How to add an envelope to this filter design?

[Stabby]

I'm building a Korg MS20 filter and I'd like to add a simple envelope (attack, decay, perhaps amount). Is there a simple envelope design I could use and how should the connected to the filter? Can it simply be connected to the CV point (see schematic)?

Frequencycentral has made a filter design based on the same chip (LM13700) and it has an envelope, but that doesn't help me out since the whole design is different (different resistor and cap values)

I would be okay with an envelope generator with fixed values, much like the envelope in the Moogerfooger filter. Does anyone have a schematic for this?

This is the filter design I'm using, which has the option of switching between LP and HP:

[frequencycentral]

Have you got this on breadboard? It should be no problem to add an envelope via a resistor into 'point A for LFO'. I'm sure those two 100k resistor could be reduced in value for more sweep too. The envelope I used wasn't particularly good, loadsa ripple. Still a fun pedal though. I'm planning an upgraded version sometime, and will defo be stealing some ideas from here: http://dl.dropbox.com/u/967492/MOOF_MF-101.png

[Stabby]

Hi Frequencycentral, glad you joined in. This MS20 filter design is being discussed on a Dutch forum and there is great interest in this filter. So far almost 100 PCBs have been ordered that will soon go in production. There is no demand for an envelope generator though, I guess that has to do with it being a synthesizer forum. I would love to make a couple of these for bass and guitar. It shouldn't be hard to build an envelope on a separate printboard and connect it to the PCB.

The Moogerfooger seems to have a pretty simple envelope generator and would be a great match. I don't know how to make a design based on the schematic of the Moogerfooger though.

It would be great if you'd share your design once you come up with it. If you want, I could send you one of those filter PCBs at no cost (I've got plenty on order).

[soggybag]

Take a look at the DOD FX 25. Here's a link to a site with a whole project: http://topopiccione.atspace.com/PJ11DODfx25.html

[Stabby]

Take a look at the DOD FX 25. Here's a link to a site with a whole project: http://topopiccione.atspace.com/PJ11DODfx25.html

Looks interesting, but I have no idea how to design and envelope generator for the filter I'm building based on this.

[Stabby]

Could this possibly work? I removed R9 from the DOD schematic (1M value) because I assume it's the resistor that fixes the filter frequency. The MS20 filter uses a 100k pot for frequency, so the much higher resistor value in the DOD layout seems suspicious to me.

[soggybag]

There is a thread on this forum about the DOD FX 25. search for it. The site I posted gives a successful build report. I notice there is a DOD FX 25b with 3 knobs. Looks like the third knob is labeled blend.

The DOD FX 25 is surprisingly few parts. The envelope looks almost exactly like the envelope from the Dr. Q.

[Stabby]

There is a thread on this forum about the DOD FX 25. search for it. The site I posted gives a successful build report. I notice there is a DOD FX 25b with 3 knobs. Looks like the third knob is labeled blend.

The DOD FX 25 is surprisingly few parts. The envelope looks almost exactly like the envelope from the Dr. Q.

I'm not really interested in building the DOD FX25. It's just a bypass filter while the MS20 is LP/HP. It's like the Moogerfooger but with the raw MS20 filter. Also, I have these MS20 filter PCBs on order, so it's more practical.