Low-pass filter for anything!

SineHacker · August 26, 2012, 12:34:43 PM

just found this online, been looking for something this simple for ages!

http://ericarcher.net/devices/diy-lpf/

DIY Analog VC-LPF filter

noob666 · October 15, 2012, 06:52:25 AM

Yeah I've been meaning to build this for a while! Anyone know of other simple filter projects?

Gordonjcp · October 15, 2012, 10:23:16 PM

The Sallen-Key filter used in the Gakken SX150 is about as simple as you can go. The Steiner Synthacon VCF is a slightly more complex variation on the same theme, with some neat features like having highpass, lowpass and bandpass *inputs* so you can do some interesting tricks using it like a frequency-controlling crossfader ;-)

nochtanseenspecht · October 16, 2012, 07:02:36 PM

hey thats interesting Gordon, do you mean you route the signal with a pot instead of switches at the input to the differend filter modes?

Gordonjcp · October 17, 2012, 09:49:11 PM

No, what I mean is that you can mix two different signals with one going to the highpass and one going to the lowpass input. As you sweep the filter, it will cut more of one and open up the other.

Here's a demo of my prototype Synthacon filter driven by a Roland HS60 and a novation Xiosynth, driven by the same sequence and randomly switching patches at the top of each sweep:

http://www.gjcp.net/~gordonjcp/mp3s/hilow.ogg

and a pic of the prototype filter, built using bird's-nest construction:
http://www.gjcp.net/media/filter.jpg

nochtanseenspecht · October 18, 2012, 07:12:33 PM

well that sounds really nice

Synthetech · November 05, 2012, 09:26:31 PM

Quote from: Gordonjcp on October 17, 2012, 09:49:11 PM
No, what I mean is that you can mix two different signals with one going to the highpass and one going to the lowpass input. As you sweep the filter, it will cut more of one and open up the other.

Do you think something like that can be done with the NJM2069 filter on the Korg Poly800?

Here's the chip

I'm just curious what the output would sound like if you merged the two of those signals together.. then hear the filter run thru it's EG and LFO application.

I always wonder why Korg chose not to use that output..

Synthetech · January 20, 2013, 06:34:43 PM

Well now I've got seven Korg 2069 filter chips and getting an itch to build an external filter fx box/circuit.

Anyone ever do this or have any pointers on how to build one.. preferrably with some kind of LFO driver to make it auto sweep open/close.. perhaps using a 555 chip?

I've tried googling a specific diy schematic, but I just cant seem to locate one.

noob666 · March 01, 2013, 07:06:42 PM

Quote from: Synthetech on January 20, 2013, 06:34:43 PM
Well now I've got seven Korg 2069 filter chips and getting an itch to build an external filter fx box/circuit.

Anyone ever do this or have any pointers on how to build one.. preferrably with some kind of LFO driver to make it auto sweep open/close.. perhaps using a 555 chip?

I've tried googling a specific diy schematic, but I just cant seem to locate one.

wow how did you wind up with that many? If your interested in selling one, what's your price?

Circuitbenders · March 01, 2013, 10:21:44 PM

I don't think a full datasheet for the 2069 exists, so its going to be difficult.

They were used in several korg machines of that era. I think the DSS & DSM samplers have 8 in them.

Synthetech · March 02, 2013, 06:05:59 PM

I got the chips off a DW-8000 filter board.

I think I'm going to make a bunch of PICs to control it via CV's

Check this site-

http://www.electricdruid.net/

Tom has open source code for PIC16's. A LFO chip, an ADSR chip and even a VCO chip (with it's own built in filter).

Get a $25 programmer, a handfull of $2 each PIC16's and make about 4 LFO's, 4 ADSR's and maybe even a few VCO's.. all really cheap.
Add in the supporting circuitry for the PICs, the V+/V- supplies and op amps for the PIC's PWM outputs and finally you have CV's you can get control of.

Cross modulate LFO's to ADSR's or reverse.. imagine there's many different ways to conduct the CV's before they reach the control pins on the 2069's.

VCF can be modulated as well as Resonance and the VCA.

As for a datasheet.. all you need is the schematic I posted above and look at a Korg synth that uses the chips to see how they hooked it up with supporting components.

Jaytee · March 18, 2015, 09:25:13 PM

FWIW, you don't even need the Korg synth to compare against; just grab a service manual for one of them and check out how they implemented the chips.

I have a bunch of 2069s I'm trying to find a good project for as well. I am building a filter bank for my Casio SKs, but I can't decide whether it's easiest to build discrete filters from known working circuits or try to wire up my own implementation around these 2069 chips...