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Old 16th April 2012, 09:34 AM   #1
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Default First desktop OB speaker project (need advice)

Hi all,
I had the OB idea from gainphile's blog and I have read many post and article.
I have to confess I skip the theory part at this stage.

I would love to try the open baffled speaker for my home theater setup but I prefer to try it with smaller project.

Because the speaker need to fit my current amp (Qinpu Q-5, 2x2W @ 8 ohm).
2W smaller fullranger driver become my limitation.

I have two different idea, one is each OB have one 2W driver.
And one OB have two 1W driver.

The structure will be super simple since it is my first DIY project.
I attached the photos.
I am not a draw software expert, I only use powerpoint to finish it.

I live in Townsville, QLD Australia, so jaycar probably is my easy option.
I am happy to purchase oversea, but the postage will be another problem.

Any advice about which driver or the prototype need some change will be appreciated.

Cheers
Colin
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Old 16th April 2012, 10:30 AM   #2
borispm is offline borispm  Hong Kong
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Hi Colin,

I have tried to create a compact passive open baffle speaker before, and here's some comments:
1. The power handling of the driver should not be the top concern, but you need to focus more on the sensitivity since your amp is a low power one... And you can feed a driver with power handling of 100W by a 1W amp as long as it's not clipping.

2. Drivers with high Qts is preferred, say as close to 1 as possible.

3. Speaking from the experience with a passive design, I would say you need a way more powerful amp to deliver reasonable SPL and bass (since the passive circuit works by attenuating the mids and treble instead of boosting the bass).

Regards,
Boris
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Old 16th April 2012, 11:43 AM   #3
sreten is offline sreten  United Kingdom
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Hi,

for the theory have a look at : http://www.quarter-wave.com/OBs/OB_Design.pdf

The potted version of the theory is you will get a baffle hump around 3KHz
due to the 10cm baffle width and below ~ 1KHz suffer 6dB/octave baffle
loss progressively down into the bass. This isn't just a driver roll-off.

Baffle loss throws away driver excursion. If your driver has 12dB baffle
loss when you EQ back to flat you have effectively quartered its excursion
capability because the baffle loss still occurs, and the driver needs to move
4 times as much as it would in a sealed box for the same SPL level.
The lower you go, the worse it gets.

For a 10cm wide baffle you'll be hard pushed to go any lower than
200Hz to 300Hz for this reason, and as Boris implies, you'll need a
lot more power and a small driver with good excursion capability.

rgds, sreten.

A typical 3" full range has a Vas of about 1 litre. In a 2 to 3L sealed box it will produce oodles
more bass than a small open baffle, with progressively much higher efficiency in the bass also.
http://av.darcher.com.au/index.php?r...&product_id=26
Also see : http://www.zaphaudio.com/audio-speaker18.html
But for computer desktop speakers do all the EQ in software, don't build the filters.
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When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 16th April 2012 at 12:03 PM.
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Old 16th April 2012, 11:45 AM   #4
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Hi,

did you simulate your baffle with "Edge"? With only one driver I guess you'll get problems with the first floor reflection which will be around 1.5kHz to 2kHz. This first interference is critical because it will be recognized. With two drivers you have two different frequencies the first floor reflection interference occurs.

Greets
Sven
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Old 16th April 2012, 02:15 PM   #5
borispm is offline borispm  Hong Kong
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Hi Colin,

One more tip: Try a wider baffle or add wings as 10cm is really way too small. Mine's 20cm and I've got an efficient 6" Aurasound woofer there, and the sensitivity of the system's just around 80dB/W @1m. Some sacrifices must be made...
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Old 16th April 2012, 02:49 PM   #6
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Thanks for all's quick reply.
@boris
Thanks for sharing your experience, I will try to remember and revise my prototype

@sreten
I have downloaded the theory PDF from martin king, but can not finish it due to the difficult terms are everywhere.
Thanks for the briefing, but I still kind of half understanding only.
Need to do more reading.

