OHM-y God! Please explain ohms to me

[Hekbass]

I'm about to get a TC Electronic RH 450 head.

http://www.tcelectronic.com/rh450/

Can someone explain to me like I'm a 6 y/o the whole bit about impedance?

The suggested cab combinations on the TC website completely baffle me. Are they saying that I can't use that head with just one of those cabs? I must use more than one cab?

In fact, I currently own a TC RS 210, which I'm using with a GK MB 210 combo as an extension cab - it works great.

I'm thinking of getting a different 2 X 10 (I want a lighter one) and either a 1 X 12 or a 1 X 15 to go with my new amp head.

So, my second question is - is that a good idea? Aren't bigger speakers slower, hence potentially causing phase problems? I'm pretty sure I've seen combinations like that (bigger and smaller speakers), but I'm still curious.

THANKS in advance!

[B-string]

You can use one 4 ohm cab, one 8 ohm cab (reduced power amp output) or two 8 ohm cabs (when combined equal a 4 ohm load). You can not use two 4 ohm cabs or a 4 ohm cab with a 8 ohm cab. When you add cabs the number goes down. Anything more than that the stickys cover your question in depth, you can read them at any time.

[two fingers]

For anyone thinking of reading any further, this entire thread broke down into yet ANOTHER argument over ohms, drivers, mixing driver sizes, what's "right and wrong" about mixing drivers, etc. Unless you like to bang your head against walls, I suggest you turn around right now. It's a useless exercise that has been beaten into the ground here WAAAAAY too many times.


READ THE STICKY POSTS and QUIT ARGUING ABOUT IT in THREADS PLEASE for the love of (enter deity of your choice here)!!!!!!!!!!

[bass10bfb]

It says that amp will push a minimum 4 ohm load. So, you can use one 4 ohm cabinet, or you can use two 8 ohm cabinets (that would be the maximum load). Either way will be fine; that head will accept either combination. You can use one 8 ohm cab, but you only be pushing 8 ohms that way.
There are threads that go in depth about mixing cabs and speaker sizes; I won't go into that part of it. I am sure there are a lot of other people that can comment more thoroughly than me on that subject.
Whatever cabs you decide to use need to match the ohm selection I mentioned earlier; one 4 ohm cabinet or two 8 ohm cabs.
Hope I helped some!!!
Barry

[Hekbass]

THANKS TO ALL OF YOU! I think I now really have a handle on this.

-H

[makohund]

Of course you got confused, looking at that website.

First, they don't tell you the impedence of the cabs. They used to... I think most are 8 ohm, not sure about the 410.

But then they say you can use three with one of their 4 ohm minimum heads. Wait, what?! Yep, they say you can.

Likely because they've scoped out the actual impedence curves of the cabs (a bit more complex than the simple nominal rating), and it might be that their heads are actually safe at a little bit less than 4 ohms. So while typical calculation for three 8 ohm cabs is 2.67ish ohms (which would fry a 4ohm min head), these might come out a little higher. And their head can handle it.

So... yeah, a nice outlier to confuse the hell out of people. Thanks, TC!

There was considerable discussion here a while back about the heads, the cabs, and that whole "3 is ok" thing.

[BassmanPaul]

I hope you realize that the RH450 is not a 450W amplifier. it's actually 236W limited by the power management system.

[Hekbass]

Originally Posted by BassmanPaul

I hope you realize that the RH450 is not a 450W amplifier. it's actually 236W limited by the power management system.

Is that if I run it with an 8ohm load?
But, it should go up to its max when using it with a 4ohm load, right (hooking up 2 cabs of 8ohms each)?

[CL400Peavey]

Originally Posted by Hekbass

Is that if I run it with an 8ohm load?
But, it should go up to its max when using it with a 4ohm load, right (hooking up 2 cabs of 8ohms each)?

No thats the max power rating at 4 Ohms. TC wildly fudges their specs.

[B-string]

Originally Posted by Hekbass

Is that if I run it with an 8ohm load?
But, it should go up to its max when using it with a 4ohm load, right (hooking up 2 cabs of 8ohms each)?

Nope. Compression routines are used to make it "sound like" a 450 watt amp. Bass Gear Magazine did actual power tests and in the same issue explains the compression scheme.

[cjp3044]

Originally Posted by two fingers

Don't mix drivers. Get matching cabs and just add another when you need more volume.

