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Old 14th February 2012, 04:52 AM   #1
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Default 1st project requires assistance

I've been a member for a short while now and have done some basic research but I haven't quite been able to find the answer I'm looking for. I'm buidling an extremely basic boombox with a velleman 2x5w kit and some 3 inch full ranges salvaged from a set of harman kardon 19.5 computer speakers. I've determined the volume of the factory housing is roughly 0.032 cubic feet. I was going to build bass reflex enclosures out of 1/2 inch plywood. Problem is I cannot find any info on the drivers to determine port tuning and crossover frequency. The factory put what seems like a high pass filter on them with a 1000uf cap and a 1.2 kohm resistor and a small green led??? Can I just replicate these filters with my own components without the led? Also what should I tune the port to? Will 1/2 inch pvc work for the port? I'm not looking for audiophile quality. Just a decent sounding mp3 boombox.
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Old 14th February 2012, 05:05 AM   #2
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I made two double reflex boxes with similar speakers ,and they sound good .
Yes , I used one inch ( ext diameter ) ducts from 1st to 2nd chamber ( second chamber being half volume than 1 st chamber where the speaker is located ) and some stuffing helped . Also shape mattered , being an half-trapezoid (base) prism .
Regarding the nonsense of the capacitor ( probably it's the output coupling cap of the amplifier ) and the led and resistor ( more probably is the power ON indicator ) that act as an high pass filter , and the more non-sense that you are looking for good bass and talk of limiting it...
Check the circuit again !!
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Old 14th February 2012, 05:14 AM   #3
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Hahaha. Okay thanks for clearing that up. I wasnt sure if I'd be wasting power trying to reproduce low frequency bass with the 3 inch drivers. But yes I want decent bass response. What am I exactly looking for concerning the filter circuit? I'm pretty new when it comes to audio electronics. Also what should I tune the box to? 80hz?
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Old 14th February 2012, 05:39 AM   #4
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the fact is , you shouldn't look for any 'boom' as in a boom-box ,but a well balanced sound.
The tuning would then be close approximately to the resonance frequency of the speaker itself , probably and hopefully an octave lower , so 80 Hz may be right .
Put some acrylic stuffing inside the box ( like the one you find in some pillows ) to reduce internal reflections and to allow a virtual volume enlargement ( around 20 %).
An high-pass filter might help to decrease distortion at high volume ,to prevent the driver to go over its excursion . The simpler ( and often..the better ) is to reduce the value of the input coupling capacitor of the amplifier . So get some good capacitors ( not electrolytic ) and try....it depends on the input impedance ( also...resistance ) of the amplifier ,to make an C-R filter , formula is Fh=1/2πRC...values you'll need will range from few nanofarads to 0.1 nF .
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Old 14th February 2012, 06:17 AM   #5
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Yes a well balanced sound is exactly what Im looking for. I was going to glue foam to the insides of the box but acrylic stuffing seems way cheaper. Im going to go with 80hz for the box. Looks like its going to be a port length of around 2.25 inches with 1/2inch ID tubing. I wish I knew the resonant freq of the driver. Yes I noticed a some distortion near max volume. I would like to reduce this if possible. I dont know what a C-R filter is but Im assuming it filters the incoming line signal from the mp3 player? That seems more efficient too? Ill have to research that. Thank you for the awesome help.
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Old 14th February 2012, 06:22 AM   #6
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Also if you could explain what the values are in that formula that would be awesome. Sorry like I said I'm very new to diy audio projects. Thanks again.
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Old 14th February 2012, 08:07 AM   #7
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Well , I guess you could not use the computer just to chat with friends
If you apply the magic words to any research motor... ( hint :high pass filter )
the world would magically disclose...
But , I also guess that if distortion arises , it is not to blame the poor speaker ;
you should look at more powerful drivers ...I always blow things like that
The capacitor at the input of the amplifier , together with the resistance , acts as an high pass filter ; they are calculated do give sufficient bandwidth to the next stages without significant phase rotation ;usually the min and max points are around 10 Hz and beyond 20 KHz ; by just changing ( with lower values )
the input capacitor on the amp you rise the HP frequency .
Regarding the box construction...it's another thing ! Maybe you should go with the foam ; it has to stay firm...because stuffing and placing of it ,it's another thing
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Old 14th February 2012, 01:38 PM   #8
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Yes I think its time for me to research what the individual components on the amplifier board actually do. I think then I'll have a better understanding of what changing input capacitors actually does. But just to verify. The ONLY component I'll be changing is the input capacitor. Correct?
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Old 14th February 2012, 01:42 PM   #9
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Also how do I determine which capacitor I need for a cutoff around 50-60hz?
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