@Sven
I had the software "Edge" when I googled around.
Honestly, I was fooling around by press this bottom and change that value.
I have no idea what I was doing.
Again, prob need to read the tutorial first.


Sum up.
I feel like the size I design (or create, since its very close to rubbish) is not close to realistic OB speaker.
The reason I use that size just because of my messy desk did not have too much space.
And my amp is super tinny. I dont think it can run some decent speaker well.

Thanks again for all,
I will keep all the advice in mind and seeking the solution.
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Old 16th April 2012, 07:52 PM   #7
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Hi colinwen,

I did a simulation in Boxsim to get a rough result for the 2-driver-approach. I attached the response curves and the correction circuit. This is only 30 minutes I've invested so there's still some fine tuning to do .
The drivers are Vifa / Peerless 9 NE 125/4 (NE95W-4) and far from cheap (here in Germany). They do have a fs at 98 Hz which is pretty rare in the 2"-driver-field. The are on 25 cm and 12 cm on the baffle. The whole system is missing a big driver for the frequencies below 500 Hz.
Just to get the idea...


Greets
Sven
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Old 16th April 2012, 11:33 PM   #8
sreten is offline sreten  United Kingdom
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Hi,

FWIW I know its not DIY but these are what I use with my laptop desktop :

Logitech X-140 Speakers at $29.99 in Speakers

Click the image to open in full size.

2.5" full range, 3" passive radiator, 2.5 W rms per channel, sounds like more
in the midrange due to active EQ, and a very effective adaptive bass end.

Best budget PC speakers I've come across by a long chalk, I've tried a few.
Not perfect, but they do some things very well, and no obvious failings.
When i bought them on a whim, looking for a printer, I was amazed
how good they are, especially the bass end, its very surprising.
Critically they seem to be very well voiced for human voice.

One thing I know, not a chance of building anything comparable for the price.

rgds, sreten.

Listening to them at the moment, still massively impressed for 15.
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When your only tool is a hammer, every problem looks like a nail - Abraham Maslow

Last edited by sreten; 16th April 2012 at 11:52 PM.
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Old 17th April 2012, 04:13 AM   #9
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@ Sven,
Thanks, I will try to understand the theory about size of buffle and feature of driver.
I dont mind if the bass is not strong enough. Lately I start to listen to more soft music.
I will see what driver I can find here in Australia have similar feature you recommend.

@sreten,
In fact, I have a set of Altec-Lansing MX5021.
But I feel like to switch to small stereo system instead of computer speaker.
That's why I am thinking about a OB DIY project.
Still appreciate your info.

Colin
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Old 17th April 2012, 07:19 AM   #10
kctess5 is offline kctess5  United States
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Here's an idea. If you could bi amp it you may want to consider having a high quality full range driver that will work with your amp, such as a high sensitivity fostex driver or something and a high excursion driver that would need a larger amplifier of lesser quality.

I'm thinking a sort of 1.5 way system with something like this dayton with crazy high excursion to handle the lower frequencies, crossed in at whatever frequency the other driver will start to roll off. You could power it with whatever, I've had good luck with the super cheap Lepai T-amps on not too sensitive drivers.

This still wouldn't have much bass, but it will have a lot more than one low excursion high efficiency driver for sure. Pretty nothing you do with a baffle that small other than having a portion of it an actual box will result in any appreciable amount of bass though, a sub, even a small one, would be required depending on the kind of music you want to listen to on it.

Also a way you could keep around the same size but push the rolloff down in frequency a bit would be to do a U shaped baffle, even if it just went as far as the back of your driver it would help.

Good luck! I do love open baffle, I've been listening to some Fostex Fe108e sigmas (courtesy of planet10) on a makeshift baffle with my sub running for a while and I'm loving the soundstage, despite the massive gap in midrange from the configuration. If you don't mind some serious bass deficiency its totally worth it
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