Understanding how to match cabinet/head ohms

[PDGood]

Please help clear up my fuzzy thinking.
If I have an amp rated at 350 watts rms into an 8 ohm load and I buy an 8ohm cabinet rated at 350 then I can operate them safely together. But If I buy the same cabinet except it's a 4 ohm cabinet, then my amp operates at around 500 watts and is no longer suitable to run safely through the cabinet - right?
Or would the 4ohm cabinet (same brand/model etc) have a higher rating than the 8ohm version?
And if I added another cabinet to handle the extra power, then presuming they are wired in parallel, that would further lower the ohm rating and (assuming the power amp can handle the load) would increase the power further which would still not solve the problem of safe operation. So in other words, the 8ohm cabinet would be the only choice unless I began with a lower wattage amplifier. Is this thinking correct? Somehow it doesn't sound right.

[Wes Whitmore]

Don't worry about the numbers entirely. Most of them are bogus, and it isn't an exact science. Those numbers are generally for marketing. Use common sense, and your ear. I run a 500 watt amp on a 100 watt 1x10 lots of the time, but I know what it can do, and what it cant.

Some cabs are available in two ohm loads, but have the same power handling. If you amp puts out 350 watts (which is a good amount and enough to run most multiple woofer cabs well), then get the 8 ohm cab. You won't really hear that extra hundred or so watts into the 4ohm version, and most of that extra power will just be excess heat in the voicecoils. Most 4x10s and decent 2x12s will match up really well with that 350w power at 8 ohm. If you get that, and notice that you are really cranking that cab and it starts to sound bad (regardless of what the volume knob says), either get another identical 8 ohm cab, or find a way to plug into the PA. Doubling of cone area is the fastest way to get a lot louder. It's more to carry, but you don't always have to run both cabs. Numbers just get you in the ballpark. You still have to know the limits of your equipment.
Wes

[Von Felgenhauer]

Dont tthink about it blowing your speakers. If the head is rated at more watts then the cab then just don't turn up all the way. Always use your ears to hear when it starts clipping and distorting your speakers

[PDGood]

Thanks guys.
FYI I'm looking at a 1-12 cab.

Relying on distortion is scary for a couple of reasons. One is that if you play hard, the pickups can break up and it's hard to tell that from amp break up at a low level. The other is that the boutique cabinet builders on TB have made it clear that turning a 500watt amp down halfway does not give you 250 watts because of transients. So I'd like to know that my system is compatible and I'm not relying on a variable (me) to protect it.

[Von Felgenhauer]

IMO you should be the ONLY variable to decide if your amp is sounding good or bad. Turning down the volume does not decrease Watts but it also limits the distance the speaker travels(xmax) which means the driver wont be pushed hard enough to blow. Its very easy to just use your ears to hear when you are pushing your speakers to there limits.

[Von Felgenhauer]

Originally Posted by PDGood

Thanks guys.
FYI I'm looking at a 1-12 cab.

Relying on distortion is scary for a couple of reasons. One is that if you play hard, the pickups can break up and it's hard to tell that from amp break up at a low level. The other is that the boutique cabinet builders on TB have made it clear that turning a 500watt amp down halfway does not give you 250 watts because of transients. So I'd like to know that my system is compatible and I'm not relying on a variable (me) to protect it.

Also playing hard might be playing wrong. You shouldn't need to dig in to be heard, but dig in to get a different tone. If you are afraid the cab wont be loud enough to hear in a band mix, your only hurting yourself by playing harder.

[Wes Whitmore]

Well, it's not that bad. In a isolated practice setting with just you and your bass, turn your volume knob all the way up on your bass. Leave your eqs flat. Set your preamp on your amp to just before clipping. Use the pad if you need to. A bass should be able to put out a distortion free signal. You are probably just clipping the amps pre circuit. Crank it up for a while, get used to what you are hearing, and maybe goose it a little more to see what it will do. When it starts to sound bad, don't turn it up that far again. Now understand that your cab should be fine to that point of volume. If you crank the bass knob up on your bass or amp, it won't be able to go as loud on the volume knob. Keep listening...always.

If 1x12 is providing enough volume for you, I think all you really need is to keep an ear open and don't expect too much. Most 1x12 might not fill the bar everywhere, so don't expect it to. Use a PA, or get more 12s. It's always smart to have a little more cone than you need.

If you are worried about transients, remember that heat and over excursion are what blow woofers. If the speaker is barking, or smelling, you don't have enough cab for what you need. At least with 8ohm cabs, you can add another without scrapping the first.
Wes

[Warrior_Bass]

Originally Posted by PDGood

Thanks guys.
FYI I'm looking at a 1-12 cab.

