Home > SDIY, Synths > SEM Modifications – VCF/Filter Section

SEM Modifications – VCF/Filter Section

The filter section needs less modification for my project than both the oscillators (because there is only one VCF) and the LFO section.  I love the SEM filter as it is and as such, I really have no desire to upgrade any of the components or make any modifications that might alter the sound in any way.  Like the oscillator section, I do want more modulation sources than the SEM provides with the stock config on the rear Molex connectors.  The two CV inputs to the VCF are found on pins H1 and H2 while the EXT modulation source is on H3.  Just like the oscillators, I want the inputs on one of the pots of the front panel to be switchable from going straight to the VCF modulation or routed to the EXT modulation input.  See the post on the oscillators for more about how to make the leads, soldering, et.c

The only other wrinkle is that I have also added a waveshaping circuit that takes the outputs from the oscillators and outputs both triangle and sine waves.  I’ll have more information on this circuit in a future post.  I want these to have their own dedicated volume control and feed directly into the filter just like the pulse and saw waveforms.  I love the alpha pots with the push pull switches like the ones I used for the oscillator section to route the cv to either the modulation or EXT modulation inputs.  I use these all the time and they will be used on the panel to control the volume of the triangle/sine waveforms and also to select which is being feed to the filter for each oscillator.  Unfortunately, these pots only come in a linear configuration so I’ll have a later post showing how to make them behave with an audio taper by adding a resistor to change the curve of th pot response.  Looking over the schematics and probing around a bit on the SEM shows that there is a group of 100k resistors on the upper left part of the board where we will need to add a 100k resistor in parallel with the ones already on the board to add the additional CV input to the VCF.  Towards the middle-left part of the board, there is a 220k resistor where we will need to add 2 additional resistors for the sub-oscillator inputs.

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Categories: SDIY, Synths Tags: ,
  1. July 21, 2012 at 12:46 pm

    Any more news on that waveshaping circuit ?

    cheers,
    _g

    • July 21, 2012 at 1:47 pm

      I actually finished racking up two SEMs but I’ve fallen behind on updating the info here. I’ll try and move it up the list. The waveshaping circuit works fine and having triangle and sine outputs really opens open some options for the SEM.

      • July 22, 2012 at 3:36 am

        Having a Sine and/or Triangle for the VCO’s would be a wonderful addition indeed. I wonder which shaping circuit you’re using… anything “known” amidst SDIY circles ?
        I’m currently rebooting the process — with a 4-year delay only — of reviving my 2 SEM’s, and adding the patch panel.
        Right now staring at the VCO sync part of the circuitry, which is kinda weird, and asking myself if I’d dare to leave out the Sync Out socket… I can’t find any reference to the sync switch in the schematics, though it’s clear that VCO 1’s sync out is used here.

        Nice work you’ve done here !

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