I am about to order speakers for a 6x10 bass cab. I can get the speakers in 4,8,16 or 32 ohm ratings and I need to wire the cab for a total load of 4 ohms. I have a couple ideas, but I was wondering if any experts out there can contribute some wiring ideas. Thanks
Six, 8-ohm speakers, when wired in series-parallel, will yield about 5.4 ohms, which is about as close as you can get. Three, 8-ohm speakers in parallel will give you about 2.7 ohms. Put two of those in series, and you have about 5.4.
Jim, do you think that 5.4 ohms will be safe with a 4 ohm load out of a 70's ampeg SVT tube head? I'm guessing it will be.
You'd be better off getting some real technical advice, as far as that particular rig goes, but if the OT can't handle a mismatch of less than 2 ohms, I'd be surprised. With 6, 4-ohm speakers, wired series-parallel, you'd get a total of about 2.6 ohms. It is actually preferable to have a load that is less than the OT expects rather than greater than the OT expects, so the 4-ohm speakers are probably a safer choice, but we aren't really talking about a huge difference, here.
What Jim says. Given the degree of mismatching (only 33% in either direction) it probably doesn't matter whether the mismatch is high or low really. Fender used the series-parallel arrangement with 8-ohm speakers in their Super Six amps of the 1970s to give a 5.33 ohm load with a 4-ohm transformer, FWIW. Another possibility would be six 32-ohm speakers all in parallel, which arguably might sound better for bass (the normal SVT cab has eight parallel-wired 32s as you probably know), but that would be less flexible if you wanted to use the same speakers for some other purpose later.
Mark and I looked at this problem yesterday and found a cool site that calculates everything for you quickly. Arguably unnecessary for those of us who can do math! But fun anyway! http://www.eatel.net/~amptech/elecdisc/spkrmlti.htm I agree that the 32-Ohm per-speaker choice is the lesser option. That is a difficult thing to find in the case of a blown speaker! Thanks for the help guys!---Peter
Ok, I looked at another possibility - running 4 8ohm speakers in parallel for 2 ohms and then a pair of 4ohm speakers in parallel for 2 ohms. Put them together in series and you get 4ohms. Do you guys think there would be any problems with this, such as having some speakers taking on a greater load than others?
Yes. Power divides according to the impedance, so each 2-ohm combination will take the same amount of power, half of the total from the amp. Thus the two 4-ohm speakers will each be taking twice the power (and 1/4 of the total) of any of the 8-ohm speakers. Generally it isn't a good idea to mix up impedances and power loadings like this - better to go with a slight impedance mismatch on the amp.