Un-matched cab impedance - need help

[LowEddie]

I understand ohms law to some extent and I've searched the stickies and didn't find what I'm looking for.

I have a solid state, 4 ohm head running a single 4 ohm 4x10. Today I picked up an 8 ohm 1x15 cab. (I did the math in my head backwards when I bought it).

If I combine these cabs I'll get 2.667 ohms which is less than the rating of my amp (4 ohm).

I know that there is a possibility that this will hurt/fry my amp and at the very least it will be unhappy.

Can someone explain to me why?

Also, I don't know the what the ohm rating is on the individual speakers in the 4x10 cab but I'm assuming that it could be rewired as an 8 ohm cab. Is this correct? Is there an easy way to add resistance to my 4x10 cab.

Now before everyone says upgrade I want to see what I can do with what I have.

Thanks in advance.

[BurningSkies]

You cannot rewire the 4/10 to be an 8ohm cab... If that were so, people would be re-wiring cabs all the time or putting in switching to make a cab either 4 or 8 ohms.

Indeed your impedance is too low and your amp is in danger if you do use it as such.

[BassMaster4s]

All you can do with your current 410 is exchange the speakers for the proper impedance, which will be about the cost of a new cab. Don't run your head below the minimum rating.

[KJung]

Originally Posted by BassMaster4s

All you can do with your current 410 is exchange the speakers for the proper impedance, which will be about the cost of a new cab. Don't run your head below the minimum rating.

+1

[BassmanPaul]

A SS amp will try to output twice the power into half the load. Thus, for example, a 100W at 8Ω amp will try to put 200W into a 4Ω load and 400W into 2Ω. You generally won't get those exact power levels due to losses in the output devices and limitations of the power supply. If the amp has been designed for 2Ω it will have more output devices and a beefier power supply than one designed for 4Ω as most bass amps are. By dropping below the minimum for the amp you are asking the output stage to handle more than it was designed for and it will fail.

Paul

[LowEddie]

Originally Posted by BassmanPaul

A SS amp will try to output twice the power into half the load. Thus, for example, a 100W at 8Ω amp will try to put 200W into a 4Ω load and 400W into 2Ω. You generally won't get those exact power levels due to losses in the output devices and limitations of the power supply. If the amp has been designed for 2Ω it will have more output devices and a beefier power supply than one designed for 4Ω as most bass amps are. By dropping below the minimum for the amp you are asking the output stage to handle more than it was designed for and it will fail.

Paul

Ok, this makes sense.

[Crockettnj]

V=IR. Or, I=V/R

So if you drop the impedance (R in this equation) your current will go up. Too much current from amps not designed for such will cause too much heat and it will fail.

Some can handle abuse beyond what they are spec'd for, prbably due to conservative initial specs. Others can't. Either way, great advice above... dont run your amp into a load below the minimum rating.




(but if you do, dont do it for long, dont push it hard, and keep it really cool if possible! ;-)

[Crockettnj]

Incidentally, the impedance listed are only nominal, since impedance of a cab varies with frequency. So , depending upon how the amp is EQ'd, how hard its driven, and the type of input signal, one guy could get away with running below an amps minimum impedance while another cant.

since you dont (or cant) know, its best to heed the advice given.

In the instances I run my 4ohm min head into 3 eight ohm cabs, I cut a lot of sub-bass and a little lows.

[4-stringB]

Yo dude, you CAN re-wire a cab to suit whatever ohm need you have, within limits , of course. Your 410, assuming the cab at 4 ohms the drivers are 4ohm each. this would be two drivers wired in parallel, producing 2ohms, two of these 2ohm/2 driver setups in series together will make 4ohms. Whew!..So, if you run your 8ohm/115 and your 410/4ohm, you should get somewhere around 6 ohms, not enough to totally cramp the output of your amp, but enough resistance to safely run loud and proud. This is assuming you are/or can run these cabs in parallel.All the math whizzes can insert their 2 cents here....Cheers...

[will33]

You can rewire a 4ohm cab to be a 16ohm cab bringing the 16+8 in parallel to 5.33ohms, safe for a 4ohm amp.

As far as the "ohms going too low" thing. Impedance and resistance are not the same thing but imagine your speaker cab as a certain resistance for your amp to push against. If that resistance is too low than your amp is designed for, it won't have as much resistance pushing back against it as it's designed to have. That will allow more juice to come through the amp than it's designed to work with, thereby heating up parts too much, making things produce more power than they're designed to and basically burning the thing up.

This is a very simple way of thinking about it. A real engineer would laugh at it, but it'll keep you from burning up amps.....amps = $.

Tube power sections are a different animal but for solid state (that means SS amps with a tube in the preamp as well), you don't want to go below the rated minimum ohms.

[Bass_Pounder]

Originally Posted by 4-stringB

Yo dude, you CAN re-wire a cab to suit whatever ohm need you have, within limits , of course. Your 410, assuming the cab at 4 ohms the drivers are 4ohm each. this would be two drivers wired in parallel, producing 2ohms, two of these 2ohm/2 driver setups in series together will make 4ohms. Whew!..So, if you run your 8ohm/115 and your 410/4ohm, you should get somewhere around 6 ohms, not enough to totally cramp the output of your amp, but enough resistance to safely run loud and proud. This is assuming you are/or can run these cabs in parallel.All the math whizzes can insert their 2 cents here....Cheers...

He can run the two cabs together for a total of 6 ohms if he runs them in SERIES, not parallel. Parallel is 2.67 ohms.

[4-stringB]

Originally Posted by Bass_Pounder

He can run the two cabs together for a total of 6 ohms if he runs them in SERIES, not parallel. Parallel is 2.67 ohms.

Jeez, it's bad enough the math whizzes can run circles around me, now I'm screwing up this stuff too (guess I'll have to talk to you guys B4 Beer time)..rats...Cheers...

[Bassamatic]

Sometimes it is not clear how the impedances stack up.

Think of it like water and hoses. A larger hose impedes the flow of water less, so it has a lower impedance. If you are feeding 2 hoses at once from a single faucet, the total water handling capability will always be greater than either of the hoses, allowing more water to flow (lower impedance to the water flow). AND - if they are not the same size, more water (current) goes to the larger one with the lower impedance. Does that make sense?

[BBEgo]

Basically it's like using a toaster and a hair dryer on the same electrical circuit at the same time.

Both are high current devices, essentially passing electricity through a low resistance load, generating heat.

If you run them both at the same time, too much current flows and you overload the circuit. When that happens, the circuit breaker pops... only in this case, your amp is the circuit breaker.

[tom-g]

8 ohms + 4 ohms in series equals 12 ohms.