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 Forum index » DIY Hardware and Software
simplified moog type filter
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vladosh



Joined: Aug 02, 2010
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PostPosted: Fri Nov 11, 2011 10:17 am    Post subject: simplified moog type filter Reply with quote  Mark this post and the followings unread

Greetings fellow diyers ..
Found this VCF from an old EFM schematic ,it's a strip down version of a transistor ladder filter , and being simple is what i like about it , however i see some difference with other moog clones as is Q11 transistor being connected to ground instead to - v via resistor ..i was hoping if someone has built this can share their experience ,or any advice is very welcome .
Thanks a lot !
Here is the link : http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/3500er/3522_VCF2.pdf
Vlad
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JingleJoe



Joined: Nov 10, 2011
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PostPosted: Sun Nov 13, 2011 7:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for sharing this diagram. It's much clearer than those old original moog diagrams I keep finding around the net, they look like they've been photocopied a hundred times and you can barely read them anymore!

I'll build this circuit eventually but someone else will almost certainly get there before me. Rolling Eyes
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Cynosure
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Joined: Dec 11, 2010
Posts: 657
Location: Toronto, Ontario - Canada
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PostPosted: Sun Nov 13, 2011 8:50 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes - a great find. I saved this one. I will make it when I get around to working on a modular.
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vladosh



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PostPosted: Mon Nov 14, 2011 5:03 am    Post subject: Reply with quote  Mark this post and the followings unread

I think i'll try few other designs first but wanna make this one too , share how the build went , i changed the cv input buffer thing .. to the one used in wildcat as that Q11 is in all other schematics tied to - v via very small resistor it's on every other transistor ladder schematic so it may be a mistake as people say there are mistakes in EFM schematics , btw here's a stripboard layout if you use this type of stripboard .. http://www.flickr.com/photos/62167965@N08/6304294244/
Good luck with it !
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fonik



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PostPosted: Mon Nov 14, 2011 10:23 am    Post subject: Reply with quote  Mark this post and the followings unread

regarding EFM designs/documentations, there are three important rules IMHO:
1. follow schematic, no matter what BOM says
2. don't be annoyed when it does not work as you expected it to work
3. change resistor values to get the CV response YOU want/need
Wink

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vladosh



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PostPosted: Mon Nov 14, 2011 1:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Fonic for the advice ,I've seen the BOMs are messed up ,one BOM for another schematic ,but so far all i've built from EFM(not much) works .. and thanks for documenting all that .. there are whole bunch of circuits there
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widdly



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PostPosted: Tue Nov 15, 2011 2:16 am    Post subject: Reply with quote  Mark this post and the followings unread

I've built a moog 24db based on one of the efm schematics.

The input section is pretty borked. The input needs to be attenuated a LOT, especially if you want lots of resonance. I don't think you can use more than one input at once either. Next time I'd put an opamp summer there with less than unity gain. You can check the yusynth moog lpf for a better input section.

I like the efm version Oberheim SEM filter. It uses only 1k, 10k and 100k resistors which is great in my book. The resonance is a little insane though.
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fonik



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PostPosted: Tue Nov 15, 2011 2:50 am    Post subject: Reply with quote  Mark this post and the followings unread

widdly wrote:
I like the efm version Oberheim SEM filter. It uses only 1k, 10k and 100k resistors which is great in my book. The resonance is a little insane though.

i used this version on my 8 stage stepped filter (breakdown filter) and experience exactly the same - have you been able to tame it?

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widdly



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PostPosted: Wed Nov 16, 2011 1:33 am    Post subject: Reply with quote  Mark this post and the followings unread

Good to know yours is the same. Mine sounds ok until about 3 o'clock on the resonance knob after which it starts to get a bit out of hand and loud.

It is still on the breadboard so I might try the back-to-back diode arrangement like in the original sem vcf schematic.

To be honest, I kinda like having a resonance knob that goes from none to far too much.
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fonik



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PostPosted: Wed Nov 16, 2011 1:43 am    Post subject: Reply with quote  Mark this post and the followings unread

widdly wrote:
Good to know yours is the same. Mine sounds ok until about 3 o'clock on the resonance knob after which it starts to get a bit out of hand and loud.

exactly.

Quote:
It is still on the breadboard so I might try the back-to-back diode arrangement like in the original sem vcf schematic.

if this worked i would have to modify eight (8!) filters...

Quote:
To be honest, I kinda like having a resonance knob that goes from none to far too much.

maybe i decide to like it too - would save me some time.

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JingleJoe



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PostPosted: Fri Nov 18, 2011 2:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

Maybe you could add a resdonance fine tune control? A pot one tenth of the total resistance of the initial resonance pot in series with it should allow more control of around the point of most desireable resonance Smile
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