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Underpowering Cabinets....

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padgettohio

Apr 13, 2006
67
0
Cincinnati
#1
Hello fellow Bass Players,

This is my first post, so let me start by saying "hello".

I've been reading posts on this site for a short while now, soaking up various information, and due to the fact that my job actually pays me to work and not spend my days searching on forums, I finally registered so that I could ask some questions that have plagued me for a short while now. Here are two that I am confused about....

My first question is about running an amp into multiple speakers. Okay, yes I've found a million posts on this subject, and I'm sure to be bombarded with the standard "dude, we discussed this", but I gotta be honest. Most of the replies get into so much detail that my head starts spinning before I get to the end of the thread and I just want to be sure I'm understanding the whole thing. So anyway, here goes...

1. ) Let's say I have the following: One amp, rated to have an output of 1200w bridged @ 4ohms, and 900w bridged @ 8ohms, and two 700w 8ohm 4x10 cabs. If I were to connect only one of the cabinets, I understand this means I would be running 900w (8ohms) into the speaker, and would be slightly overpowering the cabinet. Now if I were to run both cabinets "daisy-chained" together, I understand that I would be running 1200w (4 ohms) into both speakers because running two 8ohms cabs = 4ohms for the amp. Am I correct so far? Good. So my question is when running daisy-chained, will that be 1200w to each speaker, or 600w each; with the two 410's essentially being one 1400w 4ohm 8x10? In that case I would be slightly underpowering the cabinets, correct?

This now brings me to my next question....

2.) In my many searches, I always seem to run into threads about "underpowering your cabinet". While the whole underpowering thing makes sense, I can't help but wonder: How is it that one of the most popular Bass amp setups, the SVT-CL all tube head connected to the SVT 8x10 cabinet, works so well? The 8x10 cabinet is rated to handle up to 800w RMS, and the SVT-CL head is only rated to have an RMS output of 300w. This is supposedly "the sound" (as I read somewhere), but it just doesn't match up with the argument. Please explain this to me or even point me to the thread that can give me the answer to this. Your input is appreciated.

Sorry for the long post. Been holding those in for a little while. Funny how hard it is to stay on track when your typing a post.....looking forward to hearing what y'all have to say.
 
#2
Dude, we *have* discussed this a million times. Please have the courtesy to search. If you won't put in the work to read what people have already taken considerable trouble to type, why should people put in the work to repeat themselves for the umpteenth time? Please read what's already been said first, then ask if there's something specific you don't understand.

Sorry, but it just bugs me when someone says, okay, I did a search, but I didn't want to actually read through all that stuff, so just explain it to me all over again.

Hint: if you really think the underpowering concept makes sense, you can't have read the threads that discussed it.
Second hint: look for posts by Mark Reccord and Bob Lee.
 
#3
well I've never said (or wrote) it here before so...

when you daisy chain two 8 ohm cabs, and achieve a 4-ohm total load, you take that 1200 bridged watts @ 4 ohms and send it to both speaker cabs (split) not each of them.

when they talk about 1200 watts and such, it is in terms of Solid State or MosFet amps. Tube amps like the SVT-CL are a totally different beast. With a tube amp, all you need is 300 watts to stop the earth from rotating. I'd say probably 1200 MosFet watts are equal to 300 Tube watts.
Now I don't know the real science behind it, but that's what I know...
if you want more technical advice, the you have to do the search...sorry...
also, this sticky is very helpful
http://www.talkbass.com/forum/showthread.php?t=166225
 
#4
1) With a 1200 watt amp going to two 8 ohm speakers, they would be getting 600 watts each. This assumes the two speaker cabinets have the same actual impedance. If not, the lower impedance one would get a little more power. In my opinion, this is a “good” match and underpowering or overpowering is not really an issue. If you had these two cabinets in your bedroom with a 25 watt amp, I would not even call that underpowered. I would say that with most bass setups, the speaker rating and the amp rating do not equal exactly. It does not really matter. What if the 700 watt speaker rating is a little over-rated and it actually is only 560 watts?

Now if you got on an outdoor stage with these two cabinets with a 50 watt amp, that could be called under-powering. And if you played real loudly, and had a 2800 watt amp, that might be called over-powering.

But 700 watt speakers with an amp of 600 or 900 watts output is not a problem. The ratings are close enough that it doesn’t really matter.

2) Using a 300 watt head with an 800 watt speaker is pretty common. Again, the ratings do not have to be equal. 300 watts is all a lot of people need, and having higher rated speakers gives a little safety margin. Even on an outdoor stage, I would not call this under-powering.

