 ohm rating. That is also why bridged output ratings typically never have a power rating at as low a load impedance as for single channel operation.

    - TIm