High Pass and Low Pass Filters


Jun 27, 2020
High Pass and Low Pass Filters
  • High Pass Filters (HPFs) and Low Pass Filters (LPFs) are tailored equalizers that:
    note: all copyrighted info is displayed under "fair use" rules

    1) Are different than amp tone controls and pedal/rack equalizers that we all know and love. These type of equalizers, for each control, have what is known as "Q", i.e., a certain width and limitation to the frequencies controlled as demonstrated in the following graph(which shows boost):
    Q plot.jpg
    Note: Parametric equalizers Cut/boost and control which frequency is centered and variable Q. Semi-parametric equalizers Cut/boost, control frequency center and have a fixed Q.

    Whereas, an HPF graph looks like this where frequencies below a threshold are cut based on a slope, usually 6 to 24db per octave. The steeper the slope the deeper and quicker the cuts.
    HPF graph.jpg

    And an LPF graph looks like this:
    lpf graph.jpg

    2) Also known as a low-cut filter, rumble filter or sub-sonic filter, it could also be called an anti-Distortion filter or equalizer. It cuts low frequencies your speakers can't reproduce or reproduce well.

    3) Significantly attenuates (make much quieter) Muddy/sub-sonic lows (HPFs) and/or clanky/janky highs(LPFs) produced by your amp/ speakers, allowing your rig to sit better in the FOH (Front of House)/PA (Public Address) mix.

    4) Help tame bad/boomy acoustics for rehearsals, stages and venues (in addition to putting your rig on a stand or a pad or tilting it back, which I also recommend, to disconnect from stage) giving you better stage volume and clarity.

    5) Tame transient spikes to increase protection for speakers from mechanical over excursion (speaker cone movement beyond its limits - Xmax).

    6) Will help control 5 and 6 string and other down-tuning bass's low frequencies and help your rig go louder and sound clearer and cleaner.

    7) Keeps your amp from working so hard and gives more headroom by cutting unneeded/unnecessary frequencies (below port tuning or speaker ) from being reproduced.

    8) Decreases speaker "floppyness" by reducing mechanical movement, especially in ported cabs. Still, it works for sealed cabs too.

    9) Change the Q of your passive or active bass guitar tone controls and allows them to be more effective by allowing them to boost or cut frequencies that matter. If you add a LPF (low pass filter), depending on your style, you can tame the highs as well.

    10) Allows you to control what you send to the FOH mixing board. Why send mud to the FOH? Same for LPF. Per @JimmyM, you can cut enough lows and highs out of the PA signal (DI-direct interface) to keep the sound engineer from over emphasizing either one so that the bass player better controls his/her tone that is in the FOH mix.

    11) For home practice, an adjustable HPF can cut enough bass tone to not bother others yet you can hear yourself.

    12) Sets the "playing field" that all other tone and eq adjustments work within and allows your bass amp tone control(s) to boost or cut frequencies that your speaker can reproduce and matter most to your FOH tone. If you add a LPF (low pass filter), depending on your style, you can tame the highs as well by cutting fret noise and finger scrapes..

    13) Helps you sit in the mix better because your rig is not reproducing Muddy and/or less efficient lows (below tuning frequency) even if it sounds good on stage

    14) Help balances string to string volume.

    15) A lesser known fact (and becoming more widely known as the word gets out) is that PAs, guitar players, keyboard/synth players, keybass/foot pedal players and kick drums(FOH or run through their own amp) can improve their tone and reliability (and the mix) by using an HPF/LPF. Many PAs have an HPF available for each input channel. For some in the know, especially in sound reinforcement, HPFs have been available for decades.

    16) Guitar players would also benefit from an adjustable HPF but set much higher than a bass player, around 100hz-120hz. The benefit is also for the band mix, keeping the guitar(s) from stepping on the bass and kick drum frequencies, by reducing lows that only muddy the mix, especially guitar rigs with 12s in them.

    17) Controls and protects speakers from effects that produce sub-octave tones. Can also improve sub-octave tracking along with improving the tone of effects that produce too much low and mid-lows, especially Distortion effects. Some pedals come with an HPF and some come with an internal splitter that creates both distortion and Clean, allowing you to blend both for best effect before outputting to your amp.

    18) Also known as a low-cut filter, rumble filter or sub-sonic filter, it could also be called an anti-distortion filter or equalizer. It cuts low frequencies your speakers can't reproduce or reproduce well.

    19) Helps keep your rig from stepping on the kick drum's slot.

    20) May increase Output up to 3db. Louder is better! Like adding another speaker cab.

    21) HPFs can be fixed, some at around 30hz, and may be built in your amp already to protect the speakers from sub-sonic lows. Check with the manufacturer if it's not included with published specifications. Others are adjustable, around 25hz-330hz and give the user more options for modifying the signal. LPFs are adjustable.

