Miss-matched cross-over points?

[kringle77]

A bass player friend of mine is looking into building a cab using some 15" woofer in a sealed compartment and a Jensen mod50-70 in another sealed compartment. He was thinking about low passing the 15" at around 800hz, and high passing the 10" at 200hz. The idea is to use the 15" to get some bottom end and fill in the mid dip of the jensen. The 10" would be high passed to protect it from too much low end. Both drivers are 8 ohms. He is going to be using an ampeg v4 and wants a 4 ohm cab. It seems to me like the amp would only see 4 ohms from 200hz to 800hz. Am I right about this? I suggested running the 15 full range and just high passing the jensen which I think would give you 4 ohms from 200hz up. Im really not sure how it works. I have read a link that BFM posted many times but really don't understand if it really matters above a certain frequency.

[billfitzmaurice]

Originally Posted by kringle77

The 10" would be high passed to protect it from too much low end. Both drivers are 8 ohms. He is going to be using an ampeg v4 and wants a 4 ohm cab. It seems to me like the amp would only see 4 ohms from 200hz to 800hz. Am I right about this?

Yes.

[staindbass]

speakers have different impedance at different frequencies. google speaker impedance plot, they vary quite a bit. its normal, but i am not sure of the implications. johnny a.

[kringle77]

Thanks Bill. Am I also correct in saying that higher frequencies will not matter as much because they need fewer watts? If thats true, could he use a 4 ohm 15 and a 8ohm 10, therefore having a 2 ohm load above 200hz? Would it bother most amps?

[billfitzmaurice]

Originally Posted by kringle77

Thanks Bill. Am I also correct in saying that higher frequencies will not matter as much because they need fewer watts?

Yes and no. There's less current draw because there's less power density, there's also less current draw due to the rising impedance of the driver due to the inductance of the voice coil.

If thats true, could he use a 4 ohm 15 and a 8ohm 10, therefore having a 2 ohm load above 200hz? Would it bother most amps?

The load above 200 Hz is no problem, tube amps are perfectly happy with low impedance loads. High impedance loads, especially below 300 Hz or so, are a problem. The difficulty with using a 4 ohm 15 and an 8 ohm ten lies in the sensitivity mis-match. If it was me I'd get around that with a pair of BP102 tens for the lows and a 4 ohm mod for the highs. If they don't make a four ohm Mod use two. That also opens the possibility of using the BP102s sealed, as tubes prefer the lower impedance of sealed alignments. BP102 is happy in 1.5 cu ft each. Put the Mod in as small a sub-chamber as can be made.

[kringle77]

The bp102 was a serious consideration but, the sensativity is pretty low for the V4. He is looking for small, sealed and sensative. Otherwise he would just use and 8x10, which is pretty great with that old v4 of his. I use an old svt and therefore don't need quite the sensativity.

[billfitzmaurice]

Originally Posted by kringle77

The bp102 was a serious consideration but, the sensativity is pretty low for the V4.

Have him model a 2xBP102 in 3 cu ft sealed and then whatever 4 ohm fifteens he can find.