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Passive Low Pass Gate - Vactrols
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Passive Low Pass Gate - Vactrols
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/passivac-2-hp-passive-low-pass-g ate-lpg?ref=shop_home_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
Have you found a description for building this lowpass gate?
bsmith
?! wow!

so here's a picture of it on the dude's blog
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)?
a100user
The YouTube vids sound good and at <24 sterling.

Nice job
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.
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!!"

http://www.youtube.com/watch?v=90r7Nnirokc
http://www.youtube.com/watch?v=yjmCJqUgQ_E
http://www.youtube.com/watch?v=wV7KGadMvqQ
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.
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
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
CLee wrote:
Just wish he wouldn't call it an LPG then.
+1 on this, and what you said in your first post.
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.
ben_hex
Cheap and cheerful, is it a little kit or a pre-built thing?
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!
Navs
Time for some Homebrew Vactrols!

https://www.muffwiggler.com/forum/viewtopic.php?p=796018
bsmith
a little bit of shitty iphone vid of one I made last night normalled to an or combiner....
electrolobotomy
Nice work bsmith!
snaper
After the pictures from Electrolobotomys site.
Is that correct?
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.)
snaper
V means Vactrol in my drawing, the visible LED is just a signal indicator like in the original passivac.
Dogue
OK, that makes sense. Note that the response will likely be slightly different with two LEDs in parallel rather than just one.
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
In the example you drew, there is an LED inside the homemade vactrol, and this LED is in parallel with the indicator LED.
snaper
I see.
Regarding the video it could be a great little addition to the rack:)
ray
I would like to hear a video where the sound with/without the vactrol-"lpg" module is more obvious?
Anyone?
AAG
Does anyone know if this VC attenuator thing is good for attenuating control voltages?
ba1
Can anyone recommend what LDR, LEDs and resistor values to use for this?

Cynthia Webster says that LDRs respond best to yellow LEDs. I'm assuming the brighter the better.

Thanks!
EarlJemmings
So what am I doing wrong here?




the LED which is in parallel blinks, and I can hear the signal attenuated from from the output, but CV seems to have no effect.

In the Pictures, red is in, white is out, and green is CV

Appreciate any help, I'm lost waah
extra testicle
sounds like you have your diode connection backwards.

they usually call these opto isolators or photocouplers lol the theory is that the modulator can't actually get into your audio signal.

fun to combine with passive ring mod while you have the bread board out. smile
EarlJemmings
extra testicle wrote:
sounds like you have your diode connection backwards.

they usually call these opto isolators or photocouplers lol the theory is that the modulator can't actually get into your audio signal.

fun to combine with passive ring mod while you have the bread board out. smile


Turns out I did have the vactrol hooked up backwards on the led side, but adjusting that didn't seem to fix it.
I see in the other schematics and builds some other components (resistors, caps) but it seems that it should work at it's heart without them. Am I mistaken in this?
qp
EarlJemmings wrote:


Turns out I did have the vactrol hooked up backwards on the led side, but adjusting that didn't seem to fix it.
I see in the other schematics and builds some other components (resistors, caps) but it seems that it should work at it's heart without them. Am I mistaken in this?


The red and white is the audio in/out? I'm thinking you have that wrong. The white dot should be that cathode side of the LED. also, I found you need a curent limiting resistor on the led otherwise it ends up pulling too much current from the CV source and won't work. I've had good luck with 4.7k.
EarlJemmings
qp wrote:
The red and white is the audio in/out? I'm thinking you have that wrong. The white dot should be that cathode side of the LED. also, I found you need a curent limiting resistor on the led otherwise it ends up pulling too much current from the CV source and won't work. I've had good luck with 4.7k.


d'oh! d'oh! d'oh!

Yup, this was it!
Turned it around and re-jigged, and it works perfectly.
I guess I took notes from the data sheet when I first got these vactrols in the mail, and it was a bit late in the day. Just wrote down which side was the LED wrong.

Anyway, it sounds great, these NSL-32SR3's have a nice long ring to em, and a decent thunk.

Thanks a tonne gp, Extra Testicle
and I really like the idea of bsmith's OR combiner mixed in. Think I'll do up a couple like that Guinness ftw!
ben_hex
EarlJemmings care to share a pic? I have a 2U utility panel that's drilled and has jacks and pots ready for me to make a load of passive goodies to accompany the modular. Thinking mults, passivev mixer, attenuators, format jumbler and some of these hopefully.

