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2nd order high pass filter for tube bass amp

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  • 2nd order high pass filter for tube bass amp

    I’d like to implement a steep high pass filter around 30hZ or so into the front end of a tube bass amp. Idea is to roll off any subsonic junk for 5 string and protect drivers in a ported cab. Is it as simple as two series passive filters? It Would be cool to make adjustable or at least switchable, but not sure how I would do it with simple passive filters. Thanks for any ideas!

    Gaz
    Last edited by Gaz; 11-20-2018, 08:19 AM.

  • #2
    How about a schematic of the amp in question?

    I find it hard to believe that the output transformer works down that low.

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    • #3
      It’s a clone of a Hiwatt DR201. I’ve used a pedal like a Fdeck. HPF, Thumpinator, or Broughton HPF, and was curious if I could build something similar into the front end of the amp without opamps.

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      • #4
        Originally posted by Gaz View Post
        It’s a clone of a Hiwatt DR201. I’ve used a pedal like a Fdeck. HPF, Thumpinator, or Broughton HPF, and was curious if I could build something similar into the front end of the amp without opamps.
        To get 'steepness' in the HP Filter (3rd order/18dB/Oct or 4th order/24dB/Oct), using passive components, you'd need inductors along with the caps, and it has to be high impedance so it doesn't load down your instrument. There are charts out there for bypassing the messy part of the math to aid in the calculations, though what's readily available in off-the-self inductors is is somewhat limited. You'd probably have to wind your own. Nevertheless, it's certainly worth the exercise. Granted, I haven't looked into the pedal references you cited to see what they've done.
        Last edited by nevetslab; 11-20-2018, 05:36 PM.
        Logic is an organized way of going wrong with confidence

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        • #5
          this is something like a Fdeck,see if you can make something out of it
          Click image for larger version

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          • #6
            i don't think you need much complexity nor use inductors which are obsolete since you can make any order of filter with opamps.
            A simple gyrator like you find on graphic equalizers should be enough.

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            • #7
              I only suggested the LC passive approach, as the stated desire was to NOT use Op Amps (or active circuits, though moving the active filter past the front end takes the noise issue out, for the most part. My mind got caught up with it between the input jack and the first stage. Dumb idea, yeah?
              Logic is an organized way of going wrong with confidence

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              • #8
                Yeah, after the first stage is what I was imaging. I suppose I meant to say that an active tube circuit. Or is it as simple as two series RC filters (.0047uF/1M —> .0047uF/1M)?

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                • #9
                  If you cut at 30 hz, you are actually cutting fundamental tone of the low B string played open.

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                  • #10
                    Yes I realize that. This what the pedals I mentioned do and it’s virtually inaudible, just makes it a lot easier on speakers and a more controlled sound. There is way more going on an octave above the fundamental, plus the cut off is not a cliff, however a steep roll off

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                    • #11
                      One method:
                      See the "TIGHT" Switch on the Clean/Crunch Channels Input Stage of the Hughes and Kettner Triamp.

                      An active filter is driven by a pickoff from the anode and fedback into the cathode of teh 12AX7 triode.

                      http://www.freeinfosociety.com/media/pdf/3216.pdf

                      Cheers,
                      Ian

                      Cheers,
                      Ian

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                      • #12
                        Thanks, Ian... but I can’t find it in the linked schematic. Is it in a different version or am I missing it?

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                        • #13
                          The tight filter is on p8 of the pdf, pE of the schematic, it links back to the 2nd stage on p4 / pA via cable FCC5.
                          My band:- http://www.youtube.com/user/RedwingBand

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                          • #14
                            Originally posted by Raybob View Post
                            If you cut at 30 hz, you are actually cutting fundamental tone of the low B string played open.
                            True, but we don´t hear much, if at all at 30Hz and to boot no Bass Guitar cabinet reaches that low, no way, (a Home Hi Fi speaker might) and in any case we listen to the harmonics.

                            A fact many ignore , and is very counter intuitive, is that "famous" Bass enclosures drop like a brick below 100Hz or so , yet we are happy with that.

                            this is what I found to be the definitive Bass Guitar cabinet page (so far), with mind opening results, all actual measurements, not simulations
                            https://billfitzmaurice.info/forum/viewtopic.php?t=2357

                            such as:

                            8x10, 9 cubic feet, sealed, loaded with Eminence Alpha 10 :


                            Eminence Legend B810 10" 32 Ohm :


                            8x10, 9 cubic feet, vented, tuned to 50 Hz, loaded with Alpha 10


                            Does a huge magnet give better response? Yes… and no. Here’s an 8x10, vented, tuned to 50 Hz, loaded with 16 pound B&C 10PLB76 drivers


                            "Compared to the 8x10 loaded with Alpha 10s it does have better midrange sensitivity, but below 500 Hz it’s much less sensitive. The good news is that an 8x10 loaded with these monsters will handle 3200 watts. The bad news is that, with 6dB higher average sensitivity, the Alpha 10 loaded cab only needs 800 watts to get the same low frequency output. "
                            That said, the B&C goes one full octave lower than the Eminences.
                            Yellow cone B&C speakers were the secret behind MarkBass amplifiers ... but now they cheat , big way; for a couple years now they are being cheaply made in Indonesia, instead of expensive Italy, and use cheap "Yellow Series" stamped frame Indonesian made speakers ... people at TalkBass are starting to comment that "new amp, nominally same as old (and with same price tag) does nowhere sound like the old one bought 8 years ago"
                            I call that killing the golden eggs goose.

                            Since speaker response is the real deal, and we see how limited it actually is, it pays to have a "reasonable" highpass around 40/50Hz (yes, that high) and call it a day.
                            Yes, even with 5/6 string Bass Guitars .
                            Last edited by J M Fahey; 11-21-2018, 03:30 PM.
                            Juan Manuel Fahey

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                            • #15
                              Mhmm, one might try to get the 2nd order by choosing the coupling caps of two stages appropriately, right? What about combining 40Hz and 70 Hz? Should be a 2n order butterworth with a corner freq of 40 Hz or so?

                              BTW: another nice option for a DR201 might be a pair of TL606 which are tuned to 55 Hz and sound good with 5 string basses.

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