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technical question on power

Discussion in 'Amps and Cabs [BG]' started by Javy, Oct 12, 2000.

  1. Javy


    Sep 15, 2000
    Let's say I have two bass combos that are the same in every way except that one produces 100W and the other produces 200W of power. Common sense tells me that the 200W combo is going to be louder than the other one, although not twice as loud due to the nonlinear relationship between power and perceived volume.

    However, it seems that 100W would be more than enough power to move the speaker cone to its maximum excursion, especially if the sound being amplified had a lot of very low frequency content. Maybe this is where my thinking is off, but if not, then how would the 200W amp manage to generate a higher sound pressure level from the same speaker?

    On a related note, would you generally get more volume by increasing the amount of power driving a bass cabinet or by adding an extra cabinet to an existing amp (assuming you couldn't do both)?
  2. embellisher

    embellisher Holy Ghost filled Bass Player Supporting Member

    Javy, I'm not the most techical guy in the world(Joris or Psycho? You out there?;) ) but I know the answer to the last part.

    Kinda sketchy on the first part, but I believe that if you add power, it takes 50% more power to increase the SPL 3db. If you add 100% more power, you will increase the SPL 6db(most people have difficulty noticing a difference in a 3db increase).

    OTOH, if you add a second cabinet to the same amp, providing the amp will handle the lower resistance, you pull more wattage out of the amp, and with the addition of another cabinet, you have more square inches of speaker cone moving air.

    For example, an SWR Bass 750 head puts out 450 watts @ 8 ohms, and 750 watts @ 4 ohms.

    So if you hook up a 4x10 8 ohm cabinet to it, you get 450 watts.

    But if add say a 1x15 8 ohm cabinet to it, you get 300 more watts, plus that extra 15 pushing air as well.

    The concern in adding cabinets is whether the amp can handle the lower load.
  3. Christopher


    Apr 28, 2000
    New York, NY
    The mathematic relationship between dB and power is

    (delta) dB = 10 log (Psub1/Psub0)

    Thus, doubling the power (ie. increasing it by 200%) gives you a change of 3 db (10 log 2). Ceteris paribus, the 200W combo at full volume won't be significantly louder than the 100 W combo at full volume.
  4. Matthias


    May 30, 2000
    Vienna, Austria
    You are speaking of combo amps of one brand wich are available with different power ratings?

    I guess the main reason is 'headroom', not sound pressure level. The higher powered amp can put out more clean power, so you get less distortion at high volume levels.

  5. thayer182


    Oct 1, 2000
    ok I see all these descriptions of bass amps, but I have yet to figure out what ohms has to do w/ it. i know ohms is resistence to electronic flow, but how does that affect your sound? it would help me out tremendously seeing as how I'm looking into buying a new amp.
  6. Dave S...

    Dave S...

    Oct 13, 2000

    Mostly, ohms are kinda like shoes...you buy the ones that fit your feet. You won't play basketball better in a size 12 Reeboks when you wear a size 9 1/2.

    The impedance (ohms) of a speaker does not dramatically affect the tone of a speaker. The most important function of ohms is providing the proper size "shoe" for the amplifier output.

    Depending on the circuit design of the amp, it will produce different power levels at different impedances. It's not always along the lines of 100 watts @ 8 ohms/ 200 watts @ 4 ohms/ 400 watts @ 2 ohms, etc... You reach a point of 'diminishing returns.' Some amps are designed to be real efficient at higher ohms levels (usually 8 ohms)--sacrificing the ability to operate at very low (2 ohm) loads.

    Take Peavey's old standard 'Combo 300' It provides 210 watts RMS @ 4 ohms (with their DDT compression *which tends to artificially raise RMS ratings a bit*) and is rated @ 300 watts RMS at 2 ohms. Not a huge difference in terms of POWER, but nice in terms of FLEXIBILITY (meaning you could hook up four 8-ohm cabinets in paralell to the amp [provided you disconnect the internal speaker])

    Now if you had another mfg's amp that would only operate above 4 ohms, but provide 350 watts... you'd lose the ability to run multiple 4 ohm cabinets, (or a bunch of 8 ohm cabs) but in most reasonable situations you'd only use 1 or 2 cabs max. (which works better, anyway..)

    SO...figure out what ohms your cabinets are (or what kind of cabs you'd want to eventually use...) and make sure you new amp will operate into that impedance. THEN figure out what wattage your amp will produce at that load. (remember, if you've got two 8 ohm cabs--that's 4 ohms total. AND the amp's wattage will be split evenly between both cabs...half and half.)

    Lastly, if your amp is rated at 8 ohms-and you give it 16...you'll have noticably less power ( and USUALLY no ill-effects with SHORT-TERM usage. Don't push it, though!) If your amp is rated at 8 ohms MINIMUM and you use a 4 ohm cab, you WILL overheat the amp and cause damage. Even though the amp is facing 'less resistance' with a 4 ohm load in this case, the circuits and components were not designed to handle it. REMEMBER: an electrical 'short' means NO resistance. The lower an impedance, the closer it is to an electrical short in the amp's mind. Some amps are designed to handle low impedences, where as some amps are intended to have owners who are aware of the differences available.

    REAL sorry that this was so long-winded! I'm not a tech, so if there's one around here- and I've mis-stated any of this stuff, please post. My mis-statement wouldn't be worth some guy toasting 2 paychecks on an amp.


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