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High Pass Filter (HPF) and Low Pass Filter (LPF) Mega Thread

Discussion in 'Amps and Cabs [BG]' started by Stumbo, Mar 12, 2018.

  1. Stumbo

    Stumbo Wherever you go, there you are. Supporting Member Commercial User

    Feb 11, 2008
    Intergalactic Mind Space
    Song Surgeon sofware
    With all the interest in HPFs and LPFs, it's time to put together all the info TBrs need to make an informed decision about purchasing one or both.

    HPFs and LPFs are tailored EQs. that significantly attenuate (make much quieter) muddy/sub-sonic lows(HPFs) or clanky/janky highs(LPFs), help adapt to boomy venue acoustics, help protect speakers from over excursion (speaker cone movement beyond its limits) and will get you to love your B string or down tuning even more.

    If you would be so kind to post your success stories (or not), pics, settings, rigs, amps, cabs, applications, basses, builders, prices, links, etc. it will be very helpful and educational.

    All questions, comments, fans, owners, future owners and passerbys are welcome!

    I thank you as do all TBrs learning about these great tools in our quest for the best tone for metal! I keed, I keed. :)

    Maybe not. :whistle:

    HPFs are also known as Rumble, Sub-Sonic or Low-Cut Filters. Filter as in "filter out". Sub-sonic means below an acceptable audible level.

    The terms High Pass Filter and Low Pass Filter are somewhat unintuitive.

    HPFs do not just deal with highs or treble. An HPF lets frequencies above a set level pass. For bass, it's usually frequencies around 35hz for fixed level HPFs and between 25hz-160hz for adjustable HPFs. These frequencies are chosen by the designer/builder.

    LPFs do not just deal with lows or bass. An LPF lets frequencies below a chosen level pass, usually between 4k hz to 15khz. They let lower frequencies than those pass.

    LPFs are usually adjustable from what I have read. These frequencies are chosen by the designer/builder. For the user, depending on your sonic goals, your ears pretty much decide on which frequency to use.

    For adjustable filters, the frequency level you choose depends on a few variables: bass, amp, cab, band instrumentation, venue, effects and your ears.

    Limiter: Different than an HPF/LPF, is a circuit that limits the maximum level of a signal. It's a circuit that allows signals below a specified input power or level to pass unaffected while attenuating the peaks of stronger signals that exceed this threshold. Limiting is a type of dynamic range compression. Clipping (audio) is an extreme version of limiting. Many amps have Limiters built in.

    HPF/LPS run on very small amounts of current in pedal format (switchable or always on) and are usually hooked up between the instrument and the input jack, in the amp effects loop or between a preamp and power amp. They are usually powered by batteries or low-voltage power supplies(wall warts). Many amps have fixed frequency always on HPFs built in, some being adjustable with a knob on the front of the amp. Some effects have an HPF or LPF built in.

    If you have other effects, placement in the signal chain depends on your sonic goals and your ears. Check out the TB Effects Forum for more info on this topic.

    Why you need an HPF
    Part 1

    Mike Arnpol, MAS Cabs
    From: Is this good? Boom Bass Cabinet

    Most cabs that say 30hz---are down more like 20db at 30hz. But--your perception about 30hz extension is valid . The reason is that almost all cabs claiming this are reflex cabs. Here's the problem with that extension with a reflex cab:*

    1. most of the information at that frequency is coming out of the port. That quality of bass is very poor and uncontrolled. The great home hifi speaker designer John Dunlavy told me " I don't want to hear any bass coming out of a port." Ported enclosures are a cool compromise--you can get loud, have a small enclosure, and have good efficiency. A compromise is in the bottom octave or so. A sealed box that actually goes that low will have a much better quality of bass---but they will be huge, won't go as loud, and would be low efficiency.

    2.All of the drivers that I know that aren't designed exclusively for subs--all of the woofers used in bass cabs---have a free air resonance of 40 to 50 hz. They don't work well under that.

    3.Most reflex cabs are tuned (typically) in the 42 to 50 hz range. What this means is that any frequency under that tuning frequency--the speaker behaves as in free air. Which means that it's flopping around aimlessly.

    What this means is that when you play a note under that tuning frequency--typically under 42hz (or higher in many cases) The speaker is just flopping around trying to reproduce that note.

    What happens is that since the cone movement is uncontrolled, the second harmonic (octave) is being reproduced poorly as the cone is uncontrolled.

    The second harmonic is what we key off of for pitch. That second harmonic then is boomy and uncontrolled.

    This is the reason that many guys (correctly) say that having that extension causes too many problems on stage---not the fact of reproducing 30hz.

    People should use a HPF set to around 40hz. All of a sudden the B string tightens up and you lose the boom. Running a reflex cab without a HPF will result in a crappy B string sound.

