I have a 4 ohm 2x10 cab and would like to add a 4 ohm 1x15 cab to my setup. My Ampeg B5R is capable of running at 2 ohms. Would I be cuttin it close? Should I go with a 8 ohm 1x15? Any suggestions would be appreciated.

Adding an 8 ohm cabinet to a 4 ohm cabinet would still be 2.67 ohms. I would go with another 4 ohm cabinet if the amp will handle it.

Nope. Two 4-ohm cabs does equal a 2-ohm load. If you're Ampeg is rated to run at 2-ohms, you won't have any problems.

Actually, I heard that mixing a 4ohm cabinet with an 8ohm cabinet is bad. It can blow the speakers or something. Not sure if that is totally true, but I don't like to take risks with my gear.

Actually, it's totally untrue Back to the original subject: as said, there's no reason to believe you are imposing any threat to the amp playing it at 2 ohms, if it's rated to do just that (why even doubt?)

I worried because a friend of mine never runs at the max his amp does so I thought maybe there was a reason. Thanks for the advice, guess I'll be adding a 4ohm 15 soon.

Yes, the 4 ohms cab will get twice as much power as the 8 ohms, but I can't see a problem there, and it sure won't hurt anything, unless: - the amp has a 4 ohms minimum load, which the B5R hasn't. - you exceed the max power of one of the cabs, but you should ALWAYS check that, not only in this situation. If the 8 ohms cab happens to be 3 dB (equals twice the power) more efficient, then both cabinets will sound equally loud.

I did a search and found this thread. Have the same question. My B4r is rated at 500 watts per channel at 2 ohms per channel. I was wondering if it was ok to run a 4 ohm goliath 210 from one channel with a 4 ohm goliath 210 and an 8 ohm Bag end 15 from the other channel. From the info in this thread it looks like I'm safe doing this. I tried this earlier today and it sounded great. Thought I'd just bump this thread incase anyone else is lloking for same info.

If you are shaving it down that close, best to find out exactly what your minimum impedance is. Impedance varies by frequency. The lowest impedance in a vented cabinet will be at the tuning frequency of the box. It rises to a double peak, above and below the tuning frequency. Here's the important part: At the tuning frequency, the impedance is nearly that of the DC resistance of the driver itself. A 4 ohm driver such as the Eminence Delta 15LFc has a nominal 4 ohm impedance and a 3.27 ohm DC resistance. Two of these in parallel is a 1.63 ohm load. The only way to know for sure is by measuring the impedance at the tuning frequency.

IMO that is a wise decision. When I need to run 4 subs full blast, I originally thought about running them as 2 ohms in a single channel. After considering the huge power draw of the 2-ohm load, I concluded it is smarter running two subs on each channel and using a separate amp for the highs. Much less strain on the equipment.

when you mix impedances, the cab with the lower impedance will draw more power... example: 600w into a 4 ohm and 8 ohm cab... the 4 ohm cab will draw 400w and the 8 ohm cab will draw 200w higher impedance means less current, so higher impedance will draw less power. plus all the power peaks will hit your lower ohm cabinet 8 times as hard as the high impedance cab, so theres a bigger possiblility of blowing it out. end.

Ehr, didn't you just say the 4 ohms gets twice as many watts as the 8 ohms? Wouldn't the peaks be twice as high as well then?

With the posters setup, the 2 10 at 4 ohm probably has 2 8 ohm speakers plus the 1 15 is 8 ohms. Each of the speakers (not cabs) is receiving the same power.

...suddenly you're talking about current instead of power. And while we're at it: you say it gets squared, which still means 4 times as much, not 8. I repeat my question: with the 4 ohms cab receiving twice as much power, wouldn't the peaks be just as well be twice as high? (Look, I could tell you to your face that you're wrong, but that wouldn't look very polite, now would it?)

Does running two 8ohm cabs series or parallel change anything in a mono amp? Wouldn't it still be 4ohms either way? please pardon my temporary stupidity.

Yes. Series and parallel are two very different things. If you had 2 8ohm cabs wired in series, the amp would see a 16ohm load. If you were to wire them in parallel, the amp would see a 4ohm load. Keep in mind that in order to actually run your cabs in series, you'll need to make a special cable for yourself.