Originally Posted by mangl
howdy, i did a mediocre search through talkbass looking for threads on under-powering your cabs, consequently the results were also mediocre.
im thinking of buying an SVP-Preamp with a CrownXLS 402 (260watt @ 8 ohm) to power my old peavey 410tx cab (Rated 350rms 8ohm).. i know that severely underpowering speakers can cause damage, but i dont have a perspective on what defines 'severe'.
looking for both advice on the purchase of the ampeg and crown, and on whether the crown is too weak for my peavey tx cabs.
~highs through hitting lows
Underpowering speakers is not what causes damage. Having the wrong tool for the job can cause damage. If you have to crank your amplifier until you've exceeded the limits of either your power amplifier or your speakers and continue to use the rig under those conditions, damage can occur. An extreme example is if you have a 50 watt amplifier and a speaker cabinet like your Peavey rated to handle 350 watts RMS (root mean square - a mathematical way of saying "average" in reference to AC power). It's probably not going to be loud enough for a big loud gig, so if you crank up that amplifier into extreme clipping (distortion), the very most you'd get out of it on any constant basis is still well below the RMS rating of the Peavey cabinet. The cabinet will just sit there cranking out whatever distorted sound the amplifer generates. *CLIPPED WAVEFORMS ALONE DO NOT CAUSE SPEAKER DAMAGE*
IF your cabinet has a high frequency driver (tweeter or horn), it can handle considerably less power than the woofers. In some cases, the severely distorted sound can contain a lot of high frequency harmonic content (distortion is a harmonic series dependent on both the input sound and the characteristics of the amplifer). In rare cases, there can be enough high frequency energy even from a low-powered amplifier like our example to exceed the power rating of the tweeter. This is rare, and most good cabinets (like the Peavey if I'm not mistaken) employ some method of overpower protection for the tweeter. I seem to recall Peavey using a light bulb to dissipate extra power.
If your amplifer, with 260 watts RMS output, combined with that Peavey 4x10, is not loud enough for a gig, you may drive the amplifier into clipping. In this case, clipping can actually bring the power output of the amplifer ABOVE the RMS power handling of the cabinet. It would be VERY difficult to do this on any sort of continuous basis because even very compressed bass playing is still quite dynamic. One would have to be very busy to keep the amplifier clipping constantly enough. Still, it does happen, and when it does, the speakers may overheat. This is thermal failure, and is caused by the voice coil trying to dissipate TOO MUCH HEAT. In other words, the speakers were OVERPOWERED.
Often the solution to this possible problem (which is caused by too little volume for the gig, not by simply running an amplifier with a lower rated output than the cab's RMS power handling) is to run an amplifier with approximately twice the RMS output that the cabinet is rated for. In your case, this would be a 700 watt amplifier. Now when you play out, your amplifier may in fact at times deliver that full 700 watts for peaks in your playing. It is not long enough to cause thermal failure (which requires time for the voice coil to heat up and melt). You are not overdriving/clipping the amplifer so your signal is not unintentionally compressed, and chances are you won't even manage an average of 1/3 the power your amplifer can supply. This is again because of the dynamic nature of playing the bass.
You can damage your cabinet with this setup too, though. Play loud enough and it's pretty easy to cook the speakers, particularly if this setup still is not loud enough for the gig.
As a side note, keep in mind that EQ is powerful. Remember that to increase the volume by 3dB (a noticeable change in volume; 1dB is considered to be the approximate minimum change for most humans to notice) requires twice the power. This DOES APPLY TO EQ. If you boost your low end (let's say the 50hz area) by 3dB, that's suddenly asking twice the power from your amplifer for any content that the EQ effects. Boost another 3dB for a total of 6dB bass boost and you've suddenly doubled again. You're now asking for 4 times the power (in that frequency range that the EQ knob you're adjusting controls) without increasing your volume or gain knobs at all! See how it adds up quickly and can run you into distorting/clipping your amplifier?
So, as a moral to all of this, make sure your setup is enough for the job. In fact, to be safe, make sure it's a little bit excessive so you have some wiggle room in case a guitarist gets happy with his master volume. If your rig is enough, you won't have any magical explosions due to "underpowering" your speakers.
Hopefully this clarifies a bit. There are a lot of misconceptions about power in audio and this particular area seems tough to get a hold of on these boards. If you're interested in further explanations about power, any reading that isn't manufacturing propaganda will be of help. There is a lot to be learned from high-school and 1st/2nd year university courses in electronics, particularly those pertaining to AC signals.