Big Bass Ob

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In some earlier threads I've mentioned this. Now I finally got them. Some pictures for sharing:

Front, for 3 x 18" woofers, overall about 1.2m wide by 1.1m high.

Braces on the back:

Trial fit with Oris 150 mid horn:

With drivers, two Eminence Sigma Pro18, and one EV DL18W:

Overall view (without mid-high):

53" TV looks so small in between these monsters. And this tiny living room is also way too small, oh well...

I'll need some more time and effort to get them work. To be continued....
Partly because of the shooting angle, partly because the "suspension" is not completed.

That was just put on the frame, and about several cm below the target position.

The frequency range of those woofers is below 160Hz, and the horn mouth is actually ahead of the woofer cones by almost 1 foot, so I don't worry about a little overlap.
Thanks, glad you all like it:D

I'm married, and have a 5 y-o son (who is often louder than my speakers:xeye: )

This system is going to be actively xover'ed, 2way at this stage (for mid/low @ about 160Hz). I have built and used a tube preamp with simple xover built-in. I modified it a little to suit this OB bass. (Previously the woofers are in big cabs, so the responses are surely different.)

Previous tweeter is Audax PR120, now I'd like to try Eminence APT with modified Dayton WG (8" opening in the pictures, black stuff above the big white mid horn).

Due to the change on tweeter, the mid/high xover will be changed and not done yet. By the bigger mouth of the WG, I'll bring down the xover point to about 3kHz region and see if any good. I'll keep the xover network as simple as possible, so hopefully I won't need much time & effort. I got some spare amps around, so maybe I'll try full active 3way in the future.

The midrange is obsolete Focal Audiom 7K, which has already served in my system for 13 years. One of the woofer - EV DL18W is at the same age. The other Eminence woofers are slightly younger. So basically they are all old stuff, and I just put them together because I moved. Turns out pretty good I guess (to look at, for now).

I'll keep working on the xover today and hope it at least makes some sound later.

To be continued...
Two suggestions to save time and make better sound - the eminence tweeter is good but not below 3.5k - try B&C DE10 - the focal may not have the motor to drive the mid horn in the low treble look for a higher efficiency mid or 'fullrange' driver with a rising response that has fs below 80 cycles (for big mid horn)
Thanks a lot for the tips :) For this big mid horn, actually I've considered B&C 8PE21 (8" mid) for a while, and Fostex FF225 is also on the short list... They look very good on paper.

Years ago, I once tried Lowther PM4 with this horn, it was very good overall. However, Focal stays. PM4 lacks the high SPL capability I wanted at that time. When volume gets higher, over a particular point, it somewhat breaks apart. While Focal can keep singing louder and louder, no sweat. Except for the top octave, difference in sensitivity and high SPL capability, they sounded amazingly similar.

For now, I don't need that kind of SPL. So if I can afford, the sound of PM4 with this horn pleases me very much.

While the Focal looks not so big and strong, it is actually very capable. Seems not many people familiar with this old mid, here are the specs:

Fs 119.91 Hz
Qts 0.396
Qes 0.414
Qms 9.115
Sd 0.0165 m2
Mms 0.0073 kg
BL 8.94

Diameter (mm) 135
Weight (g) 1132
Flux Density = B (T) 1.52
Gap Height (mm) 6

2.8V/1m (dB) 98

Though the Qes seems not low enough for horn loaded, the BL vs Mms is very good indeed.

It also has a rising response (and extend very high for a mid):

(scanned of the catalog published by Focal)

On axis, it extends over 10kHz. It actually dose that in my living room. For some music, I find it doing good without a tweeter. And I ran it fullrage previouly with Audax "super" tweeter for the very top. (the dispersion integration is not good, though)

Making a driver with 120Hz fs doing 160Hz in a horn is indeed stretching it a bit. But I find no evidence of constrained sound in home use, even doning HT stuff, and even with 0.7w flea amp.

The "power handling capability" of the whole system is limited by my own ears (and kind enduring of my families and neighbors), not the hardware ;)

So in my own particular usage (low SPL), I find it Ok to "stretch" the drivers a bit.

Yesterday I ran errands around downtown, got stuck in some frustrating traffic jams... So the progress of this project is delayed somewhat.

Last night I managed to make it sing, but that was already midnight, so I could only play it gently.

First impression, bass heavy! :cannotbe: Tunning down the gain of the bass amp solved some of the problem. I listened for only about an hour.

That bass heaviness must came from the active xover. I planned it by the OB simulation of <Edge>:


Red line is the repsonse of OB itself.

Blue is the planned xover compensation for the OB drop, 6dB/oct slope LP, -3dB @ 40Hz.

Green line is the target response.

According to the prediction, I need about 6dB more gain to make up the loss. The tube preamp/xover comes with OPT, so I brought up the gain by rearranging the OPT's secondary windings. 4 windings were changed from paralleled to series, gives me 4 times of output voltage, thus 12dB. Well, it exceeded, I know. I want some margin for future adjustment.

And the gain is indeed too much, thus bass heavy sound. Fortunately the power amps have input gain control, so no big deal.

This morning, I did some rough measurement and adjustment by Behringer DEQ2496:



Actully I like this kind of downtilt response. I find it warmer and more effortless to my ears, especially for high SPL or long period of listening.

The APT tweeter is rated 105dB/w, now you may see how it goes on the RTA. The mid horn and OB bass are NOT weak at all! :smash:

Some more fine tuning will be going on....

I don't have more sophisticated sim tool to plan this project, just <Edge>, again:eek:

The Edge can not sim 2 passive (attenuated) circuit, so I took the one for boosting as the xover, and the other attenuating circuit for newly added compensation:


Green line is OB with 1st order low pass -3dB @ 26Hz.
Red is added with another 21-42Hz 6dB compensation.

With the 21-42Hz 6dB compensation added, the mid-bass region is further suppressed. And with surplus amplification gain, I get away with less digital EQ.

However, I observed the in room RTA response (by DEQ2496), the bottom octave is not so promising as the sim above. And it seemed sucked out by the room (or else?), output from the OB hardly responsed to the EQ boosting. I mean, even if the 20~32Hz sliders were pulled very high, the measured SPL of that region didn't reflect that boosting. It just stayed pretty much the same way.

However, by ears, the bass is pretty good. Big, deep, clean enough, but TBH not the best I've heard.

Maybe the space is just way too small for them, and I also can not pull them far enough from the back wall:( ....

Anyway, I'll keep working on it. And I've not yet tried all my favorited materials.
Joined 2004
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Very impressive! Nice work.

I like your approach to the OB. And I like your transformer gain boost (I work with transformers, too).

Like you, I have found that even massive amounts of low end correction still does not bring the lowest octaves to where they ought to be - in OB. But it still sounds great.

Your 40Hz shelf seems to be about right, and it looks like you have done a better job at keeping the low end full than I have. There does seem to be a point at which the room just won't let you go lower. Unique to OB? Probably not.

FWIW, I run 1x15" per side for bass with Altec 806 drivers on papier maché horns for the highs. >900Hz.

Keep up the good work; let us know how it all turns out.
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