Prevent Small Full Range Driver From Overextending Itself - diyAudio
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Old 16th November 2012, 07:00 AM   #1
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Default Prevent Small Full Range Driver From Overextending Itself

What is the best way, aside perhaps from using a filter, to prevent a small full range driver, such as the Fostex FF85WK from overextending itself?

Open baffle, or small sealed box, say 1 Liter to provide back pressure?

The idea is to prevent any bass from emitting from it, or the driver making any effort to reproduce same.... wondering if it is possible just with enclosure design (or lack thereof in the case of OB) without a capacitor inline....or is a capacitor the only way.... I can't use an active filter.... thanks!

The goal would be to allow said driver to handle loud rock music if required.
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Old 16th November 2012, 07:19 AM   #2
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You would need to do a sealed box to get away with no extra filter (sealed acts as a 2nd order HP), but if you want any base a cap in front of the amp to act as a high pass filter.

dave
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Old 16th November 2012, 07:51 AM   #3
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Thanks Dave! I thought an active filter goes in between the preamp and the amp... as mentioned I, unfortunately, can't use an active filter (because the source is a receiver without pre outs and fed via HDMI).

So, looks like a passive filter (a cap at least and maybe an inductor for 2nd order), or, the small sealed box, which would hopefully prevent damage to the driver just as well as the filter (?).

I see on your screenshot you modeled at 0.34 liters... gosh, that's small. And, I don't know if the hump around 300 Hz would be problematic in practice (?). So many questions.
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Old 16th November 2012, 08:34 AM   #4
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Problem is the bass power would still reach the coil in the driver. It would only be made into heat instead of sound. So you'd have to choose a driver that could take the punishment and you wouldn't be able to play as loud as with a filter.

I'd be interested to hear/see if anyone has actually made a setup augmented with bass drivers like this (nonfiltered).
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Old 16th November 2012, 08:49 AM   #5
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Quote:
Originally Posted by Ben_Tech View Post
Thanks Dave! I thought an active filter goes in between the preamp and the amp... as mentioned I, unfortunately, can't use an active filter (because the source is a receiver without pre outs and fed via HDMI).
Put the cap in the tape loop.

Quote:
So, looks like a passive filter (a cap at least and maybe an inductor for 2nd order), or, the small sealed box, which would hopefully prevent damage to the driver just as well as the filter (?).
After the amp before the speaker is not very effective because of the rise in impedance at resonance.

Quote:
I see on your screenshot you modeled at 0.34 liters... gosh, that's small. And, I don't know if the hump around 300 Hz would be problematic in practice (?). So many questions.
I would target 0.707 Q or a bit higher.

dave
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Old 16th November 2012, 08:58 AM   #6
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Originally Posted by Squeak View Post
It would only be made into heat instead of sound. So you'd have to choose a driver that could take the punishment and you wouldn't be able to play as loud as with a filter.
That much heat could be a problem in such a small sealed space made out of wood.

Most full range drivers can't take much punishment which is why I'm thinking about this at all.

However, I firmly believe "less is more" with audio in general, thus hoping someone has tried an unfiltered full ranger in a small cabinet or some other creative enclosure to curtail it sufficiently to do a FAST of sorts without a filter on the FR. I know most FAST use bi-amping with an active filter before the amps. However, I can't do that.
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Old 16th November 2012, 09:22 AM   #7
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Quote:
Originally Posted by Ben_Tech View Post
I know most FAST use bi-amping with an active filter before the amps. However, I can't do that.
Bi-amping just makes it easier. In our original FF85k + 2 x EL166 the goal was passive all the way and we achieved results beyond expectation.

The current project is uFonkenSET + twin Peerless woofT. It works great bi-amped, working on the passive.

The same will be true of our latest, a passive XO.

Click the image to open in full size.

If you are doing a FAST you don't need to worry about producing bass,

One of the keys is to get the XO high enuff. The original MTM was about 350 Hz with the FF85s resonance peak heavily attentuated by an aperiodic TL, the same line is being used in effort 3.

There is a trade-off. Sealed produces a natural 2nd order (with the FF85 lower than you'd like) but any passive has to deal with a big peak. Aperiodic flattens the impedance but the filter can work better. If well done it also has advantages wrt reducing boxy colourations.

dave
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Old 16th November 2012, 04:31 PM   #8
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I remember you showing a before and after of the Fs peak with sealed and aperiodic, Dave. Was really slick. I gotta leane how to do that.

Ben, as Dave has been saying, the impedance resonance peak of these fostex are quite high q and magnitude. A single cap doesn't offer much power handling right as the peak because the cap is ineffective at such high impedances. In the only FAST I've ever done, I was pushed into 2nd order on the FE83eN. I don't know any of the fancy box work to alleviate this issue.

Alternatively to 2nd order you can use an LCR impedance compensation network if really stuck on 1st order. But this is more parts, more complex, more expensive, and less power handling. It would be a true 6db/oct transfer function though. Combined with sealed roll off may be 18db.
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Old 16th November 2012, 05:34 PM   #9
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Quote:
Originally Posted by tuxedocivic View Post
I remember you showing a before and after of the Fs peak with sealed and aperiodic, Dave. Was really slick. I gotta leane how to do that.
Original Tysen, IIRC.
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Old 16th November 2012, 07:56 PM   #10
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Originally Posted by planet10 View Post
Click the image to open in full size.

One of the keys is to get the XO high enuff. The original MTM was about 350 Hz with the FF85s resonance peak heavily attentuated by an aperiodic TL, the same line is being used in effort 3.

dave
Thanks for the responses guys (Dave).... I have a question about that design you pictured.... it looks as though the chamber for the FF85 is open at the back? I assume you are heavily stuffing that small space to make it aperiodic? Or is it more complex than that?

You are also correct that 200 Hz on the lower end is lower than I'd like... 300-400 would be better. Can the small aperiodic space achieve that or are we still needing a filter?

Any drivers better than FF85 for this application? I.e., still offering "the best" mids and highs but higher acoustic roll off?

Is the heat dissipation issue with the amp bass power still being applied too much for safety?

Thanks again.
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