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Head/Cab Ohm/wattage question


jasonmassacre

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Hello, I was wondering if someone could help me with a head and cabinet question. I have been playing bass for about 3 years and have always used a combo amp. I've decided to upgrade to a separate head and cabinet. I bought a BEHRINGER BX3000T HEAD (300 watts / takes 4 Ohms), and a GALLIEN-KRUEGER 410SBX (400 watts / 8 Ohm) CABINET w/Horn.

 

My question is if I have to worry about frying anything, or popping speakers. I've played it a handfull of times. I've had the Gain up at the half way point, and the output a bit more than half way. That sucker is loud compared to my Fender Bassman 100 combo, and I just don't really know how loud is too loud. Should I worry about damaging the head - or the cabinet/speakers...

 

Any input would be great. The Head manual said it's to be used with a cabinet output of 4 Ohm, and mine is 8 Ohm. I am going to buy a 15" GK rated at 400 watts/ 8 Ohms, so I assume the 2 8 Ohms running paralell will give me the 4 Ohms for the head... but how about the wattage. 2 400 watt cabs to my 300 watt head... will I blow anything up? How does the wattage work? Any info on how this works, and if i'm in danger of overloading something would be really appreciated. Thank you in advance.

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The short answer is that you'll be fine.

 

Conventional wisdom suggests you should have more power (your amp) than your cabinets are rated.

 

If, for example, you have a combination of cabinets that are rated a total of 800 watts at 4 ohms, one might suggest having an amp rated at 1,000-1,600 watts at 4-ohms.

 

That said, the stated ratings on most amplifiers and cabinets are suspect. If it's working, don't mess with it.

 

Just make sure you don't boost the bass frequencies too much (if you want a bassier sound, cut the treble) and you don't push the gain or master output to a clipping level.

 

Good luck, and welcome to the LDL.

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Having a cabinet with a higher ohms rating than the head is never an issue unless you have a tube power section in your amp. The "underpowering" issue comes from pushing a smaller power amp into clipping trying to get more volume you think you should have because of all the speakers. If you don't hear any distorsion, you most likely are not clipping and everything will be fine. Two cabs in parallel with a total load of four ohms will let you get the most out of your amp.

You can stop now -jeremyc

STOP QUOTING EVERY THING I SAY!!! -Bass_god_offspring

lug, you should add that statement to you signature.-Tenstrum

I'm not sure any argument can top lug's. - Sweet Willie

 

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First off - thanks for the replies. Very helpful.

 

So, now I understand the Ohms part of it, what about the wattage. The amp is 300, the 2 cabs together are 800... How does that work. I assume blowing the speakers would be hard to do considering i'd be pushing only 300 into 800 total... but will the 500 i'm lacking in amp power cause the amp to cook because it's trying to pull too much power (watts)?

 

Sorry if I sound like a fool. I've been playing bass for about 4 years, but always used a combo amp so I didn't look into this. Thanks again for your input. I appreciate it.

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One of the resident gurus can talk in technical terms, but I'll dumb this down to my level for you. I bet we're on a level field:

 

You would think if a cabinet could handle 800 watts and you used a 300 watt amplifier, you'd be in the clear, right?

 

Wrong.

 

There is something called clipping. This happens when you push an amp so hard, it starts distorting (from the power section, not the preamp section).

 

Clipping can cause your amplifier to send a very harsh signal to a speaker, and make it move in ways it should not. This is bad.

 

How do you combat clipping? You get more power. So, for example, I power my two Acme Low B-2 cabinets (each rated at 350 watts) with approximately 650 watts. Lots of power.

 

However, they you have to worry about overloading the speaker and frying the voice-coil. Power handling refers to the speakers ability to disapate heat; too much power = too much heat = release of the magic smoke.

 

However, like I alluded to earlier, beware of the ratings on speakers and amplifiers. It's hard to find honest ratings.

 

If things sound good and nothing sounds like it's about to break, you should be fine.

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maury has covered the bases pretty well, but i'm going to step even further into "easyland" for an explanation.

 

your 400W cabinet can handle a 300W head in much the same way that a 16oz. glass can handle a 12oz. can of Mr. PiBB. no problem. but be careful! if you pour too quickly or if the can is warm and fizzy, you may overflow the glass, in much the same way that if you push the amp too hard, thus causing it to clip, you may burn up a speaker (most likely the tweeter).

 

but you should be safe.

 

robb.

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I'm confused about this.

 

I always thought that if you run an amp with greater wattage than your speakers could handle, then you were in danger of blowing the speaker.

 

I hear what you're saying about clipping, and damaging the cabs by trying to run the amp too hot, and that makes sense too.

 

Are we saying that it's OK to run an amp with significantly greater capacity than the cabs, and if that's the case, how much greater in amp capacity can you go before there is a risk of blowing the cabs?

