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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Help with Utility LFO
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matimatil



Joined: May 17, 2020
Posts: 2
Location: Argentina

PostPosted: Mon Aug 03, 2020 7:19 am    Post subject: Help with Utility LFO Reply with quote  Mark this post and the followings unread

Hi! I've built the great CGS58, also known as the utility LFO. It works great but I consider it rather fast for a LFO. I didn't have a 500k for the Shape, so I usead a 100k. I was trying to add caps in parallel to the standard 47n of the circuit so I added a 0.22uF one (tried both polarities, ended up with negative on the output of the opamp). It did slow down the LFO to about 0.3Hz, that is still quiet fast. The problem is that depending of the position of the shape knob, in this slow speeds, the LFO is stalling either high or low. I know people that ended up putting capacitor much larger than the one I tried so i don't understand.
Any ideas?
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Qrchack



Joined: Sep 28, 2017
Posts: 2
Location: Poland

PostPosted: Sat Aug 15, 2020 8:17 am    Post subject: Re: Help with Utility LFO Reply with quote  Mark this post and the followings unread

matimatil wrote:
Hi! I've built the great CGS58, also known as the utility LFO. It works great but I consider it rather fast for a LFO. I didn't have a 500k for the Shape, so I usead a 100k. I was trying to add caps in parallel to the standard 47n of the circuit so I added a 0.22uF one (tried both polarities, ended up with negative on the output of the opamp). It did slow down the LFO to about 0.3Hz, that is still quiet fast. The problem is that depending of the position of the shape knob, in this slow speeds, the LFO is stalling either high or low. I know people that ended up putting capacitor much larger than the one I tried so i don't understand.
Any ideas?

Look at the shape knob. There are diodes in there. This means current flows both directions - rising and falling. Your voltage thus goes up and down. This is going through the 10k resistor to the capacitors you have in place. By putting a polarized capacitor in there, you likely put negative voltage on the cap. This destroys the dielectric in your capacitor and it's likely to fail and become a short with time. Besides, it now only works as a capacitor in one half of your LFO. The shape knob is likely 500k because it was easier to use a higher value potentiometer to decrease the integrator (= lowpass filter) frequency that way instead of using a bigger capacitor. Both influence the cutoff frequency defined as 1/2piRC.

I guess it would be easier to just get a 500k pot. 470k will work as well if it's easier to source for you.
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