Adding a CEM3328 Filter

edited August 2009 in WTPA1 Mods
I was checkin out WTPA demos on youtube and saw this vid:

I'd wanna add a CEM to my WTPA.

I'm a novice, so if anybody can give me some pointers on how to do this
I'd much appreciate it.




  • Wow, that's killer!!
  • What the funk, man?

    The dude turned an $80 project into a $400 monster!
  • Whoa. That guy done went and got all pro-gear pro-attitude! That looks great and the filter sounds bangin'.

    @qmulus --

    I've never used one of the CEM chips before, as it's been a personal rule of mine for a long time to use rare/expensive/obsolete parts in my designs. Part of the fun of designing audio gear nowadays if figuring out how to do cool stuff with the tools that you have.

    That having been said I've made VCFs before with the LM13700. Not sure if you're interested, but if you download the datasheet you'll get the idea.
    A quick google search turned up the following:
    Which might be what you're looking for w/r/t the CEM.

    Hope this helps, and thanks for that link! That video rules. That guy is a madman.

  • edited August 2009
    I think the CEM runs off a split rail +12V-0V-12V power supply, so you won't be able to have it permanently wired to the WTPA and running off its power supply, if you were hoping to build it into it. You can make it a standalone unit with dedicated power supply, of course, in the modular tradition.

    If however, you were looking for a 9V DC powered CV filter with resonance, I made one from a schematic by Tom Escobedo called the "Quick and Dirty VCF". It's a really simple circuit to make - you can make it on veroboard, or even just use cardboard like I did! I adapted it for use in my Casio SK8 because the SK8 runs internal CV lines for its voices (I made it into a 4-voice polyphonic filter called the "Phat Philterbank" using 4 units). You can read about it here - and there is also the original Q&D schematic at the bottom of that page:,499.0.html

    It's exactly in the tradition you prescribe to, Todd - made out of common bits lying around, and an op-amp. That's it. Just a handful of components for a couple of dollars will get you a single CV VCF module very suitable for the WTPA. It's rough and nasty and would sound in the "spirit" of WTPA, hahah! (It actually sounds pretty decent - quite clean and will also self-oscillate). And best of all, it runs off a 9V battery very happily, with low drain (20-30mA). You would just have to adapt the input attenuation to suit the WTPA by trying out a few different resistor values (or use a trimpot) and maybe capacitor values.

    Todd, how do I put up a jpg into a post, like you did above? I'll post the PCB layout here for people, rather than them having to join the Yahoo group just to download it.

    Cheers, Graham
  • Aren't CEM chip hard to come by now? I thought at least some (all?) of them were...

    @gmeredith: to include an image, just post the URL of the image file, and when you post the message, the URL will be replaced with the image, inline.

  • That looks amazing! And it sounds great with that filter too..

  • Dan,

    Ah, ok. Unfortunately I don't have the PBC online anywhere that I can URL it to. I'll have to get a photobucket account or something. If anyone wants the PCB drawing in the meantime let me know and I'll email it to you.

    Cheers, graham
  • @Todd - not to add to your list of things to do (!), but I agree that being able to upload an image directly to the forum would help things... Is there a Vanilla plugin that would allow you to easily add that functionality?
  • @gmeredith

    I am down to check it out: a l t i tu de 909 at g mail

    I did some experiments yesterday with my warp 9 (WASP clone filter). It's actually a great device for the wtpa since it has an EG which is triggered by midi so it also adds a VCA and LFO to the mix..


    using the external filter made me notice a feature that I did not think about earlier, I am using via midi keyboard and once you take you finger off the key, the sample stops playing. What would be useful (and as a midi function only) would be a mode to have the sample play all the way through when triggered by midi. My 9090 has this for all the sampled instruments, it is simply a sysex string which changes the play back mode from stop on release to play all the way through when triggered.
  • edited August 2009

    The PBC layout has been sent to your gmail.

    I've now posted the PCB up on that site now, so anyone can directly download it from there.
    It's about half way down the page:,499.0.html

    Cheers, Graham
  • edited August 2009
    looks real straight forward and I love the single supply. Might have to put it together on the protoboard..
  • edited September 2009
    **ack, i missed todd posting the schem.. :P

    To add w/r/t the power supply needs:
  • Thanks Graham, it does look real easy - I may try too if I can find some time!

    FWIW, I have been on another thread at the electro-music forums, where an interesting bit of info from Scott Stites was contributed about how to easily create a split rail linear supply using just a DC outputting wall wart, using just the wall wart and a (pricey) DC-DC converter chip available from Mouser. Here's the info - might be useful for other projects people might be considering!

  • dan, nice find!
  • @Altitude --
    You're right. There is a "single shot" feature which works in Caveman mode, but which will stop if it receives a NOTE_OFF in MIDI mode. If I can think of an easy way to make this happen with CC messages, I'll implement it.
  • been searching around for an analog filter suitable and easy to implement and I think the ARP one is the one for me:

    Bridechamber stocks the kits with the Japanese parts for $55
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