Clipping/underpowering/overpowering

[tgriley62]

Is this correct on my part. A 4x10 cab rated at 800W RMS @ 8 ohms will have 4 speakers that are rated at 200W RMS each (4 x 200W RMS = 800W RMS) & if the amp head puts out 250W @ 8 ohms each 200W speaker is getting 62.5W (4 x 62.5W = 250W) but a 4x10 rated at 400W @ 8 ohms has 4 speakers rated at 100W @ 8 ohms. If this is the correct way in which cabs work which is better for reliabilty and longivity, a cab that is using close to its RMS rating or one that is using much less

[BassmanPaul]

You have forgotten, or may not know, that cabinet power ratings are seldom RMS. They are usually the thermal limits of the drivers or the point at which they start to melt. In the real world the drivers can use about half that power before they run out of excursion. After that point any added power will be converted to heat and the cabinet will get no louder.

[tgriley62]

Originally Posted by BassmanPaul

You have forgotten, or may not know, that cabinet power ratings are seldom RMS. They are usually the thermal limits of the drivers or the point at which they start to melt. In the real world the drivers can use about half that power before they run out of excursion. After that point any added power will be converted to heat and the cabinet will get no louder.

So does that mean a 100W driver should not be driven with more than 50W? Sorry if seems stupid but I have read so much trying to get a handle on ohms/watts/underpowerd/overpowered and such that I have confused myself more!

[bongomania]

First: drop the "underpowered" part from your vocabulary, because it is not a real thing. Check the FAQ to learn about that. There is no such thing as underpowering.

[bongomania]

I think what Paul was essentially getting at was that you can't rely on wattage ratings for either amps or cabs. The actual reality of how much wattage is produced or used, for what periods of time, and in which frequency ranges, varies so dramatically from moment to moment and situation to situation, that the numbers become essentially meaningless except as a rough guideline. For example, I would not expect a clean, undistorted 200W amp to be very loud. But that doesn't mean the 200W is a literal factual value of what's happening in reality. For another example, I would not expect a cab rated for 200W to survive for long if I crank a 1000W amp into it at high volume for long periods. But again, that doesn't mean anything literal about the values "200" or "1000"--they are just like signs on the freeway saying "gas, food, lodging, next exit".

[tgriley62]

Originally Posted by bongomania

I think what Paul was essentially getting at was that you can't rely on wattage ratings for either amps or cabs. The actual reality of how much wattage is produced or used, for what periods of time, and in which frequency ranges, varies so dramatically from moment to moment and situation to situation, that the numbers become essentially meaningless except as a rough guideline. For example, I would not expect a clean, undistorted 200W amp to be very loud. But that doesn't mean the 200W is a literal factual value of what's happening in reality. For another example, I would not expect a cab rated for 200W to survive for long if I crank a 1000W amp into it at high volume for long periods. But again, that doesn't mean anything literal about the values "200" or "1000"--they are just like signs on the freeway saying "gas, food, lodging, next exit".

I guess what I am really needing is advise on which setup would be best for my first head/cab purchase. I am looking at buying either a 400w or 500w head and using only a 4x10 with opition of adding an identical 4x10 latter on

Again thanks for all the info

[Bass_Pounder]

As already stated, the "rating" you see of a cab is the thermal rating only.

Actual physical limits are about 1/2 the thermal rating.

All things being equal, I would go for the 800 watt cab before the 400 watt cab.

[bongomania]

Say you get a 500 W amp, and an 8 ohm cab rated to handle 400 W. Turns out the amp only produces like 300 W into 8 ohms! It only puts out 400 at 4 ohms. So your speakers are mostly safe. Then later you get another 8 ohm cab rated for 400W; now you're nominally getting the 400 W, and your cabs are only getting around 200 each, so you are again/still safe from damaging speakers.

I'm simplifying, but that's a normal/common/good approach to take.

It's true that a lot of wattage is wasted as heat, but that really doesn't help answer your question. It's also true that speakers can't always handle what they're advertised to handle, but it doesn't help much to worry about that.

[GrowlerBox]

The power ratings of any commercially available cab are marginally useful at best. Different EQ settings, for example, create different stresses on speakers -- lots of bass requires lots of driver excursion and risks "farting" and damage even at relatively low volumes. Their ability to generate the tone you like with your head is infinitely more important. Once you have the cabinet you like, listen to it as you adjust your volume and EQ settings, and back off if it starts to complain. If you can't make it loud enough, save up for another of the same cabinet -- more volume, more bass extension, more happy .

[billfitzmaurice]

Originally Posted by Bass_Pounder

All things being equal, I would go for the 800 watt cab before the 400 watt cab.

The problem there is that the 800 watt cab may have exactly the same total driver displacement as the 400 watt cab. For that matter it may have less, and it may have less low frequency sensitivity as well. The bottom line is that wattage ratings alone don't tell you anything useful, and what you do need to know most manufacturers don't tell you. The only way to really know what you're going to get is to try before you buy.

[wideload]

If its a first purchase, most manufacturers have amp/cab pairings that work together (Ampeg, GK, Markbass, Peavey for example). Buy an amp and cab with the ability to add another matching cab down the line, if needed. If you find you need more volume, more speakers helps more than more power. A 4x10 is almost standard, and leaves room for growth.