VCA with Vactrol (or LPG that can bypass its filter)

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marlow
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VCA with Vactrol (or LPG that can bypass its filter)

Post by marlow » Fri Jan 24, 2020 10:48 am

Hi

I want to generate an amp contour that will be slewed by a vactrol and as I understand it and LPG will do this but it seems it will also be affected by its filter and I don't want this. I want to connect this in series to another filter which I may use in low pass/high pass mode and I don't want the filtering of the LPG but I want the vactrol response. It seems the Circuit Abbey VacVac can do this but I think its discontinued and I can't find any alternatives. Your help is appreciated.

Thanks

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fac
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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by fac » Fri Jan 24, 2020 11:00 am

Many LPGs can act as VCAs. At least the ones I have can. Malekko Borg filters in euro and Megaohm CdS in MU. Go to modulargrid and select "Low Pass Gate" in the Function menu. You'll see dozens of choices and many of them have a VCA/Lowpass switch.

For instance:

https://www.modulargrid.net/e/erica-synths-lpg

https://www.modulargrid.net/e/malekko-h ... -dual-borg

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by marlow » Fri Jan 24, 2020 11:28 am

Thank you, the erica synths one should do the job

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by moremagic » Fri Jan 24, 2020 3:50 pm

the ptg is quite handy for this

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by galanter2 » Fri Jan 24, 2020 10:44 pm

This uses an analog circuit that has a vactrol-like response that is more stable and consistent. It has a VCA-only mode, Filter-only mode, and both (ie lowpass gate) mode. I have one and rather like it.

https://pittsburghmodular.com/store/lif ... lse-filter

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by Navs » Sat Jan 25, 2020 3:30 am

marlow wrote:
Fri Jan 24, 2020 10:48 am
I want to generate an amp contour that will be slewed by a vactrol ...
You could use a slew limiter to give you the 'laggy' envelope contour you want.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by cptnal » Sat Jan 25, 2020 3:46 am

Feed a static offset to the input of any vactrol LPG, and your envelope into its CV. The output will be a vactrolized version of your envelope. Or hit it with a short trigger and the output will be a CV version of its ping response.

(Not my idea. I read it here, obvs.)

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by marlow » Sat Jan 25, 2020 12:28 pm

Lots of good info here, thanks everyone

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by marlow » Sat Jan 25, 2020 12:46 pm

With regards to feeding a static offset what do I need to do to do this? Also would I need a seperate VCA to actually amplify the sound with this method?

Thanks

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by cptnal » Sat Jan 25, 2020 1:58 pm

marlow wrote:
Sat Jan 25, 2020 12:46 pm
With regards to feeding a static offset what do I need to do to do this? Also would I need a seperate VCA to actually amplify the sound with this method?

Thanks
A static voltage of (say) five or ten volts goes into your LPG. It'll do nothing until you hit the LPG with an envelope or trigger. Then the output of the LPG is a voltage that rises and falls according to that envelope or trigger, and the vactrol's response. Take the output of the LPG to the CV input of a VCA your audio is travelling through.

I know - we're not used to passing CV through LPGs, usually audio. But this method allows you to pick up some of that vactrol flavour and apply it to something else.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by Chopper » Sat Jan 25, 2020 2:01 pm

The doepfer LPG is one of the most underrated module ever. It does what you ask and more. It can work as a resonant filter as well. With gate control of the mode selection ( VCA, LPG or VCF).... And it sounds good.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by lisa » Sat Jan 25, 2020 2:02 pm

Doepfer Vactrol Low Pass Gate A-101-2 comes to mind. It works as a LPG, a VCA and a VCF.

EDIT: Chopper beat me to it. :)
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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by cptnal » Sat Jan 25, 2020 3:01 pm

lisa wrote:
Sat Jan 25, 2020 2:02 pm
Doepfer Vactrol Low Pass Gate A-101-2 comes to mind. It works as a LPG, a VCA and a VCF.

