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  • Active XO w/o op amps

    Hi all long time since I've posted, but I'm going crazy trying to remember an old active cross over design I got back in the 80's.

    I'm not much on the electronics end, but I recall I got a design with a few resistors and a cap to make a low pass filter for my car sub woofer amp.

    I'd like to play arround with the active filter designs if I can find some decent how to literature and examples that are not using op amps and a power supply.

    Seems like FMOD is a manufacturer that is making something similar?

    Thanks for any help
    John H

    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

  • #2
    Re: Active XO w/o op amps

    Actually what you are describing is a passive line level crossover. GOOGLE "Passive Line Level Crossover" and a lot of links will pop up. Here is one example.
    http://www.t-linespeakers.org/tech/f...ssiveHLxo.html

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    • #3
      Re: Active XO w/o op amps

      That's it!

      Thanks

      John H
      John H

      Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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      • #4
        Re: Active XO w/o op amps

        Had not seen this particular page before.

        Thanks for posting it I always wondered why no one did it that way, but I guess they do
        Thanks'

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        • #5
          Re: Active XO w/o op amps

          Originally posted by Texan View Post
          Had not seen this particular page before.

          Thanks for posting it I always wondered why no one did it that way, but I guess they do
          i think the biggest reason it's not used more often is because you have to know, and account for, the input impedance of the amp it will be used with. so it's not very versatile, since you can't necessarily pick it up and take it to another amp. (well... you can, but you won't necessarily be crossing at the same frequency).

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          • #6
            Re: Active XO w/o op amps

            Originally posted by absolootbs View Post
            i think the biggest reason it's not used more often is because you have to know, and account for, the input impedance of the amp it will be used with. so it's not very versatile, since you can't necessarily pick it up and take it to another amp. (well... you can, but you won't necessarily be crossing at the same frequency).
            True, but one possibility might be to place it between two gain stages in a preamp. This way you could use the initial gain to help with insertion loss and provide a high input impedance for the player. The second gain stage could have a set high input impedance, and low output impedance. This may be advantageous if the desire is to build with discrete components, since it can be difficult to get enough gain to make as effective of use of negative feedback type filters.

            I guess this is not the best method, but rather just a method
            Thanks'

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            • #7
              Re: Active XO w/o op amps

              Most amps have at least 10k input impedance, if not 100k. For a C-R highpass filter, you can just size the R to be an order of magnitude smaller than the imput impedance of the amplifier.

              For the R-C lowpass filter, you will need to shrink the R so that the insertion loss doesn't become huge, as the R and the imput impedance resistor will form a voltage divider.

              You can build a crossover without fets. Use tubes, or use discrete transistors.

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              • #8
                Re: Active XO w/o op amps

                Originally posted by paulbirkeland View Post
                You can build a crossover without fets. Use tubes, or use discrete transistors.
                True, but I'm not sure doing so will yield improvements. The current crop of opamps are quite transparent.

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                • #9
                  Re: Active XO w/o op amps

                  Adding to the use of Opamps - using a well regulated/low noise power supply is crucial. Moreso with a discrete design because of very the low PSRR of many "lightly engineered" designs. ;)

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