MY Alpair 7 QWTL design - 'Krypton' - diyAudio
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Old 11th August 2011, 12:29 AM   #1
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Default MY Alpair 7 QWTL design - 'Krypton'

Hello and thank you for looking at my design here. This is part of a range of speakers i am developing to put up onto my own website once i am happy with how it looks. This is the pair created using the Alpair 7 Gen.2 from mark Audio, so thought i would share with you all. All feedback is appreciated.
Now my purpose was to produce a single driver, full range speaker that wasn’t too large, but could produce a fair amount of power in the bass region (22 years old – enjoy some drum n bass). It didn’t need to work beyond 85dB however and in most cases, would be run day to day around the 70-75dB region.

The Speaker i call ‘Krypton’ As i am naming them after noble gases. It is a folded transmission line. I have created the pipe to the same Sd as the Alpair 7 (50cm2) and made the pipe to a quarter wavelength of approximately 40hz (2.14m). To dampen out the natural resonances above this, i added a extra chamber at the from approximately 35cm long, with a internal volume of about 1.7l of air. I have then through testing stuffed it with aquarium filter wool i had at hand as well too satisfied with the sound output. 300Hz was the worst, of which the dampening with wool brought it down by 8dB!

Anyway, enough of the chat, here are some picture for you. Wood used was 6mm MDF, and the black material you see is this akasa sound deadening material to quieten the internal of a PC. Have my fish keeping and PC modding hobbies as well, i used things from both of them.


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This first pic is of the internals of the enclosure, before wiring and the padding material was fitted.

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I am very very happy with the outcome of them. They actually have a slight raise in the bass of about 3db around 50 to 60hz range, it drops back to the average SPL level for 40HZ and then at 40hz there is a 2nd order HP passive filter added. In my case it is actually done internally by my AMD HD 6870 Graphics card which outputs the sound to my amp. Tested and works well on other designs.
I mentioned in the previous post about the excursion and adding a filter, especially for Transmission lines and Horns. Below will be images of Hornreps which i used to help me, and show exactly why this is!

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As you can see on this picture, the excursions jumps up and down and hits a low point at the resonant frequency of the pipe it is in, 40hz. It then jumps very quickly to a peak at approximately 28-30hz of 5mm!!!! This is at only 2.83v (approximately 2.2W), so not a great result. Now each order is going to drop 6db roughly per octave, but the next octave is 20hz, so at 30hz it will be around the 3db mark per octave, where that huge spike is.

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Dropping the voltage so that is equates to a 6dB drop overall, we can see the drop is now to about 2.3mm if it was driven at 2.83v. This still for me isn’t enough of drop, and so i ended up choosing the second order option. Considering that the Transmission line will drop down now at 6db a octave, and we have achieved 40hz, it seems reasonable to go 2nd order as well. This looks like:

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Finally to finish off, 2 more pictures. One showing the Impendance and the other being what the response looks like from the pipe, if you discard the use of a chamber and stuffing.

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Click the image to open in full size.

I hope you like the design and as said up top, any advice, tips, thoughts or questions, feel free to ask.

Paul
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Old 11th August 2011, 01:17 AM   #2
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Hi Paul,
Great looking build project, well done!
So you're 22 years old? This is excellent work for a young Diy guy!

See if you can bring your audio and music friends to come and listen. Maybe they can feedback on this thread.

Please let me know when your website is ready. The Markaudio site is under new development. I'd like to make a link page for project makers.

Cheers
Mark.
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Old 11th August 2011, 01:29 AM   #3
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Thank you for your quick post there Mark and your positive feedback. I started properly trying to build speakers 2 years back after a set of THX certified loitech speakers still didn't satisify what i was looking for.

Last year i made a pair of studio monitors for a local DJ and friends heard them and was impressed. Since i have kept it low key until i have them all ready to play out. I test using both a Onkyo amp only a year old, but also a Sanyo Class AB from 1978 i got my hands on.

Will let you know when website is up. I am actually wait for yours to come back up so i can have a nosey Hopefully if things go well i will be happy with the coding and layout i have made in the next month of two.

For anyone looking at possibly building a transmissionline speaker like the above, the 40Hz 2nd order crossover would need a 1,100uF non-polar capacitor and a
14.3mH inductor.

If you can't get exact to those, choose a lower rated capacitor or higher rated inductor. This will produce a higher cut off frequency.

Paul
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Old 12th August 2011, 11:21 AM   #4
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nnnnnnniiiccceeeeee
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Old 26th April 2012, 12:21 AM   #5
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Well for some feedback i took these and kept these at work for 2 months. Let people play different music through the PC, and run them from a TA2024 amplifier. Not a single bad thing was said.

One of my elder colleagues, who has build his own speakers in the past, and fitted out cars, stated he feels there better than his £700 pair of pioneers. Another person asked where the subwoofer was placed!

A friend of mine then took them for 2 weeks, and is now asking what price i would sell them for, and if they could possibly be finished in metallic black. So overall i have had positive outcomes.

Will update this thread again soon, as i have myself a Dayton EMM-6 condenser mic with calibration file and phantom amp, along with some software to meaure frequency response. Will test in both in room and outside in the garden (64x64ft open space) and put up the results on here

Paul
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Old 26th April 2012, 02:43 AM   #6
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Hi Paul,
Great project. Glad its worked out well and your friends have also enjoyed using your system.

One suggestion, invest in a more accurate higher spec. mic. While the Dayton is Ok for general experimentation, you could do much better using an Earthworks M23 (or similar):

M23 | Earthworks Audio

Cheers
Mark.
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