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Project- Passive LPF and Comb Filtering Abatement

Discussion in 'Amps and Cabs [BG]' started by coyoteboy, Mar 27, 2009.


  1. coyoteboy

    coyoteboy easy there, Ned Supporting Member

    Mar 29, 2000
    Sactomato, CA
    Hi TB, long time listener.

    Expanding a bit on discussions here and there regarding vertical stacking and comb filtering and various issues with driver placement in bass guitar speaker enclosures, I thought a little project in this area might be fun and informative, especially for me having yet to get my feet wet with electronics. Basically, the application is a low pass filter at the proper frequency and slope to one vertical pair of drivers in a 4x speaker enclosure in order to remove comb filtering interference and improve midrange and upper frequency dispersion.

    Pics will be forthcoming as this project hopefully progresses, but for now, a pic of the speaker in question is easily found, The Phil Jones Bass 4B extension. I like this speaker a lot, and am not trying to fix something that's broken, only see if I can make a great little speaker better with a relatively simple application of circuitry. A few reasons I think this speaker is a good candidate for this project:

    - I have two, so I have a control.
    - No existing crossover nor pesky HF driver to deal with.
    - The driver arrangement is exactly square, though I'm uncertain if this will make any difference.
    - These 5" speaks should have more high end content than a conventional 4x10, tweets aside, and therefore any improvements in dispersion should be more noticeable and useable. I do not have any testing equipment, but may try to work something in this regard, online software perhaps.
    - they are small and easy to move around, take on and off a table/bench, ect.
    - the large handle opening on the top of the speaker should make installation fairly straightforward despite a small cabinet and a small input plate.

    The first stage of this project is education for me, so I will accept recommendations for literature either electronic or cellulose regarding passive low pass filters. General texts about electronic circuits and speaker building are fine, but I'd like to keep this as simple as I can, and I'm sure I'll learn a bunch in the process.

    Thanks to BFM for getting me started, his reply to my question about this issue is here.

    Thanks all,

    Andrew
     
  2. Interceptor

    Interceptor

    Mar 29, 2005
    Madison, WI
  3. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
  4. Interceptor

    Interceptor

    Mar 29, 2005
    Madison, WI
    Wow, I really like Greenboy's link!

    That's an amazingly handy set of spreadsheets. They are getting added to my shop computer this weekend.
     
  5. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    The 4B looks pretty simple, with only four drivers. One vertical pair can be run full range, the other low passed at whatever frequency the one-wavelength center to center distance is. That will eliminate combing. If you pass them even lower, as low as the one wavelength distance from outside edge to outside edge of the cones, you'll get better midrange dispersion, but at some point the reduction in output would result in diminishing returns. The easiest way to experiment would be to run the two vertical pairs off a stereo power amp with a sweepable lowpass on one channel.
     
  6. coyoteboy

    coyoteboy easy there, Ned Supporting Member

    Mar 29, 2000
    Sactomato, CA
    thanks for the links, and thanks again Bill.

    per our prior Q&A, you established for me a filter frequency of 2.5KHz, based on a 5.25" measurement c to c. Below 2.5 the low-passed drivers in this case are still contributing most of their output. The outside distance is ~9" (do you include the surrounds or just the cone?). That gives us ~ 1.45khz, right? So in between is a matter of taste, really.

    Your idea of wiring the drivers separately with one side on a filter sweep is a good one, wish I still had my Alembic SF.

    thanks again.
     
  7. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    cb,

    Check out the Peavey VS 26. Or maybe that new Carvin Speaker Managment System. You wouldn't believe how handy an easy-to-operate unit that can do multi-in multi-out multiband EQ crossover compression limiting etc is for this kind of thing. Test tones and pinking the room, etc. Mine lives in the PA rack but gets used a lot for speaker stuff.
     
  8. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    They don't appear to be that close in pictures.
     
  9. coyoteboy

    coyoteboy easy there, Ned Supporting Member

    Mar 29, 2000
    Sactomato, CA
    I haven't taken the grill off, but based on a little tape measure action, it looks pretty dead on for 5.25" cone center to cone center. The speaker flanges are squared and butted up against each other and measure the same 5.25". The grey cones are just shy 3.75" and the outside of the surrounds are just shy 5".

    I've taken the top handle off for an inside look, and there's a horizontal shelf that could be as deep as the bottom port separating the top and bottom pairs each serial wired, so wiring the other way will require a little extra wire, or a hole in the shelf.
     

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