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LM3875 and speaker impedance questions

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by whiz115, Feb 2, 2007.

  1. whiz115

    whiz115 Member

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    Hi!
    my recent build is a LM3875 amp, my speakers are 2 way at 4 ohm.
    i'm about to connect them in series so i can raise the speaker impedance
    to 8 ohm as the manual says ...is it a good idea for my 4 ohm speakers?
    (no crossover exists, only a filter capacitor for the tweeter)

    why the PDF of the LM3875 says only for 8 ohm? it can't handle well 4 ohm speakers? i haven't noticed something bad all these days i'm using my amp...and i think it should deliver more power as we reduce impedance...



    thank you!
     
  2. hentai

    hentai New Member

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    hello whiz115,
    it seems lm3875 can't drive 4 ohm loads and i think its because of the internal curent limiter build as a protection.
    as for the speakers u can put them in series but if u can put the woofers in series and the tweeters in the same box and use a proper crossover u will fell the difference.
     

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  3. whiz115

    whiz115 Member

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    the internal protection circuitry limits the current draw and the IC outputs less
    watt?
    All this time i was working it with 4ohm load...this means that i haven't see the real output of the IC?

    i have this crossover can i use it with my 4 ohm speakers?

    [​IMG]
     
  4. dave

    Dave New Member

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  5. hentai

    hentai New Member

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    yes as u can see from the chart the power output drops seriously when the load is under 6 ohm or so...
    as for the xover thats strange... i think its meant a 2nd order but... on the woofer that inductor doesnt have a proper value and the capacitor should be in parallel with the woofer. for the tw the capacitor should be placed before the inductor and that seems to have a core and at high freq. i dont recommend that. Personally i wouldn't use it unless u are sure it was desinged for the drivers u have and by a competent person. If u want i can design a xover for u if u tell me what drivers ur using and what box.
    Take care,
    Hentai.
     
  6. whiz115

    whiz115 Member

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    Ι don't understand...i think is shows a rapid fall under 8ohm and from the other side it is more linear as we increase speaker impedance...is that exactly what happens?


    yes sorry...the coil for the woofer isn't 8 ohm...it has a resistance at 1,1-1,3 ohm almost the same resistance has the inductor for the tweeter.

    and the capacitor for the woofer i think it was in a wrong place...heres an updated diagram...(i'm not sure if the inductor for the tweeter is in the correct place or over the capacitor)

    [​IMG]

    yes...if you think that isn't proper...i would like you to help me design a new one...

    P.S what's the purpose of the 8 ohm/5W resistor? does it gives a hint about the watts that the crossover can handle?
     
    Last edited: Feb 3, 2007
  7. hentai

    hentai New Member

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    if u say the coil resistance is 1.1 than my guess this is a comercial xover. if it is and if u want quality music dont use comercial xover unless u modify them or ur extremly lucky. now that resistor attenuates the tw. most comercial xover put that resistor because most people tend to burn those tw very fast because they clip their amps so they attenuate the tw abit hoping there wont be any problems.
    the xover design must be based first on the speakers u use (drivers) give me the name of the woofer and the tw or the specs.
    If ur happy how the xover sounds than my suggestion is to put a zobel network for impedance egalization of the woofer, because at the xover freq rarely is 4 ohms. and if that on the tw is a 2nd order than i would reverse the polarity of the tweeter.
    for the wattage rating look also at the voltage rating of the capacitors and their working temperature.
     
  8. whiz115

    whiz115 Member

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    The capacitors are 85C at 100V

    no...when i used to have those crossovers i wasn't very happy with the sound quality...the sound is very tedious with every amp i have tryed!

    my speakers are el cheapo :D


    Tonsil Woofer

    nominal impedance: 4 ohm
    voice coil resistance: 3,3 ohm
    input power/max power: 60/120Watt
    frequency range: 50-3000Hz
    sensitivity 1w/1m: 87Hz
    resonance frequency: 34 Hz


    Tonsil tweeter

    nominal impedance: 4 ohm
    voice coil resistance: 2,8 ohm
    input power/max power: 80/160Watt
    frequency range: 4000-20000Hz
    sensitivity 1w/1m: 89Hz
    resonance frequency: 1100Hz


    do you think that this woofer with this tweeter can be a pair in the same box?
    If that zobel network can make things better...i prefer it! because it's much
    easier than a crossover.
     
