G2X and moogerfooger Low Pass Filter

varice · Last edited by varice on Sun Feb 02, 2014 11:21 pm; edited 4 times in total

EDIT 26JAN2014: Well, Clavia has been hiding a secret about the Nord Modular synths all along, that the stereo headphones outputs can pass DC signal levels Exclamation

So, you don’t have to modify your G1 or G2 to get at least two channels of DC CV output. This discovery about the G1 was posted here:

http://electro-music.com/forum/topic-60892.html

Gracias Nacho thumb up

I checked my G2 Keyboard and G2X. Both synth headphones outputs are not DC blocked and can be used for CV output. More details here:

http://electro-music.com/forum/post-395462.html#395462

The original post follows:

I decided to interface a Moog moogerfooger MF-101 Low Pass Filter with my G2X to use with mono synth patches.

I needed to get a DC control voltage signal out of the G2X to plug into the fooger external voltage control jack for the filter cutoff. But, the G2 outputs have DC blocking capacitors in the way. So, I opened up the G2X and used my o'scope to find a point before the DC blocking cap for the G2X Output 4 from which I could get a DC signal. I soldered a cable to the G2 circuit board to bring out the DC control signal to a plug for the fooger. I found that the G2 produces a signal maximum swing of less than -4 to +4 VDC. According to the fooger specifications, the MF-101 uses 0 to +5 VDC for the external control voltage. But, this turned out not to be the case, the fooger filter cutoff will work with a bipolar external signal (the fooger filter cutoff knob produces a signal swing of -5 to +5 VDC). I traced the external cutoff signal on the fooger circuit board to the circuit that buffers and mixes the signal and modified it to increase the gain to work with the -4 to +4 G2 signal.

I connected the necessary cables between the G2X and the fooger and then created a tuning patch to trim the gain of the fooger external cutoff circuit so that the filter cutoff frequency would track properly.

The moogerfooger LPF sounds great with the G2X! Maybe not as good as a minimoog or modular 904 filter, but better than the G2 "FltClassic" module (sorry Clavia!).

This is not a perfect setup. There are a couple of "issues":

1. Currently, there is no way to program the fooger filter resonance setting in a G2 patch, it must be set manually. This would have required using another G2 output for this CV signal, leaving only one for the patch audio output signal (no G2 stereo output, too high a price to pay).

2. The G2 Master Level and the Patch Level settings also affect the DC CV signal output level. The workaround is to set these levels at maximum and adjust the patch audio output level within the patch itself.

I think that the same interface could also be made to the NM1.

Blue Hell · Posted: Sat Jul 08, 2006 4:22 am Post subject:

Wow !

Certainly a good reason to void the warranty Very Happy

Do you have some pic's of what you did ?

And it makes me wonder ... would some G2's eventually end up being circuit bent ...

mosc · Posted: Sat Jul 08, 2006 7:00 am Post subject:

OK, I gotta ask, can you send some pics of the mod to the G2, and how about some sounds?

I be this will be useful to many people with external CV controlled gear.

3phase · Posted: Sat Jul 08, 2006 7:30 am Post subject:

of cause something like phantom power can be problematic than to a g2...very problematic..the caps also protect against external voltages that might be aplied by idiots or system failure..or ...the well known phantom power that might brought to you on stage without having asked for it...
Many reasons that clavia was forced to do cap coupling...
Its also a question if the output of the last op amp is short protected..

however... interesting...

mosc · Posted: Sat Jul 08, 2006 8:11 am Post subject:

Another way to get a CV out of the G2 if you don't want to go the MIDI route some constant audio through a VCA one of the outputs, and use an outboard envelope follower circuit.

varice · Posted: Sat Jul 08, 2006 8:51 am Post subject:

Sorry - no pictures - I do not have a digital camera. There is not much to look at anyway. The only unusual thing in the G2X is the extra cable with one wire attached to common and the other to the DC side of cap C103 (the DC blocking cap for Output 4)[Edit: changed to C101 at pin 1 of LM833 opamp through a 1K resistor]. A TRS plug is used on the cable. The tip has the CV signal, the common is on the sleeve, the ring is not terminated (this is the phantom power in the MF-101 used to power a control pedal). I did not even have to modify the case to run the cable out, there is a small opening at the bottom corner that I ran the cable through. The other connections use the standard jacks on the G2X and the MF-101 - raw audio comes out G2X Output 3 and is connected to the MF-101 Audio In, MF-101 Audio Out is connected to G2X Input 3.

Yes, a better interface mod would be to build a buffer circuit with output protection and drill a hole in the case to mount a jack for the CV output, but that would have been more work - and I am too lazy! I do not play live and my kit stays in my home studio, so I should not have a problem with the robustness of this interface. I know to be careful with the CV cable while the G2X and MF-101 are powered up.

I will post some sound clips soon - I'm still experimenting at the moment.

varice · Posted: Sat Jul 08, 2006 9:05 am Post subject:

3phase · Posted: Sat Jul 08, 2006 1:08 pm Post subject:

but one question...
I was wondering about the influence of smd capaciators on the sound of devices like the NM1,G2 but also RME and other soundcards...
They all share a certain sonic signature.
could you do me, us the favor to do some listening tests with your direct output and tell us if there is a difference and how it sounds compared to the capaciator coupled outputs?

