BIC Venturi Formula 4 recap and restoration

Discussion in 'Speakers' started by Jeremy Pare, May 22, 2018.

  1. toxcrusadr

    toxcrusadr Omelette au Fromage Subscriber

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    Hmm, probably best not to crank them up again until you figure out why that would be.

    What are you using for an amp? How high are you cranking the volume control?

    Are the BICs 4 or 8 ohm?

    Are all the drivers playing, and is each driver at reasonably similar volume on both channels?
     

     

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  2. JayPare

    JayPare AK Subscriber Subscriber

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    The BICs are rated 6 ohms. My amp is a SX-737.

    While reading, I found that capacitor have an ESR that should be taken into account:
    http://audiokarma.org/forums/index.php?threads/crossover-caps-and-esr.216201/

    How can I know if all drivers are playing? Can I disconnect for example the woofer and the horn and try to play something with only the tweeter plugged and so on?
     
    Last edited: Feb 11, 2019 at 3:45 PM
  3. RTally

    RTally Speaker addict Subscriber

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    Easy way to test if a driver is playing is to get a paper towel roll or a toilet paper roll. Hold one of the tube up to the driver and the other to your ear. If the driver is playing music, you will hear something. To verify, you can move the end of the roll away from the driver to verify that the volume drops.

    Another way is to use a tone generator app on your phone connected to your amp. While running a sweep, listen with your ear close to the baffle. You should be able to hear the tone move from one driver to the next. If your hearing drops off like mine (nothing over 12 kHz), this method will not work for supertweeters.
     
  4. JayPare

    JayPare AK Subscriber Subscriber

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    Thanks @RTally. I'm not at home during the weeks but I will test this this weekend! :)
     
  5. RTally

    RTally Speaker addict Subscriber

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    Since the BIC has a pot that can be adjusted, it is safe to ignore cap ESR because the pot resistance will be much greater than any ESR for either an electrolytic or film cap.
     
  6. JayPare

    JayPare AK Subscriber Subscriber

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    However the tweeter signal doesn't seems to pass through the potentiometer. I will try to post an improved picture of the schematic later on.
     

     

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  7. JayPare

    JayPare AK Subscriber Subscriber

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    @RTally my electrical engineering skills are far but I think it looks like this:
    39647c3e-acf0-4473-85a1-28552a6b2ca3.png
    EDIT: Pot value is not the real value.
     
  8. RTally

    RTally Speaker addict Subscriber

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    957
    Location:
    Knoxville, Tennessee
    Very close. You left out the thermistor and the switch that selects between the thermistor and the pot.

    I am an electrical engineer, but not one very skilled in crossover networks. But I will make an attempt to analyze the circuit. Hopefully someone more knowledgable will chip in. That is a really odd crossover circuit.

    The .5 mH inductor and 16 uF cap form a 1800 Hz resonant (tank) circuit going into the woofer. The switched pot/thermistor are in parallel with the resonant circuit. I imagine that the switched pot/thermistor have little impact on the woofer signal because the inductor impedance is likely much less than the pot/thermistor, therefore the full bass signal is going to the woofer. I would assume that the woofer has a mechanical, acoustic rolloff above the resonant frequency.

    The mid-range horn has a second order high pass filter from the woofer. That is, the input to the horn's second order circuit is after the tank circuit and the switched pot/thermistor. I am guessing that the switched pot/thermistor have a lower impedance than the tank circuit (primarily the 16 uF cap) above the resonant frequency, therefore the pot/thermistor conduct a lot of the horn signal.

    I assume the thermistor is a positive temperature coefficient (PTC) thermistor. That is, the resistance increases as the temperature rises, thereby lowering the current and saving the horn ??

    The tweeter has a conventional second order high pass filter with no attenuation.
     
  9. RTally

    RTally Speaker addict Subscriber

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    957
    Location:
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    I stand corrected. :D I made an assumption about the crossover and it was wrong.

    Since there is no pot in line with the tweeter cap, the recapped tweeter may be brighter than when original, depending upon the ESR of the original electrolytic cap. The best way to determine if you need to attenuate the tweeter would be to temporarily put a pot in series and adjust it for a flat speaker response. Then you could replace the pot with a fixed resistor equal to the setting of the adjusted pot.

