# How cab and head wattage equal?



## DimebagTributer (Aug 12, 2008)

Ok im wondering how does a head and a cab equal up in wattage... to work together.. 

like does a 100Watt head only go with 100W Cab?

Why is there wattage for the head...

What happens if u use a 100 watt head.. for a 300W Cab?

Can you hook up more than one cab to a head? if so how many?


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## Guest (Aug 14, 2008)

DimebagTributer said:


> like does a 100Watt head only go with 100W Cab?


Doesn't have to be a perfect match like this. The power rating on the cab is a guideline -- if you don't want to tear your speakers apart you'll want to make sure the cab can handle the power put out by your amp. If you don't want your speakers to fail and clip you'll want a really high power rating on the cab. If you're after some speaker clipping you'll want a lower power rating on your cab (and you'll have to be sure you don't crank your amp and drive them to the point of total meltdown).



> Why is there wattage for the head...


Watts are a measure of electrical potential, or energy, the power rating on a head is usually the root mean square power output the head is capable of producing. What does this mean in practical terms? It's the sustained amount of power that comes out of the amp. You want to know this because running a higher power head with a really lower power cab at full tilt will rip the speakers apart. Possibly melt things. Start fires. Fun!

But that's the extreme case.

Some people purposely mis-match the power handling of speakers and amps so they get speaker breakup -- this is where the speaker is handling more power than it's designed to handle so it's hitting the extreme limits of it's motion and technically failing. It's a whole world of tone and it's what makes cabinet and speaker selection as important as amp selection. It's 50% of the equation, cab and speaker selection, and shouldn't be overlooked.

Some people go the other way and spec their speakers so they never fail under the maximum output load of their amp.



> What happens if u use a 100 watt head.. for a 300W Cab?


Good things. Usually this is how stuff is paired up: you run a head with a load that can handle more than the RMS power output of the amp. If you wanted the clean sound coming out of your amp to be reproduced by your cabinet, at high volume, without the speakers imparting any of their own distortion or breakup you'd want your cab to be able to handle more than the RMS power rating on your amp -- because the peak output will be higher than the RMS rating. If you start to hear fizzing, flubbing type stuff when you crank your amp on clean it might be speaker failure you're hearing.

It's a whole art the amp-cab-speaker pairing.



> Can you hook up more than one cab to a head? if so how many?


You bet. Depends entirely on the amp and the speaker load configuration in the cabs.


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## greco (Jul 15, 2007)

Deleted post...information provided was not 100% accurate.

Wild Bill explains this concept in detail in his Post (#15).

Dave


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## DimebagTributer (Aug 12, 2008)

umm... haha im 16... what are ohm's... like 8 ohm and 4 ohm... and wut is RMS or solid state amp head

So whats better a 100W head or 50W? 

so basically as long as the cab is higher wattage then the head.. its all good?


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## greco (Jul 15, 2007)

*DimebagTributer:*

I deleted my post. 

I was an a$$hole for writing those inappropriate comments. 

I sincerely apologize to you and, in addition, to all the members of this good forum that I might have upset.

You are trying to learn (as we all are) and I should be trying to help you...not be critical. I hope you will all accept my apology.

I'l do better in the future. I think my migraine headache was "talking"...not me.

It looks like you are getting all this watts, load, cab, head, speakers, power, etc stuff sorted out. Way to go. It isn't easy. 

Dave


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## DimebagTributer (Aug 12, 2008)

ya i understand man, sorry just new to everything and everything ive ever looked up on the net is just confusing as hell every question i try to get ansered just gives me another 100 questions more to try to get ansered... i know im taking up lots of peoples time but im tryin to contribute back wherever else i can on this site. thanks all for the help your providing tho


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## DimebagTributer (Aug 12, 2008)

So a low impedance is something that will either produce or consume a small amount of energy? 

High impedance is the opposit?

And for ohms..is that as if the lower the ohms.. thats basically low impedance... so 1 ohm... would be low impedance... and low impedance is better than higher impedance?


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## bagpipe (Sep 19, 2006)

Try this:

http://www.prestonelectronics.com/audio/Impedance.htm


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## DimebagTributer (Aug 12, 2008)

ok.

So a 100W head.. with say 10V... would be able to play with a cab of 4 speakers that have 8 ohms. or a cab of 2 speakers at 4 ohms?

If thats so.. what i dont get is how the wattage really has much to do with it then..

RMS is Root Mean Square... and this is done by adding the number of your speakers squared.. then divided by the number of ur speakers i think?

2 4 ohms equal and 8 ohm... 10v/2 ohm.. would be 5v per ohm?


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## hollowbody (Jan 15, 2008)

There is no set rule here. Some amps put out 100w at 8ohms, some at 4ohms. A Fender Super Reverb outputs to 2ohms, which is really low.

