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Pseudo .5 291 Bandpass Filter Board
MUFF WIGGLER Forum Index -> Music Tech DIY Goto page 1, 2, 3 ... 26, 27, 28  Next [all]
Author Pseudo .5 291 Bandpass Filter Board



Just thought I'd move discussion of this to a new thread. I've got a board around 75% laid out now. (first pass, so there will need to be some optimization, fixes, and I'm planning some additions/mod-space) I plan to order prototypes within the next couple of weeks.

Basically it's a single 291 bandpass filter. I'm looking into tapping/mixing other responses now too.

I've decided to do one filter per board using a smaller board. They'll be stackable with thru-power header like the .5 258 boards. This should also help with putting multiple boards into smaller formats. I'm planning a quad-board module myself that can be used as a resonant eq, or formant filter.

Anyway, I'll start updating this thread with any major progress, and hope to have a final design ready within a month or so.

Here's the basic BOM:

Bill Of Materials

QTY  PART-REFS           VALUE                         
---  ---------           -----                                       
2    R1,R45              47k                                               
1    R2                  2.2M                                             
4    R3,R8,R12,R31       6.8k                                             
2    R4,R13              33k                                               
4    R5,R15,R25,R32      10k                                               
1    R6                  5.6k                                             
2    R9,R14              10M                                               
2    R18,R19             4.7k                                             
4    R20,R21,R28,R29     1.5k                                             
1    R22                 1M                                               
2    R23,R26             15K                                               
1    R24                 68R                                           
5    R27,R33,R34,R48,R49 1k                                               
1    R30                 4.7M                                             
1    R35                 100k                                             
1    R36                 220R                                             
2    R38,R39             62K                                               
2    R42,R43             68K

1    C1                  1000pf                                           
2    C2,C5               .022                                             
1    C3                  220pF                                             
1    C4                  15uF                                             
1    C6                  470pF                                             
1    C7                  47pF                                             
1    C8                  .47


Integrated Circuits
1    U1                  TL071                                             
1    U2                  TL074                                             
1    U3                  TL072                                             

2    Q1,Q2               2N3904

Q1,Q2=2N5089 (sounds a little nicer to me anyway)

1    D1                  DIODE-ZEN 3.9V                                       
1    D2                  1N4148                                           

1    Q                   100k
4    CF,CF CV,WIDTH,     10k                                               
1    FM                  50K                                                                                                                                   
4    VT1-VT4             OptoCoupler                                               
1    WIDTH TRIM          5k   

2 10uF or higher electrolytic caps for power filtering.
4 .01uF bypass capacitors (labeled BP1-BP4) *


There are two small silk-screen errors on the board unfortunately. On the left side of the board where the controls attach, there are two resistors labeled R21. The top one should actually be R43.

* There seems to be a "bonus" bypass cap on the board. You can populate it or not. (it's an unlabeled cap) Just drop a .01 in there if you feel like it. The original layout didn't have bypass caps at all, so they are optional.

** This is a silly one, but basically the "Q" header at the top-left of the board, is more or less useless. This was on the schematics that I was working from, so I included it. The real resonance/Q trim is the 5K WTrim pot, which will adjust the resonance for the Width control. This is really something that I feel is best left to a trim-pot. Just set it the way you like it, and you're good. I'm actually not sure what the original intent of the 100K pot on the input is. While it does make very slight changes to the signal, it's definitely not audible, so it's entirely optional.

Also, here is the schematic for those interested:


cool. this may be the extra filter I'm looking for for my DIY synth, especially if a highpass output could be tapped.


yes, yes, yes....... please.

4 boards.

rico loverde



great - i really like my 291s
i would like to have a bank of them


Thank you J3RK! I'm in: 2 please.


yes! x4 sounds good to me. I was thinking about building a cgs serge resonant equalizer, but I think a j3rk pseudo 291 is first in order.


Uh huh, yup.


screaming goo yo

Junk Rhythm

You can count me in for 4 PCBs.


Count me in for at least 2 PCBs


I'll take around 4!

Actually, I'll just take 2-4 of anything, come to think of it!

Thanks J3RK.


Ill take a couple!


yes please!


Awesome! Count me in for 4.


Yes, please include the lowpass and highpass outputs! Check that EM thread as one of them, IIRC the LPF, is out of phase.

Very cool!



Rod Serling Fan Club

I agree, all 3 outputs and I'm down for 2-4 depending on price.


Sounds tempting!

Will the PCBs be Euro compatible (Voltage/PSU Header etc)?

How big will they be?
If they were small enough that you could parallel mount/stack 2 or 3 behind, say, a 14HP panel, that would be great!




edit: i'd take at least 2, maybe more
great project, J3RK


Me is in 4 4.


Cool! Looks like there is a bit of interest for this. I now have the basic layout for the board done. I just learned where to tap the LP and HP from, and will add the connections for these tomorrow. There is a nice broad space for mod space that can be used for a mixer to mix the signals. I will leave that to the builder, however, this time I will add some default traces and SIL headers for controls. This way if someone wants to add a pot or two and a couple of jacks or switches, they won't need to run wires all over the place. I should be able to order a few protos by next week.

I plan to build a small mixer circuit in the mod space, so I will test it, and post my design. Since it will be so simple, I think it's a good fit or this. If someone doesn't need it, they can use the space for something else. I will make sure it will easily accommodate it, and a good set of controls. I need to order some VTL5C3s to test with. I do have some NSL32SR3s though, which should work for some initial testing. (with a possible difference in character of course.)


if euro compatible i'm in for a few.


Right now the board is around 3x3" or so. After I get the mixer area laid out, I will see if I can compact it further. I will test on 12v, but can't think of any reason it shouldn't work. Depending on what happens in the mod space, I'll see if I can add a pin header for Euro power.


I take 4.


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