So pics and any information you can share is appreciated.
EarlJemmings
ben_hex wrote:
EarlJemmings care to share a pic?


Sure thing!
So I ended up just going for a dual vactrol vca, I'll make an 8:1 OR another time, so I can use it better with my Sequence 8.

Here's an instagram video of it working: http://instagram.com/p/vyFp9sRPpw/
Seriously, these things have like 3-4 seconds of ring for a really short maths envelope (the video cuts out some of the tail end of the decay)

I found with these that I didn't even need any more components. The LED did cause a slight attenuation, but I decided to deal with it for the brightness it granted.

Front Panel:


Top view:


Side:


I did also a while ago make a 2:1 OR combiner with various types of diodes, with the idea of using it on audio. One Germanium, one an IC, one silicon. It does make a slight difference, especially in the germanium, but really, they're best for trigger combining.
All you need to do is put diodes to the signal in of the jack, and connect grounds.

Front panel:


Components:


Hope that helps some. If you need larger pictures you can see em here (click the download icon and you can see different sizes) and feel free to pm me if you have any questions.
Shoggoth
I made two of these with some homemade vactrols and took some cues from the electro-music thread to make a selectable vactrol "vca" or "vcf".

Right before the output I connected the middle lug of a 100k linear pot, then 1uf cap to ground on lug 1 and a .1uf cap to ground on lug 3. The middle of the pot turn is just the vca-type sound with no discernible filtering. Full CW and full CCW are the differing filtered versions. I could have used a on-off-on switch but didn't have one handy. Cool to have a range of sounds with such a simple circuit. Probably would work better with a log pot since the action happens right at full CCW/CW on a linear pot.

I also normaled the output of channel 1 to the input of channel 2 because I love anything to do with switching jacks.
ben_hex
Cheers EarlJemmings
szabomate
I used PERKIN ELMER A906013 LDR, from TME.
http://www.tme.eu/sk/details/a906013/fotorezistory/perkin-elmer/#


snaper
szabomate wrote:


Nice one! grin
...another builder from the same region? grin
szabomate
snaper
helóbeló Mr. Green
ba1
I don't have a clue what I'm doing at all...

Got this from a thread on electromusic.com,

http://electro-music.com/forum/topic-42365.html

Luckily it seems to work fine, but the 100k resistor doesn't seem to have any effect. In fact I don't even have to connect the audio or CV inputs to ground. As is, there's no bleed, I can get really crisp plucky sounds. Can anyone tell what the 100k is supposed to be doing? Or if I've got it wrong?

Sorry for the shitty pic!

Thanks

[/img]
ba1
My only complaint is that the sustain from the asdr is almost either on or off with very little gradation in between. The other envelope settings work as expected. I don't have much experience with vactrols, let alone diy versions. Does this seem like normal behavior?
diablojoy
to get a better response other then on /off you would need a LED driver circuit
look at the scott stites mutant vactrol filter and how the LED in the vactol is being driven for a good example
of course that gets you totally away from it being passive.


oh szabomate
you really should add a current limiting resistor between the LED cathode and ground something like 470R- 1K
Your LED's probably wont live long without it.
Annelog
Sounds normal with va trols
indigoid
szabomate wrote:
I used PERKIN ELMER A906013 LDR, from TME.
http://www.tme.eu/sk/details/a906013/fotorezistory/perkin-elmer/#


So I built one of these this evening in a 1U tile for my 54hp lunchbox case. I skipped the extra LED and used an NSL32 vactrol. Seems to work nicely, though I also forgot to add the current-limiting resistor, will do that tomorrow night.



Not showing the other side because my soldering is awful angry
mckenic
Have some extra Vactrols so tried this last week - works GREAT!
Used a faceplate from a 4ms module I 'Greyscaled' hihi





cheapmachines
mckenic wrote:
Have some extra Vactrols so tried this last week - works GREAT!
Used a faceplate from a 4ms module I 'Greyscaled' hihi


This looks like a really nice way to do these, what are the pin outs/connections from the vactrols to the jacks? Can't really make out a "schematic" from the photos smile

Thanks for any help.
Sandrine
I thought I'd throw in a tip
With vactrols, a low impedance circuit is better for speed. The "full on" impedance may be 100 ohms, whereas the full off may be near 1 megohm.
This doesn't mean the impedance can be 1 megohm. It'd be better 10k-47k.