Ideally, you would have a perfect match between your amp and cab in terms of ohms, but most amps can output into different loads (4,8,16ohms) so it's not a big deal.

General rule is that if an amp is rated into 8 ohms, its safe(-ish) to run it into a higher impedance cabinet (16ohms), but NOT a lower impedance once (4 or less).

In order to avoid catastrophic failures on the cab side, make sure the cab covers the wattage of the amp, unless, as iaresee mentioned, you're specifically looking for a speaker-distortion effect. However, you seem pretty new to this stuff, so I'm guessing it will be better off for you to get used to how a amp and cab combination sounds in the first place before you go off tinkering with distortions. Besides, you can get all the distortion you want from your amp or effects.

As mentioned as well, tube amps are a little more robust than solid state. Solid state means it's powered by transistors. Tubes are like little light bulbs that they used in old-school technology like the ENIAC computer. Most musicians (but not all) think that tube (also known as "valve") technology sounds more musical than solid-state transistor based amps. I happen to agree. Just know that tube amps are generally more expensive and require a little more maintenance on the owner's part than solid state.



DimebagTributer said:


> ok.
> 
> So a 100W head.. with say 10V... would be able to play with a cab of 4 speakers that have 8 ohms. or a cab of 2 speakers at 4 ohms?
> 
> ...


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## DimebagTributer (Aug 12, 2008)

so its best to make sure that 8ohm head is with either a 8ohm cab or 16 ohm... 

so 8ohm cab means each speaker in it is 8 ohm.. or all of them together is 8 ohm.. 

but best to do that.. match the ohms for the head and cab. and make sure the wattage for a cab.. is higher then the wattage for the head.
a good match would be a 100W 8ohm head with a 300W 8ohm cab?


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## hollowbody (Jan 15, 2008)

DimebagTributer said:


> so 8ohm cab means each speaker in it is 8 ohm.. or all of them together is 8 ohm..


Close. Depending on whether they are connected in series or parallel, you're technically right, but at the end of the day, yes, all the speakers together will add up to 8ohms. Check this page, it's super helpful.



DimebagTributer said:


> but best to do that.. match the ohms for the head and cab. and make sure the wattage for a cab.. is higher then the wattage for the head.
> a good match would be a 100W 8ohm head with a 300W 8ohm cab?


Yeah, that would be fine. It's ok to cut it a little closer too, like 100w amp into a 150w cab, but keep in mind that many amps rated outputs are not their actual _maximum_ output. The rated outputs are usually the RMS output, which is their maximum sustained output over a period of time. Many amps can put out peak outputs much higher than their RMS output for a brief duration (a second or two), but this is sometimes enough to tear your speakers to hell.


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## greco (Jul 15, 2007)

greco said:


> *DimebagTributer:*
> 
> I deleted my post.
> 
> ...


This is the new post #5


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## Ripper (Jul 1, 2006)

One thing to remember too is that sometimes the higher wattage cabs won't give you the break-up you may want if you are running a small amp at lower volumes through them, but that brings up a whole new game for you, the art and science of speakers!

When you really have to worry about speakers and the ratings is if you are driving your amp near full all the time. I've ran lots of 50 watt amps over the years with 25 or 30 watt speakers and never had a problem.


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## Wild Bill (May 3, 2006)

Just thought I'd clarify a couple of things.

First, a 100 watt speaker or cab should be ok with a 100 watt amp, DEPENDING ON WHO MADE THE SPEAKER!

Reputable manufacturers are well aware of this question and rate their speakers by honest methods. I've never heard of a Weber or an Eminence speaker failing in this sort of situation.

There are other manufacturers that will let the marketing boys do the power rating tests. They find ways to measure that are not really good engineering but give a higher number. 

This has been going on probably as long as they've been making speakers! For an extreme example, does anyone really believe their little "girly man" computer speakers are rated for 300 watts? Or how about those Chinese-made car stereo speakers? You know, an 8" speaker rated for 500 watts! Or a car stereo 500 watt amplifier, for that matter.

Second, solid state amps are actually far more forgiving about speaker matching than tubes. It doesn't matter at all! There IS NO matching to worry about!

EXCEPT...don't go too low! You can burn out the amp!

Solid state amplifiers put less power out with higher loads. So it's all a question of the initial design. The engineer knows that his design will put out more power as the load goes down. He knows the ratings of his transistors and his power supply so he knows that at a certain point it would not be safe or reliable to run the amp. So he sets a lowest limit and plasters it on the back of the amp, usually near the speaker jacks. "DO NOT USE WITH LOADS LOWER THAN 4 OHMS!" is commonly seen.

