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Old 14th November 2010, 12:38 AM   #11
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I'm playing right now with TB 1772's running OB and driven by the First Watt F-1. Very pleasing!! Several guys were in tonight and just going wild over how it sounds.

In my case I made some smaller baffles that acoustically make it down to a measured 200hz. Fooling around right now with bass coming from Avantgarde woofers, which nicely make up high enough (being more woofers than subwoofers!). Adding in a HP crossover doesn't seem to change performance as much as I'd have though at the levels I usually listen at. Higher SPL would certainly differ.

The F-1 gives a very nice big picture with great sound stage all the way across. Super nice - Very dynamic!

Likely the 1808 is a better idea OB. These are just what I happen to have. I've gotten lots of complements on the result!

Mark
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Old 14th November 2010, 01:03 AM   #12
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Quote:
Originally Posted by knutn View Post
So If I understand you right, it is not a first goal to look for drivers with low mechanical Q. It is, as Nelson says, maybe necessary to compensate for the high resonant impedance somehow. But to my knowledge it is easyer to compensate for a low than a high Q.
First & foremost you have to keep in mind that dipole rolloff can be considered compensation for the lift in frequency response from the interaction of current drive and rising response. If you can get one to perfectly counter the other you have an ideal situation.

In a voltage drive OB we look to use high Qt to compensate for the dipole rolloff... in a current drive OB you look at the Qm.

Also if one surveys the Qm of available drivers, <2 is actually a low Q.

dave
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Old 14th November 2010, 08:23 AM   #13
el`Ol is offline el`Ol  Germany
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I doubt that a fullrange small-size OB-solution (except for desktops) is possible because the displacement limits compensation of the dipole rolloff. But you could try what can be done with drivers that are already OB-suitable with voltage drive driven by a current drive amp. Such drivers (e.g. from Sica) are not expensive, could be fun to try.
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Old 14th November 2010, 02:31 PM   #14
Vix is offline Vix  Yugoslavia
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The best compensation is no compensation...in terms of OB. In other words, the bigger the baffle, the less compensation is needed, which makes it easier to get the correct balance. On the other hand, the bigger the baffle, the lower the WAF , (so you have to compensate elsewhere)
Most of us here would have a hard time to accommodate baffle sizes that Nelson utilizes, so we must turn to compensation. It is interesting to see how Mr. Linkwitz does it via shelving filter, which compensates at the rate of 6 db/oct, from about 10 Hz up to 300 Hz (this would depend on the width of the baffle). But then it makes it necessary to use biamping, going active..more complicated...so..the quest continues

Last edited by Vix; 14th November 2010 at 02:34 PM.
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Old 14th November 2010, 04:49 PM   #15
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I find that the issue of compensation appears after the
baffle is built and you take a listen. Often it is independent
of baffle size because the compensation we are talking about
is up in the upper frequencies - 3 KHz and beyond. Your
ordinary baffle step is in a much lower range, and I don't
seem to run into that much. It looks to me like it has more
to do with the shape of the baffle than the size - the edge
effect when you have two identical waves cancelling is
different than having the baffle surface turn at a right angle.

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Old 14th November 2010, 05:09 PM   #16
Vix is offline Vix  Yugoslavia
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Interesting. I was mostly concerned with the compensation in the bass range 300 hz and downwards. I don't have an idea about the >3Khz compensation, other than to notice that midrange gets more pronounced with on the Open baffle. Then, I scratch my head and wonder if Linkwitz 3 Khz notch is something we should look for (or, other means to shape a frequency response). By the way, sorry for going almost off topic...
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Old 14th November 2010, 05:55 PM   #17
el`Ol is offline el`Ol  Germany
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Quote:
Originally Posted by Vix View Post
...so..the quest continues
Currently I am working on BLOB (backloaded OB). Conical baffle, opening towards the rear, and placed in front of a room corner. Is in a very early stage however.
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Old 14th November 2010, 06:10 PM   #18
knutn is offline knutn  Norway
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Default Compensation

I would think that a possible compensation would be easier to do actively; it is a much cheaper solution, and it is easier to modify than a passive solution. As Pass says, this can be done after the baffle is built. I would guess that a rate of 6 db/oct or maybe less should be sufficient. I don't think a notch filter would be the way to go.
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Old 14th November 2010, 06:20 PM   #19
Speak is offline Speak  Sweden
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Many years ago I built open baffle design with 3x 10” seas driver Qts around 0,28 if I don’t remember wrong. With 15 Ohm in series on each driver, crazy thing to do but it worked very well. Fastest open baffle design I have heard. No active compensation, decent SPL/W quick bas with good low frequency performance and you could play rely loud. We play with a 200W amplifier and also 50W and both worked well.
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Old 15th November 2010, 10:50 AM   #20
el`Ol is offline el`Ol  Germany
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I also think series resistors are bad per se. My main 3-way system runs with a low Qts wideranger and 3.3 Ohm series resistor that brings it to Qts = 0.5. Sounds much less stressed, much more relaxed.
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