A few examples- my band’s PA is 300 watts a side, our speakers are 200 watts per side – we have had the same 200 watt EV woofers for 11 years- never blown them. My setup is 225 per channel – one speaker is 300w and the other 600w – seems to be a good match.
 
#5
Mr. Lindsey: Sorry, I didn't mean to be discourteous. I actually have performed searches on this subject. Most of my post was to see if I'd understood it correctly. The additional questions were things either I hadn't found completly or just plain didn't understand from what I'd read. I'm still learning how to get around efficiently on this site (usually while at work, as I explained), so hopefully, when I reach the amount of posts you veterans have, we won't have to deal with my ignorance. Again, I'm sorry, and I hope I didn't irritate you too much.

That being said, Thank you for directing me to posts by Mark Reccord and Bob Lee!! I'll be sure to look for them.

As for eastcoasteddie and LesS: Thank you gentlemen for your clarification about the watts to speakers. You gave me exactly what I was lookin for, with both explanations and threads to check out. I thought a watt was a watt, but I will be sure to read the sticky you posted eddie, and look for more on Tube vs. Solid state.

Thanks again to all of you for making my first post a good one.
 
#7
padgettohio said:
Mr. Lindsey: Sorry, I didn't mean to be discourteous. I actually have performed searches on this subject. Most of my post was to see if I'd understood it correctly. The additional questions were things either I hadn't found completly or just plain didn't understand from what I'd read. I'm still learning how to get around efficiently on this site (usually while at work, as I explained), so hopefully, when I reach the amount of posts you veterans have, we won't have to deal with my ignorance. Again, I'm sorry, and I hope I didn't irritate you too much.

That being said, Thank you for directing me to posts by Mark Reccord and Bob Lee!! I'll be sure to look for them.
Click to expand...

No problem. And sorry for being a bit cranky. I was finishing my taxes that day.

It's just that I know that there are a ton of useful and clear posts *already posted* on exactly the topics you're asking about. It's sort of a "why reinvent the wheel" kinda thing, you know? Definitely check out what Mark and Bob have said.
 
#8
And this for the umpteenth time:

THERE IS NO SUCH THING AS "UNDERPOWERING" A CABINET.

If you have an 800 watt capable cab and drive it with an 800 watt amp, and only turn the amp up to "one" on the volume knob where the cab only sees 40 watts or so playing quietly, you are doing no damage to your speakers.

The term "underpowering speakers" is totally erroneous, and at best a severe misuse of the english language.
 
#9
Taxes are enough to ruin anyone's day. Totally understand. I just need to get better at performing good searches on this forum. I just sometimes end up taking up a full work day reading all the info....

I guess my question of "overpowering" or "underpowering" speakers had to do with loud volumes when an amp could start clipping. It would be nice to play at softer levels and not have to worry about any of it; but I play in a band where the volumes are borderline absurd, and I have blown a few speakers in the past, so I'm just trying to get a better understanding of setting up a good rig. I've actually found a couple of good threads since posting this, so I'll keep searching. I reckon I just need to be a little more patient. Still, I am appreciating all of the good feedback.
 
#10
padgettohio said:
I guess my question of "overpowering" or "underpowering" speakers had to do with loud volumes when an amp could start clipping. It would be nice to play at softer levels and not have to worry about any of it; but I play in a band where the volumes are borderline absurd, and I have blown a few speakers in the past, so I'm just trying to get a better understanding of setting up a good rig.
Click to expand...

I think this is a reasonable concern. There is such thing as underpowering a cabinet, and this is one aspect of it. You're much more likely to blow a speaker by clipping an underpowered amp than you are by overpowering it with a monster. Clipping can fry a speaker quickly; too much juice usually warns you first by sounding bad.

Another element of underpowering a cab has to do with getting it to sound as good as it might. My epifani cab doesn't sound good with a 300 watt amp, even if it's cranked. The little amp just doesn't seem to be able to control the speakers, so the sound is loose and flabby.

On top of all this, some speakers just don't sound good at low volumes. This has more to do with choosing the right speaker for the right gig, but it's related. Some cabs will be at their best playing loudly with a powerful amp, and under other circumstances you won't be getting the best out of them.
 
#11
The problem is that the commonly used term "underpowering" is an unfortunate misnomer.

Running a lower rated amp into clipping actually overpowers the cabinet, as a clipped amp can produce up to double its rated RMS power when severely clipped. A 500W amp could produce up to 1000W of clipped power.