More simply, the amp might be rated at 300 watts into a 4 ohm speaker load. If you run an 8 ohm load you might get only 200 watts. 16 ohms would be even less. You can run with NO speaker load just for servicing and not hurt anything!

If you try to run a 2 ohm load the amp will attempt to put out MORE than 300 watts. Things will start to get very, very, hot!

Hope this helps!

:food-smiley-004:


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## caaustin02 (Nov 1, 2007)

DimebagTributer said:


> ya i understand man, sorry just new to everything and everything ive ever looked up on the net is just confusing as hell every question i try to get ansered just gives me another 100 questions more to try to get ansered... i know im taking up lots of peoples time but im tryin to contribute back wherever else i can on this site. thanks all for the help your providing tho


Hey DBT:

Don't worry about asking questions. You are not wasting anybody's time. Exchange of info is one of the purposes of this forum. The stuff you are talking about is complicating, even if you have learned about it school. Just remember that if you are wondering about it, many other people are as well. There are a good number of people with a lot of knowledge that chime in when you ask a question , and even though many of us don't post on a particular thread, we still read the Q/A and learn. You just have the guts to ask the question, and that is just fine. In my biology classroom in highschool there was a sign that read "The foolish wonder, but the wise asks".

Think of electricity as flowing water. The Voltage at the wall is like a waterfall to get the water flowing. The higher the Volts, the higher the waterfall. Voltage is also called potential. Current, like a river current is the flow of electricity along wires (conductors). Electrical current flow is measured in Amperes (Amps for short, but that is confusing in this discussion). When you multiply current x voltage you get power in Watts. (more on that later)

Speakers are loads, and loads resist the current flow Think of something that is put in a river that can prevent the water from flowing, but since water and electrical current can't pile up, in our analogy the load in the river slow the water all the way back to the waterfall. A higher load speaker will prevent electrical current flow all the way back from the wall socket and beyond, current can't 'pile up' anywhere. Loads are measured in resistance or impedance, which for us can be considered the same thing. Impedance is measured in Ohms. 

So...A load with a smaller impedance draws more current from the source, or in our river example allows more current to pass through.

When talking about parallel or series wiring for more than one speaker. Think of the river splitting in 2 branches with a load in each branch for parallel, and for series the river doesn't split, and has more than one load in the same branch. This is why parallel circuits has less overall impedance than series with the separate loads being equal in impedance and number.

The voltage at the wall stays constant. Different devices use different power by how much current they draw (current is what Hydro One uses to measure how much power a house consumes). An amp that is connected to a 2 Ohm load will be using more current (and outputting more power) than when it is connected to a 4 Ohm load, and so on.

There are flaws in above river analogy, but you can use it to visualize what is going on at a very basic level. It is the picture that helped me when I had to study circuits in school. I hope that helps and didn't confuse you further.


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## DimebagTributer (Aug 12, 2008)

holy shit... i think i get it!! haha wow!!

Moment u said speaker is the load. Now i know that its basically if the amp it 8 ohm....going through the wire to the speakers, and the the higher the ohm.. the higher resistance. And the higher the voltage the faster the current.

So if u get a 4ohm head.. make sure the speakers arent below 4 ohm.. because if u do it doesnt have enough resistance.. So it will allow to much current and cause damage because the speaker will try to produce more sound then its potential. Make sure the speaker is never lower then the head.. because meltdown may happen.

the lower the ohm... the less resistance which is allowing more current to go through... resulting as a Louder speaker(if its meant to handle it). 

As to where a speaker is 16 ohm.... then its resistance is much more stronger, meaning less current being consumed by the speaker... 

But when u have an 8ohm head with a 2 ohm speaker... The head is producing more than the speaker can consume.. so the Speaker may blow... or the head may fry.. either way.. isnt all that good..


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## mhammer (Nov 30, 2007)

OK, my turn.

Wattage ratings on amps are essentially a reflection of how much current they can feed to a given output. If the rating is honest, then with a specific input signal level, and no internal attenuation (i.e., nothing in the amp is "turned down"), the amp is supposed to be able to pass that much current and accompish that much "work" (current going into a load accomplishes "work").

Not all such ratings are honest, however. I'm sure somewhere in a bargain store you will be able to find $5 computer speakers that are 3" and powered by 4 AA batteries that brag about 180W PMP or some such rot. These are extremely hypothetical numbers, relying on "if this and this and that are juuuuust so, and you don't count this and this and that....". It's a bit like saying that if I lost 40lbs, worked out more, had my height changed surgically, and had a bit of facial reconstructive surgery, I'd look just like Tom Cruise. Yeah, I suppose I would, but I don't right now under these circumstances.

