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Old 11th June 2012, 10:06 PM   #1
zmyrna is offline zmyrna  United Kingdom
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Default First OB with vintage Seas

I finally started building my first OBs.
The drivers are vintage Seas paper cone alnicos from early 70s.
4 ways:
1 x 2"
1 x 4"
1 x 8" mid (fabric surround)
3 x 8" woofers (rubber surround)
For the first experiment, I used a 3 way Seas crossover for the upper 3 drivers (2000-5000). This CO works fine for the same drivers in a box. But on OB, upper mids are a bit too pronounced. Can anyone guess or explain why?
However, the bigger problem is the bass.
The woofers are 15ohm each, wired in parallel to give about a 5ohm load. Each woofer should be around 89-90db efficient with Fs=30Hz and Qts of 0.4-0.5
The 8" mid driver should be about 92-93db, and Qts~0.7
I have just a 10mH coil on the woofers and no high pass on the mid.
I don't have any measurement gear, but purely by ear I do not think there is much going on under 100hz at the listening position (more bass as you get closer though). I even tried adding a 4th woofer, but still weak bass.
I have another eight 10" woofers from the same series (these were used in the famous Dynaco A-25). My original idea was to build separate H frame subs with the 10" woofers. Now I am in doubt. The bass might never be to my liking. Plus they will probably take too much space in my living room.
Any comments on the CO of the main speaker and the idea of H frame subs?
I think I probably should put the woofers on the main speaker in a closed box instead of adding subs.
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Old 11th June 2012, 10:11 PM   #2
djn is offline djn  United States
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Might be getting bass canclation because of the narrow baffle. If you bi amp you can bring up the bass
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Old 11th June 2012, 10:24 PM   #3
ra7 is offline ra7  United States
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There is a 12 to 18db/octave bass roll off on an OB depending on the baffle size. The corner frequency depends on how far the rear wave has to travel before it starts cancelling the front wave. A bigger baffle will push the cancellation frequency downwards, and your bass will start rolling off later than that on a small baffle.

To counter this loss, you must add a big coil to the crossover, such that the falling bass is compensated by a rolled off mid. You can simulate this in Edge. Another way to counter the loss is to get lots of cone area, something like twin 15" woofers will do it. Higher efficiency units also work better, because you have efficiency to throw away in the bass.

At the end of the day, you are trading off efficiency. To get a flat response, you must EQ your mid and midbass to the bass, which has that steep rolloff due to the OB.
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Old 11th June 2012, 11:17 PM   #4
ra7 is offline ra7  United States
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See the crossover design and explanation in this one:
Fast, fun, Inexpensive OB project
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Old 11th June 2012, 11:22 PM   #5
Pano is offline Pano  United States
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Yes - what ra7 said. OB crossovers are different. You will be fighting the rising response of the woofer(s). To get the proper tonal balance you need to set the woofer crossover much, much lower than you would in a box. Electrically it may be very low, but acoustically it will be right.

You've gone in the right direction with the 10mH coil on a 5 ohm load, but you may still need a big cap in there (second order) to fight the rise. If the load really is 5 ohms, you should have about an 80Hz low pass now. You'll loose a lot of SPL doing that, but it's the trade off you have to make.

You can also try some shallow side wings to help the bass. Making them asymmetrical will help.
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Old 11th June 2012, 11:53 PM   #6
Bigun is offline Bigun  Canada
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I like the overall approach and those drivers look really nice. I'm hoping to learn more about OBs.

Given how much effort you are putting into this, it's worth buying a router.
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Old 12th June 2012, 05:25 AM   #7
DrBoar is offline DrBoar  Sweden
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You will need some baffle step compensation for any resonably sized baffle. With such narrow baffle more than a wide baffle such as OBL11.

With no mean of measure it is really hard to get things right. If you have an other one of the 4" units you can use (BLACKCONES) and then you have the midrange and tweeter range set. I am thinking about building something along with the OBL11 using four Seas 10" high Q drivers and then the 8" and 4" units of the black cone.
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Old 12th June 2012, 12:50 PM   #8
djn is offline djn  United States
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If you want to experiment a bit: take a piece of cardboard the same height as your baffles and 2 or 3 feet wide, and tape it to one side of your speakers and run it backward. So it would look like an "L" if you looked down on it. Then see how much you are getting.
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Old 12th June 2012, 08:22 PM   #9
DrBoar is offline DrBoar  Sweden
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I think you have to reduce the output of the top 8" driver. at least the 4 Ohm version of Seas 21 TV-G is about 96 dB and actually about 100 dB for 2.83 Volt in the 1.5-4 kHz range. I assume that the orginal box was wider than the open baffle. A 30 cm wide baffle drop up to 6 dB below about 450 Hz the original design with a wider baffle would have support to a lower frequency. A reduction in output 450 to say 300-200 Hz compared to previous design would explain part of the lean sound.

10 mH gives a crossover at about 80 Hz. Here and below the woofers are into cancelation between front and back so there is no way you are getting 92 dB. I would try to mute all 3 top drivers by 6 dB and see if you like the balance better
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Old 13th June 2012, 09:57 PM   #10
zmyrna is offline zmyrna  United Kingdom
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Quote:
Originally Posted by djn View Post
Might be getting bass canclation because of the narrow baffle. If you bi amp you can bring up the bass
my amp is an integrated digital amp with digital input and no pre-out.
so going active is complicated.
i was eyeing digital plate amps with line level input but those are expensive and hard to get in the UK.
i was also hoping to avoid wings on main baffle (hence the idea of separate H frame subs).
experimenting with cardboard extensions to the baffle is a very good idea. thank you.
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