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FMOD High Pass Filters - Where to run them?

Discussion in 'Amps and Cabs [BG]' started by teedub, May 1, 2006.

  1. Hi all,
    I'm thinking about adding an FMOD 30 Hz High Pass Filter to my rig, and have heard of people running them through the FX loop, and also in the line out from the pre. Is there an advantage to running them one way over the other? Are these little devices a signal eater? I searched, but there wasn't a whole lot of info about these devices on these boards.

    Also, if anyone had an extra one (they're sold in pairs) I'd gladly buy it from them. The shipping to Canada from Parts Express is little silly for such a small package. They could be shipped in an envelope!


    On the same note, does anyone prefer to run it at -24dB/octave (with the 3dB attenuator), over the single -12dB/octave? Do you find it cuts too much out at -24dB?

    Lots of questions, I know.....
    Hopefully you guys have the answers. :)

    (Thanks in advance to bikertrash82, and remo for your help and suggestions.:) )
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Both would work, but.... why? A HP filter is a necessity for PA, but is about as useful as a screen door on a submarine when it comes to bass.
  3. Eric Moesle

    Eric Moesle

    Sep 21, 2001
    Columbus OH
    Those FMODS are made for home stereo use, that's why they are RCA adaptors. Don't use them for bass rigs.

    If you really need a 30HZ high pass filter for a bass rig, either get a power amp with such a feature built-in, like the QSC's, or get a proper crossover rated for such an application.
  4. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Don't agree....

    A suitable HP filter can really clean up the sound..... Especially for someone who likes to boost everything that says "low" on it......

    By the time a couple of shelving controls are boosted, there can be a lot of power used shuttling air in and out of the cab at inaudible frequencies.
  5. seamonkey


    Aug 6, 2004
    Sadly the QSC PLX 1804 doesn't have a rumble filter.

    Another option is to pick up an active crossover. Besides the rumble filter you can use it to split the frequencies between the cabinets. No sense sending highs or lows to a cabinet that can't reproduce them. It sure can be a transparent sound.
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Not at 30 Hz. 60 Hz I could see.
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Yeah, "suitable" is the operative word.....

    I can see 40 Hz even......

    Misinterpreted you to be suggesting ALL HP filters were of no use......
  8. Exactly! Thanks Jerrold!

    You can actually pass subsonic freq's just by thumping the strings. Turn up the amp, tap the strings, and watch the speakers jump. I set my preamp EQ flat but it's still got a lot of low end. The main reason I would like to this would be to allow my power amp to be pushing freq's that are 100% usable, and not wasting valuable wattage pushing air that can't be heard, and could possibly damage the speakers. Subsonic freq's soak up a good percentage of your power, especially when it's time to floor it.

    Sad indeed, I think that's a feature they should include on every model.

    Been the bi-amp route before, and wasn't that keen on it. Both my cabs now are full range and can handle all the appropriate freqs. The less options, the better for me. (But you'd never be able to tell......;)) Thanks for the ideas, though. The active crossover is also a possibility, but I'd prefer the line-level FMod if possible. It's a relatively simple, inexpensive solution.

    Would you recommend that over the 30Hz? I initially chose 30Hz because the freqs above it are generally safe and audible, and I didn't want to cut too much out.
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    I think 30 is a litle lower than you'd ever need, and probably doesn't do as much for the speakers as it could.

    The suggestion of 60 is good, it might work fine, but I suspect some people might start noticing a difference, depending on what they want.

    40 or 50 is sorta middle-of-the-road.....

    Best is probably somethng with a knob on it..... so you can set where you want. I'd go with as high as you can stand, simply because that should put more of your available power in audible sounds.

    The whole EQ thing is an issue..... the low is usually shelving, or extended response, anyhow, and what does nice things in the more audible range may cause trouble in lower non-audible frequencies. Even a parametric isn't the only answer, because it often can't get the low cutoff if it is set wide enough not to sound odd.

    Hmmm..... I think I have something to do now......
  10. Steve


    Aug 10, 2001
    If you're like me and use acme cabinets and you really don't wan't the fundamental of the 5 string... or if you want to make it really difficult for a less than excellent sound man to mud you to death..or you play a normally voiced bass cabinet and you don't like to see it beat itself to death trying to reproduce frequencies it really can't...

    Then those 50hz FMODS are a gem.