Relying on distortion is scary for a couple of reasons. One is that if you play hard, the pickups can break up and it's hard to tell that from amp break up at a low level. The other is that the boutique cabinet builders on TB have made it clear that turning a 500watt amp down halfway does not give you 250 watts because of transients. So I'd like to know that my system is compatible and I'm not relying on a variable (me) to protect it.

Sending 350W at a 1-12 will likely make it distort at max volume, but you'll easily distinguish this from the natural overdrive of your pickups. The speaker distortion sounds like it is working too hard, lacks low end, and frankly sounds like a fart. Unless you're cranking the low end constantly and ignoring the warning signs, you should be fine. Another 1-12 later on will increase your volume, should you need it, and reduce the workload on the one speaker.

[PDGood]

Thanks for the replies guys.
I actually do have other cabs for louder settings, I'm just trying to down size where possible and the new super 112s are becoming very tempting. I normally run a pair of cabinets and have never had to worry about any of these issues, but I'm trying to understand the ohm situation so that if I'm able to purchase one of the 112s I'll know which one to get. From what you're saying sounds like either one.

So maybe I should rephrase the question a different way. When would you choose a 4ohm cab and when would you choose an 8ohm cab?

[Wes Whitmore]

8

[Warrior_Bass]

Originally Posted by PDGood

So maybe I should rephrase the question a different way. When would you choose a 4ohm cab and when would you choose an 8ohm cab?

Depends on your amplifier's load rating. If it's rated for 4 ohms, then:

Choose a 4 ohm cab if you're certain only one cab will be needed
Choose an 8 ohm cab if you want the option to add another one later.

If your amp's rated for 2 ohms, then:
Choose a 4 ohm cab. You can add another for a 2 ohm load if you need it.
You could run four 8 ohm cabs, but some might consider that overly burdensome.

[PDGood]

Aaah. Now I get it. Thanks!

[makohund]

Originally Posted by PDGood

So maybe I should rephrase the question a different way. When would you choose a 4ohm cab and when would you choose an 8ohm cab?

Pretty much what Warrior Bass said.

The only time I'd choose a 4 ohm cab is when I know I'll pretty much never need a second cab. For me that means only big and/or "superpower" cabs... those that would be considered a "full stack" by themselves. (And can keep up with guitards playing full stacks.) Even if my amp will push 2 ohms (it can when not bridged), I'll stick to planning for a 4 ohm load... who knows if my next favorite amp down the road will be able to handle 2 ohms or not.

So for me, "half-stacks":
1x10, 1x12, 1x15 = always 8 ohm
2x10, 4x10, and any "typical" 2x12 or 2x15 = always 8 ohm
"super" 1x12, 1x15 = always 8 ohm

And "full stacks":
6x10, 8x10, 4x15 = 4 ohms
"super" 2x12, 2x15 = 4 ohms
4x12 = depends on the cab, and how loud it can get by itself

Full disclosure, I play a "super" 2x12 + mid/tweet, and it is 4 ohms. But it easily hangs by itself in very loud rock/metal stage situations, even without PA support. My amp could reasonably run another if I ever really needed to... a thought my mind boggles at. (What... 2 drummers, 4 guitars, maybe some keyboards, and an entire brass band? With no PA for me? Not gonna happen.)

The only way I'd choose a 4 ohm "half-stack" is if it were very small/high output cab, and was only needed for low/medium volume stuff. (No loud shows without PA support.) A secondary cab option , to allow leaving the "big one" at home when it isn't required. (A ThunderChild 112 comes to mind as a good example.)

Some folks have different volume requirements, and may never need anything bigger than a "half stack". If that means you... then a 4 ohm version may be fine for you, and squeak out a teeny tiny bit of extra volume from your amp over the 8 ohm. But if the cab I really wanted was available in an 8 ohm version, I'd probably still choose that one anyway. Just in case.

[PDGood]

Very nicely explained makohund. The Thunderchild is one under consideration, but I can't say for certainty that I'd never want to put it with another cabinet. One of my heads will go to 2ohms though.

[RickenBoogie]

I'd always go with an 8 ohm cab with a single driver. Fact is, most cabs won't take the full wattage running at 8 ohms, forget 4. Always leave room to breathe, or expand in this case.

[PDGood]

That's kind of my thinking. I thought it strange when I read on one of those 112 sites where the builder said most people ordered the 4ohm.

[Selta]

Originally Posted by PDGood

That's kind of my thinking. I thought it strange when I read on one of those 112 sites where the builder said most people ordered the 4ohm.

I'm sure most DO order the 4 ohm, for all the wrong reasons.