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Dual voice coil speaker: Each coil get an inductor?

  1. Dec 25, 2014 #1
    1st order crossover desired low-pass frequency: 100hz

    At 8 ohms, that'd be about 12mH.

    But what happens with a DVC speaker? Each coil get a 12mH inductor? I'd like to think so...

    Speaker is 8+8 ohms and will be parallelled to a set of 8 ohm extended range speakers showing my amplifier 4 ohms per channel.


    *Edit: Oh my, a 12mH inductor is nearly $40.00 USD!

    Can I use an unpolarized electrolytic capacitor in parallel to each coil? They are only $3.75 each.
     
    Last edited: Dec 25, 2014
  2. jcsd
  3. Dec 26, 2014 #2

    Baluncore

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    If a DVC speaker is 8+8 then you can run it's two coils in parallel as 4 ohm, in series as 16 ohm or by using only one of the coils as an 8 ohm speaker with half the power rating. Any which way, you will only need one crossover network, unless there is a crossover power handling problem. We do not know the power involved here.
    That does not seem to make sense. You are missunderstanding something or confusing us by mixing two channel stereo discussion with one channel discussion.

    The series inductor required to block higher frequencies should be in series with the woofer. You could use a capacitor across the VC but that possibility would depend on the circuit you are using for your crossover network and the possibility of self-resonances.
     
  4. Dec 26, 2014 #3

    meBigGuy

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    What power levels do you want to run at? Post a sketch of the hookup you are proposing. How many channels, how many speakers, where you want the crossover.
     
  5. Dec 26, 2014 #4

    Averagesupernova

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    Not uncommon at all to run dual voice coil subs with one coil per channel. A properly set up system with crossovers will not let the amp see your setup as 4 ohms.
     
  6. Dec 26, 2014 #5

    jim hardy

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    yep.
     
  7. Dec 27, 2014 #6
    Amp: 2 x 20 Watts RMS

    Extended/full range speakers: 8 ohms each (single voice coil or SVC for short)


    The manual that came with my amplifier didn't specify whether or not that 2 x 20 Watts figure was at 4 or 8 ohms. I assume 4 ohms as it'd make a good selling point (20 x 2 at 4 ohms sounds better than 10 x 2 at 8 ohms). Manual just stated the amplifier was stable down to 4 ohms.

    Anyways, the full range speakers sounded pretty good alone, but I wasn't getting anything under 100hz or at least not very much, so I decided to add a subwoofer.

    The ideal thing would be to purchase another (more powerful) amplifier, but I'm broke and my room is only 12 x 12 so it's not going to take much. I'll just use the amplifier I have.

    I have a few options, but the ideal thing to do is to run 1 DVC 8+8 ohm subwoofer. Take one 8 ohm coil from my subwoofer and parallel it to one of my 8 ohm full ranges and connect it to one of my amplifier's outputs. This would show my amplifier 4 ohms which it can safely be ran at.

    Now do the same thing for the other channel.

    I can parallel the coils of my subwoofer and run it to either of my amps outputs (either left or right) but that'd only give me bass from left or right channel. Same Applies if I'd wire it in series.

    I opted for a dvc 8 ohm so I'd only have to buy 1 subwoofer. It'd get me both left and right, and get me the most power my amp can put out.

    I can't afford to buy two 12mH inductors to wire to each coil, so I'm asking if I can use two electrolytic capacitors in parallel to each coil of my subwoofer. This would save me $74 usd.

    Also, take that 20 x 2 with a grain of salt. According to the audio gurus over on diyaudio, it's more like 12 watts and swapping out the 12v, 2 amp supplied power supply with a 12v, 5 amp psu is recommended.



    I forgot to mention. I will be wiring a 500uf cap in series to each full range.

    My iPod is refusing to show an upright image.
     

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    Last edited: Dec 27, 2014
  8. Dec 27, 2014 #7

    jim hardy

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    Is my arithmetic right, that 200 uf would be 8 ohms at 100 hz?
    That'd be the value to put in series with your smaller speakers.

    The object as you know is to force the lf into that woofer (which i presume you'll put in a good enclosure).

    Impedance in series with speaker adds to its 8 ohms. So with a good crossover your amp sees more or less same impedance at all frequencies, 8 ohms not 4. Series inductor makes woofer "disappear" at high frequency, series capacitor makes tweeter disappear at low frequency.

    That's why you cant just bypass your woofer with a capacitor, it'll look to the amplifier like a short at high frequency.
    12 millihenries will be somewhere around 300 feet of 18awg on a 1 inch form. Here's a calculator to tinker with..
    http://www.diyaudioandvideo.com/Calculator/Inductor/

    That much magnet wire will cost about the same as the inductors you considered.
    http://www.powerwerx.com/wire-cable...15&plc={placement}&pkw=+magnetic +wire&pmt|=p

    Your dual voicecoil sub is a good idea. Maybe you can scrounge enough magnet wire to wind your own inductors.
     
  9. Dec 27, 2014 #8

    meBigGuy

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    Hopefully you know that you need non-polarized capacitors.

    To build an RC lowpass you need a series resistor for the capacitor to work against. That resistor consumes power. The resistor will appear across the amp at high frequencies, and in series with the sub at low. It will consume significant power. The smaller you make it, the more power you can get to the sub, but the less power you will get to the other speaker before the amp clips.

    You have a low power system, so you can maybe wind with 22 gauge wire. It is 200 ft of wire, and 3.6 ohms on a 0.5 dia X 1in core.