    The 30hz are IMO, pretty useless as Bill said.
  11. joelb79


    Mar 22, 2006
    Lansing, Michigan

    You would want a 24db / octave at 30hz. Unless you play a Low F, your not going to need anything below 30hz anyways. and i can bet some money that your cabinets are already down a few dB's at 30hz anyways.
  12. seamonkey


    Aug 6, 2004
    If you're good with a soldering iron, you could check out:

    It'll take you through a general purpose input filter.
    You can change the values to provide different tunings.
  13. I'm a hack with a soldering iron. That's sort of why I was hoping to go the "plug and play" route.

    I only play a 4 string and don't plan on extending to a low B anytime soon. I have been known to drop the E to a C, but rarely, only when called upon.

    Is it recommended to always run the two filters in-line with the -3dB attenuator, (giving -24dB/octave)? Is there ever a reason to just use the one, (giving -12dB/octave)?

    I guess I'd be leaning towards the 40-50Hz ones then.......

    Problem being, Avatar doesn't list the freq. response of their cabs on the site. I assume that they are both full range, but how low will they go? I guess that's a crucial point in deciding which filter to get.
  14. remo


    Jan 15, 2005
    I just purchased a pair of 30Hz HP FMODS directly from Hlabs. Just call them up (number is on website). I will experiment with running it between the preamp (Alembic F-1X) and the poweramp and through the preamp FX loop. I went for 30Hz because this is ballpark where most poweramps opt at rolling the lows off. I also play a 5 string so I want the fundamental of the low B to come through.. I'm only worried about stopping the really sub stuff.

    here's a thread I started in the Eden forum about the same issue.. I was trying to find out if normal bass amps cut the subsonics.


    Just because the FMOD is designed for car audio and has RCA connectors does not mean it won't work for bass guitar.. Just as long as you run it at a line level juncture of your rig ie. pre -> FMOD -> Poweramp or Preamp FX loop. You just need to make up or buy some connectors for it (I soldered my own).. simple.
  15. Remo:
    Did you order the -3dB attenuator as well? I think they mentioned on their site that you would need that in order to stack them.

    I guess I'll wait for you to give a full review before I make any decisions. :D
  16. remo


    Jan 15, 2005
    no.. i didn't order any attenuator.. just plan on running the 30Hz HP solo! I won't be getting my PLX2 1804 until the 18th of this month so I'll bump this thread then with a review!
  17. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    By and large yes, though not necessarily intentionally so. Since bass amps don't require flat response from 5Hz to 100kHz few manufacturers spend the extra $ required to get it.
  18. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Actually, it costs little to get the response, a little more to get it in a "meaningful" way..... and quite a bit more to get enough power to USE the low frequencies. (the high end... well....)

    The issue is that even if the amp naturally rolls off the "useless" lows, as all I know about do, if you then apply a couple low end boosts, that tends to "undo" some of the natural cuts.

    So if you use a low boost on the basic EQ, and turn up the low band on a graphic EQ, you may have just cancelled out some of the effect of a 12 dB / octave low-cut filter.

    You can't cancel it entirely, but you might easily have extended the lows as much as an octave. That may make speakers unhappy, and can run you out of power faster.

    Tube amplifiers automatically have a low rolloff due to the transformer, not to mention all the capacitors between stages inside. So some controls that are very usable on a tube amp, may need to be used a bit differently with an SS amp.
  19. remo


    Jan 15, 2005
    ok so I'm finally running the QSC PLX2 1804 with the 30Hz HP FMOD from Harrison Labs.. I tested the setup with and without the FMOD and to my ear there was no difference. With the FMOD in my signal path my rig sounds just as fat as ever. So anyone considering going down this path with an '04 model PLX2 I can say it works FINE!

    My new PLX2 absolutly creams the Mackie 1400 I was running, and it is just heavenly to pickup now.. like a feather in comparison! I did all the cabling myself (1/4" to RCA male to FMOD to RCA female to XLR). It was pretty easy you just have to keep your polarities correct and don't forget to wire pins correctly when going from the female RCA to the XLR:

    Pin 1 and 3 Ground (shield, common)
    Pin 2 Positive (Signal, hot)

    I have to shorten the cable so it's not so unwieldy.. but here it is...

  20. We're rigmates now!:hyper: (Except you've got that tasty F-1X..........)

    Glad to hear that it's working out for you! Serious power, eh?
    And thanks for the wiring tips!

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