[sdiy] SEM VCF Mod

[Michael Bacich]

On Oct 22, 2006, at 10:33 AM, prototek wrote:

>     I want to bring out the "VCF Mod In" on my SEM so I can do S&H  
> type
> stuff with the filter. I'm just confused on one bit. In the back of  
> the
> manual it lists three different VCF options on that Molex. They are:
>
> Control Voltage Input #1-VCF
> Control Voltage Input #2-VCF
> External Modulation Input-VCF
>
>     I'm assuming I'd want to bring out the third one listed? Are  
> the first
> two for precise 1/V Oct stuff? I just wan to be sure before going  
> in there.


Hello John,

Which input you use depends on how you want that input to work.   
Inputs #1 and @2 are connected directly to the filter's CV input  
summing bus (via those 100K resistors).  If your filter is properly  
calibrated, then either of those CV inputs should have 1V/octave  
response to incoming CV.  Input #3 is connected to that 3-position  
slide switch that says "LFO / Ext. / Env."  In order to use any CV  
that's connected to input #3, you must have that switch in the "Ext."  
position.  Whatever CV source you have selected with that switch then  
goes through the 50K Mod Depth pot, which can be turned either  
positive or negative.   Typically, you would probably keep the switch  
on the "Env." setting, so you could use the envelope to modulate your  
filter's attack and decay, or on the "LFO" setting for filter LFO  
effects.

If you want to be able to use the envelope or the LFO at the same  
that you are using your external CV signal (which is likely), then  
you should connect your external CV to either input #1 or #2.   
However, those inputs do not have an adjustable depth control. so you  
would need to adjust their depth externally (with a pot or a mixer).   
If you want to use the filter mod depth pot that's already there on  
the SEM, then you should use input #3, set the switch to the "Ext."  
position.  Personally, I would probably go with input #1 or #2, so I  
could still use the Envelope and LFO along with my external CV.

Michael Bacich