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Sallen Key filter with LM13700
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Dimitree



Joined: Feb 09, 2011
Posts: 14
Location: Italy

PostPosted: Tue Jul 29, 2014 7:12 am    Post subject: Sallen Key filter with LM13700 Reply with quote  Mark this post and the followings unread

hello everyone
my aim is to re-build the Korg MS20 "External Signal Processor" using LM13700s.

This is the schematic (quite edited):
http://www.ed-sounds.com/Immagine.png

as you can see there are 3 LPF and 1 HPF (no resonance).
They use 4 gang potentiometers. I have those pots, but I'd like to slim the whole thing using standard 9mm knobless pots.

So looking at the LM13700 datasheet and the article "Using Otas" by Ray Marston, I found this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

so it looks like I could replace the original filters simply using this LM13700 application example (2 of them in series for a 2 pole filter).
But I have some doubts:

1) is it true that I can do that?
2) how should I calculate the right voltage that I need to supply to the LM13700 in order to get the narrowed frequency cutoff range as the original? (50Hz -> 2500Hz as opposed to 45Hz -> 45kHz) ?
3) since the original uses a reverse log taper pot, can I simply use a standard log taper and use it in reverse? (I don't want to build a "lin to anti-log" circuit.
4) I don't need C1 on the application example circuit, right? or maybe the capacitor C12 on the original schematic is a DC blocking cap so I need C1 too? (if yes, what value)?
5) how do I replace the trimmer on the last LPF of the original circuit?

thank you Smile
Dimitri
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JovianPyx



Joined: Nov 20, 2007
Posts: 1225
Location: West Red Spot, Jupiter
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PostPosted: Wed Jul 30, 2014 11:13 am    Post subject: Reply with quote  Mark this post and the followings unread

I see "Sallen Key Filter" in the title of your post, but I don't see much about it in the post text. Those two filter images are not of Sallen Key - they are simple single pole VCFs.

While a Sallen Key is a two pole filter, it is (in my opinion) a poor choice for a synthesizer. This is because the Q is restricted to values of 3 or less. That won't create much of a resonance peak. The major advantage of the Sallen Key is that it uses only one amplifier - which costs less.

A better two pole filter that is easy to make with LM13700 is the state variable filter. With a SVF, you can get crazy high adjustable Q, low pass, high pass, band pass, notch and band reject all from one circuit. You can find a schematic for an SVF built from an LM13700 in the LM13700 datasheet.

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Dimitree



Joined: Feb 09, 2011
Posts: 14
Location: Italy

PostPosted: Wed Jul 30, 2014 12:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

thank you for the reply,
I said sallen key because that's the type of filter used in the original schematic I'm trying to convert..

http://www.ed-sounds.com/Immagine.png

are not that filter Sallen Key type filters?
Now I need to remove those filters and use a LM13700 to make equivalent filters, so I don't need resonance, and these filters are not applied to usual VCOs but to audio signals. I need the OTA because I want them to be voltage controllable.
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JovianPyx



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Posts: 1225
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PostPosted: Wed Jul 30, 2014 12:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

The filters in the image link of the original MS20 schematic are Sallen-Key.

The schematic you posted in your first post are not Sallen-Key, they are simple single pole filters.

As for the other questions, there are good books on filters (The Active Filter Cook Book is one) that will show how to configure them. You can probably find the information using Google. Using an OTA adds a level of complexity in that the resistor values depend on the cap values and on the control voltage and what range of Fc you need.

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Cynosure
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Joined: Dec 11, 2010
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PostPosted: Fri Aug 01, 2014 7:44 am    Post subject: Reply with quote  Mark this post and the followings unread

This has been done before:

http://www.schmitzbits.de/ms20.html

http://electro-music.com/forum/topic-43085.html

http://www.diystompboxes.com/smfforum/forums.html?PHPSESSID=d8a245857e524fb670a1569d3526dc53&topic=85279.msg712884#msg712884
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