PLAN B analog blog

Theoretical situation. I have a VCO plugged into IN 1 and it's set to FILT with the Offset 
turned all the way down. If I put an LFO into it's VC In will the Offset knob behave like a 
normal VCF's Freq knob? In other words will the LFO sweep the frequency range? I dont have 
an M13 in my hands yet so I cant try this myself.
Regards,
John

Yes, with the following exception:

The control input to a vactrol is a LED.  In order for a LED to  
turn on, there must be current flow through the device -  the cathode  
must be biased higher than the anode, which in the case of the M13  
(and most other vac appliications) is shorted to ground. The net  
effect is the LED in M13 will only turn on (open the filter) when  
the  control signal is higher than ground.  We did this because we  
wanted the M13 to behave like a VCA.   Using an LFO to drive it will  
work fine, but only the positive side of the waveshape  (that above  
zero) will actually open the filter.

If you want the full cycle of the LFO to modulate the Model 13,  
you'll need to put a M14 in-between to create the necessary positive  
offset so that both the peak and trough are above ground.



--- In PLAN_B_analog_blog@yahoogroups.com, "j_inform3r" <inform3r@...> wrote:
>
> Theoretical situation. I have a VCO plugged into IN 1 and it's set to FILT with the Offset 
> turned all the way down. If I put an LFO into it's VC In will the Offset knob behave like a 
> normal VCF's Freq knob? In other words will the LFO sweep the frequency range? I dont 
have 

> an M13 in my hands yet so I cant try this myself.
> Regards,
> John
>