Too much clipped power from a smaller amp heats up voice coils and burns them out, while too much clean power from a bigger amp will cause overexcursion and bottoming out. Either way, the damage is from too much power, not too little.

Hence, underpowering is not the problem. Overpowering is the problem.
 
#12
What 12bass said ... and consider WHAT kind of signal you are using ... really low ground shaking bass uses lots of power ... I have a Modulus Q-5 with a 35 inch scale neck and Bart pickups ... that low B takes way more headroom and power to reproduce a clean sound than using the same rig to get a good tone with punchy midrange passive P bass.
 
#13
paulraphael said:
I think this is a reasonable concern. There is such thing as underpowering a cabinet, and this is one aspect of it. You're much more likely to blow a speaker by clipping an underpowered amp than you are by overpowering it with a monster. Clipping can fry a speaker quickly; too much juice usually warns you first by sounding bad.
Click to expand...


This is not really the case.

What does thermal damage to speakers is average power. Waveform is all but irrelevant.

Cases in point:

Given a speaker that can handle, say, 500W continuous.

Easy enough to blow it up with a 1000W amp without ever clipping it. Run a sine-wave signal into it at, say, 600W and see what happens. You can get away with using a more powerful amp than the speaker rating because dynamics make it difficult to put the full power of the amp into the speaker on an average basis.

It's possible to damage it with an amp rated at 250W+, though the probability goes down as the power rating goes down. This is because clipped waveforms have more energy than unclipped ones at the same amplitude (voltage). Every amp can put out a maximum voltage. This voltage is the clip point. If the amp is driven beyond this point, the peak voltage will not increase beyond it. However, the flattening off of the wave decreases the peak-average ratio, so there is more average power in the clipped signal even though the voltage is the same. A pure square wave has 2x the power of a pure sine wave with the same voltage.

Below 250, the chances of thermal damage are slim to none no matter how much you clip the amp. The amp just can't put enough power into the driver to damage it.

Some people seem to believe you can blow up a 500W speaker with a 10W amp, but this just isn't the case. You'd be more likely to be hit by a satellite.

Some cabinets require lots of power to sound their best, typically because they demand a lot of current at low frequencies and with big phase angles. These types of loads can be difficult for an amp to drive if it's not optimally designed.
 
#16
"Below 250, the chances of thermal damage are slim to none no matter how much you clip the amp. The amp just can't put enough power into the driver to damage it."

That's probably true with a radically underpowered amp, but not with one that's likely to be used for bass in the first place. When an amplifier clips, the waveform becomes a square wave. In the early stages of clipping, when just the peaks of the waveform clip, you hear nasty distortion, but that's it. When the waveform gets radically clipped, you are essentially sending gobs of D.C. current to the voice coils for a stretch of time, and then reversing the polarity for another stretch of time. This is asking the cone to be at full excusion in one direction under high current, pause, then go to full excuresion in the other direction under high current and to pause (if it even has time to move at all ... depending on the frequency it may just get stuck in place and kind of flutter). At the moments when the cone isn't moving, the driver is generating no back EMF. So the speaker's impedence drops down well past its rating, and the current going through the coils spikes.

You can think of a driver as an a.c. motor. An a.c. motor can be damaged by surprisingly little d.c. power. It can also be damaged by a square wave (this happens sometimes--lower priced uninteruptable power supplies often produce a square wave when the battery backup kicks in. The assumption is that the connected equipment, like computer power supplies, can take the abuse, but some things like motors can get fried).
 
#17
paulraphael said:
"Below 250, the chances of thermal damage are slim to none no matter how much you clip the amp. The amp just can't put enough power into the driver to damage it."

That's probably true with a radically underpowered amp, but not with one that's likely to be used for bass in the first place. When an amplifier clips, the waveform becomes a square wave. In the early stages of clipping, when just the peaks of the waveform clip, you hear nasty distortion, but that's it. When the waveform gets radically clipped, you are essentially sending gobs of D.C. current to the voice coils for a stretch of time, and then reversing the polarity for another stretch of time. This is asking the cone to be at full excusion in one direction under high current, pause, then go to full excuresion in the other direction under high current and to pause (if it even has time to move at all ... depending on the frequency it may just get stuck in place and kind of flutter). At the moments when the cone isn't moving, the driver is generating no back EMF. So the speaker's impedence drops down well past its rating, and the current going through the coils spikes.

You can think of a driver as an a.c. motor. An a.c. motor can be damaged by surprisingly little d.c. power. It can also be damaged by a square wave (this happens sometimes--lower priced uninteruptable power supplies often produce a square wave when the battery backup kicks in. The assumption is that the connected equipment, like computer power supplies, can take the abuse, but some things like motors can get fried).
Click to expand...

