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Old 14th June 2008, 08:57 PM   #1
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Default am I cheating? Bi-pole 127E

I recently built the Fostex 127e sealed bi-pole and I was more than pleased with the results, but I keep trying to get these mama's to love my EL34 SET, so every milliwatt counts.

The point of a bi-pole is to eliminate the need for a baffle step correction circuit, right? I heard the idea to roll off the back driver above the baffle step point, but I had never had any idea on how to do it with just one amplifier, then I thought of it!

Put the two drivers in series, then put a capacitor in the middle that shunts the high frequencies to ground!

so I scrapped an old pair of speakers that had dry rotted (one of the boxes now holds my two 8" subs), and took the caps out of the crossover, I currently have 18uf between the two drivers shunting everything 1300hz-ish up right to ground AFTER it goes through the first driver. This makes the front driver have all the High frequencies through it, sparing power from getting dumped out the back. Just to be absorbed by my ad-hock room treatment.

So far, so good! I think it makes the Midrange "shout" worse, (why wouldn't it? its getting twice the power), but I think it has superior imaging on the high highs than previously, its cranking out a cool 92db (with help from the subs crossed at 100hz) on Pink Floyd's Animals ( a smooth, but demanding album).

I think my only problem is that the XO point is too high! which is where I need some guidance, I have no idea where I should set the cross over point between the two drivers? anyone have any theories? However, I am glad that I can squeeze every milliwatt I can out of those EL34s (they are obviously not powering the subs.)

any other help/comments are welcome!

-Moose
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Old 15th June 2008, 02:25 PM   #2
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Hi Moose,

Sounds like you are having fun!

Couple of things, with the bipole you either can have a 4ohm load or a 16 ohm load. What loads can your SET amp take? If your amp can take a 4 ohm load an EL34 should be able to get some decent spl out of these.

Rolling off the back driver should reduce room refections and thus improve imaging as you have noted. However, it may make them sound smaller. See which you prefer.

However, I'm not sure that rolling off the rear driver will make more power available to the front. I would think it is the bass frequencies that use the majority of power.

As for the baffle step point, it is a function of the baffle width. Assuming you used a 7" baffle, F3 is ~650Hz, so you will need a larger cap.

Since you are playing around, you can also try disconnecting the rear driver and using it is a passive radiator.

Cheers
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Old 15th June 2008, 08:26 PM   #3
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Thanks for the response!

I ran it under both a 16ohm load, and a 4ohm load, however, when I did such an adjustment, I did not adjust the primary, so under a 16ohm load the EL34 sits at 10k, and under a 4ohm it sits at 2.5k, I liked the sound of the 16ohm more, and without the two drivers in series, I can't do my trick with the capacitor.

I thought about it like this: Two resistors of equal resistance (or impedance) if there is one 8 ohm resistor (as a high frequency would see), its going to dissipate the same amount of energy through the one 8ohm load, as a low frequency would through a 16ohm load, the only reason it is louder, is because all of the energy is aimed at me, not half of it.

I did use a 7 inch baffle (its almost like we have an identical pair of speakers...) so if f3 is ~650Hz, I need to make sure that the Low pass filter is flat by 650hz, if I use a first order filter, that means I need to have a crossover point at ((650/2)+650)= 775hz) minimum maybe i'll round it to 800hz, just to be safe.

should I bump it up to a 12db/octave filter, should I calculate the inductor value for 8, or 16 ohms??? I assume that I should use 8 when calculating the capacitor, but you know what happens when you assume...

-Moose
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Old 15th June 2008, 10:38 PM   #4
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I don't know the topology of your amp, but I would expect about 2.5 to 3k would be a good load for a SE EL34.

I'm not convinced that this trick will make more power available, but I don't know for sure. The way I see it, your are almost always going to have musical content below the XO point. Have you noticed more available power when trying this?

I wouldn't worry about 2nd order or an exact XO until you are happy this is getting you better results.

Let us know how you make out.
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Old 15th June 2008, 11:30 PM   #5
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I would like to point out that I am very sorry with the horrible math I preformed two posts ago 650/2 + 650 is 775, it is 975

Just wanted to know that, I am going to keep tweaking the values, and see what works the best.
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