3phase · Posted: Sat Jul 08, 2006 1:18 pm Post subject:

might be an interesting modification to have an extra set of 4 switched jack sockets that over bridge the output cap... in a controled enviroment where the next stage is the input transformer or input cap of a desk channel there is no harm... and you can decide for as many channels to act as cv outputs as you want just be deciding for the other jack connector..

just space wise this sounds to a good mod for my NM1 rack

the 4 extra jacks also can act as terminals for big high quality output caps...
so there would be an improvement for the standard outjacks aswell...

3phase · Posted: Sat Jul 08, 2006 1:49 pm Post subject:

and one security suggestion..when you connect the un coupled output to another device you should messure ac and dc voltage between the external machine and your g2...sometimes there are devices with switched power supplys like laptops or so that can create nasty sneaking voltages...
I think thats the only danger for an un coupled output to run into some unwanted voltages... when i understand the data sheet wright the op amp should be short protected within the possible voltage ratios of the g2

varice · Posted: Sun Jul 09, 2006 4:13 pm Post subject:

Here are some sound clips of the G2X with the MF-101 LPF. There is no EQ used except on the LFO modulation clip - it has a 2 Band EQ module set to +2db on the bass with a shelf freq 110Hz, and -2db on the treble with the shelf at 6KHz. The MF-101 is driven moderately hard to get a little bit of overdrive. The resonance knob is set at 7.

Comments welcome.

G2 and MF-101 Filter Sweep.wav

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G2 and MF-101 Filter Sweep.wav

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G2 and MF-101 Filter LFO modulation.wav

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G2 and MF-101 Filter LFO modulation.wav

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6.09 MB

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G2 and MF-101 Filter audio rate modulation.wav

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G2 and MF-101 Filter audio rate modulation.wav

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7.11 MB

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varice · Posted: Sun Jul 09, 2006 4:29 pm Post subject:

mosc · Posted: Sun Jul 09, 2006 7:01 pm Post subject:

Yes, if you don't have one, you will find it a valuable addition to your kit. Cool

davep · Posted: Mon Jul 10, 2006 3:37 pm Post subject:

Hey, do you think this mod would work for the original NM1 as well? Since the NM1 has no midi out modules, a mod like this could be very VERY cool for folks who have an NM1 but no G2. It would allow you to control your analog modular synths from control signals generated in the NM1.

3phase · Posted: Mon Jul 10, 2006 4:12 pm Post subject:

sure it would work..its just the coupling capaciator in the output that prevents the opamp to be able to deliver DC output.

I still would like to know if ther is an audioable difference... bat maybe i ve to test it myself...

varice · Posted: Mon Jul 10, 2006 5:58 pm Post subject:

Here is a short sound clip. This music was generated by a single mono patch with three oscillators each driven by a separate simple algorithmic note generator. Several patch variations are manually selected.

G2 and MF-101 Filter Seq10 Demo.wav

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G2 and MF-101 Filter Seq10 Demo.wav

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8.78 MB

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varice · Posted: Mon Jul 10, 2006 6:10 pm Post subject:

varice · Posted: Mon Jul 10, 2006 6:22 pm Post subject:

richardnixon · Posted: Tue Jul 11, 2006 10:17 pm Post subject:

Awesome! Thanks for sharing this. I just got one of the Frostwave Resonator (MS-20 filter clones) and was looking to figure out some way to get a control signal out of the G2 in order to control the Resonator...I've become fond of using the Resonator as the main filter of a G2 patch, running the audio out to the filter and then back into the nord, but I can't live without some kind of filter envelope applied to my sounds.

I had been debating a MIDI-CV converter, but was unsure of possible latency problems. Mosc have you even noticed anything along these lines with MIDI-CV converters that you have used? Would they easily be suitable for tight filter envelope control and such?

Thanks.

mosc · Posted: Wed Jul 12, 2006 7:17 am Post subject:

MIDI isn't suitable for really fast transistions, but I would certainly give it a try before trying more exotic solutions. I'd guess it would work well enough for all but fast attack times. If you have any CV gear, you should get a MIDI/CV converter.

richardnixon · Posted: Wed Jul 12, 2006 12:24 pm Post subject:

Thanks for the info Howard, I'll try one out.

mosc · Posted: Wed Jul 12, 2006 12:56 pm Post subject:

Curiosity killed the cat.

I just hooked up my Moog modular to my G2 via the MIDI/CV converter and yes, you can use it for envelopes. You can use MIDI for LFOs too. At a little over 4 Hz, you start getting some irregularies though. They sound interesting. This is just my experience with my MIDI gear. YMMV. Very Happy

richardnixon · Posted: Wed Jul 12, 2006 1:19 pm Post subject:

Interesting! Which MIDI-CV converter do you have by the way?

mosc · Posted: Wed Jul 12, 2006 1:41 pm Post subject:

I have one of the first ones, Roland MPU 101.

varice · Posted: Wed Jul 12, 2006 4:35 pm Post subject: