CV inverter?

[Beneficial]

I have a 2 channel analog filter (Mutronics Mutator) that has 1 CV input per channel. The CV controls the filter sweep. I want to use the Moog MP-201 CV pedal unit to generate CV and use it with the Mutator filter... BUT I want to send one normal CV signal into Channel 1 and on inverted CV signal into Channel 2 so that I can get stereo pan sweeps and stuff like that. Is there a way to invert control voltage modulation to do this? I don't think the Moog unit has the option of inverting CV out so I think I might need something external to do that.

[Beermaster]

CV inverters are quite common in modular gear not that helps you

[Beneficial]

Is anyone aware of a stand alone inverter? or know if the Moog MP-201 can do that?

[T71]

Originally Posted by Beneficial

Is anyone aware of a stand alone inverter? or know if the Moog MP-201 can do that?

Ask Amos over at Moog Music. His address is: amos at moogmusic.com

He will likely have a quick answer.

[ignatius]

you could get a tiny doepfer beauty case and put one in it. there are many CV processors you might like.

a n a l o g u e h a v e n

[shadowfac]

Does the mutator CV inputs take positive and negative voltages, or only positive?

If they work with negative voltages, all you have to do is make a special cable with the wires inverted (e.g. tip goes to ring and viceversa), so the voltage is actually inverted.

[nofi]

Originally Posted by shadowfac

If they work with negative voltages, all you have to do is make a special cable with the wires inverted (e.g. tip goes to ring and viceversa), so the voltage is actually inverted.

naaah.

This will just short your CV signal, because ground is common....

[Oli]

One could build a pretty simple circuit for this, if you feel like getting your hands dirty. You would need a power supply (or access to bipolar rails) and an opamp in an inverting config.

[Beneficial]

I reached out to Amos at Moog... real cool guy and super responsive. He said CV inversion or inverted LFO is not a feature currently available in the MP201 BUT it will be available with a firmware update some time next year. In the meantime he said I could just use an external CV inverter. I'm trying to figure out how to make one but I'm really new to doing any kind of electronics work. I was really hoping to find someone or a company who is selling just a simple external CV inverter.

[golden beers]

call me a c-word but couldn't you do this by routing a cv through an audio mixer and pressing the reverse-phase button? you'd have to set the correct level on the channel obviously. could set up a 'head-phone mix' using a cannel and the aux.

[Beneficial]

Ok great idea. For some reason I was thinking there was more to inverting the CV signal than just phase swapping. Going to see if I can find a stand alone phase swap box or something.

[Oli]

Are you sure a phase inversion will work? I think that is intended for AC signals. One may need DC on a CV signal, right? It might work for this application, provided the mixer can handle the required levels.

[golden beers]

i've got no idea

[DSPaudio]

Most desks won't pass CV signals as they move to slow and are filtered out along with DC offset. Do you have a MOTU audio interface?

[Beneficial]

I have a RME Fireface 400 interface. I'm surprised there's no standalone CV inverter out there with how much CV was used to control synths before midi was around. There has to be a simple way. It's cool Moog will be able to implement this with a firmware change next year though.

[Doktorfuture]

Inverting CV is easy enough with an op-amp.

Here is a good how-to for an inverting amplifier. Note, you don't need to 'amplify' - you can just invert (ratio Rf and Rin is 1:1 so buy some accurate resistors)

Here is a good link:
The Basic Op Amp Inverter

That should be good to get you started. Also go to wikipedia for 'operational amplifier'.

OP Amp's are great to learn and have zillions of audio applications. You can make gain stages, EQ's and filters, envelope followers, summers, etc...

Originally Posted by Beneficial

I reached out to Amos at Moog... real cool guy and super responsive. He said CV inversion or inverted LFO is not a feature currently available in the MP201 BUT it will be available with a firmware update some time next year. In the meantime he said I could just use an external CV inverter. I'm trying to figure out how to make one but I'm really new to doing any kind of electronics work. I was really hoping to find someone or a company who is selling just a simple external CV inverter.

[Oli]

I think there would be a few things for a newb to consider for this sort of thing.

One is op amp selection. Get one with good DC performance. Don't go for high speed etc.

Also the power supply is key. If you build your inverter into the case of another device, you may be able to tap into an existing bipolar supply. Depending on the op amp, one may need a power supply range of ~0.5 volt greater than the signal range.

There are other things one can do such as input protection, or general circuit protection.

It's not really a big deal, but can be a bit more involved than it first seems. If working inside another device, always try to be aware of what you are doing. It's no good blowing a synth for the sake of saving $50 on a used converter. If one has friends experienced in electronics, it may be a good time to start shouting for beers.

[Electric Motel]

For this application I don't think a traditional inverter circuit is whats required.

A basic op-amp inverter circuit with a gain of 1 will simply multiply your input voltage by -1. I don't think this is what you want.

For example..If you were to send a cv of 5V for say max cutoff that circuit would output -5V. Most CV stuff stays above 0V. I'm not sure about your Mutator pedal though.

What you would need is a circuit that would output 0V with a CV input of 5V, and 1V with a CV input of 4V etc. For this you would require a traditional subtractor circuit with the positive input referenced to whatever your CV max is for full cutoff operation of your Mutator.

The circuit is very similar to the traditional inverter circuit shown earlier and easy to find on the interweb.

Best of luck