HPF, Subsonics, & center freq. for bass control

Discussion in 'Amps [BG]' started by Jim C, Jan 13, 2013.


  1. Jim C

    Jim C Supporting Member

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    Nov 29, 2008
    I have been surprised at how many here have found the f'Deck HPF to be so useful even with amps that are not known to have low freq. issues (i.e. amps that don't reproduce down to the subsonic level).

    My assumption is that its' use is more for boomy room EQ rather than fixing an amp design issue (like a SWR SM400 & others).

    This got me thinking as to what is the center frequency for the bass knob on modern amps. Here are a few:

    Shuttle 9.0 - 80 Hz
    Streamliner - 55 Hz
    MB2 500 - 60 Hz
    700 / 1001 - 60 Hz

    and then
    Fusion 550 - 40 Hz
    TH500 - 40 Hz

    If Francis's HPF starts at 35 Hz, isn't 40 Hz a bit low for the center freq. for the bass knob? I would think that this control also has a fairly wide Q meaning that there is also the potential to boost signal below this center point.

    Could one make the assumption that a HPF would be a required tool for any amp that has a bass knob centered below 50-60 Hz in order to allow the bass to fit in cleanly with a band situation?

    For that matter, what is the reason that some manufacturers select such a low set point?
  2. Passinwind

    Passinwind Charlie Escher Supporting Member

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    Some food for thought here, near the bottom. Most of those specs are not for the center point per se.
  3. KJung

    KJung

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    This topic interests me, ever since I had the 'lifechanging' experience of adding a hi pass filter to the unusually low end extended SWR SM400 back in the day.

    First, most bass controls are shelving (not all, but most), meaning that they impact frequencies below but not above (or not much above). The 'slope' of this impact is much more gentle than a hi pass filter, which really chops off EVERYTHING below the set point.

    Also, the shelving (or in some cases the center point) of a bass EQ has little to do with 'how deep the amp goes'. That is the impact of internal hi passing, or other circuitry that limits the absolute low end output of the amp. So, those very low shelving point bass controls, like the 30hz of the TecAmp Puma, are IMO meant mostly to cut, not to particularly boost. So, kind of a 'poor man's hi pass'

    +1 that many use the FDeck as a tone control (i.e., literally changing the sound of the amp) versus the more classic use of a hi pass, which is meant to eliminate sub bass to protect speakers and make a rig more efficient, with virtually no change to 'tone'. Either is a totally legit use of a hi pass filter.

    The beauty of combining a hi pass like the FDeck with a shelving bass control, even with amps that are well behaved down low like the Genz amps (which are hi passed around 35hz with the variable hi pass/bass boost off) is that you can sculpt the low end very accurately. For example, with that 80hz shelving on the Genz, you can hi pass at 50hz, allowing you to crank that bass control for a fat tone, and still not 'wasting' power and speaker mechanical capability below the typical tuning point, etc.

    Lots of uses for that little box, even with amps that don't really need 'classic' hi passing (i.e., elimination of useless sub bass).

    IMO!
  4. Jim C

    Jim C Supporting Member

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    Nov 29, 2008
    Francis,
    Thanks for this explanation; I just assumed that the frequency for the bass control was in fact the center freq. for the bass control.

    Is this how these numbers are normally represented in audio or is this just MI nomenclature (bigger is better cause it sells more amps)?

    BTW, nice new avatar.

    Edit: Simulpost with Ken; also a good explanation
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  6. KJung

    KJung

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    + Francis (edit: and Charlie!) is kind of a 'god' of this stuff, and I too dig the new avatar.

    It does get a bit confusing as to what those bass controls actually do, and also the true low end performance/cut off of a given amplifier. For example, the Markbass bass control is actually NOT a shelving control, but a very wide, and very asymmetrical control with a 'peak' at 40hz. Markbass used to have the plots of the tone control voicing/impact in the back of the manuals for their heads. From what I remember, the bass control peaked at 40hz, impacted probably 5hz above that and quickly dropped off, and then had a gentle slope all the way through 30hz and below.

    The nice thing about a variable hi pass like FDeck's Hi Pass III is that you know exactly what it is doing... hard chopping (-12db) every frequency below the variable frequency point, and then adding another -12db at 35hz, no matter where the knob is set. So, it kind of serves as a tone control and 'safety/efficiency' hi pass all in one little box:bassist:
  7. Passinwind

    Passinwind Charlie Escher Supporting Member

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    "Don't call me Francis !
    " :cool:

    In all the heroes of legend and song-o
    There's none more brave than El KaBongo.
  8. dmrogers

    dmrogers Supporting Member

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    The main reason I purchased the Series 3:
    A nice use of the Series 3:
    I didn't really think I "needed" an HPF, but I wanted one, for the reason below. Improved efficiency.
    fDeck's HPF is a really nice addition to my rig. I was happy with my power and tone before the purchase, but I'm even happier now. It's not like buying an effects box or trying a different set of strings. It is subtle, yet effective. Once you get used to it, you can't imagine not using it.
  9. Bmorefoozler

    Bmorefoozler

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    Ya, I thought Francis was Fdeck, and Passinwind was Charlie??? Charlie has a nice new avatar...
  10. KJung

    KJung

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    Hah, I was thinking of the Hi Pass, and responding to your post, and out came 'Francis'. Well, the only redeeming thing to the name blunder is that you are both 'gods at this':D
  11. agedhorse

    agedhorse Development Engineer-Mesa, Product Support-Genz Supporting Member

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    Interesting and very relevant discussion IMO.

