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 Forum index » DIY Hardware and Software » Developers' Corner
Sallen Key vactrol filter values
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Rolbista



Joined: Nov 17, 2012
Posts: 22
Location: Poland

PostPosted: Tue Apr 16, 2013 5:24 am    Post subject: Sallen Key vactrol filter values Reply with quote  Mark this post and the followings unread

Ok, so after some fighting with a Steiner filter I decided to give up and just put together a Sallen Key and add vactrols for cutoff. I'm using the schematic form doepfer site: http://www.doepfer.de/a1011_tec.htm and a circuit from Buchla low pass gate for LED driver, the driver works nicely, but when I tried the filter on the breadboard it doesn't really filter anything, just lowers the volume and makes a big jump in it at the end of the pot travel, so I wonder what would be an appropriate value for two cutoff R's and for the resonance pot. I used two 1M pots fot cutoff and I have 50k for Q. It could be that the breadboard is loose or something, but before i commit this project to a board, I'd like some advice on best values to use here.

EDIT I also put a unity gain buffer in before the filter and an amplifier with gain of 10 on the output
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L´Andratté



Joined: Sep 23, 2012
Posts: 61
Location: Hamburg, Germany
Audio files: 5

PostPosted: Sun May 19, 2013 3:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Rolbista!

I guess I´m probably too late (;
Please do the Steiner, it´s basically a Sallen-Key with 3 extra resistors (1k) for the different inputs! So not much more work but a lot more feature...

I didn´t build the Buchla driver, I think it´s overcomplicated and I don´t understand it fully, but I would like to direct you to the led-driver of the Mutant-Vactrol-Filter:http://www.birthofasynth.com/Scott_Stites/PDF/Mutant/mutant_filter.pdf


That one leaves nothing to be desired-
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Rolbista



Joined: Nov 17, 2012
Posts: 22
Location: Poland

PostPosted: Mon May 20, 2013 8:31 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm afraid I ditched the Steiner completely in favor of a state variable 12 dB filter. I left the Buchla driver and it works as far as the cutoff knob is concerned but it hardly reacts to external CV from LFO, not sure why that is
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L´Andratté



Joined: Sep 23, 2012
Posts: 61
Location: Hamburg, Germany
Audio files: 5

PostPosted: Mon May 20, 2013 11:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Then you could do the steiner next Smile

I don´t know your skills so I take the liberty to try to explain (learning myself):
I looked at the Buchla and I think your problem can´t be too hard to solve:
the cutoff-knob gives just a voltage between 0/+15V via the 150k resistor to the inv. opamp input, so your cv should be just another variable voltage mixing with that via the 100k, so check everything around that point thrice...
If the one works the other should also...

Also notice that if your cutoff-knob is a) fully open/+15V b) fully closed/0V then a) pos.cv or b) neg. cv would achieve nothing!

Did u use this one?
http://www.electro-music.com/forum/phpbb-files/thumbs/t_buchla_292_clone_191.gif
Did you try the 10k at the cv input?
btw. what value has the zener protection diode?

& don´t give up on it, you´ll laugh at last!
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Rolbista



Joined: Nov 17, 2012
Posts: 22
Location: Poland

PostPosted: Mon May 20, 2013 12:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

yes, the schematic from your link is what I used, first I had a 100k in the place you mention, but after it didn't react much i soldered a 10k resistor in parallel to the 100k, still, the response didn't change. Maybe it's the LFO's fault? Or a faulty jack? Of course I will check everynthing over once again but right now this filter is sitting safely in a rack playing gigs, so I wouldn't like to mess it up. I guess I could try feeding a battery through a pot to the CV input to check. BTW, i'm using an adapted 12V computer psu.
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