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  1. #1

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    Default Understanding LSI25 schematic

    Hi all,
    I am working on replacing the Lsi25 crossover components and would like to understand the schematic more. The schematic can be found here:
    http://www.polkaudio.com/forums/show...l=1#post434888

    On the tweeter section, I see a typical 3rd order highpass (C2, C1, C3).
    On the upper driver (D1) section, I see a bandpass (C4, L2) and a Zobel (C5, R2). On the lower driver (D2) section, I see a Zobel (C6, R3).

    My questions are:
    1) What's R1 and C1 in parallel for? Is it some kind of a highpass filter? I found it in Lsi9 but not LsiC or Lsi15.
    2) Why did Polk change R1 from 2.25 to 1.75ohm? Is it to make the tweeter louder?
    2) What's L3 and R4 in series for? Is it some kind of a lowpass filter? I don't find it in any other Lsi.
    3) What's the effect of connecting the two Zobel together on D2? Or, are they not Zobel at all?
    4) Are D1 and D2 in opposite phase because D2+ is same as D1-?
    5) Is D2 a baffle step compensation to D1? I listened to D1 and D2 separately and the volume from D2 is so much lower compared to D1 when playing the same low frequencies.

    My last question is if I can list all those components in the order of importance regarding improving the sound quality? The reason for this is that I want to spend the money wisely and choosing the components correctly.
    My thinking is by this order below in groups:
    1) C2, C3 for the tweeter
    2) C4, L2 for the midrange
    3) C5, R2, C6, R3 for the two Zobel
    4) the rest that I am not sure (R1, C1, L1, L3, R4)

    My other thoughts are that the stock tweeter is too bright, so may not want to upgrade L1 to make it even brighter. And, since D2 sounds very low, it may make no difference to upgrade L3 and R4.

    Sorry for the loaded questions. Your thoughts, advice or similar experiences would be appreciated.
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  2. #2

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    I've been studying recently, so let me try to answer some my own questions. I can't say everything is correct, however.

    1) What's R1 and C1 in parallel for? Is it some kind of a highpass filter? I found it in Lsi9 but not LsiC or Lsi15.
    Taken from this website: http://www.ceb.cam.ac.uk/pages/resis...-parallel.html
    With resistor and capacitor in parallel the phase angle is seen to change from 0° at low frequencies where the current flows almost completely through the resistor arm to -90° at high frequencies where the current flow is through the branch containing the capacitor.... If the current is alternating, the resistor behaves the same, but the capacitor's effect changes with frequency. At very high frequencies, it is essentially a short-circuit that bypasses the capacitor. At low frequencies, it flows very little, forcing the current to essentially flow only in the resistor. An RC combination can be used as a filter that passes high frequencies better than low.

    2) Why did Polk change R1 from 2.25 to 1.75ohm? Is it to make the tweeter louder?
    In addition to the above. It is often referred to as a compensation circuit to attenuate the tweeter, lower the crossover point and remove signal components below that crossover frequency. A lot of what it does and how different values can make positive or negative differences can be found here http://www.pispeakers.com/Speaker_Crossover.pdf

    2) What's L3 and R4 in series for? Is it some kind of a lowpass filter? I don't find it in any other Lsi.
    C4 and L2 forms a band pass, C4 and L3 also forms a 2nd order LP filter, L3 and L2 forms a LP filter as well (probalby also attenuates). So they all work together. R4 is to attenuate the lower driver further.

    3) What's the effect of connecting the two Zobel together on D2? Or, are they not Zobel at all?
    I believe it is just a fancy way of a Zobel configuration for both D1 and D2. Taken from http://www.trueaudio.com/st_zobel.htm
    Adding a Zobel to a woofer will allow the passive crossover to work more effectively. The impedance of the woofer will also be restored to the driver’s nominal impedance throughout the high frequency range. This technique allows the use of crossovers designed for resistive loads to operate as intended.

    4) Are D1 and D2 in opposite phase because D2+ is same as D1-?
    Every capacitor and inductor introduces a 90 degree phase shift in the network with capacitors and inductors go opposite ways. When two drivers are 180 degrees out of phase they cancel out each other. So, without digging deeper, I believe it this way due to phase shifts.

    5) Is D2 a baffle step compensation to D1? I listened to D1 and D2 separately and the volume from D2 is so much lower compared to D1 when playing the same low frequencies.
    A baffer step compensation only requires about 6dB. We say Lsi25 is a 3.5way with D2 is the 0.5way to augment the lower end (80 - 800Hz) of D1 (80 - 2400Hz). 6dB is not much to hear so D2 does not have to be loud at all (this is the same on LsiC, 9 and 15).
    Last edited by sk88; 05-07-2013 at 05:03 PM.
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  3. #3

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    Quote Originally Posted by sk88 View Post
    I've been studying recently, so let me try to answer some my own questions. I can't say everything is correct, however.

    1) What's R1 and C1 in parallel for?
    Looks like they are using this contour network to provide some hi frequency lift at the top end of the tweeters opeating range.The cap is shorting the resistor at higher frequencies so it will not be attenuating the signal as it would in the lower treble.

    2) Why did Polk change R1 from 2.25 to 1.75ohm? Is it to make the tweeter louder?
    Yes it would be to adjust tweeter level.

    3) What's L3 and R4 in series for? Is it some kind of a lowpass filter? I don't find it in any other Lsi.
    That is the shunt leg of the 2nd order hi pass filter.I assume the resistor was added to help hit the target roll off shape.


    D2 will not be for baffle step compensation since it is serires connected with D1 and share the same 1.5mh low passinductor.Though the differing RC values across the voice coils is intriguing.

    The Lsi 9's extra woofer would provide baffle step compensation since it is operated in parallel with it's own low pass filter that limits it's bandwidth to the lower mids and bass,thus adding some boost in that range.
    Last edited by FTGV; 05-07-2013 at 06:02 PM.

  4. #4

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    FTGV, thanks for your input. You provided several points where I didn't think of before and in an easy understandable way.

    Interesting to see the two woofers are in parallel in Lsi9 while they are in serial in Lsi15 and 25.
    Since Lsi9 doesn't have a HPF for the two woofers but the HPF in Lsi15 is 150Hz and it's 80Hz in Lsi25. I am starting to think that L3 and R4 are necessary only on Lsi15 in order to lower the crossover to 80Hz.

    There must be also somthing there (e.g. R2, R3?) to balance the output between the two woofers as it always runs a risk of interference (i.e. comb filtering effect) when two drivers are near to each other and share some frequency range. Maybe this is all part of what Polk called a "cascade tapered array".
    http://www.polkaudio.com/polk-univer...rray-crossover
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