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Below you see Sweef. It’s an embarrasingly simple attempt to build an inexpensive variable passive bandpass filter for my own live setup. It does a nice job on Ecco, and you can build it yourself, too. Doesn’t work with instrument impedances (no direct guitar input).
Sweef’s biggest flaw is, that there is no electrical reason, why it should work. You can see that from the schematic below. I do have a rev2, but i didn’t solder it together yet. At least that rev2 is electrically seen better and only slightly more complex. Sweef in the displayed form is an experiment, that isn’t supposed to work, but actually does, sometimes ;)

Comments (at end of page) are appreciated.

sweef - by jayrope 2014

Sweef needs no power, can be bypassed and otherwise gets you almost any round sounding emphasis from sub bassy on the left side of the dial to almost ultra sounding on the right. I tend to use it on extreme settings, it doesn’t do much in the center, although kind of resembles a 1970s broadcast setting.
The filter sweep is very musical/smooth, without much noticeable filter resonation on the respective bandpass ends.

Schematic
sweef - by jayrope 2014

See part list at end of this article.
Values for the pot, C1 and C2 are mainly the result of experiments with spare parts (1k log pots with solder ends, a short dial and a small body size seem hard to get in Germany), mostly not calculation.

Precautions
1. Sweef has no flat frequency response other than bypassed,
2. If Sweef is on the overall signal level drops noticeably, well, it’s passive.

Other flaws (out of phase, impedance chaos etc.) are very possible.
I chose a pretty low impedance setting, for this wouldn’t steal too much audible level, when turned on.

Possible changes
1. If you don’t like the sudden drop towards the highest frequency level you can insert a 4.7 Ohms resistor (adjust to taste) before C1. This minimizes the range of the pot.
2. You’ll get even more bass by increasing the value of C2. But then you will probably also have to increase the value of C1, as otherwise there’ll be a significant level bump somewhere in the lower mids. See 1. for what happens then.
3. I might build an active version sometime. Don’t ask me when.

I am not a tech expert, i work by ear. If you’d like to give me audio expert tech advice regarding this, please comment.

Thank you and enjoy.

+++
Part list
Aluminium case type A – www.musikding.de – 4,50 €
Pot – www.conrad.de – 3,99 € (any is good as long as 5k log)
Cap MKP10 – www.sly.de – 1,09 €
Cap SHR – www.musikding.de – 0,40 €
Mono 1/4” sockets (2x) – www.musikding.de – 2,50 €

Addendum
Sweef is apparently an Africaans word, and it means “to float”. Hell yeah, it does.


Comments: 3

  1. Ben

    Cool filter! Just to let you know it’s a band-reject or bandstop filter and not a bandpass :) kind of similar to a Twin-T.

  2. Jay Kroon

    Hi. I was looking for something like this, I’m gonna build a stereo version. Did you have any luck with v2?
    Thanks, Jay.

  3. Schmitt trigger

    I can give a hint: the problem with totally passive filters, as a fact any passive filter topology, is that its response is wholly dependent on both the source and load impedance.
    That is the single most important reason to use input and output buffers. Even a simple NPN source follower will improve things tremendously.


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