 

:confused:

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Well. I'm totally comfortable running a 3400 watt power amp into a speaker rated at 100 watts RMS. I mean, the rating of the amp is presumptive of being driven to full output. I don't need to try that. I can actually tell when the speaker is raching its safe limit just by listening and knowing a little bit about it.
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[qb]Are we saying that it's OK to run an amp with significantly greater capacity than the cabs, and if that's the case, how much greater in amp capacity can you go before there is a risk of blowing the cabs?[qb]

 

You can go a lot higher in amp capacity before you have to worry, eg the suggestions posted in this topic or 2-3 times if you want my opinion. Speakers are rated in RMS watts and speaker ratings are essentially heat disapation capacity -- so big(ish) transients are OK. Amps clip when the transients get too high, this is typical of a "clean" bass signal. If you use heaps of distorsion or compression you'll reduce the size of these transients.

 

So, you can run transients that would be 300WRMS -- if they were sustained -- through a 100WRMS cab and you'll be OK.

 

DISCLAIMER -- please only use this as a starting point for some further reSEARCH...there is a search function on this site. ;)

A man is not usually called upon to have an opinion of his own talents at all; he can very well go on improving them to the best of his ability without deciding on his own precise niche in the temple of Fame. -- C.S.Lewis
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Not to mention that speakers with high RMS ratings may be terrible at bass - indicated often by a very low xmax figure in the Thiele-Small specs. An example that's faily common: 300 watts RMS, but can't handle a 150 watt sine wave at 40 Hz. Summary: there are speakers that are better at bass, and there are those that SUCK.
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Originally posted by greenboy:

Well. I'm totally comfortable running a 3400 watt power amp into a speaker rated at 100 watts RMS. I mean, the rating of the amp is presumptive of being driven to full output. I don't need to try that. I can actually tell when the speaker is raching its safe limit just by listening and knowing a little bit about it.

So, have an amp rated at higher than your cab, but just don't use all the power...

 

By doing so you reduce the likelyhood of clipping as you don't have to run the amp hard to gain the required output. This gives plenty of headroom.

 

But, if you do choose to run the amp harder and it's capacity is significantly greater than that of the cabs, you risk damaging the cabs.

 

???

 

I think that's about it?

 

:thu:

 

...there is a search function on this site
Yep, seen that up there, and used it a few times too ;)
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Essentially. But again note that many more speakers are probably overheated, wapred, and burned because the amps driving them are being constantly clipped in a vain attempt to get more power into the cabinets.
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Originally posted by greenboy:

Well. I'm totally comfortable running a 3400 watt power amp into a speaker rated at 100 watts RMS. I mean, the rating of the amp is presumptive of being driven to full output. I don't need to try that. I can actually tell when the speaker is raching its safe limit just by listening and knowing a little bit about it.

You might be comfortable but you're wasting a lot of amp.

 

The problem is that what you're really looking for is loud. More power more loud. Clipping happens when you don't have enough amp to get as loud as you want. So you start with lot's of amp power. So next you want to get a speaker that will take whatever the amp puts out plus more. Power rating at RMS is fine but they have to handle the peaks.

 

If you're blowing a 3400 amp into 100 watt speaker you'll do fine until you do something to increase your amps output. You have to keep in mind that the amp's output isn't constant. It's dependant upon input. A little feedback, pop a connector out... pzzzttt... sizzle... instant fried cab.

 

David

"Life's journey is not to arrive at the grave safely in a well preserved body,

but rather to skid in sideways, totally worn out, shouting "&$%^, what a ride". - Doug Berlin

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PS: it's probably a typo like so many of mine. But LOTS is a plural and not a possessive and thus does not require an apostrophe.*

 

* how did frank zappa foretell of the dreaded internet apostrophe disease? was he related to nostradamus?

 

EDIT: whew! there goes some more incapassotated typing!

.
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A rule of thumb is: if there is no distotion, you're gear is safe.

 

If you need more volume (remember: without distortion) the amp, cabs, or both need to be larger.

 

Guitarists overdrive the PREamp for their distortion. There is an appreciation for driving the speakers to the edge also, but that is a trade-off. Premium tone for replacing speakers every once in a while. Driving the POWER amp to distortion isn't normally done for purposes of tone.

 

For bass, I do like the sound of a speaker in the grey area between full clean and full distortion on occassion. A nice, rough, growl is worth a speaker every five years or more. I've only replaced two in my life.

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"Delete overly sensitive reaction". :)

 

Have a bassically nice day.

David

"Life's journey is not to arrive at the grave safely in a well preserved body,

but rather to skid in sideways, totally worn out, shouting "&$%^, what a ride". - Doug Berlin

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Batman,

 

You're reading way too much into this. Now back on topic:

 

Perhaps you should ask yourself why I used the 3400 into a 100 watt RMS speaker example... there are a couple reasons implicit if not fully stated. That does NOT mean I routinely go around buying expensive megawatt power amps to drive cheapie nearfield monitors or whatever.