    At you low power levels you could try an iron core inductor.

    8 ohms, 10 watts = 1.11 amps. No idea how to design that with an iron or ferrite core.

    Here is a 10mH 3.8ohm ferrite crossover coil for $10.
    http://www.mcmelectronics.com/product/555-25635

    10mH 1.2 ohms is a bit more
    http://www.mcmelectronics.com/product/555-25615
     
  10. Dec 27, 2014 #9
    The 100hz crossover point is for my woofer. The 500uF capacitors are for my full ranges. The full ranges I have in my possession are a set of Aurasound ns-3's. These Aura's + the 500uF capacitors are called Wolf's PC Speakers. It's a simple design from a member on diyaudio. I already had the Auras and capacitors in my possession. When I purchased the woofer, the tech recommended that I cross the woofer over around 100hz and call it a day.


    When you say I can't just bypass my woofer with a capacitor, you mean I can't wire a capacitor in parallel to either Coil of my woofer with the intent to low pass the woofer at a desired frequency?

    I'm aware a capacitor in series with a loudspeaker will shun low-er frequencies and will act as a high pass filter.

    I'm also aware that an inductor in series with loudspeaker will shun high-er frequencies and will act as a low pass filter.

    But I thought that a capacitor in PARALLEL with a loudspeaker would serve as a low pass? And an inductor in PARALLEL would serve as a high pass?


    I assume when you say short, you saying that ...uh, an escape route for electrons to flow ...so they (high frequencies) would flow through the speaker (and not the cap). So there would be no low pass filter here.

    So what's the purpose of paralleling capacitors and inductors to speakers?
     
  11. Dec 28, 2014 #10

    jim hardy

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    Put yourself in your amplifier's shoes for a minute.... what load does it see?


    The speaker is somewhere around eight ohms , and that's what the amp is designed to drive. It can produce enough current to make sound with a four or eight ohm load.

    If you connect a sizeable capacitor right across the speaker terminals the poor amp sees a load that gets progressively smaller as frequency increases. That 200 uf that's 8 ohms t 100hz is only 0.8 ohms at 1khz and only 0.08 ohms at 10khz.....
    The capacitor will bypass all the hihg frequency current your little amp can produce, and bypass it right around all your speakers.

    You can bypass the woofer with a cap provided there's an inductor between your cap and the amplifier.

    It's important that you think like an amplifier.
    Are you familiar with calculating series and parallel impedances?

    We haven't any way to know your level of expertise except by guessing from what you post.
    If you intended to bypass that woofer on the speaker side of the inductor you said you couldn't afford , well, that'd work.
    But i figured you were suggesting the cap instead of the inductor not in addition to it.

    old jim
     
  12. Jan 4, 2015 #11
    I was suggesting the cap instead of the inductor as a low-pass filter...to save money. I knew that couldn't be right though (substituting a series inductor with a parallelled capacitor) otherwise everyone would be doing it, and I've never seen it done before.

    There are certainly holes in my understanding and no I am not familiar with calculating series/parallel impedance -I'm still stuck on understanding how a capacitor "blocks" low frequencies. But anyways I see what you're talking about. It was tricky for me because whenever I see/think/hear of a capacitor, I think of a simple 1st order on a tweeter. The whole point of this is to PASS high frequencies "through" it, and block low ones of course. But in our scenerio, the capacitor has a higher resistance so current takes the least path of resistance and goes through the speaker. So here high frequencies are actually bypassed (not passed through as how I usually see it). That is correct, right?
     
  13. Jan 4, 2015 #12

    jim hardy

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    How to approach this....
    Electricity seems baffling at first because the basic units are all named after people, not something that'd help you remember them.

    The analogy of water in pipes is popular but can lead one to some misconceptions...
    so let's stick to electrical charges in the wires.
    You can think of charge as sort of a fluid. We measure it in a strange unit "Coulombs
    a Coulomb being the amount of charge that would be carried by 6X10^18 electrons...
    it's large but not huge... imagine a waterglass full of charges and call it a Coulomb.
    Look up who Coulomb was and you'll see why they picked that name.


    Impedance is just what it sounds like - something that impedes the movement of charge.
    A copper wire allows almost unimpeded movement of charge. So it has not much impedance.
    We measure impedance in Ohms, a measure of the difficulty with which charges are pushed through something.
    A typical loudspeaker has four or eight ohms which isn't a lot.

    When you connect two impedances in series they numerically add. (it's a vector addition but save that detail for later)
    When you connect two impedances in parallel, you must add their reciprocals and take the reciprocal of that result. We call that reciprocal "Mhos', Ohms spelled backward(no kidding)

    So: 2 four ohm loads in series gives four ohms plus four ohms = 8 ohms
    likewise 2 four ohm loads in parallel gives 1/4mho + 1/4mho = 1/2 mho, and 1/(1/2mho) = 2 ohms

    Now a capacitor's impedance is directly related to frequency
    the equation is :
    ohms = 1/(2 X pi X frequency X Farads)

    So your 500 uf capacitor has what impedance at 100 hz?
    1/( 2 X 3.14 X 100 X 500X10^-6) = 3.18 ohms

    Can you figure what it'd be at 1000 hz? and at 10 hz?

    And why it'd short out your amp at high frequency were it connected across speaker output terminals?

    I hope you get interested and dig into electronics.

    old jim
     
    Last edited: Jan 4, 2015
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