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Electronics DIY Boombox Project

Discussion in 'Modding' started by mattthegamer463, 14 Jul 2008.

  1. mattthegamer463

    mattthegamer463 What's a Dremel?

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    I'm building a boombox using some old parts from a old boombox from 1983 that went kaput, but still had many good components in it. The units only audio source will be a stereo line in though. The speakers I removed from the original boombox are JVC 8 ohm 15W main speakers and JVC 8 ohm 15W tweeters. I've been looking for a speaker amp circuit to use and I found this from a local store:

    http://sayalhobbies.com/CatalogDetail.asp?link=205537

    They list the specifications as this:
    So according to this using 8 Ohm speakers will result in 12W of sound per channel. Since the speakers are 15W they won't blow and I don't need the ultra high volume anyway, however I guess I can't run the main and tweet on the same channel, right? I recall from engineering class that when two resistors are in parallel, their resistance becomes less than the lowest resistor, as calculated by

    Rtotal = 1/[(1/R1)+(1/R2)+(1/R3)...]

    So then does this law apply with 8 ohm speakers? Would putting the main and tweet in parallel on the 20W channel be 4 ohms of resistance rather than 16 ohms?

    If that stereo amp is not suitable for my project, would this one be better?

    http://sayalhobbies.com/CatalogDetail.asp?link=204270

    But with 50W of power would I then be running the risk that if someone turns the volume to 10 that the speakers will blow? I'd rather be too quiet than blowing my speakers.

    Thanks for the help.

    EDIT

    im an idiot. this would be better suited to go in Modding instead of tech support.
     
    Last edited: 2 Aug 2008
  2. spazmochad

    spazmochad Minimodder

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    Parallel will derease the load, in your case giving 4ohm. Wiring in series will do the opposite. Are you using a crossover between the main driver and the tweeter?
     
  3. cpemma

    cpemma Ecky thump

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    Advantage of a bridged amplifier (first link) is that it gives four times the output from a given supply voltage compared to single-ended, so they're great for getting plenty of volume from a 12V battery, in-car applications. One problem is that it's more difficult to add a headphone socket if required (there's no "speaker ground" common to both channels to suit the headphone 3-pin plug).
     
  4. modgodtanvir

    modgodtanvir Prepare - for Mortal Bumbat!

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  5. mattthegamer463

    mattthegamer463 What's a Dremel?

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    Do you mean adding a headphone jack as input or are you talking about using the amp to drive headphones? If you're talking about the latter then theres no problem, I don't need a headphone out on this thing.

    I'll draw up a wiring diagram in a second to see if my idea will actually work. I assume that I will need a crossover even though I was debating about really needing one, but I'm sure they are crucial to good sound, yes? I think I have a pair of speakers I don't care about that I could pillage the crossover from and see if it will suffice.

    [​IMG]
     
    Last edited: 15 Jul 2008
  6. lost_modder

    lost_modder What's a Dremel?

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    I would just put a capacitor inline for the tweeters. That's all the "bass blockers" in stores are. The main speakers won't be able to do really high stuff anyways and at 12w they will be just fine.
     
  7. SteveyG

    SteveyG Electromodder

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    An 8 ohm tweeter and 8 ohm midrange/woofer won't place a 4 ohm load on the amplifier - this is only the case for pure resistance, not impedances. If you connect the two in parallel, it'll still be 8 ohms, because these impedances are valid only for the centre frequency of the driver.
     
  8. mvagusta

    mvagusta Did a skid that went for two weeks.

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    An 8 ohm tweeter & 8 ohm woofer will only give an ~8 ohm load if a crossover is used, or if piezo tweeters are used, which are like capacitors.

    Using a small capacitor in series with the tweeter, say ~4uf is often ok, but it depends on the tweeter & woofer.. it all should match, and this method puts the two speakers out of phase, so the sound quality isn't that good.

    I like using 12db crossovers instead, as reversing the polarity of one of the speakers has everything in phase :thumb:
     
  9. mattthegamer463

    mattthegamer463 What's a Dremel?

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    I finally got a hold of the speakers I had.

    [​IMG]

    [​IMG]

    [​IMG]

    Apparently the regular output is 7W but the max is 40W, so this crossover should work fine I think. Thoughts?
     
  10. cpemma

    cpemma Ecky thump

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    You're better using the crossover with the speakers it's been made for, but it should work with other 8R speakers.
     
  11. mattthegamer463

    mattthegamer463 What's a Dremel?

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    Normally I would, and I had debated that fact for a minute, but these ones really have NO bass whatsoever, and the ones I want to use are close to twice the diameter, so they cover the bass and midrange much much better than these ones. I personally don't like these ones at all, they sound horrendous on their own.
     
