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 Forum index » DIY Hardware and Software » Developers' Corner
Sallen key filter with a dual-gang pot?
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okelk



Joined: May 08, 2014
Posts: 29
Location: Vienna

PostPosted: Sat Aug 16, 2014 4:15 pm    Post subject: Sallen key filter with a dual-gang pot? Reply with quote  Mark this post and the followings unread

Hi!
I need a simple filter without CV control.
I thought about implementing a simple sallen key filter with a dual-gang pot.
Maybe something with clipping diodes like the korg35/ms20 filter.
like here:
http://www.doepfer.de/a1011_tec.htm
or here with the pot instead of the npn transistors. :
http://www.jhaible.com/jh_720_vcf.gif

What pots should I use, or what is the lowest/highest resistance I want?
and they should be linear ...right?

Also If I want to use a filter calculator like this:
http://sim.okawa-denshi.jp/en/OPseikiLowkeisan.htm
what are typical min/max cutoff and Q values for synth purposes?
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JovianPyx



Joined: Nov 20, 2007
Posts: 1247
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Sun Aug 17, 2014 5:28 am    Post subject: Reply with quote  Mark this post and the followings unread

Sallen-Key is a filter that was designed to be economical by using only one opamp. For synth use, it's major problem is that it's Q is limited to about 3. Most synth filters go far beyond that.

My suggestion for a better filter would be a state variable. I'm not sure what the high end Q for SVF is but it is far above 3. Normally, a SVF is CV controlled, but it's easy to create a control voltage with a single pot (no dual gang required). A SVF schematic can be found in the datasheet for LM13600 and LM13700.

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okelk



Joined: May 08, 2014
Posts: 29
Location: Vienna

PostPosted: Sun Aug 17, 2014 7:58 am    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
Sallen-Key is a filter that was designed to be economical by using only one opamp. For synth use, it's major problem is that it's Q is limited to about 3. Most synth filters go far beyond that.


The low parts count is actually the reason I thought about a sallen key...
I need something simple for a little noisemaker but I want something more capable than a WSG-filter...
I didn't think that the Q factor could be a problem since the korg35, ms20, synthacon and buchla 292c filters are all (kind of..) sallen key filters, and some are capable of pretty nasty resonance sounds...
I think my problem is more about how to replace a voltage controlled resistor/gain cell with a simple potentiometer in an existing filter design...

JovianPyx wrote:

My suggestion for a better filter would be a state variable. I'm not sure what the high end Q for SVF is but it is far above 3. Normally, a SVF is CV controlled, but it's easy to create a control voltage with a single pot (no dual gang required). A SVF schematic can be found in the datasheet for LM13600 and LM13700.


I'd rather not use an OTA, but I'm open to other filter designs/ideas.
to fit well to the small noise box I'm designing it would be nice if it uses 1-3 opamps...
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JovianPyx



Joined: Nov 20, 2007
Posts: 1247
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Sun Aug 17, 2014 8:47 am    Post subject: Reply with quote  Mark this post and the followings unread

An LM13700 is a dual OTA, so a SVF can be built with just one IC (low parts count) and 2 pots, one for frequency, the other for Q.

The OTA is what makes it possible to avoid a dual gang pot. Dual gang pots are more expensive than singles. And dual gang pots don't offer as many selections for resistance value.

However, you can make a SVF with opamps too. It's just that you can't tune it without a dual gang pot. You can also control Q with a single pot (which is true of both OTA and opamp circuits).

For an opamp based SVF, you will still need the dual gang pot for frequency.

Another advantage of the SVF is that it can produce low pass, high pass, band pass and notch outputs from one circuit.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
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L´Andratté



Joined: Sep 23, 2012
Posts: 60
Location: Hamburg, Germany
Audio files: 5

PostPosted: Mon Aug 18, 2014 1:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Replace the LDR´s (light grey background circuit) with your dual pot sections and you´re ready to go (or rather don´t, build it with vactrols!)
http://tristandabbles.wordpress.com/2011/06/15/the-steiner-resonant-filter-a-new-adaptation/
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