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Old 15th February 2009, 08:43 AM   #1
CLS is offline CLS  Taiwan
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Default Multiple Dipole Subs

Hi there,

I finally gave my big old TV away and bought a new one which is physically much smaller.

This, leads to the theme I'd like to share -- a center OB 'sub'. Combined with my original OB speakers, now in the bass, there're 3 of them -- multiple dipole subs:

Click the image to open in full size.

The center channel 'sub' is made as V shape with 120 degree between the 2 woofers.

Click the image to open in full size.


The center OB bass is now wired as mono, driven by a plate amp, and via a set of T-bass circuit of course. The setting of T-bass and the integrated xover of the plate amp make it work only under 40Hz.

All 3 OB's are indeed too close to the wall, but I can't do anything about it. Can't aford a bigger place.

I wired them up for only half a day, the settup is not optimum yet. Preliminary listening showed some potential but also something obviously wrong. By the RTA reading, the frequency range of the whole system extends lower and flatter, but I didn't 'feel' any improvement of the bass sound. Actually I feel it somewhat 'unfocused' on some materials.

Squatting just in fornt of the center OB, there can be a very strong sense of low bass -- feel of vibration instead of 'sound'. However I can not detect it on the listening position. I guess some cancellations happen here.

There's a knob marked as 'phase' on the plate amp with dial of 0~180 degree. I'm not quite sure its 'real' function. The difference of path lengths of the center and the main is measured as 23 cm (the center channel is closer to listener). At 40Hz, this 23 cm difference makes a 9.6 degree of phase angle shift. But where's the exactly position on the knob can give me this? It can not be examined.

By only ears and a simple RTA, I'll need more time to get it right....
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Old 17th February 2009, 06:00 AM   #2
CLS is offline CLS  Taiwan
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Over 100 views and no reply ....

OK, here are some updates.

I re-adjusted the whole system -- brought up the active xover point of the woofer (on main OB), and raised the overall gain of mid-high. This gave several dB lift to all frequecies from about 50Hz and up.

So the add-in center sub would not give a 'heavy-footed' effect due to the uneven response and unsmooth transition from lower mid to upper bass to lower bass. (And now it's a mix of 3.5way and 2.1 channels.)

This made the whole system sound extended and lively. And the previous 'unfocused' phenomenon improved a lot. Now the impact and sense of vibration are stronger and more lifelike. Did I mention it sounds big? It's indeed no replacement for displacement! Big cone area works!

As to the 'phase problem', I leave that knob very close to 0 (or sometimes right at the 0). I can sense a little difference on some materials when the knob is at 0 or not. Setting it slightly off 0 is mostly a better choice with most of my collections.

I think of other problems on this. Even if the phase knob is "properly working", it's still questionable. Reflecting to the 23cm difference of path lengths, the phase angle at 40Hz and 30Hz and even 20Hz are all different! Which point should I aim for? In the end, I probably converge them in one frequency and miss all others. So, what I need should be a constant time delay instead of a phase angle. Am I right? (or wrong?)

In addition, I'm not sure about what exactly the built-in 2nd order xover of the plate amp and the T-bass circuit do to the final acoustic phase. So I have no idea if the 3.5way structure itself is coherent or not. Let alone this 'tiny' path length difference. I'm basically shooting in the dark here. And, afterall, related to the wavelengths of 40Hz and down, 23cm is just so small. Maybe I should just ignore it?

Any comments?

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Old 17th February 2009, 07:19 AM   #3
jamikl is offline jamikl  Australia
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Hi CLS, I like the look of what you have done. I take it that the subs are solid enough not to cause any vibration problems to the TV. What speakers are yhou using in the mains and the subs now?

I have four Beta 15As but am concerned they run out of xmax so quickly. Maybe I should forget sims and just do it. Right now I am listening to two open baffles with four 6" speakers in each, not ideally located on the baffle. They are on an old JVC amp which has loudness comp. switch which is on. they are rated at 1.5 W each speaker and the voloume is on 2.5 on a dial which goes up to 10 or 12. I am about 36 feet away in another room and can hear them quite loud. Stan Getz is sounding sooooooo good. So maybe I can disreguard xmax issues on the Betas.
jamikl
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Old 17th February 2009, 08:03 AM   #4
CLS is offline CLS  Taiwan
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Thanks for the compliment I'm now using Eminence Sigma Pro18 on center sub and also on main baffle. Actually the center sub still vibrates. (Its overall depth is very shallow) After all, the cones and the air they pump is massive. Luckily there're foam pads under the seats of TV for some damping, so it's not seriouly shaken. I've once built a W baffle, with face to face arrangement, it cancels the vibration much more effectively. However this V baffle is better looking

I'd say don't worry about the Xmax. My woofers barely move most of the time. The motion can only be seen within very short distance. When the motion is visible, I'd begin to worry if I'm bothering the neighbors. It's no way to see them move at the listening position. They're just so loud with so less moves. Even with your slightly smaller 15"ers, I think it's no problem at all. Just go for it.

And, please also try T-bass circuit on your OB bass:
'T'-bass drive for OB LF drivers.
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Old 17th February 2009, 10:03 PM   #5
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Nice setup, are you (still) married? ..


I tested linkwitz transform on OB and was very pleased with the result. I'm sure LT and heaps of woofers can give extraordinary SPL at 20Hz
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Old 18th February 2009, 03:00 AM   #6
CLS is offline CLS  Taiwan
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LOL!

My system is really getting smaller and smaller, for wife and kid. And they love the new plasma TV, which also helps a lot to the whole thing...

As to the LT, I think it's also a kind of EQ, isn't it? Did you try T-bass?
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Old 18th February 2009, 04:24 AM   #7
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Yes, LT is 12db/oct driver equalization which moves the poles. For example Eminence Alpha has Fs=41Hz and Qt=1.2, with LT we can tune it to Fs=20Hz and Qt=0.7. Perfect!

http://linkwitzlab.com/filters.htm#9

So in an OB we use dipole equalization (6db/oct) plus LT to obtain flat response to very low frequency.

I have heard of T-Bass, but it seems to complex where the active xo solution is more precise and simple.


Click the image to open in full size.


Click the image to open in full size.
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Old 18th February 2009, 07:43 AM   #8
CLS is offline CLS  Taiwan
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I've not yet tried a LT, can't comment.

In my active xover, I once used very low xover point (40Hz, or even lower) with higher gain (+12dB) to offset the baffle loss, and fine tuned the whole thing with a digital EQ. This brought me a frequency response quite flat and very extended, but I was not so happy with the real world sound quality. Until I tried the T-bass.

It's actually not that complex and really worth a try.
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Old 18th February 2009, 03:10 PM   #9
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I noticed that bass boost requires very powerful amplifier. IMO better solution is to add more low passed woofers, 18" goldwoods are cheap
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Old 18th February 2009, 03:43 PM   #10
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Not at all actually. Driving OB woofers takes very little power. My 50watt gainclone copes really well driving Eminence Alpha. Note that ALpha has high Q=1.21 at Fs=41Hz, which peak is flatten so in fact it takes more SPL with LT.

Of course I agree with you that more is better and 18" is better than 15". Everyone knows that except the ministry of war and finance.
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