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Multi-Way Conventional loudspeakers with crossovers

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Old 8th October 2012, 02:43 PM   #21
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the twetter is 4 ohm i linked you the 8ohm one because i can't find the datasheet of the 4 ohm but the model is the same
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Old 8th October 2012, 02:44 PM   #22
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how can i find the Qts of the woofer?
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Old 8th October 2012, 03:26 PM   #23
T101 is offline T101  Bulgaria
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It doesn't matter. Go for a nice looking 30L box filled with recycled cloth (main content wool) wadding in a see-through cloth. It looks like a loose pillow.

Apply the active crossovers and have a listen. If it is too dark and boomy, buy 10-12 cheap ~100uf electrolitic capacitors (not less than 20V rating) per speaker and try different arrangements as follows:

In all cases you connect two capacitors in series with opposite polarity to each other in the woofer + terminal.

With polarised eclectrolitic capacitors 100+100 in series is again 100uf - remember that.

When you connect two capacitors in parallel the capacitance doubles.

The required capacitance in series with the woofer will be somewhere in the 300 to 600 uf range.

So you should start with 6 ~100 uf capacitors grouped 3 by 3 in parallel and the two 3's in series to each other.
If the capacitance is too low, the speakers will sound tiny, if it is too much, you won't be able to hear difference. Make it just right and it will sound sweet with well articulated bass.

The trick with the capacitors is for overcoming the nasty peak in the LF that every system with a driver with Qts above 0.7 produces. Visaton, KEF and etc. use that approach shamelessly for the past 30-40 or so years...

If I am wrong with the capacitance values for 4 ohm driver, let some of the more knowledgeable members correct me.

Best regards and go for your extremely cheap and hopefully good sounding system!

p.s. since I like cone tweeters and since this one is regarded in a magasine, I wouldn't hesitate to indulge myself with a $~50 system that sounds funnily good

Last edited by T101; 8th October 2012 at 03:29 PM.
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Old 8th October 2012, 04:52 PM   #24
balerit is offline balerit  South Africa
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Two 100uf caps in series = 50uf.

Mac

http://novelbooks.weebly.com/crossovers.html

Last edited by balerit; 8th October 2012 at 05:13 PM.
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Old 8th October 2012, 04:57 PM   #25
Dissi is online now Dissi  Switzerland
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Quote:
Originally Posted by davidel94 View Post
how can i find the Qts of the woofer?
Measuring Loudspeaker Driver Parameters
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Old 8th October 2012, 06:52 PM   #26
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That's a great link for measuring speaker parameters, but very technical and most people don't have the needed equipment to do this. There are some "old school" ways to get a general idea of the speakers characteristics:
Gently push on the woofer cone (evenly) and "feel" how stiff the suspension is. It takes experience to judge if this feels "stiff" or "loose" (or compare to another woofer of known specs) This helps choose a box size ("stiff" needs larger box and lots of stuffing material) Now short out the speaker wire terminals with something metal (key) and "feel" the stiffness again. It should feel a bit stiffer and more "damped". The more change you feel, the better electrical dampening. A stiff speaker and poor damping is not a good thing. Now make a box (see link for picture) and "feel" how stiff the woofer is when sitting on box. These are very crude methods, but easy to do and cheap! Finally, you can use a test CD (or make one) that has different tones and check for strong peaks (both free air and when sitting on box) This will get you close enough to choose a box size
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Old 9th October 2012, 10:02 AM   #27
T101 is offline T101  Bulgaria
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Quote:
Originally Posted by balerit View Post
Not sure it applies to polarized electrolytic capacitors. Even if it does, he will arrive at the required capacitance in the end. Either way, this is just for cost saving. I don't think it is a good idea to buy 300 to 600 uf non polarized non electrolytic foil capacitors as they will cost more than the system. Reputable brands have done it this way with the opposite polarity electrolytes.

If he finds non polarized electrolytes it would be better and the series connection will be avoided.

Best regards!
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Old 10th October 2012, 12:14 PM   #28
T101 is offline T101  Bulgaria
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Here is the science behind that:

Can you make a non-polar electrolytic capacitor out of two regular electrolytic capacitors? - Electrical Engineering

Quote:
If two, same-value, aluminum electrolytic capacitors are connected in series, back-to-back with the positive terminals or the negative terminals connected, the resulting single capacitor is a non-polar capacitor with half the capacitance.
From:
http://electrochem.cwru.edu/encycl/m...4-appguide.pdf - linked in the above article.

So, you are right and I am right as well

For those who got confused by the capacitor saga.The series capacitor is used to cure the output of a system that not only has Qtc of over 0.707, but is employing a driver with Qts of over 0.707. Then the peak in the response is very exaggerated:
Click the image to open in full size.
Allot more than the violet curve and falls steeper below Fc.

Quote:
Qtc:
Qtc is the total Q of the speaker in an enclosure including all system resistances. A Qtc of .707 is the most common and generally produces the flattest frequency response with approximately a 6dB/octave rolloff. Higher values of Qtc will give a peak in the output with a sharper rolloff. A lower Qtc will start to roll off earlier and will roll off at a slower rate. If you don't know what Qtc you need, start with a Qtc of .707.
Source: Speaker Box Calculations

The Qtc of a system cannot be lower than the Qts of the driver.

Further, there is a common misunderstanding that high Qts drivers are only suited for acoustic suspension/sealed box. This is wrong.
- relatively low Qts, heavy membrane and low Fs drivers are best suited for sealed enclosure! And the EBP (Fs/Qes) is the best other reference value whether a driver is suitable for sealed enclosure. A 0.28 Qts, 0.3 Qes driver with 17 Hz resonance will have an EBP of 59 - merely above the absolute minimum for vented enclosure... which makes it better suited for sealed despite the lowish Qts.

High Qts drivers are suitable for midranges, open baffle, H and U frame (vintage radio receivers), infinite baffle and lossy boxes (aperiodic/labirinth/vibrating thin walls).
There is no contradiction between good sounding driver and high Qts, so some designers and hobbyists use high Qts drivers in conventional sealed enclosures by making use of the capacitor trick.
The capacitor trick as already said cures the nasty peak above and around Fc in the response, makes the roll off less steep and can even bring F-3db below Fc.

So again, go for it with the uberly cheap drivers and share the results

Best regards!

Last edited by T101; 10th October 2012 at 12:19 PM.
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Old 10th October 2012, 02:07 PM   #29
balerit is offline balerit  South Africa
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@T101: Not sure what you mean but to clarify equal value caps in series halve while the overall voltage rating doubles. To make a bipolar cap connect the 2 positive leads together.
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Old 10th October 2012, 03:45 PM   #30
T101 is offline T101  Bulgaria
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Quote:
Originally Posted by balerit View Post
@T101: Not sure what you mean but to clarify equal value caps in series halve while the overall voltage rating doubles. To make a bipolar cap connect the 2 positive leads together.
In the article it is stated that those are two common false beliefs.
The voltage rating remains the same as the one of single capacitor.
And there is no rule whether you connect positive or negative terminals together since it is AC current.

The capacitance halves indeed

Anyway, it seems that davidel94 got confused and stopped posting.
A young man should never be discouraged to put a cheap ''8 in a 30L box and see what happens!
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