    The biggest difficulty in defining a realistic, or real world shelving eq turnover point is that while in theory a 3dB point would traditionally be used as a filter response point, the filter response and this any potential 3dB point will vary with the amount of cut or boost. In reality this point will shift left or right (down or up in freq). In practice, some nominal or real world extrapolation of what a range of typical users would experience is necessary if the number is to be even remotely useful to the typical user. This is part of the feel of one particular eq choice over another. Different, not better or worse, but one might work better for one player over another. This is why playing different amps, as well as discussions like this can be especially helpful.
  12. Tuned

    Tuned

    Joined:
    Dec 6, 2007
    Because the slope of the bass shelf EQ is rather low and the HPF slope rather high, the bass knob will still have considerable effect even if it and the HPF are at the same frequency.

    Something to keep in mind is that bass amps already have a HPF, though it's fixed at an unknown frequency. If you're maxing out a speaker cabinet a variable HPF is definitely an asset, but let's not ignore there is a fixed HPF already there. If you don't believe me, use an EQ pedal (has no HPF) as a preamp into your power amp input, and watch them cones fly.

    Actually if you're setting the HPF to 80Hz or higher, you can then use PA speakers safely to their full rated program power, even in small enclosures. If you've got subwoofer support from the PA. I've got a wedge monitor with an Eminence Delta Pro 15A rated 800W program, I could bridge my IPR 1600 (1000W into 8 ohms) with the crossover set to 100HPF and see what happens. Don't really care if it fries (7 year warranty), but it's so damn efficient I can't imagine needing more than about 300W.
  13. Jim C

    Jim C Supporting Member

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    Is it fair to say that the bass frequency listed in an amp's spec sheet is not particularly useful as the 3dB point shifts with the amount of boost or cut (which I never knew)? It also looks like comparing these number from maker to maker is irrelevant due to different slopes.

    "Ya, I thought Francis was Fdeck, and Passinwind was Charlie??? Charlie has a nice new avatar..."

    Sorry, I get all you smart guys mixed up and appreciate the input from Agedhorse at Begantino ;)
  14. agedhorse

    agedhorse Development Engineer-Mesa, Product Support-Genz Supporting Member

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    Yes, this is in general accurate. The effective 3dB point (or it's extrapolation) will vary with the amount of boost and cut. For example, with 2dB of ultimate shelf boost, the 3dB point is never encountered...
  15. agedhorse

    agedhorse Development Engineer-Mesa, Product Support-Genz Supporting Member

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    It's important to quantify how the particular HPF works. There are many bass amps that have no (or very low slope) HPF's that are essentially useless, especially combined with LF eq boost. Also, it's not uncommon for an outboard eq to incorporate a HPF.

    I would like to point out that this is not always the case. When considering driver design, program power must be taken into consideration with program dynamics as well as the specific cooling design that the driver incorporates. Many LF drivers, including the driver you mention, depend on a portion of the LF displacement to dissipate some of the thermal energy from the voice coil. When you remove the displacement, you change this thermal equation and generally derate the thermal power handling of the driver... maybe by as much as 50%, so the speaker may ony be capable of a 200 or 250 watt thermal rating. The same thing occurs when the program material is heavily compressed. The increased average power increases the thermal energy that needs to be dissipated, and without the displacement, you may find that the necessaary derating skews any published rating that is based on a wider frequency range test signal (which is usually specifically badwidth limited noise with a defined crest factor).

    Regarding the 7 year warranty, I recommend that you read carefully any manufacturer's warranty for what it does and does not cover. I suspect that this warranty excludes mechanical or thermal damage which might cause you to care more rather than less. ;)
  16. bongomania

    bongomania Gold Supporting Member

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    Small qualification, "most" bass amps have that HPF, but not "all". And even accounting for the "most", one of the reasons an external HPF like the FDeck or Thumpinator can still be useful is because of the second part of your statement, that the built-in HPF is fixed at an unknown freq (and slope). If the corner freq or slope are non-ideal, we may still benefit from adding an ideal one.

    Of course since that all hinges on things that are unknown (and usually not found in the amps' documentation), the results or benefits will vary widely, from none to a little to a lot, depending on the amp.
  17. jnewmark

    jnewmark Supporting Member

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    For someone who plays 4 string, and has a 50hz slider on a graphic eq, would sliding that to the bottom do more or less that same thing ?
  18. bongomania

    bongomania Gold Supporting Member

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    No, because (a) the graphic EQ slider controls a bandpass filter that cuts along a bell curve, and (b) its slope will usually be a lot shallower than the one in the specialized HPF units. What this means is by the time you've cut out the unwanted lows, you've also gutted the lows you meant to keep.
  19. jnewmark

    jnewmark Supporting Member

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    So, for a 4 string guy like me, where would I set the HPF ?
  20. bongomania

    bongomania Gold Supporting Member

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    Assuming you've got the FDeck or something similar, with a very steep slope and an adjustable corner frequency, you would probably start with it set near 30 Hz, and then sweep upward little by little until you've found the "sweet spot" compromise, where your lows are tight but not weak.
  21. jnewmark

    jnewmark Supporting Member

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    Thanks. This might come in handy in boomy rooms.

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