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Old 19th March 2005, 12:23 PM   #1
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Default pioneer b20fu20-51fw

I'm new to speaker building. I was thinking that this summer I would start out with a cheap project using the Pioneer B20FU20-51FW, mostly because it's so cheap and it can play fairly low for the price.

I used a program called WinISD to design a very simple bass reflex enclosure of about 106 liters. (12"x15"x36" inside) My question is this: is there a better design out there that would be more suitable for this driver?

I should qualify my question by saying that I am not a very good carpenter, so a complicated folding rear horn is probably beyond my ability.
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Old 19th March 2005, 05:32 PM   #2
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Open Baffle.
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Old 19th March 2005, 06:36 PM   #3
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Old 20th March 2005, 01:30 AM   #4
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Greets!

While an OB is about as simple as it gets, this driver is like most FR drivers in that it needs all the acoustic help down low it can get, so I prefer straight pipe loading as a nice trade-off between simplicity and performance. Taking the T/S max flat specs and converting it to a pipe (all dims are inside (i.d.)):

L = 50.75"
w = 14.375"
d = 8.875"
driver down 19.25"
vent = 4" dia x 2" long positioned 1" from the bottom

Lining the top, one side, and back with fiberglass insulation or a loose fill of polyfil from the top down to just below the driver is usually sufficient.

Since this driver is somewhat lacking in HF BW, a super tweeter is required for best performance. Since piezos like used in the referenced sealed design are no longer made, this inexpensive Vifa should work well: http://www.pispeakers.com/Components...X25TG05-04.pdf

I recommend packing the area between the whizzer and diaphragm with polyfil (AKA $0.98 tweak) before dialing in the super tweeter XO values since it smooths some of its rather ragged/peaky HF response. There's a number of other tweaks that improve this driver, but most folks prefer to just upgrade to a more expensive one that has them already built in.

GM
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Old 27th March 2005, 05:37 PM   #5
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I originally shied away from open baffle because it seemed that decent low-end response would require an enourmous baffle or a subwoofer--neither of which I want, if possible.

Quote:
driver down 19.25"
Just to clarify, this means the center of the driver should be 19.25 inches from the top of the inside of the box, correct?

Also, how deep do you think this design could play usefully?

As for the tweeter, I think I'll build the single driver boxes and add the a super tweeter later on if I feel like something is missing. In music, I can't hear much above 16khz anyhow, so I'm not sure if it's worth it to add super tweeters...it may sound just as good to my ears to fix the peaky highs by putting some kind of lowpass filter on the driver so it just doesn't play anything above 16 or 17 khz.
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Old 27th March 2005, 06:56 PM   #6
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Greets!

Correct. Aneochoic Fb = ~34 Hz using PE's specs so reproducing all but 64 ft organ pipes and some LFE special effects isn't out of the question.

GM
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Old 31st March 2005, 08:53 PM   #7
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Thanks for all the help.

I've been doing some more reading and I think I will go ahead and use a super tweeter. However, I don't know how to design the crossover.

Assuming I use the tweeter you reccommended, what should the crossover frequency be? 14khz? How difficult would it be to build this crossover my self?

If all that is involved is putting a low frequency cutoff and attenuation on the tweeter, it seems like it might be pretty easy...but I really have no idea how to go about this. Any good links for crossover design?

Would these tweeters work? They are about half the price of the ones you recommended.
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Old 31st March 2005, 10:43 PM   #8
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Greets!

You're welcome!

Normally, a super tweeter is just capped off, so there's only one component in series with the ST's (+) connection. You may want to try a textbook 2nd order on it though, or two components. Depending on your hearing, you may even need to add an L-pad to shelve it down a bit, or another two components.

Here's a 2nd order calculator: http://ccs.exl.info/calc_cr.html#second

For example, starting with a 15 kHz XO point and just looking at the published impedance plots, it looks like the B20's impedance has risen to ~40 ohms (low pass) and the tweeter you suggest is at ~8 ohms (high pass), so punch these numbers in to get the component values as a first approximation. Most folks use Butterworth, but FWIW I prefer L-R.

Without measuring equipment you will have to try different values if you don't like how it sounds, so it's a good idea to build it on a piece of PC or peg board and put it somewhere convenient to get to until finalized.

L-pad: http://ccs.exl.info/calc_cr.html#lpad

I haven't read any reviews of this tweeter and it's FR implies that it probably won't sound as smooth as the Vifa, so I guess it depends on how good the HF hearing is of all the folks likely to hear them.

GM
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Old 12th April 2005, 05:02 PM   #9
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Okay cool.

I think I'm going to try a few things. I will test out a 15khz filter on the tweeter, a 10khz filter, then maybe a 5khz full two-way crossover. (All 12db/oct.) I'll probably put a 4db L-pad on as well. Maybe only 2 or 3 decibels with the full 2-way.

So here's another question: what kind of power handling will these end up having? The full-range can take 60 watts, and the tweeter 70 watts. How much wattage should my amplifier put into these speakers? And what will the impedance be (full-range is 8 ohm, tweeter is 6 ohm)?

I want to make sure my amplifier won't fry these speakers or vice-versa.
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Old 18th April 2005, 04:29 PM   #10
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Not to put you off your plan, but you might want to consider stepping up to a little higher priced driver. I look at it this way: You're going to spend $50 on a pair of pioneers, maybe another 40+? on tweeters, and probably a fair chunk on various inductors, capacitors and resistors for the X over. If you build a project using for exaple the Fostex FE167E you probably won't need the tweeter and X over, and the cost is comparable.

I have a pair of the poineer drivers, and they aren't bad (I'm sure they would be much better with a tweeter)-but I doubt they could beat the 167-for my needs anyway. Just my $.02

Best of luck with your project!
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