Passive Low Pass Gate - Vactrols

petkost · Post by **petkost** » Sun Mar 09, 2014 5:14 am

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.

NS4W · Post by **NS4W** » Sun Mar 09, 2014 8:52 am

Have you found a description for building this lowpass gate?

bsmith · Post by **bsmith** » Sun Mar 09, 2014 9:03 am

?! wow!

so here's a picture of it on the dude's blog

Igor Berlin · Post by **Igor Berlin** » Sun Mar 09, 2014 9:13 am

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)?

a100user · Post by **a100user** » Sun Mar 09, 2014 9:20 am

The YouTube vids sound good and at <24 sterling.

Nice job

CLee · Post by **CLee** » Sun Mar 09, 2014 9:25 am

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.

petkost · Post by **petkost** » Sun Mar 09, 2014 9:44 am

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!!"

petkost · Post by **petkost** » Sun Mar 09, 2014 10:27 am

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.

CLee · Post by **CLee** » Sun Mar 09, 2014 10:35 am

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 · Post by **Dogue** » Sun Mar 09, 2014 10:50 am

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

Tim Stinchcombe · Post by **Tim Stinchcombe** » Sun Mar 09, 2014 11:42 am

CLee wrote:Just wish he wouldn't call it an LPG then.

+1 on this, and what you said in your first post.

petkost · Post by **petkost** » Sun Mar 09, 2014 6:03 pm

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.

ben_hex · Post by **ben_hex** » Sun Mar 09, 2014 8:00 pm

Cheap and cheerful, is it a little kit or a pre-built thing?

electrolobotomy · Post by **electrolobotomy** » Mon Mar 10, 2014 7:43 am

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!

Navs · Post by **Navs** » Mon Mar 10, 2014 4:31 pm

Time for some Homebrew Vactrols!

viewtopic.php?p=796018

bsmith · Post by **bsmith** » Mon Mar 10, 2014 4:34 pm

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

electrolobotomy · Post by **electrolobotomy** » Mon Mar 10, 2014 7:28 pm

Nice work bsmith!

snaper · Post by **snaper** » Tue Mar 11, 2014 7:57 am

After the pictures from Electrolobotomys site.
Is that correct?

Dogue · Post by **Dogue** » Tue Mar 11, 2014 6:45 pm

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.)

snaper · Post by **snaper** » Tue Mar 11, 2014 11:26 pm

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

Dogue · Post by **Dogue** » Tue Mar 11, 2014 11:31 pm

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

snaper · Post by **snaper** » Wed Mar 12, 2014 9:51 am

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 · Post by **Dogue** » Wed Mar 12, 2014 9:54 am

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

snaper · Post by **snaper** » Wed Mar 12, 2014 11:31 am

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

ray · Post by **ray** » Tue Aug 12, 2014 3:37 am

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

MOD WIGGLER

MOD WIGGLER

Passive Low Pass Gate - Vactrols

Passive Low Pass Gate - Vactrols