XO for 2.1 system

Discussion in 'Home Theater Projects' started by DanielGM, Jan 7, 2004.

  1. DanielGM

    DanielGM Stunt Coordinator

    Jul 15, 2003
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    I'm going to be using a passive diy sub powered by an external amp with my 2 channel system. My preamp has dual outputs, the XLR's are going to my main amp and so there are a pair of RCA's I can use to run the sub. (this is how I was running the system with my Infinity IL120s which has a built in line-level crossover). So I am looking to get an external low-pass crossover to use the new DIY sub with the full-range outputs of my pre-amp. The sub was very affordable so I really can't justify spending more than $100 on a XO, and if I can get it done for much less that would be great. (this rules out Marchand stuff) Are there any comparably priced XO's that compete with the Paradigm X-30? With my particular application I don't need the high-pass outputs at all (because I just feed the mains with the other full-range signal from the pre-amp) so it seems like a waste to buy the X-30, but I keep hearing that products such as Harrison Labs FMOD, which seem like they would be perfect for the application, are not up to the standards of a high fidelity two channel system.

    so what's the cheapest solution with the least compromise?

    Also, if I do use something like the FMOD, can I sum the L&R outputs simply with a reversed Y-splitter, then insert the FMOD, then use another Y-splitter to split the signal and feed two amplifier channels (it's a two driver sub)?
  2. Saurav

    Saurav Cinematographer

    Feb 15, 2001
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    First off, you cannot sum signals with a Y splitter. You'll need at least a pair of resistors, and if you don't want signal degradation, you're better off using an opamp circuit.

    If you only want the low pass section, then you're really looking for a line level filter, right? Not a crossover. Marchand has solutions starting from $10 (if you can assemble a kit), which I think is less than the X-30.

    I bought John Pomann's active crossover kit for $75 and built my own crossover. If you can follow instructions and cut wires, you should be able to build it too. Search for 'John Pomann' on Google and you should find his site. I think it uses pretty high quality parts, I'm quite satisfied with it.

    I had a setup with the mains running full-range and the sub low-passed. In my experience, you'll get a much better blend if you use a full 2-way crossover and high-pass the mains. Yes, it ads another component into the signal path, but in my room the mains were exciting the room modes down in the 22-35Hz region, even though their -3dB point is somewhere around 55-60Hz. I couldn't get the sub and mains to blend well at all until I high-passed the mains. I also added an equalizer to the sub channel, that made a huge difference too.

    Take a look at this thread if you're interested (it's on a different forum):


    Hope that helps. Feel free to email me if you have any questions.
  3. BruceD

    BruceD Screenwriter

    Apr 12, 1999
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    I agree with Saurav, a complete XO (both high-pass and low-pass) is a more desirable solution.

    In addition to the room modes Saurav mentioned, you will also get multiple frequency reinforcement and cancellation when both mains and sub are outputting the same frequencies because you are running mains fullrange. This will give you very uneven frequency response.

    I'd also suggest you might try a DIY XO, but the biggest problem you face is trying to determine what the best XO frequency is for you set of speakers and your room size. Not always a simple task.

    In many cases for sub integration with mains, a 24dB/octave L-R (Linkwitz-Riley) high-pass and low-pass XO with the XO frequency set between 1/2-1 full octave above your main's low frequency -3dB response is a good working setup.

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