There is nothing wrong with mixing driver sizes in different cabs, and even in the same cabs to get a desired effect. A very common configuration is a 1x15 on the bottom with a 4x10 or 2x10 on top. Matching cab "brands" is not really an issue either if you like the sound of a certain cab vs. another. The only really important thing is impedence matching. Most bass cabs are either 4ohms or 8 ohms - if you want a 2 speaker setup, get 8 ohm cabs as 2 8ohm cabs equals a 4ohm load. If you want just one cab, get either - a 4ohm load will allow your amp to run at full power where an 8ohm load will run your amp at less watts - the difference in watts won't make much of a difference in perceived volume (depending on the cab configuration), however running the amp at full power will give you more overhead to play with. Do not run an 8ohm cabinet in conjunction with a 4ohm cabinet as that will cause an impedence mismatch and also do not run 2 4ohm cabinets together unless the amp can handle a 2ohm load - most bass amps have a 4ohm minimum, however there are bass amps that exist that will handle a 2ohm load as well.

[Sparty]

Originally Posted by two fingers

Don't mix drivers. Get matching cabs and just add another when you need more volume. There's nothing inherently more "bassy" about a 1-15 as opposed to a 2-10. It's all in the drivers used and the cabinet tuning.

As far as ohms go, most systems are in parallel. Let's just keep things simple, shall we?
Some of these calculations will seem "backwards", but they aren't.

8 ohm cab + 8 ohm cab = 4 ohm total load
8 ohm cab + 4 ohm cab = 2.66 ohm total load
4 ohm cab + 4 ohm cab = 2 ohm load
Your amp will have a MINIMUM ohm load rating on it somewhere. Do NOT go below that rating. There is also a problem with mixing driver sizes and cabs of different ohm loads because one cab (or speaker) may be getting more power than the other, rendering the lower power cab fairly useless.

The short version is this. If your amp will handle 4 ohms (like most), get two MATCHING 8 ohm cabs. Take one for smaller gigs because your amp will run fine and be plenty loud at 8 ohms. Take both for larger gigs because the combination of the amp running at its optimum power output and the increase in speaker surface area will produce more volume without the phasing problems of using different kinds of drivers, cabs, etc.

Any more info you need can come from doing a good search of this forum. This topic has been discussed millions of times here. You should be able to find more reading than you can handle on the subject.

Edit to add: Once again I try to do with a sledge hammer what B-string can do with a tack hammer.

I concur with two fingers, however I have alway made sure the the power handling of the cabs are even and are more than the amp can produce at 4 ohms.. Eg
Amp = 450w @ 4ohms then the speakers cabs would be 450w each minimum.. This doesn't mean u can't use lesser wattage cabs tho u just have to be more careful not to turn them inside out.
Sparty :0)

[Bass_Pounder]

Originally Posted by cjp3044

There is nothing wrong with mixing driver sizes in different cabs, and even in the same cabs to get a desired effect. A very common configuration is a 1x15 on the bottom with a 4x10 or 2x10 on top. Matching cab "brands" is not really an issue either if you like the sound of a certain cab vs. another. The only really important thing is impedence matching. Most bass cabs are either 4ohms or 8 ohms - if you want a 2 speaker setup, get 8 ohm cabs as 2 8ohm cabs equals a 4ohm load. If you want just one cab, get either - a 4ohm load will allow your amp to run at full power where an 8ohm load will run your amp at less watts - the difference in watts won't make much of a difference in perceived volume (depending on the cab configuration), however running the amp at full power will give you more overhead to play with. Do not run an 8ohm cabinet in conjunction with a 4ohm cabinet as that will cause an impedence mismatch and also do not run 2 4ohm cabinets together unless the amp can handle a 2ohm load - most bass amps have a 4ohm minimum, however there are bass amps that exist that will handle a 2ohm load as well.

How is one to know the "desired effect" without trying them out together ?

And once you buy them, and they suck together ?

Or - they sound good to you standing in front of your amp, but sound like ass all around the room ?

Matching cabs = predictable

Plenty of threads with advice from engineers that design speaker cabs to back up "Don't mix speaker sizes"

Also, there are times that you DO want to mismatch your cabs impedances. Classic example is mixing a 4X10 with a 2X10. If people follow your advice, they run a risk of blowing their 2X10 as the speakers in it will be getting 2X the power of the speakers in the 4X10.

[cjp3044]

Originally Posted by Hekbass

Can someone explain to me like I'm a 6 y/o the whole bit about impedance?

Your confusion is completely understandable. This is really a pretty simple topic, however it tends to be one that sometimes people try to make more complicated than it is and by doing so it tends to create more questions than answers. With the answers that have been given you make sense to at least a certain degree, then you probably understand it better than you give yourself credit for. The lower the ohm rating, the higher the load, the higher the load, the more power draw from the amp. Amps like even and divisible numbers (16ohms, 8ohms, 4ohms, 2ohms - in some cases). You don't need an electronics lesson (unless perhaps you were building your own cab with multiple drivers and trying to achieve a specific load rating, then you need to know the difference between series and parallel wiring, etc - but that's not the case here).

