Analog filter here I come!

edited November 2009 in WTPA1 Mods
So I am now committed to adding an analog low pass filter to my WTPA. The one I chose is the yusynth arp clone ( since it is a small PCB and I dig the sound (very Japanese). The +15/-15 power rails are are via wallwart and a half wave PSU from MFOS. I plan to pack everything into the PT-8 case I have and add controls to the front panel. I'll document my progress here so watch this space..


  • Document away, I'd love to see how this goes. Been wanting to build this sucker myself for a while but have been a bit reticent to take the plunge.
  • Cool! Let us know how it goes.
    That schematic describes a pretty traditional style voltage-controlled state-variable filter, except that it uses a lot of obsolete components.
    Consider using something like the
    instead of those goofy dual transistors, and consider losing the LM3900 for pretty much anything else. Or don't -- obsolete parts are a special bee in my bonnet. Some people don't mind finding them.
    A lot of designs like that are implemented with an OTA in place of those dual transistors (like the LM13700).

    My guess is that those transistor arrays have a lot to do with the thing sounding the way it does. Likely non-linear as hell, which is usually cool.

    The other thing to consider is that most State Variable filters by their very nature have a bandpass and highpass filter built in as well. Check pins 5,4, and 9 on that schematic and see what your output looks like :-)

    Please do let us know how it goes!
  • edited November 2009
    Thankfully, Bridechamber sells the parts kit with this so I am sorted component wise. I am not really bothered with whole "unobtainium" parts search from my rather compulsive obsession with old synths..

    One note on Bridechamber, that place has the most ridiculously fast processing I have ever seen. I literally got a shipping notification 13 MINUTES after paying them..

    If anyone is not familiar with BC:
  • Oh, cool! Yeah, I was always weak on the DIY sourcing sites. And their shipping puts Narrat1ve to shame :-)

    Still, do check out the output from those pins -- You might get lucky.

  • edited November 2009

    things are moving. Bride Chamber stuff is here (got here Monday). Mouser stuff will be here tomorrow so I should have everything stuffed this weekend.


    I have decided to go with 5 controls: Input level, CV level (for external control), cutoff, resonance, and a bypass

    Inputs will be CV, V/Oct, External Input (via normally closed jack so I can use the filter on it's own if I want)

    I have identified 3 possible locations for controls on the front panel that will not interfere with components on the board

    The middle seems most logical since there is a nice avenue for the wires to go out to the VCF board and the bypass will sit right on top of the audio output for easy access.

    Next issue: height clearances. The pots I am using are the same as the ones on the board but the plan is to break off the legs and make the base as short as possible then JB weld them to a plate and mount that plate to the face panel. If that does not work, i'll just JB weld them to the front panel itself but I would like to get their height as close as possible to the PCB ones so the stick out roughly the same amount..


    I may dump the external input idea since the WTPA already has a thru function in it..
  • Ok,
    Got the boards for the power and the VCF stuffed and ready for testing. I figured I should go ahead and build the potentiometer board for the controls right off the bat so:

    By cutting off some plastic flash and standoffs I was able to get the depth of the whole deal to around 7mm with the plate which gives me plenty of room (I used a 3/8" standoff) between the main faceplate and the WTPA PCB.

    Hopefully, my second kit will arrive soon so I can get some audio samples going
  • I'm watching this thread with great relish. Thinking of getting into the modular game and the ARP filter is one of the components I'd like to start with. Good luck with the build!
  • Just a tip -- make sure to ground those pot bodies (and by association, by the looks of it, the mounting plate). Those controls are right over the digital section of WTPA and with the impedances involved will be prone to noise pickup.
  • edited December 2009
    @ TB: I was thinking about taping a grounded shield right below where the pots are but if you think just grounding the bodies of the pots will be sufficient, I'll just do that since those are JB welded on and that stuff is conductive so I pretty much get that for free. I hope it is clear how this is intended to work, I will be bolting that whole assembly to the underside of the top panel hence the backwards mounting..
  • A shield would probably be OK too. Probably better than grounding the pots, although grounding them certainly wouldn't hurt.
  • That looks pretty awesome.
  • Some pics of the new boards:

    And the neat low profile caps I found:
  • Any progress on this? Curious to hear how it sounds.
  • I'm still sorting out some design issues and I just got my WTPA replacement today so hopefully I'll have something in a week or two, unfortunately I only have several hours a week to work on this stuff so progress is slow
  • edited January 2010
    Ok, so I have reached the first hurdle. Modular gear like my filter is designed to run at a much higher signal level than what I am used to and as a result, I am getting a SUPER low signal out of the filter. I tried playing around with the gain of opamp at the input stage but only got maybe 30% more volume before it started overloading and distorting. Looks like I will have to add an amplifier between the two devices and a kind soul on SDIY pointed me to Ken Stones Site ( and his stomp box to modular adapter which raises a low signal level up to a modular level. I figured I would just do the single amp stage to start.
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