Monday, 12 January 2015

HPF Technology HPF-Pre 2

I've had a few folks asking for active bass cutter circuit for couple of different uses. First, a lot of stoner rock enthusiast want to split their signal for two amps and some blues folks want to do that too. And since in cases like these, the second amp is usually a guitar amp, which will perform in a happier manner if we don't feed them the lowest frequencies. I got tipped off with HPF Technology's HPF-Pre 2. Even though the main aim for the design is to address issues with double basses, the circuit will work great on other applications as well. This design is open and for mainly that reason, i strongly recommend getting an original unit if you find use for this circuit. The price is more than just right, so your savings won't be much by building one. So. If you can, please support the orginal design.

I built one already to see if the board works with J201s and 2N3906. It does. The frequency pot should definitely be dual gang logarithmic taper.

A quick overview from the author:
HPF-Pre is a preamp with adjustable high pass filter and phase switch for use with piezoelectric pickups on the double bass. It is intended to address the primary issues faced by bassists when playing through mainstream amps and PA systems. Foremost, HPF-Pre is a preamp. Its high input impedance ensures proper buffering of the signal from piezo pickups. It is battery powered and housed in a tough plastic enclosure with internal shielding. An adjustable high pass filter tunes out the subharmonic “thump” generated by the plucked bass, which otherwise can drive ported bass speakers to over-excursion. It is also a useful tool for coping with adverse acoustical environments and PA systems with excessive subwoofer support. A phase (polarity reversal) switch is provided to help control feedback problems. It could also help when setting up systems with multiple pickups, or a pickup and a microphone. Series 2 of the preamp adds an output volume control and an LED battery status indicator. The volume control is handy when your stage setup requires your amp to be out of arm's reach. HPF-Pre is an "open" design. I sell the HPF-Pre at a modest price from my part-time home business. Otherwise, you are welcome to build your own.

Check the site and get the original from https://sites.google.com/site/hpftechllc/home

30 comments:

  1. Replies
    1. Slowly but not so surely. Main reason being the simpler schematics that are worthwhile to draw are getting really scarce...
      +m

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    2. Been looking everywhere for a Fuzz God II layout...

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  2. Is there a way to make this control-less? Like fix it at 30-35hz? I could deal with just a volume pot. I would like to implement this in the same manner as an sfx thumpinator.

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  3. omit the dual pot and solder 2 resistors (100k will give you 35 Hz) where the pot should be: ie between Freq A1 and A2&3 rows and between Freq B1 and B2&3

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  4. i've built this circuit quite a few times using another layout and 2N3819s and BC327 with success. I've just tested J201s and I wouldn't recommend usuing them, using test tones, there's a fair amount of distortion...

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    1. Strange. I didn't have any distortion on mine with J201s.
      +m

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  5. I'm talking about sine test tones, i didn't notice audible distortion with a bass guitars (then, "why bother?", you could say :D ), but when testing the frequency response, a 100 Hz sine tone came out audibly distorted, which was not the case with 2N3819s. maybe my J201s (Tayda's) are to blame, though... I'll post a comparison soundclip when I have the time

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  6. OK, so here's a soundclip:
    http://clients.cylens.net/diy/hpf.wav

    first pass is a pure 100 Hz sine tone
    second pass is through the HPF with 2n3819s
    third pass is the same circuit with J201s... notice the distorsion and 10 dB loss

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  7. do you know if a 2n5485 would work in place of the 2n4416s?

    Thanks,

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    1. They're both N channel JFETs but it really is going to be a case of try it and see. Unfortunately the results from different JFETs is unpredictable and so we just don't know how they will react in the circuit. You are more likely to have success with JFETs used as gain stages like in this though, as opposed to when used as variable resistors in phasers and similar. So hopefully you'll have good luck.

      You can get the 2N4416 from Farnell though

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  8. If I got the 2n4416 from farnell, what would the 4 lead be connected to?

    Thanks

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  9. Going to try the j201's, but would also like to compare the 2N4416 as that was what was originally used. However, the 2N4416 has 4 leads (one for the case)....what would that get connected to?

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  10. All you need are drain, source, and gate pins. Clip the fourth pin

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  11. hello,
    what is D1 ? a Zener 12v is ok ?
    thanks !

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    Replies
    1. Probably, I believe it's just for polarity protection.

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  12. Thank you Charles
    signed : Charles

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  13. No way to let it work
    here my voltage
    Q1 D 4.81
    G 4.04
    S 4.04
    Q2 D 3.69
    S 2.89
    G 2.90
    Q3 C 3.12
    B 3.68
    E 8.85
    Someone can help me?

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    1. Hey, I think the diode in use here is just for polarity protection of the circuit - I believe a 1N4001 will do fine here. They are pretty stock standard.

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  15. Hey, can anyone please point out what to do to eliminate the volume pot? Trying to fit in a small box and already can handle volume from other pedals.

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    1. just ignore vol 1 wire, and take your output from vol 3.

      otherwise you could squeeze a trimmer in there to fix your output to the level you like.

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    2. Ok, thanx! Petyxhmeno nick btw :)

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    3. thanks!i think this hobby grows really fast here in greece!!

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  16. So, another backfire there, project done, works like a charm, pots and switch response right, but...
    When I set it aside, with no in and out jacks, battery dies in 1-2 days, with no apparent (to me) reason. Can you take a look and leave any opinions?
    Note I added a little battery check led, activated when only the Out jack is inserted. I hope you can understand by the wiring drawing I provide. Also a few photos of the guts.
    http://postimg.org/gallery/16nw218hw/

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    1. Hm, now it hit me... Maybe I should run the ground first by the output jack, and then into the vero and not parallel. So the circuit closes only when the jack is in. Silly me. :)

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