If using with an existing circuit, and the drive can handle it, sometimes pulling the output side of the LDR down to ground/common/quiescent is do-able. Also, if there's audio across it and the impedance is lowered this way, then the decoupling caps may need to be increased to keep lower frequency response (1/RC)

Cheers!

-edit-
The voltage drop of an LED is around 2V, so if the CV (or whatever) bottoms out above this, then the LED will always be on. Placing a diode or more in series with the resistor (4.7k?) will drop the voltage .6V with each diode.
edwinm
Just made a double vactrol gate, works like a charm! If I wanted a longer decay would I put a cap in-between the CV in and the LED? Seems to make sense to me...
astrosound
edwinm wrote:
Just made a double vactrol gate, works like a charm! If I wanted a longer decay would I put a cap in-between the CV in and the LED? Seems to make sense to me...


If you mean put a cap across the LED (or CV input) and ground, yes. But it would also increase the attack time. Attack/decay time would then depend on the LED current limiting resistor and the output impedance of your CV/gate source (which is often a 1k resistor tacked on the output, give or take).

These vactrol gates are great - I built one early this year and use it all the time. I'd love to build a module with 4-6 of them, buffered in/out with maybe a mixer, but the passive circuit is so quick and easy...
edwinm
thumbs up
saaad
Wow ! crazy topic applause
I think that I will try to build one LPG these coming weeks.

I've found this schematic with a pot for more fun
on Davidh's blog (inspired by this topic !)


I have few VTL5C3/2 left (dual package vactrol)
is it possible to replace the VTL5C3 by a dual vactrol ?
if yes, how must I connect the 3 pins to this circuit ?
kyleg500
saaad wrote:
Wow ! crazy topic applause
I think that I will try to build one LPG these coming weeks.

I've found this schematic with a pot for more fun
on Davidh's blog (inspired by this topic !)


I have few VTL5C3/2 left (dual package vactrol)
is it possible to replace the VTL5C3 by a dual vactrol ?
if yes, how must I connect the 3 pins to this circuit ?


Try using any 2 pins and either ignore the third pin or connect it to one of the other 2. Test on a breadboard to see if there are signification differences between different connections, then pick your favorite.
thermionicjunky
saaad wrote:

I have few VTL5C3/2 left (dual package vactrol)
is it possible to replace the VTL5C3 by a dual vactrol ?
if yes, how must I connect the 3 pins to this circuit ?


Another thing to try is a resistor from the middle pin to ground. The 207 panner circuit has a 47k. It's worth experimenting with if you're using duals.
southphillysynths
I just built a passive lpg using the VTL5C3 and it sounds awesome! It is a little quiet though, do you ever find the need to amplify it a bit?
Opnotic
If you got the circuit tweaked for a nice sound but it's all coming out a bit quite, taking the project to a level of making it non-passive and feeding the output into an op amp to boost the audio out, -was where I went with the idea. This was the slippery slope that gets you having too much fun with this stuff. =p

On the Doepher DIY site under 'basic circuits' is info to create a non-inverting amplifier circuit using an op amp - with values for using a potentiometer to control gain, sufficient for boosting line level to modular level (a useful thing!).
http://www.doepfer.de/DIY/a100_diy.htm

While it doesn't give the exact pin out connections for the op amp to apply power, nearly all op amps use the same pin connection for the +12v rail, the -12v rail, and ground (and signal in / out). Google basic op amp wiring and use values provided in text on the Doepher site and bobs your uncle.
bonjourmyfriends
kyleg500 wrote:
I've found this schematic with a pot for more fun
on Davidh's blog (inspired by this topic !)


I'm going to build two of these tonight into a 2HP module. I only have two 100K ohm pots available now - how will that affect the sound in place of the 25K pot? (I believe that's a low pass filter section?) How will changing the capacitor there affect the sound?

