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Cab and head wattage

Discussion in 'Amps and Cabs [BG]' started by GDAE, Dec 16, 2002.


  1. GDAE

    GDAE

    Aug 14, 2001
    Iowa
    OK, I'm thinking of getting a new amp and I need some help figuring out what a few terms mean. First, on a cab (an Ampeg, can't remember the model number) it was listed at 200W or 400W program. What does that mean?
    Also, on a head (also an Ampeg) is was listed at 200W or 400W depending on the ohms. Again, I have no idea what that means.
    Can someone please explain to me what these things mean? Also, are there any reasons you wouldn't want a cab to have more wattage than the head powering it? I can understant why you wouldn't want a head that was more powerful than the cab, but I don't know about vice-versa. Thank you in advance to anyone who answers these questions (which are probably pretty dumb questions).
     
  2. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    Amps ae typically rated by power in watts "RMS" (root mean square). This is a number that is measureable. Often they will be rated in "program power" which is usually about 2x the RMS rating. Folks used to rate them sometimes in "peak music power" which could be pretty much anything they wanted to make up. RMS is all that counts.

    The power the amp can deliver is different depending upon the impedance (resistance) of the load (measured in ohms). Think of it as a pump- a pump may be able to push 10 gallons per minute into a 4" pipe, but 20 gallons per minute into a 6" pipe. The same pump might very well blow up without any backpressure as if it ran into a 10" pipe. It's common to rate amps at a 4 ohm load, have them make a bit less into 8 ohms, and although some will drive a 2 ohm load, lots of them will just fry.

    Often folks gert a head more powerful than their speaker cabinet, because it allows them to run their amp at a reduced setting where it has "headroom" to carry the peaks in the music without distorting. In guitar amps distortion is a "good" thing (and they commonly push 15-50 watts). For bass amps distortion is usually a bad thing since you may make 100-1000 watts which creates a lot of heat in the speaker.

    To complicate your choices even further, every speaker cabinet has a different "efficiency" rating- how sensitive it is to power. What this means is that cabinet "a" may be louder than cabinet "b" even though 'B' may be running twice the power. This is normally rated in "decibels at 1w / 1 meter" or similar (fixed power signal measured at a fixed distance in front of the cab)- 92db is a very quiet cabinet while 102db is a very loud cabinet. To make a 92db cab as loud as a 102 db cab would require 10x the power.

    Normally a less sensitive cabinet may have a lower frequency response and/or have cheaper speakers. It may sound better to your ears (or maybe not). You will discover that a cabinet can be either small, loud, or go low, but getting 2 of those will cost money and all 3 violates physics.
     
  3. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    No, it would take about 10x the power. Were you thinking 1000%?
     
  4. ESP-LTD

    ESP-LTD

    Sep 9, 2001
    Idaho
    Edited per your response.

    Actually, I wasn't thinking much at all ...
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Can I please kill a big misconception here. Everyone agrees that a 1000w amp can easilly kill a 35 watt speaker. But no-one seems to care that 35 watt amps are MORE likely to kill 1000 watt speakers.

    When an amp is forced to work too hard, if cuts off the peak of the sine wave and sends a "clipped" distorted signal to the speaker. It has a much higher temperature and will literally cook the voice coil.

    As a general rule, try and match your amp and speaker wattage as much as possible. If anything, make the amp slightly more powerful than the speakers. But whatever you do, don't let your amp clip.
     
  6. After i read this thread i got my doubts. It was pretty well discused here.
     

  7. This is absolute nonsense. Explained at length in several threads.
     
  8. Richard Lindsey

    Richard Lindsey

    Mar 25, 2000
    Metro NYC
    Um, I think the misconception is yours. 35 W amps are not gonna kill 1000 W speakers, though they can certainly kill 40 W *tweeters* (an entirely different thing). Just think about the rock guitar players of the world. Why doesn't a cranked 100 W Marshall immediately fry a 240 W cab hooked to it? Why doesn't your stereo fry when your amp reproduces severely clipped signals?