In praise of 4-pole filters

[Mark Hammer]

I bought a Sequential Circuits Pro-1 that is still in a bit of a state of disrepair, but for now it processes external signal just fine through the ext-in jack.  It has a 4-pole lowpass filter built around a Curtis CEM3320 chip, harnessed to a very decent envelope-follower, gate/trigger extractor and ADSR transient generator.

I had never played guitar through a 4-pole lowpass before, and I have to say it is REALLY nice.  And, confirming Stephen Giles' frequent suggestion, using a gate and transient generator provides a whole universe of sounds beyond what the average envelope-controlled filter will produce.  Using a slow attack, moderate envelope modulation, , and some chorus, you can get some truly lovely pads.

I have a diode-ladder ECF board from an old Japanese project 98% populated, and look forward to hooking it up later this summer, once I'm done with my other commitments.  For now, I've convinced myself that "liberating" one of the SSM2040 chips I've been sitting on since the early 80's is the smart thing to do.  Four poles is a lot more expressive that two.

[frequencycentral]

I used to have one, but bought it at the same time as an Odyssey MkI and a Roland 100M, it paled a little beside those two, and I never really liked it's filter, so I sold it. Still have the other two though. 

Here's the Pro One schematics:

http://machines.hyperreal.org/manufacturers/Sequential/Pro-One/schematics/pro1-schematics-a.gif
http://machines.hyperreal.org/manufacturers/Sequential/Pro-One/schematics/pro1-schematics-b.gif

[Mark Hammer]

Well, yeah, I guess a Pro-One would pale beside a 100M.  It's a decent synth with decent modulation capabilities, but not a 100M.

One of the things that distinguishes the Moogerfooger filter from so many others, though, IS the fact that it does 4-pole, rather than the 2-pole stunts that so many others are limited to.

We gotta fix that.

[frequencycentral]

...but you're right, one thing I do remember about it was it's excellent external signal processing capabilities (I must look at that again for the Funky MF MkII) - great for processing samplers too. That and it's very hot output signal.

[chicago_mike]

I have CEM3360's! And a LOT of CA3280's. And a few CA4570's.

So design something. Cause I want to use these!!

[aron]

I have two Pro Ones, an Odyssey, OB-1 and MicroMoog among others. Very cool units. The Pro Ones survive and prosper!!!!

[moosapotamus]

I'm another proud Pro-One owner!

But, I don't know why I have never tried putting an external signal through it. Don't you need to provide gate and/or trigger inputs along with the external audio to get the filter and amp to open up? Something like this... http://www.musicfromouterspace.com/analogsynth/signaltogate.html

~ Charlie

[chicago_mike]

Bass players used to plug into moogs back in the day. But I dont know if they needed andthing for CV inputs and such...

Give it a shot.

[Strategy]

I think if you want to play straight through you can shut off the gate that goes to the VCA and VCF for a 'drone' type effect, keyboard/gate/trigger free playing. But its been a while since I looked at Pro One, I may be misremembering.

Strategy

I'm another proud Pro-One owner!

But, I don't know why I have never tried putting an external signal through it. Don't you need to provide gate and/or trigger inputs along with the external audio to get the filter and amp to open up? Something like this... http://www.musicfromouterspace.com/analogsynth/signaltogate.html

~ Charlie

[Mark Hammer]

I'm another proud Pro-One owner!

But, I don't know why I have never tried putting an external signal through it. Don't you need to provide gate and/or trigger inputs along with the external audio to get the filter and amp to open up? Something like this... http://www.musicfromouterspace.com/analogsynth/signaltogate.html

~ Charlie

Look at the schematic and manual and you'll see that nothing more is needed beyond an audio signal.  The Pro-One does all the gate generation itself.

[R.G.]

I think that a decent four pole can be made from two LM13700s pretty easily. The SSM is a four-section OTA without buffers as I remember.

[Mark Hammer]

Tom Gamble, of EFM, posted a 9v-based diode ladder 4-pole filter some time back.  http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/vcf6-9v.pdf

I've not built it nor heard of anyone confirming its functionality, though I've not heard of anyone confirming that it didn't work, either.

What the relative merits of OTA-based vs diode-ladder filters are is well beyond the perimeter of my knowledge.  As is having an opinion over which type could behave properly, using a 9v supply.

[moosapotamus]

Tom Gamble, of EFM, posted a 9v-based diode ladder 4-pole filter some time back.  http://www.modular.fonik.de/files/EFMforum/orgEFMfiles/vcf6-9v.pdf

I've not built it nor heard of anyone confirming its functionality, though I've not heard of anyone confirming that it didn't work, either.

I tried breadboarding that once and could not get it to work. Might be worth another try, tho.

[frequencycentral]

The Roland System 100M VCF is 4 pole OTA based, using 4 x 3080's, running on +/-15v. An excellent LPF, and can easily be patched as a HPF by mixing it's output and the source signal into a mixer. You can even make a HP or LP 8 pole out of two of them, or an 8 pole BPF by chaining two in series and mixing the output of the first with the output of the second.

http://share.ovi.com/media/Melectroworks.Roland100MServi/Melectroworks.10125

[chicago_mike]

I wonder how close that is to the moogerfooger LPF pedal.

[Jarno]

Well, Bob gained fame because of transistor ladder based filters, so the ones in the moogerfooger are probably not OTA based. I think I've seen gutshots with a truckload of transistors.
Reverse engineering a moogerfooger will be a heck of a job, lots and lots of parts!

[slacker]

Bit shaky on the terminology but so I might be wrong but the OSCar synth VCF is 4 pole, and a piece of cake to convert to 9 volts.
http://www.diystompboxes.com/smfforum/index.php?topic=65918.0
The sound clips in that thread are just one section of it so only 2 pole.
Or in a similar vein stick 2 MXR envelope filters in series.

[wavley]

I used to have a Sequential Prophet 2000, it was a great board, wish I hadn't sold it.  But the disk load time, mylar buttons, and only one knob to adjust the parameters made it a pain to operate, I prefer a board a little more conducive to playing live. But I REALLY loved those filters.

[Mark Hammer]

Poked around through my files, and the filters you loved were based on these chips: http://www.synthdiy.com/files/2004/c3378pdf.pdf

These are dedicated 4-pole LP filters plus on-board VCAs.  The Pro-One uses a CEM3320 chip which is also 4 OTA sections, but it is not "committed" to lowpass function, merely configured as such.  Once I get this thing functioning properly, I'm going to mod the filter to have some other functions.

[liquids]

The Roland System 100M VCF is 4 pole OTA based, using 4 x 3080's, running on +/-15v. An excellent LPF, and can easily be patched as a HPF by mixing it's output and the source signal into a mixer. You can even make a HP or LP 8 pole out of two of them, or an 8 pole BPF by chaining two in series and mixing the output of the first with the output of the second.

http://share.ovi.com/media/Melectroworks.Roland100MServi/Melectroworks.10125

Obviously this is way after the fact, but I wanted to note that the filter shown in the schematic can easily be duplicated with 2x13700 OTAs.  Probably the most minimal component count I've come across on the filter side. Thanks Rick!

And though it takes a little bit of know-how to do so, It also, surprisingly, worked just fine off of a single sided 9v supply with a voltage divider for the center rail/Vref as a stand-alone filter when I tried it as such. 

Adding envelope control should be cut and paste if you want a quick and dirty 4-pole envelope filter, though greater than 9v is often preferable for envelope generation for those so inclined.