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 Forum index » DIY Hardware and Software
VCF for Workshop Synth
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kaputtpanzer



Joined: Nov 02, 2009
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PostPosted: Thu May 29, 2014 11:50 am    Post subject: VCF for Workshop Synth Reply with quote  Mark this post and the followings unread

Hello friends,

lately I am working on a "everything-on-one-pcb" synth, for a workshop I will give for some fellow students.

I decided to make a very simple tone generator, 2 Schmitt Trigger OSCs feed their signals into a NAND gate (pseudo ringmod Rolling Eyes ). In additional the synth will have a simple LFO to control the VCF.

I breadboarded some filters. Including the Ruin CMOS VCF (http://www.electro-music.com/forum/topic-55851.html) and the q&d VCF (http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/simplevcf.gif).

But both of them were unsuitable for my project. The CMOS VCF has a too small filter range (I changed a lot of components to make it more suitable, but it is still not so good) and the Q&D VCF starts to hum when my hand was getting closer to the op-amp. If I would use a enclosure that would be not an issue, but for this project it wont work.

I am looking for a VCF that is suitable for this pcb trim-pot synth. I breadboarded the MS-20 9V VCF a few years ago and was really impressed by it. So I think I will use something like this. Are there some other easy to build LM 13600/700 OTA VCFs? And will it hum when my hand is getting close to the IC?

The image shows a early version of the pcb layout...


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comrade_zero



Joined: Mar 05, 2009
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PostPosted: Fri May 30, 2014 1:13 am    Post subject: Reply with quote  Mark this post and the followings unread

I am assuming you want single supply? Eric Archer has a 13700 9v vcf that sounded pretty good on the demos. You could cook up a low-parts count Sallen-Key filter with a single op-amp and some LED's and LDR's. You could adapt some of the Wasp filter clones that are floating around (based on the CD4069, I seem to recall a 13700 version to replace the 3080's of the original.) Probably the simplest resonant filter on a single supply is the WSG filter from Ray Wilson (musicfromouterspace) it's a little "weak," but for a workshop synth it may fit the bill...
Good luck, hope this helps.
c_z
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kaputtpanzer



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PostPosted: Fri May 30, 2014 2:57 am    Post subject: Reply with quote  Mark this post and the followings unread

hey c_z,

yes single-supply with virtual ground.

Like I mentioned the op-amp based filters start to hum, when I get close with my hand.

But thanks for the hint with the eric archer vcf...
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kaputtpanzer



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PostPosted: Fri May 30, 2014 3:11 am    Post subject: Reply with quote  Mark this post and the followings unread

I had a look at the eric archer ota vcf. Looks nice and simple. But I was wondering about the op-amps, do i really need them? The one at the output is just an amp, I guess I can cut out this part. But what about the CV input section, I don't really understand what the circuit does there. Isn't it possible to build it like the ms 20 vcf and cut this part out, too?

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kaputtpanzer



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PostPosted: Fri May 30, 2014 3:12 am    Post subject: Reply with quote  Mark this post and the followings unread

This is the link to the eric archer schematic: http://ericarcher.net/wp-content/uploads/2011/03/9v-analog-diy-lpf.pdf
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elmegil



Joined: Mar 20, 2012
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PostPosted: Fri May 30, 2014 6:10 am    Post subject: Reply with quote  Mark this post and the followings unread

The amps do two things

1) provide a wet/dry signal mix

2) buffer the output.

If your intent is to just have raw filter as normal for modulars, you could omit the first op amp and the pot that does the wet/dry mix, but I don't think I'd recommend unbuffering the output, otherwise loading will affect the behavior of the filter.
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kaputtpanzer



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PostPosted: Fri May 30, 2014 6:48 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
The amps do two things

1) provide a wet/dry signal mix

2) buffer the output.

If your intent is to just have raw filter as normal for modulars, you could omit the first op amp and the pot that does the wet/dry mix, but I don't think I'd recommend unbuffering the output, otherwise loading will affect the behavior of the filter.


Guess my question was not clear enough, the opamp questions were related to the eric archer vcf... But thank you anyway
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elmegil



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PostPosted: Fri May 30, 2014 7:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry I was confused because there's a schematic right there in the post you asked your question in Smile


Edit:

So to Eric's schematic; the first op amp is part of the expo converter, that's required. The second amp is doing the same thing as the two op amps in the schematic I replied to before -- it's providing a wet/dry mix (you could omit some of the resistors and the pot) and a buffer for the filter. Again: if you omit the buffer, and you plan to hook the output to anything arbitrary, you could have a load change the characteristics of the filter. If the filter output goes to one fixed place, and you know the load *doesn't* alter it from your desired response, then you could omit it (though you shouldn't omit the decoupling cap).
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kaputtpanzer



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PostPosted: Fri May 30, 2014 12:44 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your help.

I think I will modify the ms20 schematic a little bit for our needs and use this. I was wondering why the ms20 filter is working with out a expo converter and the eric archers vcf needs one.
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elmegil



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PostPosted: Fri May 30, 2014 2:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

The MS-20 was a V/Hz device not a V/Oct one.... Probably the cause Smile
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