  12. modgodtanvir

    modgodtanvir Prepare - for Mortal Bumbat!

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    Excuse my noobishness but could you not achieve the same effect with bigger capacitors?
     
  13. mattthegamer463

    mattthegamer463 What's a Dremel?

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    What? Do you mean just using caps in line with the speakers instead of a premade crossover?
     
  14. cpemma

    cpemma Ecky thump

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    No. An ideal crossover is designed to send a mix of audio frequencies to the appropriate speaker and at the same time give a level frequency response with no peaks or dips at the crossover frequency. In the very simple Realistic design the capacitor is limiting the low frequencies at the tweeter, the inductor is limiting high frequencies at the woofer; bass extension is a function of woofer and cabinet design.
     
  15. mattthegamer463

    mattthegamer463 What's a Dremel?

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    My Sketchup plan for the exterior, Which I'm debating materials at the moment. Most speaker enclosures are made of plywood, particle board or MDF. I would like to use MDF but its very heavy. Would anyone have any suggestions on the best type of material to use that is relatively light, strong, paintable, and has good acoustics?

    [​IMG]

    [​IMG]

    [​IMG]


    A 1x1x1 meter box for size comparison. The unit is 500x390x160mm.
    [​IMG]

    Also, to clarify before anything explodes, running my 15W JVC speakers on my cannibalized crossovers using this ( http://sayalhobbies.com/CatalogDetail.asp?link=205537 ) amp will not result in an explosion?

    I really have very little clue about what I'm doing here so I want to double check before I throw away money. Questions are free.
     
    Last edited: 21 Jul 2008
  16. modgodtanvir

    modgodtanvir Prepare - for Mortal Bumbat!

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    Awesome dude. I see I have a rival on the design side of things here :p

    I would have thought 1cm thick plywood would make for a nice and lightweight, but strong exterior. I think, for acoustics, audiophiles prefer natural hardwood speaker enclosures, but I'm not sure how relevant that is... It'll need to be airtight. Be sure to fill up the cavity with some type of foam - it helps the acoustics.

    That amp does look rather good. I wish I could find something like that in the UK... it should work with your drivers. For my set up, I found that my speakers had "4 Ohms Impedance" and 40W Max on the sticker, so I assumed it would work... any such labellage on your old enclosures?
     
  17. mattthegamer463

    mattthegamer463 What's a Dremel?

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    No worries, it won't look anything like that by the time I'm done I'm sure.

    Actually, I was planning on making two of those 4 smaller dark holes as pressure relief holes, which I believe help th speaker attain more bass by eliminating the pressure build up being the speaker, which inhibits its movement.
     
  18. mattthegamer463

    mattthegamer463 What's a Dremel?

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    Interesting development.

    I pulled a crossover out of my sacrificial speaker set, and when connected to the speakers I was going to use the sound is like mud out of my Hi-Fi. Like absolutely terrible. From a distance its barely better than the speaker in my Zen Vision, and could be in the same category as ipod headphones on overdrive. They have no bass and if I boost the bass they start making the WHAMP noise and moving a good half inch. I guess I will use these other speakers in the end then, because they sound excellent compared to these other pieces of crap.

    I bought that dual 20W amp circuit, and I'll be assembling it tonight.

    EDIT

    Redesign #1

    Its now 500x318x136mm

    [​IMG]

    I would like to shrink its width, but I don't want to put the tweeters any closer to the box where MP3 players will be stored for fear that the high power magnets will damage them. If I had the box grounded would that protect it from EMF? Lead?
     
    Last edited: 22 Jul 2008
  19. sheninat0r

    sheninat0r What's a Dremel?

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    The thing about speaker casings is that they need to absorb extra vibrations, so they have to be stiff and dense - hence the use of MDF in a lot of speaker cabinets [and why Bose is criticized for using LDF in their "high-end" 5.1 setups].

    Plywood should work fine, though, especially since this isn't quite an audiophile setup.
     
  20. mattthegamer463

    mattthegamer463 What's a Dremel?

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    And I'm not quite an audiophile. My MP3 player doesn't support FLAC, and since there won't be a turntable built into this boombox most audiophiles will spit on it. My only concern with material is easy ways to maximize bass.

    I was wondering, is there a way I could make a simple circuit that would give me a Bass pot and a Treble pot like nearly all home theater amps have? I use those so often on my home stereo that I would love to have them on this thing.

    I bought and assembled the amplifier, and even though I've only tested one side right now, its working incredibly well and I pumped the OOMPH! as I could without getting in trouble at this time of night.

    [​IMG]

    [​IMG]
     

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