General rule of thumb:
Most bass amps work best with an 8 or 4 ohm load. Most bass amps will work with either an 8 or 4 ohm load. 2 8 ohm cabs = a 4 ohm load. So normally you can use either 1 8ohm cab, 2 8ohm cabs, or 1 4 ohm cab and not have an impedence issue.

[BassmanPaul]

Originally Posted by cjp3044

There is nothing wrong with mixing driver sizes in different cabs, and even in the same cabs to get a desired effect. A very common configuration is a 1x15 on the bottom with a 4x10 or 2x10 on top.

<sigh> This ruined an otherwise reasonable post.

Common a 4x10 + 1x15 pairing certainly is. Good it is NOT and it should be avoided.

[cjp3044]

Originally Posted by Sparty

I concur with two fingers, however I have alway made sure the the power handling of the cabs are even and are more than the amp can produce at 4 ohms.. Eg
Amp = 450w @ 4ohms then the speakers cabs would be 450w each minimum.. This doesn't mean u can't use lesser wattage cabs tho u just have to be more careful not to turn them inside out.
Sparty :0)

That also depends on the efficiency of the drivers. When dealing with higher freqs you are actually more likely to blow a mid or horn with too little wattage than with too much - the reason is that too little wattage can lead to DC input of the driver or horn at higher freqs, but yes it is a good practice to match as closely as possible the cab's power rating to the amps.

A good example of what I'm talking about. If you've ever tried to play through a Basson 8x10 cab (very inefficient cab at lower wattage) with an amp rated at less than about 750 watts into 4ohms, it will sound like ass - these are great sounding cabs if you feed them properly though - they are (or were) rated at 1600 watts, but anything less than 750 at 4ohms simply doesn't push that cab enough, so a higher rated cab is not always a good thing either. Another example is the Ampeg 1542 cab (no longer in production - it was a single cab with a 15 in the bottom and 4 10s on top) - this cab is only rated at 200 watts believe it or not, however because of it's efficiency, you could easily pump a LOT more power into it - I used one with an 1100 watt amp with no problems and another friend used one with a SWR 900 with no problems for years.

[BassmanPaul]

Originally Posted by Bass_Pounder

Also, there are times that you DO want to mismatch your cabs impedances. Classic example is mixing a 4X10 with a 2X10. If people follow your advice, they run a risk of blowing their 2X10 as the speakers in it will be getting 2X the power of the speakers in the 4X10.

Completely true. If you want a 2x10 and a 4x10 to work electrically the 2x10 needs to be TWICE the impedance of the 4x10. So 4Ω 4x10 and an 8Ω 2x10. How they actually perform together acoustically is a crapshoot.

Obviously in the case cited the amplifier would need to be able to handle a 2Ω load.

[cjp3044]

Originally Posted by BassmanPaul

<sigh> This ruined an otherwise reasonable post.

Common a 4x10 + 1x15 pairing certainly is. Good it is NOT and it should be avoided.

Why? Give me a logical explanation. I've used that combination for certain venues many, many times with great success, even used an Ampeg 1542 cab (a single cab with that configuration) for years.

You've posted YOUR opinion with nothing to back it up.

[Bass_Pounder]

Originally Posted by cjp3044

Why? Give me a logical explanation. I've used that combination for certain venues many, many times with great success, even used an Ampeg 1542 cab (a single cab with that configuration) for years.

You've posted YOUR opinion with nothing to back it up.

How about you search through all the threads regarding mixing driver sizes.

Audio engineers that actually design speaker cabs have explained the "why nots"

A 1X15 and a 4X10 pairing is terrible, and the SVT1540HE was one of the worst cabs Ampeg ever made - that is why they are not around anymore.

[cjp3044]

Originally Posted by BassmanPaul

Completely true. If you want a 2x10 and a 4x10 to work electrically the 2x10 needs to be TWICE the impedance of the 4x10. So 4Ω 4x10 and an 8Ω 2x10. How they actually perform together acoustically is a crapshoot.

That is completely wrong - using a 4ohm cab with an 8ohm cab creates an impedence mismatch. Period. AND will likely damage your amp if the amp is rated for 4ohm min. 4ohm + 8ohm = 2.66 ohms - NOT good for your amp.

How do you logically justify that a 2x10 would have to have twice the impedence of a 4x10 - that simply makes no sense.