    There are however, some cabs that are of a different design that don't unload and can reproduce the B. When you hear a cab that goes that low and play on the B string---all of a sudden it sounds and feels like the other strings. The fundamentals in that range---while not really needed for an effective B string--do add a pretty neat cushion that does not get in the way.

    *Note: A bass reflex system is a type of loudspeaker enclosure that uses a port or vent cut into the cabinet and a section of tubing or pipe affixed to the port. This port enables the sound from the rear side of the diaphragm to increase the efficiency of the system at low frequencies as compared to a typical closed box loudspeaker or an infinite baffle mounting.

    Part 2
    @agedhorse, MESA, Genz Benz
    from: 8 ohm cab question

    There are different metrics used by different manufacturers (and even different models within a brand) that might be used to describe exactly the same power.

    For example, on an amplifier that is rated at 500 watts "RMS", it is also going to be capable of delivering 1000 watts peak, because the mathematical definition of the peak voltage of a sine wave (used for ratings) is 1.414x the RMS voltage, and when this 1.414 factor is squared in the power equation, it results in 2x.

    The same thing applies to speakers, but with an added twist based on some old colloquial information that you may not be aware of.

    Speakers tend to be rated in two ways, the first being the same "RMS" (or continuous average) and peak that we discussed above for amplifiers.

    The second way, which is more common in the pro audio industry, is 'RMS", "program" and "peak... the meanings being different here.

    "RMS" is the same, program is defined as 2x the RMS value and comes from when speaker manufacturers were very conservative in their ratings and clipped/compressed program is excluded, and peak which is defined as 2x the program power or 4x the peak power.

    So, a speaker rated at 500 watts "RMS" might also be rated at 1000 watts "program" or "2000 watts peak.

    Which units that are being used is often left to the imagination (or the marketing departments). Also, most speaker power ratings are based on a "must survive, damage allowed) time period of 2 hours. When you extend this test to say 200 hours it's common to see a 1/3 reduction in power rating.

    Another factor is that the mechanical part of power handling starts falling rapidly at low frequencies (often below 60Hz), right where the biggest demands are placed on them. At say 30Hz, the power handling might be only 50% of rated speaker power.

    The challenge is that you have an amp like the Subway, which is rated at 400 watts RMS into 8 ohms, and somebody buys a "500 watt" cabinet thinking that it will be fine BUT it's 500 watts "program" or only 250 watts "RMS" and maybe not so fine. It's even worse if these numbers are peak units.

    Therefore, it's not anywhere near as easy as it seems, and this is one big reason IMO why many cabinets fail through no direct fault of the user.

    Part 3

    Why do I do this to myself...

    Why you need an HPF/LPF

    Broughton HPF LPF question
    Last edited: Mar 20, 2018 at 1:59 PM
  2. I like being able to plug into any little backline "rig'' and drop a low B bomb without farting it out.
    RandM, JaseyT, Pbassmanca and 4 others like this.
  3. bholder

    bholder Affable Sociopath Supporting Member

    Sep 2, 2001
    central NY state
    Received a gift from Sire* (see sig)
    Seems to me to be a good argument that we need better drivers. ;)
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Or better overall designs, but that increases cost.
    Dasgre0g, RandM, Pbassmanca and 3 others like this.
  5. Better drivers, more cost. And maybe not the bang for the buck you'd expect.
    I suppose there comes a point of diminishing returns.
    RandM, Pbassmanca and Stumbo like this.
  6. Wasnex


    Dec 25, 2011
    Nice to see the HPF considered in the context of cabinet design and tuning.

    About 30 years ago I used to EQ my SVT and Ampeg 810 using a couple of bands of parametric EQ to peak my open E string. I believe in total I was using about 14-18db of boost depending upon the sound I wanted. The system was crossed over at 200hz to a pair of JBL E120s. This setup felt really quick and responsive, and the sound was very tight and controlled. I received lot's of positive comments about my sound.

    Using the same SVT, E120s, and EQ technique with a pair of EVM 18Bs in small cabs ported at 50Hz produced a loose sound that a respected educator described to me as "woomy." I have a pair of 18Bs in some huge TL405s that are ported in the high 20s. They only need a few db of boost to be flat to 40hz. It's amazing how much better the 18Bs sound in the bigger cab. However, the sub frequencies can be a bit obnoxious with an amp like a Mesa M9 especially if you bypass the amp's HPF by engaging the deep switch. Adding an HPF in the low to mid 30s would probably help eliminate nasty artifacts that I suspect are below the fundamental of the low B.

    Currently I have the 18Bs in the little cabs set up to cover 50-100hz with a steep HPF (48db/octave) at about 48 hz. With a couple of mid/high packs they sound great with bass, but don't really confer the same heft and weight as the larger cabinets.
    RandM, Pbassmanca and Stumbo like this.
  7. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    I use a HPF religiously with my DB to avoid feedback. While the HPF effect is audible, it's interesting that I can set the rolloff quite high (like 100hZ or more) before the open E note starts sounding weak.