Your description of what happens under clipping is not borne out in the real world.

For one thing, no amplfier will never produce a pure square wave no matter how hard you clip it. For another, square waves are not DC and not even remotely like DC, other than the fact that the heating power is the same. Yet another thing is that the 'square wave' you see on the scope is really a combination of a lot of sine waves and does not lock the cone into place at any time. Furthermore, the inertia of the cone would allow the cone to move in a practically sinusoidal fashion anyway. Also, clipping doesn't necessarily move the cone out to maximum excursion, that's highly dependent on the amplitude and frequency of the signal. Speakers don't greatly depend on movement for cooling either. That's part of the equation but the transfer of heat across the gap and into the magnet accounts for the bulk of a speaker's cooling efficiency. If you start getting the coil out of the gap you'll quickly damage things, no matter how clean the power.

I've actually tested all this stuff experimentally. There is no significant difference in heating (or current draw) between a pure square wave and a pure sine wave set up to dissipate the same power in a speaker. I.e. the RMS voltage of the sine wave is equal to the peak voltage of the square wave for equivalent heating power. Obviously, there's a difference if the peak voltages are the same because the square wave develops twice as much power as the sine wave does.
 
#18
paulraphael said:
IAnother element of underpowering a cab has to do with getting it to sound as good as it might. My epifani cab doesn't sound good with a 300 watt amp, even if it's cranked. The little amp just doesn't seem to be able to control the speakers, so the sound is loose and flabby.
Click to expand...

This still isn't underpowering the cab, because it has nothing to do with the ratio between the amp's output and the cab's power handling. It's a simple matter of bigger amps sounding better because they provide more headroom. Of course your Epi doesn't sound good when you crank your 300 W amp. The fact that you're cranking your amp means that you don't have enough amp for the job you're trying to do. Making you underpowered for your playing situation, not underpowered for your cab. If you replace that 300 W amp with a 1000 W amp, you're going to sound better regardless of your cab's power rating (assuming the cab can take the power).

As for some cabs sounding better with more power, I've never found this to be true. My experience has been that *all* cabs sound better with bigger amps (= more headroom), within the limits of their particular capacities. It's not that, say, Acmes and Epis like headroom and Bergs and Edens don't. I don't think there's any magic there.
 
#19
Mark Reccord said:
YI've actually tested all this stuff experimentally. There is no significant difference in heating (or current draw) between a pure square wave and a pure sine wave set up to dissipate the same power in a speaker. I.e. the RMS voltage of the sine wave is equal to the peak voltage of the square wave for equivalent heating power. Obviously, there's a difference if the peak voltages are the same because the square wave develops twice as much power as the sine wave does.
Click to expand...

So does this mean that the peak power of the square wave would be equal to roughly twice the rms power of the amp?

I'd be interested in seeing more about your experiments. The explanation I gave was based purely on what a speaker engineer told me. It was in reference to stereo speakers, not bass cab speakers, but the physics seem to be the same. I've seen a lot of stereo speakers blown with amps well below the 'power rating' of the speaker, and i asked him why. I've also seen bass speakers behaving the way he described when an amp clipped hard--fluttering in place and moving erratically.

I also wasn't suggesting that the speaker dissipates its heat by moving. It creates its impedance by moving; the motion of the coils past the magnet creates the back EMF that is the primary source of resistance to the amplifier's voltage. If the speaker cone is restrained, the impedance vanishes and the coils can suddenly see way more current than they were designed for.
 
#20
Richard Lindsey said:
This still isn't underpowering the cab, because it has nothing to do with the ratio between the amp's output and the cab's power handling.
Click to expand...

of course. i don't think there's an actual phenomenon called "underpowering a cab." i'm using that as shorthand for a couple of separate reasons why too little power can be a bad thing.

Richard Lindsey said:
My experience has been that *all* cabs sound better with bigger amps (= more headroom), within the limits of their particular capacities. It's not that, say, Acmes and Epis like headroom and Bergs and Edens don't. I don't think there's any magic there.
Click to expand...

I agree in general, but in my experience some cabs sound worse with an underpowered amp than others. The Edens that I've played through still sound like Edens with a 200 or 300 watt amp. They just don't sound quite as good or as loud as they could. On the other hand my Epi sounds like ass with an underpowered amp. I barely recognize it as mine. It's loose and floppy and slow. It may have as much to do with damping factor as with power, i don't know.
 

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