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Old 14th January 2013, 11:08 PM   #1
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Default Cross Over design and Woofer choices for FAST

I love the idea of FAST. I know there's the Tysen thread, but I've wanted to start a thread like this for a while and a recent comment from Mark Fenlon has prompted me to start it because I can't seem to say what I want to say over in his play ground.

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Originally Posted by markaudio View Post

Woof No.6 (EL-166) showed what could be done (near flat/linear response) for modest money once a bit of elbow grease design effort was put in. Ask yourself this question: Why is it that so many woofs have got large peaked outputs, yet sell so well? Surely a crossover design challenge, but why do so many users put themselves through such challenges, maybe because it is a test of their skill to tame an errant driver. Or is it simply that they little alternative product choice?
Well, of course it's not about the challenge. That's just silly, but I don't think Mark meant that literally. What I want in a FAST woofer is some with a lot of Vd. So xmax and Sd are most important.

Usually that means big heavy woofers that take gobs of power to push. I'm not concerned with break up as long as it's above say 500 to 750hz. The reason is I believe in using active cross overs for FAST and about 24db/oct does it.

So, what woofers make the best FASTs ?

Does high frequency extention matter to you for low order cross overs or not?
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Old 14th January 2013, 11:28 PM   #2
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Here is a list of woofers I would like to try in a FAST

SB Acoustics SB34NRX75-6
SB Acoustics SB29SWNRX-S75-6
Dayton RS270-8
Peerless 830669 for a budget choice
Exodus Anarchy for a compact choice
Seas L26

Some of these will reach down around 20hz and still work with pretty low order cross overs.
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Old 15th January 2013, 05:21 AM   #3
TiMBoZ is offline TiMBoZ  Australia
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Can someone please define FAST? Thanks
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Old 15th January 2013, 06:08 AM   #4
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Quote:
Originally Posted by TiMBoZ View Post
Can someone please define FAST? Thanks
It stands both for "Full Range and Sub Technology" or for "Fullrange Assisted", if I am not mistaken.
The science and art of marrying a fullrange driver that takes care of most of the audio frequency range with a woofer, crossed low.

Last edited by talaerts; 15th January 2013 at 06:16 AM.
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Old 15th January 2013, 07:15 AM   #5
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Thus the necessity of evaluating both drivers that make a Fast design.
Usually little FR are in the low sensitivity range, and that might go well
with the low efficiency below 100 Hz expressed by most budget woofers, as
the passive crossover might conjugate curves'n'slopes to the best ( for that project).
But also the low ( relatively to woofer+tweeter applications) crossover point
suggests that the FR is called to work from 3-500 Hz and up, so it has to do a double job, plus the distortion rise which is much more audible than that produced by the woofer ( which, I read, can reach 10% and still be 'forgiven'),
so power factor is another thing .
Basing the project on a powerful woofer means nothing; looking for a powerful FR has some issues, as 3" is already 'big' for correct HF dispersion.
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Old 15th January 2013, 09:13 AM   #6
xrk971 is offline xrk971  United States
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There is the Manzanita OB design that uses the Peerless 830669 12 inch woofer (89 db) and a Vifa tc9fd 3.5 inch full range. That may be a good starting point for cross over design. The latest version of the design is on post #455 of this thread: Fast, fun, Inexpensive OB project

I know you are looking for a FAST design but this seems to have shared requirements. The crossover from the thread is below. FWIW, I think that an active crossover and bi-amp to avoid the rather bulky and expensive power robbing speaker level crossovers.

Click the image to open in full size.

Last edited by xrk971; 15th January 2013 at 09:21 AM.
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Old 15th January 2013, 09:28 AM   #7
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OB has another design approach, as explained by Pano in the starting posts
of that thread.
Also, what does 'power robbing' mean ?
Did you know that attaching another amp to the mains causes some 'power suction' ?
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Old 15th January 2013, 09:40 AM   #8
xrk971 is offline xrk971  United States
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Not sure what you mean by power suction? Sag in line voltage when amp hits deep bass peaks?

Power robbing refers to how the crossover resistively burns of amplifier power - that is the function of those big power resistors in the crossover. They can get hot. Rather than amplify unneeded audio power just to throw it away in the power resistor, shape it first with active crossover and send to two separate amps for more efficiency. Cost is the other big reason as the 20 mH coil is close to $40 ea. By the time you add up all the costs for the crossovers, an active crossover and second T amp can be had for about same price with the advantage of being adjustable in crossover point. If you go with DSP, the slope can be selected too.

Last edited by xrk971; 15th January 2013 at 09:42 AM.
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Old 15th January 2013, 10:29 AM   #9
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power loss is strictly correct, but a misleading reason to advocate active as a cure-all. Ive never got a 5W W/W warm let alone hot at domestic volumes, and id thank that fuse action if my OP devices ever let go. If loss is a third, it matters a great deal what the input power is before the loss can be viewed as significant. Its only voltage thats lost anyway.
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Old 15th January 2013, 12:49 PM   #10
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Quote:
Originally Posted by mondogenerator View Post
power loss is strictly correct, but a misleading reason to advocate active as a cure-all.
Absolutely. You can boost the bass (active) or cut the treble (passive), but when you run out of excursion on the bass, you run out of SPL. It will happen at the same power level either way. I use active because it is sooo much easier to implement and has other advantages.

Bob
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