Does this speaker have a botched crossover filter?

Discussion in 'Home Cinema Speakers' started by mgallimore, Jul 23, 2012.

  1. mgallimore

    mgallimore
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    Hello. I'm new to this forum so please bear with this perhaps trivial question.

    I bought a pair of floor standing speakers from a car boot sale (Jamo Sudio 170s). These are rather old but for £10 I just took the chance. They work to a degree. There's obviously music coming from both the tweeter and the woofer but apparently nothing from the midrange speakers (on both sides). To me they still sound great :)

    I took the back off to investigate. Using a multimeter and by disconnecting a few wires I've confirmed that all speakers do work in issolation (I injected each with a sine wave from my PC's sound card).

    I replaced the speaker capacitors in case they might have deteriorated over the years. Alas there was no apparent change.

    I then looked at the circuit for the crossover filter. To call it a circuit is stretching it a bit.

    The three filter mechanisms were as follows;

    Woofer - no cap or inductor. This is connected straight to the power cables.
    Tweeter - An inductor is in paralle with the speaker and a capacitor is in series with inductor / transformer combination. I recognise this as a high pass filter.
    Mid range speaker - the speaker is simply in series with a capacitor.

    So - the woofer runs all the time. The tweeter has a high pass filter. I haven't a clue what the midrange speaker in series with a capacitor does (I guess it's supposed to be a band pass filter).

    It all looks somewhat makeshift - the coil is glued to the tweeter, the caps are soldered between the speakers.

    The question - Is this how Jamo intended it? If so then does anyone have an idea why no noise comes from the midrange speaker?

    The speakers were sold by a guy who claimed he used them for DJing. Could it be that he botched this as some sort of home modification?

    Do you think it would be worthwhile simply installing a £15ish 3 way crossover filter unit that are easily available from loads of places? I don't know any inductances / impenences of the speakers to design a purpose made crossover. I'm hoping that a simple off the shelf filter will give me a step change anyway.

    This is my first foray into a bit of HiFi. I've gone from a £100 shelftop 30 watt do-eveerything unit to the following that I've picked up at bootsales in the space of 3 weeks:

    Speakers Jamo Studio 170 £10
    CD player Marantz CD5001 £10
    Amp Marantz PM240 + other speakers £15
    Subwoofer Kenwood SW-600 £5
    Spares inc repair bits £30

    This is all I can afford and so I'm hoping that the 3 way crossovers will give me a cheap fix - Any thoughts / suggestions please?

    Thanks, Mark
     
  2. AlanB43

    AlanB43
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    Hi Mark

    I have a pair of those Jamo 170 speakers and took the back off one after reading your post. I had also listened to the three speakers and also thought that the mid-range speaker was silent.

    So I was very surprised to see the capacitor wired in series with it, which is only 1 microfarad. For a mid-range 6-ohm speaker a low-frequency blocking capacitor should be 50-100 microfarads, so no wonder the thing has no sound coming out of it :).

    I will be off to Maplin on Monday morning to get a couple of 68-microfarad bipolars on Monday morning, and will wire them in asap . . . I will also rewire the mid-range speaker which is at present wired the wrong way round on the one I looked at. Thanks for the reminder.

    Alan
     
  3. BlueWizard

    BlueWizard
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    No crossover components rely on the natural roll off of ...in this case... the woofer.

    One capacitor in series is a high pass. This rolls off on the low end at 6dB/Octave. (First Order Filter)

    One Cap in series and a Coil in parallel is Second Order High Pass rolling off on the low end at 12dB/Octave.

    If the midrange only have a high pass, then likely it too relies on the natural roll off of the driver on the high end.

    If the midranges are not working in normal operation, but the drivers themselves function, then the only possible component is the Capacitor.

    However, to what extend did you verify the midrange? Did you actually pass a tone through it?

    IF the capacitor is 1 microfarad, and IF the driver is 6 ohms, then using the top calculator on the top of this page -

    Crossover Design Chart and Inductance vs. Frequency Calculator(Low-pass)

    The Mid crossover is some crazy high number ... like 20,000hz. Which makes no sense.

    If the midrange is 16 ohms, then the crossover drops between 8000hz and 10000hz, which still makes no sense.

    A common off the shelf 3 way speakers is going to be oriented toward either 8 ohm or 4 ohm, most commonly 8 ohms. When the impedance is off, the gap between the speaker widens leaving a gap.