    22) By removing sub-sonic or ultra-lows, it allows your amp to run cooler.

    23) Gives you more headroom because your amp is only amplifying what you want it to which does not include ultra-lows.

    24) If you have a Full range speaker cabinet, you can use a HPF/LPF as a speaker cabinet simulator (cab sim) by decreasing lows and highs. This is helpful for playing the blues, Motown, funk, or any music that does not need a piano sounding or more modern bass tone. Another application for Full range speaker cabs is to create a Dub/raggae tone by keeping the lows extended and cutting the highs down to around 300-800, based on your venue testing.

    25) If you use effects and other pedals, especially a compressor or octave, installing an HPF first in your signal chain will allow effects to run more smoothly because extraneous sub-lows are removed prior to the effect application especially helpful for compressors and other frequency based pedals.

    26) Running an additional HPF/LPF at the end of your signal chain will remove any tonal artifacts created by your effects and allow you to tune your tone for the room. Here's my thread: Order of effects that may help in figuring out where you want to run your HPF, LPF or HFP/LPF. Note: Spacial effects may be better placed after the HPF/LPF. Let your ear by the judge.

    For example:
    After some discussion with a few TB members, incl. Mesa amp designer @agedhorse, I decided to buy the (Broughton) always-on HPF to place at the input(beginning) of my small board and since I already had the LPF/HPF, which I wound up placing at the end.

    What I achieved was a tighter, controllable and more Focused tone (in between I typically choose between a few 'tone stack' pedals like a Broughton Silver Deluxe, a Broughton SV-Pre or an Ampeg Classic Pre. 'If' I use any spatial effects, like reverb, I'd place it after the LPF/HPF.

    Here's a pic of my Lil'Big Tone Board, lately (with my jazz/blues/rock semi-acoustic trio) I've been using HPF>Ampeg Classic Pre>Broughton Silver Deluxe>Broughton LPF/HPF so I can accommodate an AGB/EUB and jazz basses.
    View attachment 3465674

    27) By removing distortion (mud) your tone will be cleaner and tighter leading to less ear fatigue, a change in how you hear Overtones, especially in loud venues.

    For ongoing replies, application and updated information for this Wiki, please see @Stumbo's TB High Pass Filter (HPF) and Low Pass Filter (LPF) Mega Thread

    TERMS

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

    1) HPFs do not just deal with highs or treble.

    An HPF lets frequencies above a set level pass.

    For bass, it's usually frequencies above around 30-35hz for fixed level HPFs and between 25hz-160hz for adjustable HPFs. For example, if an HPF is set at 60hz, all frequencies below 60hz are significantly attenuated (reduced). Also, all frequencies above 6hz are allowed to pass without attenuation. These frequencies are chosen by the designer/builder.

    FYI: stereos and other amplifiers where turn tables (record players) were the main source of input, "rumble filter" switches were built in to reduce the low-end rumble of motors, belts and gears from being amplified.

    2) LPFs do not just deal with lows or bass.

    An LPF lets frequencies below a chosen level pass, usually between about 160hz to 15khz. They let higher lower frequencies than those pass. LPFs are usually adjustable. For the user, depending on your sonic goals, your ears pretty much decide on which frequency to use. For example, if an LPF is set at 5khz, all frequencies above 5khz are significantly attenuated (reduced). Also, all frequencies below 5khz are allowed to pass. The frequency range is chosen by the designer/builder.

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

    3. Limiters
    Limiting is a type of dynamic range compression. 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. Clipping (audio) is an extreme version of limiting. Many amps have Limiters built in.

    4. Compressors
    Compressors act like an automatic volume control with controls for how much, how fast, how soon and how long to work.

    OPERATION

    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).

    HPFs and LPFs can come separately or together in an foot pedal or a rack unit. HPF/LPS can also be part of multi-effect units in digital form.
    Many amps have fixed frequency, always-on HPFs built in, though some being adjustable with a knob on the front of the amp.

    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 Arnopol, 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, designer, 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 (speaker) 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.

    @fdeck, builder, hpf V3
    TB thread on fdeck's pre-amp/high-pass filter

    You can go as low as you want with any speaker diameter. What you give up when you try to do that, is volume.

    There's a rule of thumb in speaker design: Small, loud, low. Pick any two.

    As a result, speakers made with smaller drivers tend to sacrifice low end so they can still reach acceptable volume. Or, they use a larger number of small drivers to approximate the behavior of a single large driver.

    Part 3
    @sevenyearsdown
    From: Why do I do this to myself...

