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Capacitor question

Discussion in 'Home Cinema Building DIY' started by EasyTiger, Jun 1, 2004.

  1. EasyTiger

    EasyTiger
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    I am looking to connect some flat panel speakers directly to my amp. These speakers are meant to go via a sub which cuts out everything below 200Hz. As I'm not using the sub for the LFE (I have a Rel for this) and the speakers are just used for the rear surrounds I would like to remove the sub from the room. The only problem is that I can't use the amp's crossover at 200Hz as I have this set at 80Hz for the fronts.

    So, I think I need to put a capacitor in series, however am not quite sure. Does anyone know whether it is just a single capacitor and if so what value?

    :lease:
     
  2. eviljohn2

    eviljohn2
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    To achieve you'll need a passive high pass filter, there is plenty of information on the internet although buying a dedicated electronics book might help. In principle you can achieve this using just a capacitor and a resistor of the correct values according to a fairly simple formula.

    Just using the inductive component of a capacitor is of little use for audio frequencies, also bear in mind that you'll need components which won't be fried when a large current is passed through them! :)
     
  3. Mylo

    Mylo
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    You can use a capacitor to block low frequencies, I've done it myself. :thumbsup: In very basic terms, inductive coils block high and capacitors block low frequencies. If you look at a 3 way car speaker you will usually find a capacitor 'in series' stuck to the underside of the high frequency drivers. This is a pretty crude method and you need to work out the correct value for the required frequency.

    As John says you will probably be better off using a dedicated crossover from someone like CPC or Maplins. If you are not using the supplied sub and you don't intend on selling it, why not take the crossover out of that? That way you are guaranteed the correct frequency’s are sent to the panels.

    Cheers, Mylo :)
     
  4. EasyTiger

    EasyTiger
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    Thanks guys.

    On searching the web I found the following link that pointed me in the right direction. Looking at the table I should get a 200uF capacitor.

    Looking on the Maplin site I found this capacitor which is 220uF, 100V and 860mA.

    So the crude way is to wire it in series with the +ve.

    :thumbsup:
     
  5. eviljohn2

    eviljohn2
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    That's the idea. Make sure you get the polarity right though or bang!! :)

    Ideally you need to get some idea of the -3dB point in your subwoofer crossover so you can add the correct capacitor. This is because none of these filtering methods provide a "brick wall" filter and have a roll-off with frequency, alternatively as it's just for your rear speakers I'd consider moving the crossover point up to about 250Hz to be on the safe side. They're dirt cheap so maybe try a few and see the results for yourself.
     
  6. Killahertz

    Killahertz
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    EasyTiger,

    The use of a single capacitor to provide high pass filtering is a valid one. Indeed, many car audio drive units, and even domestic loudspeakers utilise this simple means of filtering.

    Now, few people like (even simple) formulae, but these two are incredibly simple, and useful in this application:

    F = 1 / (2 x Pi x R x C) and it's transposition C = 1 / F x (2 x Pi x R).

    [F = frequency (in Hertz, Hz), Pi = 3.14, C = capacitance (in millihenries)].

    You'll note the need for R (the nominal impedance of the speakers to be filtered usually being used here). If you run the numbers you'll see that your 220uF cap will give you a filter point of 180Hz with a 4Ohm nominal load, and that will half to 90Hz with an 8Ohm nominal load. If, then, the target speakers are indeed an 8Ohm nominal load, your filter point has shifted from that required. This halving/doubling of capacitance to filter point versus load follows, hence it then follows that for a 200Hz filter point for an 8Ohm load, you need 100uF capacitance. You can then, if need be, begin to consider filter point versus that from your sub, and the rate of attenuation offered (6dB/octave in this case, or 1st order filtering).

    The capacitor used should not be polarized (non-polar, or bi-polar) - (it will have no +ve or -ve markings), and rated at an absolute minimum of 50V. The one you linked to from Maplin is ideal in these respects. It then does not matter what way round it is installed, or into which speaker wire (pos or neg).
     
  7. EasyTiger

    EasyTiger
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    Hey Killahertz,

    Great post. Dropping the uF value means that I they're cheaper so that I can get better quality ones. I've been told to get Polypropylene if I can.

    Thanks
     
  8. Killahertz

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    Thanks. Yes, you can use the polyprop ones if you like - they generally have a lower tolerance, so you closer match your selected filter point, and they are more stable over time and temperature. That said, a modest quality electrolytic will do a perfectly good job.
     
  9. Nic Rhodes

    Nic Rhodes
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    I tried this with Wharfdale flat panel and a new sub (Rel Storm), it was a mess, what are you planning?
     
  10. EasyTiger

    EasyTiger
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    Beekeeper,

    I have 4 mission FS1's acting as the four rear surrounds (in 7.1) paired with Kef Q5/Q9c and a RelQ150mkII sub. Upto last night I was connecting the flat panels via the mission sub (switched off) to ensure the low frequencies were filtered out from the flats (I had the mission sub switched off as it was intermittedlly powering down on TV when there were no rear sounds for 2 minutes - a result on not using panels for the L & R). I wanted shot of the big silver sub that's taking up space and WAF points.

    Following this thread I decided to have a play and listen to the rear SL and SR with a normal stereo source (so that I could gauge the impact of the capacitors). I was amazed at how poor they sound with the sub switched off (30Hz - 250Hz)! Anyway, I then used a test tone CD to see when the sub took over and guess what, the sub does cut in below 250Hz but does not cut out the signal to the panels. They produced sound at 100HZ!

    So this leads me to conclude that after all this I don't need to protect them from the low frequenciesl. It also proves to me that I have to move house so that I can get some space behind my listening position so that I can have some decent rears. One step at a time though....
     

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