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Thread: M19 crossover mod question

  1. #1
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    M19 crossover mod question

    I've got my k145/511b/BMS4552 speakers up and running with a M19 crossover. All is fine (great actualy), but I find that I'm running my pots at the low end of the scale. I've already switched the R3 pot over to it's 8 ohm terminals but I find that I'm still running both pots at approx. 1ohm or less. I guess the BMS driver is more efficient than the Altec was.

    Could I increase the 3ohm R6 resistor to, for example, 6 ohms and thereby get my pots into the centre range?

    I'm presently running an 18 guage coil at L2 because I was playing with it to set the crossover for the k145. I will be replacing it with a 14 guage which should result in 0.5 ohms less DCR. Would this be a significant gain for the woofer or should I still expect to cut the horn driver somehow?

    Thanks

    Jorg
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    Quote Originally Posted by Jorg
    Could I increase the 3ohm R6 resistor to, for example, 6 ohms and thereby get my pots into the centre range?


    (i) Conceptually, your approach is what's required to add some play or "room to move" back into those variable LPads.
    - Therefore, you get an "A" for understanding the proper direction to move towards ( ie ; adding resistance to burn off excess voltage levels ).

    (ii) Functionally, you don't get a passing grade . You don't appear to understand the consequences of your simple approach. Adding inline resistance will change the load impedance that the LC components ( located earlier in the circuit ) will "see" & work into .
    - Changing the load impedance alters the F3 ( turnover ) points of the HiPass section of the crossover . Unless you want to alter the crossover point of the HiPass section / you don't want to just add inline ( or parallel / conjuagte ) resistance .

    (iii) One solution ; find an online Lpad calculator / and use it to calculate the resistor values for a simple fixed 8 ohm Lpad that's set to attenuate anywhere from 3 to 6 db . I'd try 3 db of extra padding first to see if that returns enough play ( in the variable Lpads ) .

    - Once you have the values build this fixed ( two resistor type ) Lpad into the circuit just before the driver ( after R6 ). At this point you'll notice that you have two inline resistors connected to each other ( R6 & Rwhatever). Blend the two into one ( add the values directly ) and replace the two resistors with the single blended value ..


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    Quote Originally Posted by Earl K View Post

    (iii) One solution ; find an online Lpad calculator / and use it to calculate the resistor values for a simple fixed 8 ohm Lpad that's set to attenuate anywhere from 3 to 6 db . I'd try 3 db of extra padding first to see if that returns enough play ( in the variable Lpads ) .

    - Once you have the values build this fixed ( two resistor type ) Lpad into the circuit just before the driver ( after R6 ). At this point you'll notice that you have two inline resistors connected to each other ( R6 & Rwhatever). Blend the two into one ( add the values directly ) and replace the two resistors with the single blended value ..

    I'm still struggling with electronics. The first time I looked at any of this stuff was in June when I found the schematic for the M19 crossover.

    The R2 resistor by-passes/bleeds off current from the driver, which is what I want, but would result in a lower load impedance seen by the circuit, so the R1 resistor is added to bring the load impedance back to original?

    Jorg
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    Quote Originally Posted by Jorg
    The R2 resistor by-passes/bleeds off current from the driver, which is what I want, but would result in a lower load impedance seen by the circuit, so the R1 resistor is added to bring the load impedance back to original?
    - Both R1 & R2 drop the overall db level.

    - You're correct in your thinking that R1 restores the target impedance ( 8 ohms in this case ) / that has been dropped ( below 8 ohms ) due to R2 being paralleled across the driver .


  5. #5
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    A question on the R3 and R5 "Mid" or Horn compensation.
    I'd like to get another 3db of cut in this area. If I remove the R5 resistor am I messing up the load impedance. Will removing the R5 resistor be insignificant as I have already set the 35 ohm R3 pot to zero.?
    Should I look at C4, R2&R4 instead to get more cut in this area?

    Jorg

  6. #6
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    Quote Originally Posted by Jorg
    A question on the R3 and R5 "Mid" or Horn compensation.
    - R2, R3, R4, R5 along with C4 , form a midrange suppresion filter of sorts .
    Quote Originally Posted by Jorg
    I'd like to get another 3db of cut in this area.
    - Okay


    Quote Originally Posted by Jorg
    If I remove the R5 resistor am I messing up the load impedance ?
    - Yes you would be mucking up the range of intended load impedance(s) .

    Quote Originally Posted by Jorg
    Will removing the R5 resistor be insignificant as I have already set the 35 ohm R3 pot to zero.?
    - Removing R5 is "significant" since that means R3 is no longer connected to ground and becomes inoperable .
    - Or /Removing R5 and replacing it with a direct connection to ground ( piece of wire ) is significant since now you've established a short-circuit ( to ground ). The amp is unlikely to blowup since R2 & C4 still present it with a minimal load of @ 3.6 ohms .

    Quote Originally Posted by Jorg
    Should I look at C4, R2&R4 instead to get more cut in this area?
    - You need to employ the services of someone ( such as Zilch ) who has all these parts on hand, to custom design a filter to suit your projects' needs .

    - Alternatively, you can do this filter redesign empirically ( step by step, & be prepared to risk blowing up things on your own dime ).
    (i) Keep R5 as 2 ohms
    (ii) Replace R3 with a piece of wire
    (iii) Increase R2 to 7 ohms
    (iv) Report your findings


  7. #7
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    Well I've made some of the changes with some success.

    I moved the R3 pot into an Lpad position at the tweeter to establish the amount of attenuation needed for a fixed Lpad. An additional 12db

    I then jumpered the C4 capacitor right past R6 to increase the sorta parallel notch filter attenuation.

    This seemed to work quite well by ear. I have to get the TACT back to take some readings. I think I may still add a 1mh coil to make a real LCR because I was allready getting too much attenuation at about 1000 to 1400 Hz.

    I added the extra resistance at the R4 resistor beacuse it was easy. Any speculation on the difference between adding 4 ohms to R4 only vs. adding 2 ohms to R4 and 2 ohms to R2? Is there any significant HF signal that passes through C4 making its way through R4, R3, and R5?

    Jorg

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