decent voltage controlled filter design?

[slotbot]

Hey

I was jsut wondering if anyone knew a decent design for a VCF? im trying to incorporate one into something im making. Id like to avoind anyhting that uses an LDR. And either have a low pass or a bandpass type filter.

any links would be appreciated.

scott.

[darren]

I like this one:



it uses a diode as the voltage controlled resistance element.  

[gez]

Id like to avoind anyhting that uses an LDR. And either have a low pass or a bandpass type filter.

Try an OTA state variable filter.  The LM13700 data sheet has a 'guide design'.  There's also the Penfold wah (Mark's site) and a DOD design (can't remember the details) which is drawn up wrongly but if you fix the mistakes it's reasonable.  

None of these filters 'as is' will sound that great, but if you toy with them it's possible to get an amazing sounding VCF, my favourite type...

Edit:  I think the Maestro Sample & Hold is a variation of an OTA SVF if I recall.  Haven't played with that one, but there are sound clips flying around the net and it sounds promising.

[Transmogrifox]

Go to AMZ and look up the DOD 545 envelope filter.  DOD uses an OTA based filter.  You can also get these OTA filter designs from this OTA datasheet:

http://www.onsemi.com/pub/Collateral/NE5517-D.PDF

They also provide a lot of application notes and theory that are useful for this (if you are one to understand and enjoy the theory).  Otherwise, they give some good cookie-cutter designs you can play with to  adapt to guitar filtering.


You can also take the state-variable filter and replace the resistors (for which the MUTRON uses LDRs) with a 100:1 voltage divider and either

1.  an OTA
2.  A quadrant multiplier IC.

Analog devices makes a good quadrant multiplier that you can find by searching at www.analog.com

Also, at AMZ schematic section, look at the EH zipper.  It uses JFETs as a variable resistor, and a rather interesting filter as well.  The 2 important lessons to learn from the EH zipper is that you must significantly attenuate the input signal before feeding it to the JFETs, and also take good note of their use of the op amp feedback loop to linearize the voltage to resistance conversion on the FET.  It's a relatively intriguing design.

Finally, do a search for the MXR envelope filter design, which uses a pulse-width modulation scheme to generate and "average resistance" by clocking a CMOS bilateral switch in the signal path.

I think that pretty much covers the VCFs, excepting the switched capacitor filters.  You can buy the switched cap filters as an integrated circuit.  I've heard they generally are not favored for music filter implementation due to aliasing.

[Mark Hammer]

Gadzooks!!!  Designs, schems, and even fully laid out PCBs abound for OTA-based VCFs.

Try this one from Ray Wilson: http://www.musicfromouterspace.com/analogsynth/statevariablefilter.html

Or some of these from Osamu Hoshuyama: http://www5b.biglobe.ne.jp/~houshu/synth/

Or Yves Usson's draw of the Steiner VCF here: http://www-timc.imag.fr/Yves.Usson/personnel/SDIY/Modular/EN/STEINERVCF/

Or check out some of Ken Stone's filters here: http://www.cgs.synth.net/

Or some of Tom Gamble's here: http://www.ele4music.com/

Or some of Motohiko Takeda's here: http://www.aleph.co.jp/~takeda/radio/homemadesynthE.html

Had enough? :lol:

[ExpAnonColin]

http://experimentalistsanonymous.com/diy/index.php?subdir=Schematics%2FFilters%20Wahs%20and%20VCFs&sortby=name

-Colin

[gez]

Go to AMZ and look up the DOD 545 envelope filter.  

Ahh, this seems to be the correct version of the DOD schematic, thanks for the link!