    Last edited: Feb 3, 2007
  9. hentai

    hentai New Member

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    hmm i see the tw has a resonance freq pretty nice but if the freq range is from 4k its hard to pair them. pls give me their model number so i can check up their t/s parameters and freq response curves.
     
  10. whiz115

    whiz115 Member

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    GDWK 10/80
    GDN 20/60/15

    thank you!
     
  11. Hero999

    Hero999 Banned

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    The tweeter schematic looks incorrect to me, are you sure the capacitor doesn't come before the inductor?

    Connecting two speakers in series can ruin the damping factor to some degree, it really isn't an ideal situation.
     
  12. whiz115

    whiz115 Member

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    probably...it's hard to tell...

    what is "damping factor"? this means that my speakers are not suitable to work with the LM3875? and that there's no way to work them at 8 ohm?
     
  13. hentai

    hentai New Member

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    i couldnt find the freq response curves so im quite blind but i came up with something very minimalistic but it should work better than the other one. the cut point is at 3.5 kHz 2nd order Butterworth, that means there will be a 3 db boost at 3.5kHz which compensates for the natural rolloff of the drivers (if it is there :) ). Use capacitors rated at least 250V and at temperatures preferably higher than 85 degrees Celsius and try not to use electrolits.
    If u plan on a bigger upgrade i suggest using a midrange like GDM10/60/2.
    Take care.
     

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    Last edited: Feb 3, 2007
  14. whiz115

    whiz115 Member

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    i was about to upload the frequency response diagram...but you said that your're going to find it... :D

    anyway...the schematic you send me is calculated to pair my 4 ohm speakers with my LM3875? i see it's in parallel that why i ask... :rolleyes:

    i don't have a way to know how to wind the coils so i can have the mH you specified...is it possible to translate it to wire dimensions? :D :D
     
    Last edited: Feb 3, 2007
  15. hentai

    hentai New Member

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    upload the charts if u have them
    the tweeter is in parallel with the woofer the ideea was to put to boxes (containing a woofer and a tw each) in series but its a bad practice.
    if i would have made a series xover the resulting impandance would still be 4 ohms
     
  16. Hero999

    Hero999 Banned

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    Speaker resonate which emphasises certain frequencies more than others. Amplifies have a very low output impedance to dampene the resonance of the speakers. Connecting the two speakers in series ruins this because each behaves like a series resistor increasing the output impedance and reducing the damping factor. Your speakers will still work but they won't sound as good.
     
  17. whiz115

    whiz115 Member

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    sorry for the quality...it's the best i can do...

    [​IMG]
    [​IMG]

    i think the tweeter has pretty nice frequency response...correct? :rolleyes:
     
  18. hentai

    hentai New Member

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    yes thats pretty smooth i wonder why they say its lower frequency is 4 k...
    well dont use that xover ill design another one tomorow morning with all the details about the coils :D
    take care
     
  19. whiz115

    whiz115 Member

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    ok! thanks for everything!!! :D
     
  20. audioguru

    audioguru Well-Known Member Most Helpful Member

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    I had a big argument about speakers in series with the moderator on another electronics chat forum.
    I said it ruins the excellent damping of speaker resonances provided by a modern amplifier.
    He said that Geetar speakers are frequently connected in series.
    I said that the Geetar speakers sound terrible, have the back of their enclosure open and the resonance is an effect.

    I prefer to listen to a clear acoustic guitar, not a distorted electric Geetar.
     
  21. hentai

    hentai New Member

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    ok as i promised here is the xover. the inductors can be build after this calculator http://www.lalena.com/Audio/Calculator/Inductor/ the spl of the tweeter is 2dB higher than of the woofer so i attenuated a bit also there is an attenuation in the mid region to compensate the woofer's natural rise in spl. use this with 4 ohm amps if still want to use ur LM than i sugest u take the speakers put them in series (woofer+woofer, tw+tw) and make a new box for them... and a new xover if that is ur plan let me now so i can modify this xover for 8 ohm.
     

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