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Passive subsonic filter, EE's help?

Discussion in 'Amps and Cabs [BG]' started by zagnut, Mar 16, 2009.


  1. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    Hoping this is the right place to post this.

    Sometime in the near future I'll be replacing the 18 in my homemade cab. After playing with modeling in WinISD, I want to put a SSF in there. I want to build one, buying one is not an option! (Unless it's less than $40).

    I'm quite capable of building it, but I don't know what values I need for the inductor and cap. Just looking for a simple 12/db butterworth, nothing fancy.

    New driver will be 8 ohms and eventually pushed with 300-400 watts. Looking for a x-over point of 35Hz.

    On a side note, I've seen the PFMOD's adjustable filter, but not sure how well it will work with the signal being converted from 1/4" or XLR to RCA.
     
  2. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    You don't get a lot of protection from a 2nd-order filter without also cutting up higher into usable signal content. And passively, this is going to require real hefty components. Better solutions include power amps with hp filter, more robust drivers in better enclosures or better EQ facilities at preamp/head.
     
  3. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    35Hz allows what I need and still leaves me with a -3db point of 42.67Hz as well as keeping xmax in check unless a 400 watt 32Hz signal is present, which won't be often if at all.

    Hefty components are available and shouldn't cost more than $40. Other equipment not an option due to cost and lack of space. Driver is robust, Kappalite 3015LF.
    I appreciate you're input on this, but I just need to know what values I need to build this. Believe me, if I had the money and space, investing in a pricey preamp with a SSF or outboard active filter would be my first choice.
     
  4. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    http://www.diyaudioandvideo.com/Calculator/XOver/

    You really don't need much cut into the response curve of a KAPPALITE with a typical bass guitar signal (in fact you are wasting its potential) if the cab isn't tuned way high. Most guys could use it forever and not exceed xmax unless using ungodly amounts of wattage and feeding it sub-bass signals from synths and using excessive low EQ boosts. A bass guitar just doesn't make much fundamental on low notes - in many cases it's 6, 10 or more dB down from the octave overtone, which is equivalent to quarter the power, or 1/10th the power.
     
  5. To build a passive filter will I suspect cost a bit more than $40. To build a solid state high pass filter will cost much less and work far better. Greenboy is right on the mark.

    I'm doing the same thing to my rigs to rid myself of unwanted subsonic noise from my piezo equipped instruments.

    Paul
     
  6. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    Cab will be tuned to 45Hz to get the flattest response curve I can get. Ungodly amounts of wattage and feeding sub-bass from synths is a possibility. Might go with 4th Order Linkwitz-Riley, but that would about double my cost and not be much benefit until 400 watts or more is reached.
    Thanks for the link!


    This is exactly what I'm doing.
     
  7. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    BassmanPaul is talking an active circuit in the preamp.

    But bass players with heads/preamps with the right controls can often use a combination of the facilities to get LF control and still have a bassy sound. Which is good, because most of the bass guitar cab drivers in use are not very good at long excursion ; }

    By the way, I've been using 3015LF since it came out, so a certain amount of what I'm saying is in appreciation of its use in real-world conditions.
     
  8. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    I may just have to find out for myself, as finding inductors and caps the size I would need is proving to be much harder (and more than) expected.
    So, this brings back to my original post. I mentioned something called PFMOD -
    http://store.hlabs.com/pk4/store.pl?view_product=12

    What kind (if any) problems would I run into if I were to use this on an XLR or 1/4 line, with a XLR (or 1/4) to RCA converter.
     
  9. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    What do you think is inside of that? I'm guessing inductors and coils if you are lucky - only vastly underspec'd for the amount of power you are talking about.
     
  10. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    This is line level (between signal and amp), as opposed to my original plans of using a post amp SSF (between amp and driver).
     
  11. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Oops, sorry - brainfart. I guess I should read their copy. I wouldn't be all that confident that anything with RCA on it could be all that great, though...
     
  12. Red Planet

    Red Planet

    May 29, 2005
    Atlanta
    My guess is that little box is gonna mess with your tone big time.

