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CEM3328 Low Pass VCF

 

NOTES: This documentation was written based the V2 version of the module. If you have V3 PCBs and still have questions AFTER reading the documentation, don't hesitate to contact us!

Make sure you mount the pots and jacks to the panel before you solder them to the board! This is true for any pcb mounted parallel to the panel, tolerances in panel manufacturing can make it tough if you solder the components on their own. 

It is also recommended that you place a washer (or two thin ones like the type that come with bi-tech pots) between the pots and the back of the panel to get the panel level with the PCB, as the pj-301b jacks are a few mm taller than the pots. 

 

The CEM3328 is a vintage 24dB/Oct Low Pass VCF IC.

The IC supports voltage control of resonance, and "Custom Tailoring of Volume vs Resonance", which allows the filters output level to stay more or less constant as the resonance is increased. Very handy! 

If you want to build this filter, you'll need to find the chip. They're long out of production, so they must be fished from old gear or bought from fellow enthusiasts on eBay or Muff Wiggler.

Synths utilizing these super rad filters include:

  • Crumar Bit-01     
  • Ensoniq Mirage  
  • Oberheim Prommer

While classic Oberheim gear and Bit-01s cost too much to justify disabling for their filters, the Ensoniq Mirage is relatively cheap and common! They can often be found for ~$50 to $100, and contain 8 x 3328s! Find one on craigslist, sample some junk and marvel at its charming sound, then take those bad boys outta there. 

The Sound:

 

This first one is two CEM3328s. One CEM3328 is generating a sinewave and is heavily modulated, then is fed into a second CEM3328, modestly modulated.

 

 

 

This one is a much more straightforward demo. Two simple sequences modulating the cutoff and a vco, which is fed into the signal input. 

 

Every track on the Executive Blaster Soundcloud uses this filter.

 

 

The Build:

Click here to download the datasheet - c3328.pdf

The datasheet above has a concise outline of the IC's capabilities, as well as a schematic. My PCBs are based directly off of that schematic, and I can verify that it works great. The only major addition is a TL074 quad op amp, to provide active summing for the two CV inputs. 

 

R12 and R13 on the bottom right of the TL074 are HORIZONTAL. All other resistor orientations should be intuitive.

The designations for the 3 capacitors to the right of the vcf are clipped! C3 (330pF) is the bottom right cap, followed by C2 (0.033uF) in the middle, and C1 (0.033uF) on the top.  

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Take CAUTION when connecting your power cable. V+ and V- are marked, but only on the top side. Don't get turned around! 

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Click here to download the SUPERCEM3328 LOW PASS VCF V2 Schematic

Click here to download the SUPERCEM3328 LOW PASS VCF V3 Schematic

Click here for the V2 Mouser Cart

Click here for the V3 Mouser Cart

 

 

note: if sourcing your own parts, make sure to get small caps! 2.54mm lead spacing is ideal, but some caps with 5mm lead spacing work fine (especially if they have kinked leads). Standard resistors will fit vertically, but only barely, so make sure not to leave too much slack when bending the leads.