Passive Low Pass Gate - Vactrols

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petkost
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Passive Low Pass Gate - Vactrols

Post by petkost »

Hi guys,

I am trying to get my hands dirty and make some diy modules.
And maybe, learn something about electronics.

I think this would be a useful and easy one:
http://www.etsy.com/listing/180674772/p ... e_active_1

You know, we need VCAs all the time ; )


Does anyone know a good vactrol to play with it in the eurorack world at Mouser store?
... I am finishing my order and would be great to have some of this.

Thanks.
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NS4W
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Post by NS4W »

Have you found a description for building this lowpass gate?
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bsmith
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Post by bsmith »

?! wow!

so here's a picture of it on the dude's blog
Image
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Igor Berlin
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Post by Igor Berlin »

oh :-) would love to make one of those, since I'm trying to get into diy. Does this look as if he made the vactrol himself (because of that heatshrink)?
some audio, please.... or it never happened!
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a100user
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Post by a100user »

The YouTube vids sound good and at <24 sterling.

Nice job
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CLee
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Post by CLee »

Well, I don't see where the Low Pass is in his LPG. What there seems to be is a voltage (current actually) controlled attenuator.

The LPG is a Filter/VCA built around a Sallen-Key filter design. I only see a few resistors and a home made LDR. This will only attenuate a signal overall, not filter it at the same time.

Also, being passive, there's no circuit controlling LED drive, that's dependent on the CV source you patch in. Also the attenuator will behave differently depending on how it's connected somewhat.

The Buchla LPG design is very nice and unique. Your not going to get the same thing from a Vactrol and a few resistors.
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petkost
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Post by petkost »

NS4W wrote:Have you found a description for building this lowpass gate?
Nope.

Description and videos from ElectroLobotomy:

"PassiVac is a 2 HP, vactrol based, single channel voltage controlled attenuator. It requires no power! Just feed it CV. Sounds nice with minimal bleed. Makes a great passive Low Pass Gate!!"



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petkost
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Post by petkost »

CLee wrote:Well, I don't see where the Low Pass is in his LPG. What there seems to be is a voltage (current actually) controlled attenuator.

The LPG is a Filter/VCA built around a Sallen-Key filter design. I only see a few resistors and a home made LDR. This will only attenuate a signal overall, not filter it at the same time.

Also, being passive, there's no circuit controlling LED drive, that's dependent on the CV source you patch in. Also the attenuator will behave differently depending on how it's connected somewhat.

The Buchla LPG design is very nice and unique. Your not going to get the same thing from a Vactrol and a few resistors.
Maybe not a LPG, I dont know, but seems like a nice VCA.
Thanks, really informative.
Last edited by petkost on Sun Mar 09, 2014 6:09 pm, edited 1 time in total.
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CLee
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Post by CLee »

petkost wrote:Maybe not a LPG, I dont know, but seems like a nice VCA.
Just wish he wouldn't call it an LPG then.
Dogue
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Post by Dogue »

A couple notes:

Regarding the original question, the most commonly used vactrols in analog modulars are VTL5C3/2 and VTL5C3. Silonex vactrols like the NSL32-SR3 are also used. I don't think Mouser sells either of those. The design above uses a homemade vactrol made by putting an LED and an LDR literally in touch with one another and then covering both with heat-shrink tubing.

CLee is right -- that is not a lowpass gate, it's just a passive "VCA". While Buchla's LPGs and many other designs use a Sallen-Key filter typology, it is possible to build a very simple passive variant on a low-pass gate by just putting a capacitor to ground after the vactrol's resistor side to create a simple RC lowpass filter. See this thread (and also the Silonex datasheet) for ideas:

http://electro-music.com/forum/viewtopic.php?&t=26643
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Tim Stinchcombe
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Post by Tim Stinchcombe »

CLee wrote:Just wish he wouldn't call it an LPG then.
+1 on this, and what you said in your first post.
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petkost
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Post by petkost »

Dogue wrote:A couple notes:

Regarding the original question, the most commonly used vactrols in analog modulars are VTL5C3/2 and VTL5C3. Silonex vactrols like the NSL32-SR3 are also used. I don't think Mouser sells either of those. The design above uses a homemade vactrol made by putting an LED and an LDR literally in touch with one another and then covering both with heat-shrink tubing.

CLee is right -- that is not a lowpass gate, it's just a passive "VCA". While Buchla's LPGs and many other designs use a Sallen-Key filter typology, it is possible to build a very simple passive variant on a low-pass gate by just putting a capacitor to ground after the vactrol's resistor side to create a simple RC lowpass filter. See this thread (and also the Silonex datasheet) for ideas:

http://electro-music.com/forum/viewtopic.php?&t=26643
Thanks a lot, great info.
Maybe its the time to make some diy vactrols.
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ben_hex
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Post by ben_hex »

Cheap and cheerful, is it a little kit or a pre-built thing?
All DivKid video series to be found here!
http://www.youtube.com/divkidvideo
electrolobotomy
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Post by electrolobotomy »

Dear friends,
This is not a LPG. Tho, when plucked sounds reminiscent of a LPG. Sorry for calling it that...

The Vactrol's are self made.

Cheers!
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Navs
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Post by Navs »

Time for some Homebrew Vactrols!

viewtopic.php?p=796018
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bsmith
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Post by bsmith »

a little bit of shitty iphone vid of one I made last night normalled to an or combiner....
[video][/video]
electrolobotomy
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Post by electrolobotomy »

Nice work bsmith!
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snaper
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Post by snaper »

After the pictures from Electrolobotomys site.
Is that correct?
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Dogue
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Post by Dogue »

The LDR, which you have marked "V", has only two leads, those towards the top of your diagram. (You can either use a vactrol, in which case you leave out the LED, or you can use a "homemade vactrol," which is an LED and LDR touching and heatshrinked together.)
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snaper
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Post by snaper »

V means Vactrol in my drawing, the visible LED is just a signal indicator like in the original passivac.
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Post by Dogue »

OK, that makes sense. Note that the response will likely be slightly different with two LEDs in parallel rather than just one.
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snaper
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Post by snaper »

2 indicator LEDs or 2 LEDs inside the homemade vactrol?
I would like to use a few cheap vactrols, would be a great FX unit, insted of a true "wannabe" VCA
Dogue
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Post by Dogue »

In the example you drew, there is an LED inside the homemade vactrol, and this LED is in parallel with the indicator LED.
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snaper
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Post by snaper »

I see.
Regarding the video it could be a great little addition to the rack:)
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ray
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Post by ray »

I would like to hear a video where the sound with/without the vactrol-"lpg" module is more obvious?
Anyone?
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