    I doubt youll need it. You would have to be packing some srious muscle to reproduce signals that low to a dangerous level for that driver.

    I run some sho nuff Joules to my Speaker Cabs and when they start squawking at me I turn down just a tad. :D
     
  13. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    Same here. Looks like my only other options are a cheap rackmount EQ with a non-adjustable SSF or a parametric EQ.
    Unless I can find a decent head for no more than $300 with a SSF built in, and I'm doubting a cheap head will have that feature.
     
  14. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    Found an interesting thread here at TB where the OP is looking to do the same as I, even mentioned the FMOD filters.
    After going through the 5 pages, there's a member here who used to make exactly what I'm looking for with 1/4 inputs -
    http://personalpages.tds.net/~fdeck/bass/hpfpre.htm

    The member who made these was fdeck. Not sure if he's still making them. If he isn't he has his documentation and schematics listed -
    http://personalpages.tds.net/~fdeck/bass/hpfpd.pdf

    Won't be hard to build this baby, but gonna keep looking before I make up my mind.
     
  15. i'm a bit lost as to why you feel it is necessary to have a rolloff below 35hz between the speaker and the amp, especially when using a speaker that will handle alot of lows. if it is for using a synth through it, why not making a filter that goes to the input of the amp? wouldnt be any simpler, but the parts would be alot cheaper and smaller.

    sorry if this isnt much help, but i'm struggling to figure out why you feel this is necessary or even mildly helpful.
     
  16. bongomania

    bongomania Commercial User

    Oct 17, 2005
    PDX, OR
    owner, OVNIFX and OVNILabs
    I too think a filter belongs in front of the power section, not after it. Putting the filter after might have the benefit of keeping any large subharmonic spikes from overexcursing (real word?) the cones, but by putting the filter in front of the power section you get that benefit plus the more consistently useful benefit of focusing all wattage into the audible/useful range, rather than allowing subharmonic content to suck up all your watts.
     
  17. greenboy

    greenboy

    Dec 18, 2000
    remote mountain cabin Montana
    greenboy designs: fEARful, bassic, dually, crazy88 etc
    Yep, best place for it is usually at the end of the preamp section, right before power section. This way any onboard EQ or gain accidents are handled, and anything that makes big stuff like envelope filters in effects loop stay quashed too. Very ocassionally with my V-Bass I might have put the equivalent of a HPF early in its configurable chain to keep keep a really phat signal from eating up internal bandwidth, but that's a luxury since I always have one at the end.
     
  18. zagnut

    zagnut

    Jan 4, 2009
    Back in Detroit
    Well for 1, synth may very well find it's way to this cab. 2. It is quite possible to send subharmonics through the cab with an ill placed slap or pop. Modeling for my cab shows this driver will reach xmax at 31.59Hz with a 400 watt signal. While xlim is really more of the concern, it's better safe than sorry.

    I do agree that it's best to put the filter on the pre side of things as opposed to post-amp. Passive crossovers aren't exactly known to be efficient. But I was thinking it would be cheaper to design a post-amp filter as opposed to pre-amp. After seeing the values needed for a 35Hz filter and trying to find inductors and caps that are even close to what I'd need, the cost would be non-beneficial. I'm farther ahead whether I build my own pre-amp filter or use something like a Behringer Ultragain Pro MIC2200 ($100). In either case, it would seem that filtering this before the signal hits the amp stage is going to not only be better, but easier on the wallet.

    This will also keep me from wasting precious amp power trying to reproduce anything below (practically useless anyways), which translates into less energy consumption and less heat = amp longevity. It's a win-win situation for the amp, the driver and my sanity.
     
  19. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Exactly! And the real killer with trying to design a passive high pass filter is that below the tuning frequency the impedance has a big resonant peak so you'd need to add an impedance compensation circuit to get even vaguely close to the response you need. In reality I'm not even sure it's possible to do with passive components because of this and also because at the high power levels when the protection is most needed the temperature changes in the speaker voice coil and the inductor coils will play havoc with the response.

    Alex
     

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