    For the simplest crossover to cross at 4000hz, the Cap = 5 microfarads, the Coil = 0.32 Milli-Henries.

    If the impedance changes to 6 ohms, then the Cap goes up and the Coil goes down. If we find the crossover for the new Cap/Coil values, the High shifts to 5400hz, and the low shifts to 2900hz, leaving a gap between 2900hz and 5400hz.

    You can use the calculator in this new link to make your own crossover. You can select any frequencies you want, and you can select the slopes, though I would recommend Second_Order -- 12db/Octave, Linkwitz-Riley or Butterworth.

    ERSE - Crossover Calculator - Second Order 3 Way

    For example, if we assume the woofer is 8 ohms and we assume the Mid is 6 ohms and we assume the tweeter is 8 ohms, though find out the value or each and use that value, then we choose 800 for the Bass-to-mid cross and 5000 for the Mid-to-high cross and you will have the values you need. Individual coils and capacitors can be purchased to create the crossover.

    How well that will work, I can't say because I don't really have the specs on the drivers, but 800hz and 5000hz is a common 3-way crossover, and should be within the range of each speaker.

    The advantage is that you can get components that exactly match the rated impedance of the specific drivers you have.

    Steve/bluewizard
     
  4. TomScrut

    TomScrut
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    The mid to high cross needs a bit of care IIRC as it will be very dependant on the diameter of the driver. 5000 to me sounded a bit high but if it is a small midrange driver then it could be right (I have no idea what the speakers in question even look like), but AFAIK most mid to high crossovers in 3 ways go between 3khz and 4khz
     
  5. Philip4242

    Philip4242
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    I think you'll find that it's not unusual for a mid-range speaker in a 3-speaker arrangement to be wired 'out-of-phase' relative to the tweeter and woofer, in some crossover arrangements.

    See the last sentence of the 'Amplitude' paragraph of this fairly comprehensive explanation, where it clearly states :-

    'The midrange must be wired with inverted polarity due to the phase behavior of the 2nd order filters.'

    Crossovers: The Short Version The pros and cons of crossing over. Article By Jeff Poth
     
  6. AlanB43

    AlanB43
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    Hi Philip

    On looking at it more closely I spotted the + and - signs embossed near the driver terminals, and one of them was the other way round compared to the other two, so I was wrong about it being antiphased.

    Hi BW thanks for the reply and it's all good advice

    I did not put a tone through the speaker, the only test equipment available was a Mk.1 ear pressed up against it. As far as that goes it confirmed that the 1 uF installed in series with the mid-range driver gives no detectable sound compared to the other two drivers. 1 uF is 159 ohms at 1000 kHz so large attenuation is expected on an 8-ohm speaker even if it might not be exactly 8 ohms at that frequency. The measured dc resistances are 6.2 for the 8" woofer and 7.5 ohms for the mid-range. The nameplate describes the impedance as 4-8 ohms.

    I tried wiring the mid-range in parallel with the woofer and the result is a little harsh with too much mid-range. Next idea is to try 47 uF in series with the midrange to give a high-pass curve turning over at 420 Hz, and wind a 2.7 mH inductor to go in series with the woofer to give a low-pass turning over at about the same frequency. If you say 800 Hz might be a better choice I will take that figure and it will take less wire on the inductor as well :) . . . and I will rely on the mid-range dropping off naturally around 3-4-5 kHz.

    The tweeter with 3.3 uF in series and an air-cored inductor in parallel seems to be working correctly.

    But I have just picked up some nice smaller Gale speakers second hand so am considering taking the Jamos up to the dump next week :).

    I found an interesting article on speaker impedances:
    Passive Crossover Network Design

    Alan
     
  7. BlueWizard

    BlueWizard
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    If you search Google Images, you will see that this is clearly a Woofer, Midrange, Tweeter arrangement.

    Jamo Studio 170 - Google Search

    Roughly 800hz and 5000hz are common in this arrangement. However, many modern 3-way, are NOT woofer, midrange, tweeter, rather they are Low-Bass, Mid-Bass, Tweeter. In this case, yes, both the Bass/Mid and the Mid/High crossover would be considerably lower.

    Generally, the Midrange covers 3 octaves, so relative to this, the Mid/High crossover is a bit low. The Bass/Mid crossover can be anywhere from 500hz to 1000hz, but 700hz and 800hz are very common.