I plan to use female sockets so I can try out different vactrols and one for the cap's legs too. So granted I will experiment with plugging thing in and out, but it helps to have someone who knows what they're talking about explain what's happening...
quincas
Hey Mufflers smile

I haven't been very active around here, but I thought I'd share my video on this post.
Let me know if I said anything dumb!

Enjoy

https://www.youtube.com/watch?v=8BOnvSQpoGY
Opnotic
Nice vid!
Cynosure
quincas wrote:
Hey Mufflers smile

I haven't been very active around here, but I thought I'd share my video on this post.
Let me know if I said anything dumb!

Enjoy

https://www.youtube.com/watch?v=8BOnvSQpoGY


Great video!

Has anyone ever tried making a LPG with a H11F1?
basicbasic
I've built a bunch of these and it's pretty much the perfect first DIY synth project. I suggest buying as many different LED types as possible and trying them out as you can get a wide range of different responses.
garblemoth
NS4W wrote:
Have you found a description for building this lowpass gate?


I also am wondering if anyone has a schematic, or knows what resistors are used in this, looks super simple to build, quite keen to give it a go, cheers all smile
christie
I took a stab at the passive gate this weekend.

I know there are always questions - "why use a passive gate when active are fuller featured, sound more dynamic, etc."

Here's one reason why - Most of my racks have "active" low pass gates already except two but sometimes I want to add a little vactrol squish to those. But those two cases are completely full and I don't want to waste even 2hp.

So I built the simple passive gate into a cable - so I can use it on my 3U euro serge, or in my "east coast" moog inspired system.

The first one came out like this:



Sealed up and with jacks and plugs:



Plug in audio and cv, and plug it into the destination.
Raleigh
Not sure if any of you have tried this technique using a laser printer, but it works great for printing your own PCBs as well as doing your own aluminum front plates for modules. I bought a cheap used black and white laser printer just for this purpose, actually this is the first time I've used it. Plan to put some cool artwork and graphics on all my DIY modules.

There's many tutorials on how to do this, but here's one, more geared towards PCBs though.

Happy with the results here, and this test on the Vactrol module here.

http://www.instructables.com/id/PCB-etching-using-laser-printer/

Raleigh
Hey got a serious question. How can I build a circuit using vactrols very similar to this one, except it would do exactly the opposite. As the signal comes in, it cuts the second signal. Instead of bringing up the volume when the led is lit, it cuts the sound out.

Secondly how could I get that to work better with a sound source as the CV? The circuit in this thread also will work with an audio source as the input into the CV but only at high volumes. Maybe just an opamp to up the sound source?

I have for a while wanted to be able to layer a sound on top of another sound, but the first sound becomes quieter attinuated to the second sound.

I guess this is somewhat like a side chained compressor or gate.

Any ideas?
chrisj
Raleigh wrote:
Hey got a serious question. How can I build a circuit using vactrols very similar to this one, except it would do exactly the opposite. As the signal comes in, it cuts the second signal. Instead of bringing up the volume when the led is lit, it cuts the sound out.

Secondly how could I get that to work better with a sound source as the CV? The circuit in this thread also will work with an audio source as the input into the CV but only at high volumes. Maybe just an opamp to up the sound source?

I have for a while wanted to be able to layer a sound on top of another sound, but the first sound becomes quieter attinuated to the second sound.

I guess this is somewhat like a side chained compressor or gate.

Any ideas?


I did this to add a passive sidechain effect to my old Midiverb :)

What you do, is you run the cds cell to ground (rather than run the signal through it). That way, if it's a high resistance, it's not attenuating, but if it's being lit up brightly, it's sending more signal to ground. I found places on the circuit board where I got an effect doing this, and I sent a spare input into the LED through a tiny bridge rectifier.

I wasn't getting any decay on the tiny cds cells I was using, so I hooked up large capacitors to the LED side in hopes they'd store a bunch of energy. This also slows the attack quite a lot, but it did work.

You almost certainly can't get it to clamp the output to silence (the cds cells won't go to that low a resistance) but it does work. I did a homebrew stereo attenuator this way, by gluing multiple CDS cells onto the LED. I also used a high brightness LED, and surrounded it with white paper, just to get the most extreme low resistance I could (don't assume you should use black heatshrink, you might want to reflect as much light as you possibly can in order to get a super-low resistance out of the cds cell :) )
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