EDIT: Chopper beat me to it. :)
I thought of this at first but wasn't sure whether the VCA has a vactrol. I've since checked the manual and it suggests (but doesn't explicitly state) that it does, so it should do the trick. However, vactrols…

My A101-2 has a very tight response compared to either channel of my LxD. If you have a handful of different sounding LPGs the offset trick should work on any of them, so you can grab whichever response you like. :party:

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by Blairio » Sun Jan 26, 2020 7:35 pm

cptnal wrote:
Sat Jan 25, 2020 3:01 pm
lisa wrote:
Sat Jan 25, 2020 2:02 pm
Doepfer Vactrol Low Pass Gate A-101-2 comes to mind. It works as a LPG, a VCA and a VCF.

EDIT: Chopper beat me to it. :)
I thought of this at first but wasn't sure whether the VCA has a vactrol. I've since checked the manual and it suggests (but doesn't explicitly state) that it does, so it should do the trick. However, vactrols…

My A101-2 has a very tight response compared to either channel of my LxD. If you have a handful of different sounding LPGs the offset trick should work on any of them, so you can grab whichever response you like. :party:
Vactrol performance is remarkably variable, even within the same unit. My MakeNoise Optimix is a 2 channel LPG / Vactrol device, and the two channels have a distinctly different decay 'profile'. One is longer than the other.

I also have two A101-2's. Again one has a longer 'decay' than the other. I guess it isn't a problem unless you are looking for closely matched processing on a stereo signal.

In future I will try and audition Vactrol devices in a store if possible. Personally I like ones with longer decay.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by galanter2 » Sun Jan 26, 2020 8:43 pm

The Pittsburgh module I noted above minimizes the inconsistency by replacing the vactrol with other circuitry. Just say'n...

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by Blairio » Sun Jan 26, 2020 11:30 pm

galanter2 wrote:
Sun Jan 26, 2020 8:43 pm
The Pittsburgh module I noted above minimizes the inconsistency by replacing the vactrol with other circuitry. Just say'n...
as in "virtual vactrol"?

It is a neat idea.

I would like if modules which employ vactrols mounted them on sockets so that they could easily be swapped out for vactrols of different values. THONK in the UK offer vactrols of different values, and I am sure other places do too. It would be neat to easily experiment with different values.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by arthurdent » Mon Jan 27, 2020 4:15 pm

Blairio wrote:
Sun Jan 26, 2020 11:30 pm
I would like if modules which employ vactrols mounted them on sockets so that they could easily be swapped out for vactrols of different values. THONK in the UK offer vactrols of different values, and I am sure other places do too. It would be neat to easily experiment with different values.
If you build a vactrol-based kit, you can use sockets in place of hard wiring to do exactly that. Someone here at Muff's did something similar with diodes for a ring modulator, had some nice photos, just can remember who, might have been rex coil.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by Wjbratcher » Thu Jan 30, 2020 3:29 pm

My dual borg seems to be AC coupled, so the offset trick I don’t think works with it.

The lxd by make noise is dc coupled!

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by cptnal » Fri Jan 31, 2020 3:23 am

Wjbratcher wrote:
Thu Jan 30, 2020 3:29 pm
My dual borg seems to be AC coupled, so the offset trick I don’t think works with it.

The lxd by make noise is dc coupled!
A consideration, it's true. Must make sure to include this proviso if posting the trick again.

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Re: VCA with Vactrol (or LPG that can bypass its filter)

Post by Duga-3 » Wed Feb 05, 2020 5:59 am

arthurdent wrote:
Mon Jan 27, 2020 4:15 pm
Blairio wrote:
Sun Jan 26, 2020 11:30 pm
I would like if modules which employ vactrols mounted them on sockets so that they could easily be swapped out for vactrols of different values. THONK in the UK offer vactrols of different values, and I am sure other places do too. It would be neat to easily experiment with different values.
If you build a vactrol-based kit, you can use sockets in place of hard wiring to do exactly that. Someone here at Muff's did something similar with diodes for a ring modulator, had some nice photos, just can remember who, might have been rex coil.
Vactrols have a huge variation, even if they are of the same type. Especially the "decay" phase from light to darkness varies. IIRC, the Doepfer LPG also has it's vactrols soldered to the board afterwards. Maybe they're hand selected for getting reproducible values.

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