    I've changed my rig so often that I end up dumping gear that I need later on. Discovered at two gigs recently that I once again need a HPF. I'm pushing my amp into limiting on the low end at some stupid loud gigs. After proudly proclaiming that a HPF isn't necessary, I'm now regretting the two that I've sold. Should have just left in mu rig the whole time, dummy.

    Why you need an HPF/LPF

    @J.Wolf
    From: Broughton HPF LPF question

    I'm interested in getting one of these for a couple reasons. I have and like using an HPF to tame the boom in a lot of rooms/rigs, and I have recently discovered that I like using a LPF to help me dial out some of the crispy treble, while keeping the upper midrange fullness that I like for fingerstyle playing and soloing.

    Question is: is the HPF as effective as my Fdeck V3? AND, does the LPF work in the way I mentioned?

    Can I use it to shelve out some of the string noise-y, hissy, clanky sound that is not readily tame-able with most traditional treble controls?

    How steep is the curve on the shelf?

    thanks for any input.

    @Azure Skies, Builder, Broughton Audio replies:
    The HPF and LPF are both 12 dB per octave Butterworth response filters. I use the LPF in the way that you describe: to filter out ultra high frequencies (string noise, fret buzz) but still keep the lower treble/upper mid frequencies.

    @JimmyM says:
    So how come nobody ever complains about guitarists wanting to control what goes out to the PA? Or keyboard players?

    But dang, a bass player wants to do it, and bass players line up to tell them they're egomaniacs and jerks and they need to leave everything up to the sound tech? Wow, what a beaten-down bunch of schlubs!

    How dare anyone tell me what to do! It's my show, not theirs! I refuse to be beaten down into the status quo! I'm always happy to listen to competent people if they think I'm doing something that's making their gig impossible, but I've had too many try to take me down a road that I don't want to go.

    Examples:
    ---I hate tweeters on electric bass and have told the sound tech of my disdain for them. But the sound tech loves the sound of guys like Marcus Miller and Victor Wooten (I do too but not for my thing), therefore I end up with tweeter frequencies in the PA.

    Low pass filter is for them.

    ---In our band, if you have too strong frequency response below 60 hz or so, it creates mud.

    But the sound tech is younger and grew up with hip hop and EDM and loves those frequencies and pushes them hard despite me being quite vocal about not liking them.

    High pass filter is for them.

    ---I have had sound techs tell me I need to plug my bass straight into a DI, despite my use of effects.

    When I tell them that I use effects and want them in the PA, I have been told things such as "You will not have effects in my PA" or "We'll set up a mic for your cab and blend them," then they never turn on the mic.

    Well sorry...but when you're on my gig, that is MY PA until the gig is over. So you're getting effects in the DI and you will like it.

    Don't worry...between my professional use of effects and my H and LPF, your speakers are safe, Mr. Sound Tech.

    But when you demand that the guitarist doesn't use effects, then and only then will I take your demand that I plug my bass straight into the PA seriously.

    On the other hand, our sound tech yesterday showed me his iPad with my channel pulled up...he says, "Man, I have to add a lot of bass to your channel to make it sound right out here" and shows me the graph with a hump at 100 hz. I explained to him that I usually try to send him a fairly flat signal that he can easily tweak as he needs, other than my H and LPFing.

    Then I told him that I loved how it sounded in the house (and you're darn tootin' that I go out there and check it out, so I know exactly what it sounds like), and he gave me the thumbs up.

    So you can see I'm very cooperative and leave a lot to the sound tech's discretion. But H and LPFing and effects are non-negotiable. Those who know what I want don't care, and those who do care don't get a choice in the matter.

    Isn't it enough that I gave up my awesome Heil mic and went to a DI? What do you want? Blood?

    Why you need an LPF
    @JimmyM says:
    From: NPD: Broughton Audio Low Pass Filter

    So every now and then, I lament the sorry state of what's available in the way of speaker simulation (high dollar rack stuff, sims married to larger pedals that do other things, etc.), and one day I was commenting how Markbass was missing the boat by not putting their VLE (Vintage Loudspeaker Emulator) control in a 1590a box and selling it, and I said, "First indie pedal builder who does it gets my money." Welp, Josh Broughton (Azure Skies) was the first one.

    It's a low pass filter (obviously) that gives you a gradual sloping rolloff of highs without affecting the other frequencies, allowing you to emulate the freq response of tweeterless cabs, which is a must for as much overdrive as I use.

    All the way left is full highs up to 16khz, all the way right cuts highs above 160 hz.

    I found my favorite 5k rolloff somewhere between 9:00 and 10:00 (have to experiment a little more to find it exactly), which is very similar to how the VLE behaves.

    Close enough for rock and roll...I strap it to the board and take it right to the gig.

    Other than the 5k rolloff I had dialed in, it was dead Transparent, so I was quite happy...
  • Loading...