    I have no use for a LPF, I just use the speaker's natural rolloff. I turn off the tweeters in any cabs that have one.
    Pbassmanca, LeonD, Pat Harris and 2 others like this.
  8. Wasnex


    Dec 25, 2011

    IMHO, some would find a 3K LPF useful on a DI to somewhat simulate the roll off of a speaker. When I am running sound I usually don't engage an LPF on bass unless the signal is really bright and obnoxious or perhaps extremely hissy.
    mesaplayer83 and Stumbo like this.
  9. MrLenny1


    Jan 17, 2009
    ??? Do I need a HPF or LPF ???
    Educate me, Please.
    I use Active 5 string basses and
    Eden 2x10 XST cab rated for low B IIRC.
    filwitheneff, 12BitSlab and Stumbo like this.
  10. You need a HPF to tame the lows.
    You need an LPF to tame the highs.
    Chances are you need to tame the lows more than the highs.
    ctmullins, RandM, jmone and 6 others like this.
  11. GrapeBass


    Jun 10, 2004
    Graphic designer: Yorkville Sound
    Initially I bought mine to help while recording direct but the uses for it live are great. While most cabinets don't go so low, adding something that tempers it will prevent LF content that could muddy up your sound but also interfere with other instruments (such as keys and drums). It also helps your power amp The Low Pass filter tames the HF much better than your tone can as it is not centered at a specific frequency, you dial in where is appropriate. I find this helps open up space for the drums (cymbols etc.) and other instruments that need more crispy highs.
    RandM and Stumbo like this.
  12. mesaplayer83

    mesaplayer83 Supporting Member

    Jun 27, 2017
    I have found a variable HPF to be way more useful than I ever thought it would be - with ZERO drawbacks... For those looking for an external HPF, I would strongly recommend fdeck's HPF-3...
  13. dukeorock

    dukeorock Owner BNA Audio Supporting Member Commercial User

    Mar 8, 2011
    Nashville, TN
    Authorized greenboy designs builder/Owner of BNA Audio
    I have a couple fDecks and a Broughton Audio HPF/LPF pedal. I used to strap an fDeck to my upright tailpiece to get impedance sorted as close to the source as possible. Works great, but these days I just use the HPF in my Grace Designs Felix pre.
    I use the Broughton HPF/LPF with my tube amp...cleans up some low end slip and gives me extra headroom on the amp. Just use the LPF as needed
  14. mesaplayer83

    mesaplayer83 Supporting Member

    Jun 27, 2017
    I can get highs under control quite easily without added outboard gear - after using it for a little while, I will definitely be keeping my HPF... IMO, HPFs shine their most when used with smaller rigs - mine really allows me to get the most out of my Rumble 200 Combo, for an example...
    Stumbo and dukeorock like this.
  15. I keep a Rane PE17 in a small rack of misc gear, which, in addition to the five bands of parametric EQ, also has a high pass filter and a low pass filter. Great for keeping excessive lows and irritating hiss out of unknown equipment. When needed, it usually it gets patched in at FOH. But I've also used it to clean bass amps and other problems on stage. I have another that lives permanently in my wireless acoustic rack for the low pass and to tame howlers. When you need it, there's nothing better. It's older and I'm sure discontinued, but can be found relatively cheaply. Digital mixing has meant that I'm using it less than previously at FOH.

    Last edited: Mar 14, 2018
  16. hintz


    Jun 5, 2014
    wahiawa, HI(Oahu)
    Subbed, I wanna learn more about this
    Stumbo, 12BitSlab and dukeorock like this.
  17. Roxbororob

    Roxbororob Supporting Member

    Jun 8, 2015
    My fdeck HPF was one my biggest bang for the buck gas buys. Like another TBr it tamed my SM400. It is my always on pedal with any amp even DI to FOH.
  18. I keep my FDeck HPF v3 on my pedalboard. I set the frequency knob to about 1:00, which (in the absence of any helpful markings) I suppose is somewhere around 70 Hz. This setting tightens up the low end while still giving a good, powerful sound.

    I don't use an LPF, but I don't seem to need one. Maybe if I had an amp that exaggerated the treble I would want an LPF. My typical amp setup at the moment is an Ampeg SCR-DI into the effects return of either an Eden Traveler 400 with an Eden 4x10" cab or a Fender Rumble 500. The Eden setup sounds better, but the Fender is much more portable.
    dukeorock and Stumbo like this.
  19. mesaplayer83

    mesaplayer83 Supporting Member

    Jun 27, 2017
    Yup - same here... I've also gotten excellent results using my same pedal board for my AEB gigs and on my electric bass gigs:

    Stumbo likes this.
  20. wraub


    Apr 9, 2004
    ennui, az
    deviated prevert
    The high pass filter in my pre is only 30 Hz, but it's always on. I'd (almost) never play without one.
    Stumbo and mesaplayer83 like this.