It goes beyond $5 computer speakers. Many solid-state amps use commercially-produced power amp chips. Those chips have a particular power rating, but you need to feed them with a signal of sufficient proportions, and provide them with enough of a power supply and a low enough load to meet the spec. The TDA2050 is a commonoly used chip, and while it can deliver a reasonable wattage in the double digits, you need a high enough supply voltage and big enough current delivery capacity in the supply to do that. You could run your truck's headlights off 8 C-cells too, but for how long, and how bright would they be? Same things with power ratings. That doesn't stop companies that use said chip from sticking a "50" in the name and insinuating that the amp has a 50W capability.

Speakers are a whole other matter. A speaker voice coil is essentially a fuse, made of thin wire. Pass too much current through it and it burns up, just like any other fuse. Fortunately for us, that "fuse" is engineered such that, as long as we are not making it red hot and causing it to burn up, it manages to do something useful for us by being wound in a coil around and inside a magnet structure, and affixed to something rigid that will move a cone back and forth. Lucky. 

Keep in mind, though, that the moving back and forth risks producing friction, and that friction itself generates heat. When you combine the heat generated by friction, and the heat generated by coming close to the maximum current-handling capability of the "fuse", it is easy to burn the fuse out.

So, in the case of the amp, wattage relates to the properties of what it can DO, and in the case of speakers, it relates to what the speaker can TOLERATE.

"Tolerate", however, is a function of a number of things. If heat build-up is the issue, then heat dissipation is the solution, and the mediating factor. That is why you will see ratings for "RMS" and "peak" on speakers. RMS would be a rating with respect to the amount of current it could handle on a steady state basis; i.e., as fed by an amp with a signal generator on the front. Since heat builds up and dissipates over time, though, most speakers can tolerate passing more than their RMS rating for brief periods, as long as it doesn't last too long. You can touch your skin to a hot pan VERY briefly and suffer no ill effects, but hold it there for even a couple of milliseconds longer, and you get tissue damage. Same deal with speakers.

This is one of the reasons why distortion pedals are such a problem when it comes to speakers and speaker requirements. Consider that when you were in scouts they never asked you to rub two sticks together slowly to make a fire. You had to rub them together fast so that any friction-produced heat would not have a chance to dissipate. In other words, fast rubbing allowed the generated heat to sum over successive rubs. Same is true for speakers. More high frequency content, especially if it is sustained over a long enough period, generates much more heat, and risks adding to whatever heat is created by the current being fed at higher volumes. Distortion pedals have the uncomfortable characteristic of generating lots of high frequencies and keeping them steadily pumping out for a prolonged period at the same high amplitude. It's like having a high-speed stick rubbing machine - small wonder it generates heat. The upshot is that the speaker wattage rating really ought to reflect the style of music played. Richard Thompson could play a 100W Marshall into a single 20W Greenback for years without failure, but Johnny Ramone could not..

As well, keep in mind that simply because an amp CAN pump out 100W when all the stars are aligned, doesn't mean that this is the typical signal. Indeed, under most circumstances, you probably won't be putting out more than an average of 5-8W (i.e., measured over the course of several minutes, and including the silence and moments of softer strumming), even in a metal band. So, if the "misbehaviour" lasts only very briefly, and does not recur too often, you could easily feed a 300W amp into a 5" speaker rated at 2W and it would do okay. Turn that sucker up and play hard for more than a few milliseconds and the fuse will blow.

In the case of multi-speaker cabs, the use of several speakers in parallel permits more current to pass via parallel paths, such that no single speaker/voice-coil has to pass ALL that current and handle all that heat. This is why you can stick a quartet of speakers rated at 20W each into a cab and pummel it with a 50W amp dimed and come out pretty much intact every time: the current is divided up evenly over the multiple speakers and what they get is well within their current-handling capability.

Finally, as I'm fond of pointing out, power handling is not the same as efficiency. Speaker efficiency is rated in terms of decibels sound-pressure level at 1 metre with 1W of input. Keep in mind that a 3db difference is equal to double the loudness. So, if I have one speaker rated at 30W power *handling* but with an efficiency rating of 98db, and another rated at 50W power handling but a 92db efficiency, the first one produces roughly 4 times the volume for the same 1W input. A speaker like that may only ever require you to turn a 50W amp up to 4 to achieve the desired loudness; well below its power rating when you consider what sort of amp output is needed to achieve that loudness.

It would be a bit much for folks who earn what music store employees earn to know all of this, and be able to explain it clearly to the average customer, which is why you tend to get the lowest common denominator explanation and recommendation. However, in that spirit, I will end by noting that while there are many other reasonably safe arrangements, the surest guarantee of continued performance from a speaker cab would be one where its overall RMS rating is equal to or greater than the maximum power output of the amp going into that load (8, 16, 4, etc).

Hope this clears some things up.


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