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I realize that. Your point is accurate. It doesn't matter how big the amp or how small the cab as long as you don't use more power than the cab can handle. I think what I was trying to point out is that not having enough headroom in your cabs can lead to disaster. In my opinion of course. Just because you've found a level that will let you use smaller speakers doesn't mean that you may, intentionally or not, do something to cause the amp output to spike beyond the speakers capabilities.

 

David

"Life's journey is not to arrive at the grave safely in a well preserved body,

but rather to skid in sideways, totally worn out, shouting "&$%^, what a ride". - Doug Berlin

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Originally posted by Eberbachl:

I hear what you're saying about clipping, and damaging the cabs by trying to run the amp too hot, and that makes sense too.

 

In general, driving a solid state power amp into clipping generates DC currents that get sent to the speaker. These DC currents heat the speaker coil much moreso than the transient spikes (since they're more constant than the transients). Anyway, clipping the power amp will damage the cab drivers MUCH MUCH more quickly than transients. Transients will make the drivers bottom out, but even that will do much less damage than sending DC current through the speaker coil.

 

Unless you need to get really loud, you should be fine. I would assume that you actually listen to yourself when playing ;) so if you hear your cabs sounding really bad (kind of a flatulent sound) and you've got the amp cranked, then you might be clipping the amp. Go from there. If you discover you're under-powered (which is again doubtful unless you need to be quite loud AND you like gobs and gobs of lows), then just looking a bigger amp.

 

Enjoy that new rig...adding power and cabinets will probably be quite an enjoyable sonic experience after playing through a smaller combo!

 

HTH,

Dave

Old bass players never die, they just buy lighter rigs.

- Tom Capasso, 11/9/2006

 

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Originally posted by Dave Sisk:

]]In general, driving a solid state power amp into clipping generates DC currents that get sent to the speaker.

Myth. Unless there is something wrong with a power amp, it will not put out DC. Substitute "increased average power and reduced cone control" for "DC currents" and your post hits the mark.

 

Alex

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Wow. This forum sure isn't like some other ones i'ves asked this question at - I got like one or two replies.

All of this input was great. Basically I understand. When it comes down to it, as long as I don't crank it up - i'll be fine.

 

Again, thanks for all of the help. I appreciate it!!! Thanks!

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rms means average. Your amp can put out 300 watts on average.

 

However when clipping, your amp can put out really ugly signals at say...500watts or so. Not to say that your cab cannot handle 500w, but the real question is whether or not your cab will be able to withstand that nasty signal.

 

When clipping your amp loses control of the speakers and it is possible that the speakers may crack from over extension.

 

The reverse logic that is employed here is that if you overdrive your amp in order to obtain a lerger sound than it can comfortably produce, you are more likely to send it into clipping.

 

When would this happen? If you are in a situation where you are poorly placed (next to a loud guitar amp, 10ft from your cab and very close to loud band member x) and you *think* you need more volume b/c you can barely hear yourself.

 

This has been covered extensively in other threads though.

 

jason

2cor5:21

Soli Deo Gloria

 

"it's the beauty of a community. it takes a village to raise a[n] [LLroomtempJ]." -robb

 

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rms means average. Your amp can put out 300 watts on average.
Wrong. It's not so bad thinking such things as long as you keep your thoughts to yourself, but explaining to other neophytes that such is true, tsk tsk ; }

 

When clipping your amp loses control of the speakers and it is possible that the speakers may crack from over extension.
See my comment above about xmax. What you've just said here is highly conditional.
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RMS is Root Mean Square. It's the mean (or average) of a varying value over a period of time.

 

There are many discussions on whether or not it's an accurate measure of amplifier output.

 

In researching some of this I did find something interesting Speaker Power Handling that causes me to retract my thoughts pending further study. :) )

 

David

"Life's journey is not to arrive at the grave safely in a well preserved body,

but rather to skid in sideways, totally worn out, shouting "&$%^, what a ride". - Doug Berlin

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Bat to Verse: RMS is Root Mean Square. It's the mean (or average) of a varying value over a period of time.
The key words there are where I placed emphasis: over a period of time. And for RMS by the various accpeted inductry standards, that is a very long period of time indeed, for cabinets.

 

It's more useful to think of AVERAGE as being what some companies call PROGRAM power. This is typically 3 dB above RMS, and is a better example of what the human ear (and speakers) considers average. There's some good reading on the Yorkville site, which I've pointed to here in times past.

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robb - great analogy.

greenboy - I picture you typing and making an error. Instead of one of the other colorful words most of us use, you cry out in anguish "capasso". Neighbors and friends hear it as it's use spreads to include banging your thumb or stubbing your toe. Before long there is a huge area in the northern US that has a new way to express upset and displeasure and pain. Thanks...

 

Yeah Jason - stick around. The natives are both restless and friendly.

 

Tom

www.stoneflyrocks.com

Acoustic Color

 

Be practical as well as generous in your ideals. Keep your eyes on the stars and keep your feet on the ground. - Theodore Roosevelt

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