    If the speakers do not sound too bright, I would not go to the trouble of trying to correct for ESR.
     
  10. JayPare

    JayPare AK Subscriber Subscriber

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    Yeah since the switch only replace the potentiometer with the thermistor, I didn't add this part since it complicated the circuit for nothing and it doesn't change that much in the crossover.​

    From my testing, it's a NTC thermistor:

    They are not. I was more worried about my amp cutting at a certain level which I pretty sure it was not before.​
     
    Last edited: Feb 12, 2019 at 9:41 AM
  11. JayPare

    JayPare AK Subscriber Subscriber

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    I added the Thermistor and the switch. I still need to test the potentiometer to find the resistance but when I measured the thermistor is was around 16 ohms at 25°C). Also, I realized the 4.7µF and the 1µF capacitor could be added together to simplify. Here is the resultant (please correct me if I'm wrong):

    Crossover_3.png
     
    Last edited: Feb 13, 2019 at 7:52 AM

     

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  12. StimpyWan

    StimpyWan AK Subscriber Subscriber

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    500nF cap on the tweeter? Is that correct?
     
  13. JayPare

    JayPare AK Subscriber Subscriber

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    Not sure. I read this from Wikipedia:
    Then, from the image I posted in OP, I made the assumption that MFD was standing for μF but after calculating the resonant frequency I realize the 500 nH (0.5μH) coil doesn't make sense so maybe those cap values are off too. Also, two coils uses the μH. Therefore, I guess making that "μ" was possible so they would have used it in other places instead of using M (as in MFD and Mh). Anyway, I ordered a LCR meter to measure the original caps and coils and find if I'm in the good range. I will update this weekend.
     
    Last edited: Feb 13, 2019 at 7:52 AM
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  14. toxcrusadr

    toxcrusadr Omelette au Fromage Subscriber

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    Usually crossover inductors are shown in millihenries (mH). Ranging from maybe 0.1 mH in a tweeter circuit to 2-5 mH in a woofer circuit.

    As for caps, tweeters in a 3-way speaker usually have a cap somewhere in the 2 uf or under range. 500 nf would be 0.5 uf which sounds about right.

    Crossover basics: https://www.diyaudioandvideo.com/Guide/WireSpeakerCrossover/

    It helps to think of caps as high pass filters, inductors as low pass. There's almost always a cap in series with the tweeter (high pass filter), a cap and inductor on the midrange (i.e. a bandpass filter), and an inductor in series with the woofer (low pass filter). This is a simple first order crossover, with slopes of 6 db on either side of the crossover points. If you add a cap across (parallel to) the woofer, it shunts to ground some of the higher frequencies that made it through the inductor. Net effect is making the dropoff steeper. Same with adding an inductor parallel to a tweeter. It becomes a second order xover with 12 db slopes.

    There is a more conventional layout for crossover diagrams.

    See here:

    https://www.diyaudioandvideo.com/Tutorial/DesignBuildCrossover/


    [​IMG]
     
  15. toxcrusadr

    toxcrusadr Omelette au Fromage Subscriber

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    Looking back at the wiring diagram in your original post, it's funny that BIC mixed their units on the inductors, some in mH and some in uH.

    Kinda odd that the woofer inductor is only .5 mH. Usually that would be a larger value to allow the lowest registers to get to the woofer. Far be it from me to question the designers though. :biggrin:
     
  16. JayPare

    JayPare AK Subscriber Subscriber

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    Mixing units it not a problem, but mixing meaning of "M" in coil and caps (milli in coils and micro in caps) is something really odd. I'm pretty new in audio though. May be usual for those years, IDK.
     

     

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  17. toxcrusadr

    toxcrusadr Omelette au Fromage Subscriber

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    I never have liked mfd as microfarads just because m also stands for milli. I prefer uf even though the English 'u' is not technically the correct character for mu. At least it can't be mistaken for something else.
     
    JayPare likes this.
  18. JayPare

    JayPare AK Subscriber Subscriber

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    I stand behind you for this!
     

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