    For example, starting at 800hz, the next octave is 1600hz, then 3200hz, and at a full 3 octaves, 6400hz. Most consider 6400hz a bit high, so they lower it down nearer to 5000hz.

    If we start at 700hz, the 1400hz, 2800hz, and 5600hz, with 5600hz being the 3rd octave up.

    If you use this calculator, when you fill in either the Low/Mid or the Mid/High frequency based on either a 3 octave spread, or a 3.4 octave spread, it fills in the other number.

    3-Way Crossover Designer / Calculator

    Again, from the photos, this is clearly a woofer, midrange, tweeter configuration.

    But, one of the problems is not knowing the limitations and working frequency range of the drivers. The best you can do is guess.

    Curious, does the inside of your speaker look anything like this? -

    View image: 170

    Because if so, there is something even more odd about the wiring. It looks from the photo as if both capacitors are attached to the tweeter. I'm not sure what to make of that.

    Steve/bluewizard
     
    Last edited: Aug 24, 2014
  8. AlanB43

    AlanB43
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    Hi Steve

    Yes! the interior of my Jamo 170 was exactly as in the picture.

    You can see those thin red and green wires going up from the woofer. The green continues up to the left-hand terminal on the tweeter. The left-hand 3.3 uF capacitor is connected from the red (left) terminal on the mid-range to the right-hand terminal of the tweeter. The right-hand capacitor is connected from the left of the tweeter to the right-hand terminal of the mid-range, and this is the offending 1 uF capacitor that kills the mid-range driver. It lies between the right-hand side of the mid-range and the green main power wire.

    To summarise, the woofer is directly across the power cables. The tweeter has 3.3 uF in series and that coil in parallel (across the driver terminals). The mid-range is across the main power wires but has 1 uF in series.

    So your photo confirms what the OP and I have both noticed, and the thing is a botch as far as the mid-range driver is concerned. Funnily enough I have used these speakers for maybe 10 years, running on woofer and tweeter only, without being dissatisfied by the sound :).

    I haven't used the design formulas, I have only used the basic idea for a 1st order filter that it's flat on one side and 6dB/octave on the other side, the crossover frequency (in rad/sec) being the reciprocal of the time-constant CR or L/R. It's easy on a hand calculator.

    Alan
     
    Last edited: Aug 24, 2014
  9. Michaeltairone

    Michaeltairone
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    Hola, os lo Encontrado por El Mismo Problema, los que Comprado de Segunda Mano. Ya habia oido UNOS 180 Jamo estudio y me encantaron, Pero ESTOS 170 tenian mejor aspecto m. CUANDO nota Que El Medio no se oía lo abri; Y no comprendi La Coloca
     
  10. Michaeltairone

    Michaeltairone
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    La colocacion del filtro no se comprende? Y la polarizacion inversa del medio y agudo tampoco, pero el agudo suena
     
  11. BlueWizard

    BlueWizard
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    GOOGLE TRANSLATE -

    Hello, I Found it for the same problem, which bought secondhand. And I had heard about 180 Jamo study and loved, but they looked better 170 m. WHEN note that the average opened not hear; And not understood the Place
     
  12. BlueWizard

    BlueWizard
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    GOOGLE TRANSLATE -

    Filter placement is not understood? And the reverse polarization of the medium and not sharp, but sharp sounds

    - - - - - - - - - - - - - - - - -

    I think you don't understand the Filter Placement, because the Filter Placement doesn't make any sense. If I were there I would draw out the schematic and see if that help make sense of it.

    But from the limited information I have ...no... it doesn't make sense.

    Steve/bluewizard

    Creo que no entiende la colocación de un filtro, porque la colocación del filtro no tiene ningún sentido. Si yo estuviera allí yo extraigo el esquema y ver si eso ayuda a dar sentido a.

    Pero a partir de la limitada información que tengo ... no ... no tiene sentido.

    Esteban/AzulMago
     
  13. Michaeltairone

    Michaeltairone
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    Un amigo de Wallapop me ha dado la solucion, se dedica a restaurar equipos hifi. (J.p.) quitar los condensadores. La bobina no la quiteis y soltar los hilos de alimentacion del twiter y el de medios. Dal alimentacion negativa desde el grabe a los negativos. Y dar positivo a los positivos; intercalando para el Twitter el condensador de 1 microfaradio y para el medios el de 3.3 microfaradios. Funciona muy bien.
    Darle las gracias y comprarle algo.
     

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