Friday 30 October 2020

Synthesizer Build part-35: RESONANT LOPASS GATE (Buchla 292).

 An awesome sounding combination of a Voltage Controlled Amplifier and a LowPass Filter using Vactrols. It has three modes: VCA, VCF or Both. Prepare to fall in love with this one!! 

This is one of my favourite modules on this website because A, it sounds so good and B, it's versatility.
This module is not like your conventional Lowpass Filter. It's a combination of a VCA and a VCF. It helps if you're trained a bit in your modular synthesizer knowledge to get the best out of this module. As a beginner you might be better of building some normal filters first and leave this one for later. But then again, if you're feeling adventurous, then hop to it. You will certainly learn a thing or two as I did. Plus it's quite easy to build. 
Doepfer has the Eurorack LowPass Gate for sale for around €100. It's the A-101-2. That's the same one as in this project.
There are now also Gerber files available for this project. The link is at the bottom of this article.

THE RESONANT LOPASS GATE WILL RUN EQUALLY WELL ON +/-15V AS ON +/-12V. No extra changes are necessary.

A little bit of history:
When modular synthesizers were first being developed there were two people who became prominent in this world in the United States. Don Buchla on the West Coast and Bob Moog on the East Coast of the States. While Bob Moog preferred a more conventional way of playing the synthesizer by using a black and white piano style keyboard, Don Buchla chose to go an other route and developed a touch sensitive device that would react to the pressure human fingers would impose on it. Buchla didn't even like to call his instruments synthesizers since that name connotes imitating existing sounds and/or instruments. His intentions were to make instruments for creating new sounds. He wanted unrestrained artistic expression un-bound by the conventional chromatic scale used in western music. A completely different approach to modular synthesis but one that sounds out of this world if you get it right. However, piano style keyboards are instantly recognized by musicians as something they can work with, and therefore the Moog system became the most widely adopted system in the world. This module is one from Don Buchla's stables, in fact the first one from his design philosophy on my website. (Hopefully not the last one because I really like the West Coast approach.) The addition of the resonant feedback loop and the refinement of the original Buchla design goes to the credit of Thomas White. The module I built is the Thomas White version as presented on the website Click here to visit that webpage. 
Here's the link to the NatualRythmMusic website which features the same project.
(I'm not associated with any of those websites.)

Resonant Lopass Gate:
To be honest with you, I had never heard of Resonant Lopass Gates before I held a poll on Facebook to see what people would like me to build for future projects. This was one of the options that was mentioned. It instantly intrigued me  because I didn't know what it was. So I asked for schematics, did some research and started building one. 
This module consists of three parts and there's a mode switch to switch between them. There's a voltage controlled amplifier or VCA and a lowpass filter (12dB) and the option to have both on at the same time. The VCA is nothing more than a voltage controlled attenuator and with the switch in VCA mode that is what you get. Now if you set the switch to 'Both' mode, you get that same VCA function but unlike a pure VCA not all frequencies are attenuated equally. The amplitude will change in accordance with the frequency response. Depending on the height of the Control Voltage, the filter cuts off parts of the high frequency content of the input signal. If we now switch to VCF mode we have the full function of the lowpass filter including resonance (and it can self-oscillate) and the CV voltage determins the cut-off frequency of the filter. The VCA part is no longer working in this mode but we still get a mixture of changing cutoff frequencies and changes in amplitude driven by control voltage and the CV input also affects the amount of resonance that is put on the audio signal. It's very complicated and I can't explain it very well but it makes for a very special sounding module. Because it works best with a constantly changing CV inputs, the lopass gate really shines when used in more percussive typ patches (See demo video lower down the article for sound samples) but that doesn't mean you can't use it for other purposes. It'll work equally well as a VCF module. It just begs to be experimented with.
The CV inputs can be anything from Gate signals to Envelope signals or LFO's or any combination of those. You can experiment with what sounds best. I think it's better to have signals going into both CV inputs at the same time. The CV 2 input has an inverter connected to it in the form of opamp U2-A to form an attenuverter, The more you turn it clockwise the more the CV signal gets inverted. This is one of the changes that has been made (by Thomas White) from the original design as described in the 'modularsysthesis' article in the link below here, which I incorporated into the redrawn schematic. It works very well. The CV-2 control contributes a lot to the funky sound of this module. CV-1 is the more dominant input and if it is fully opened up it will somewhat suppress the working of CV-2 so you need to find the right balance between the two CV's.
Here's the schematic drawing that I re-made from the schematic on 'modularsynthesis. It has all the changes that are suggested in the linked article implemented. (Click on the image to enlarge it and then right-click and 'Save as' to save it to your computer. Then you can zoom in on it.).

The schematic says to use VTL5C3 vactrols but VTL5C4's will work fine too. It's a matter of taste and experimenting. I used home made ones myself.
I did not use any bypass/de-coupling capacitors on the two IC's but if you want them included, or if you're having trouble with noise from the powersupply, then just put a 100nF ceramic cap between the plus and ground and one from ground to minus 15V and as close to the chips as possible  You can also put some 10µF/25V electrolytic caps on the power rails to suppress any hum. The 'Deep' switch is a normal SPDT toggle switch (ON-ON). If you turn it on, the sound will be deeper with less high tones. It has the effect of turning the 'Offset' knob counterclockwise. You can set the amount with the trimmer Tp2. The MODE switch needs to be a 3 pole ON-OFF-ON switch and I have colour-coded the connections so you can easily see what goes where. The 3 by 3 diagram with red, green and blue represents the bottom pins of the switch and the colours match up with the colours in the schematic drawing. You can see it all connected in the layout below. The switch needs to have a middle position and in that position none of the 3 connections in the schematic are made, so they are all open. This is the 'Both' mode and is how it should be although it may look a bit weird at first. You can also use a 3 position rotary switch of course but it will have to be a 3 pole, 3 position rotary switch. I myself used a vintage 6 pole 3 way switch I had in my junkbox. I had four of them and used two of those in earlier projects. One in the Digisound 80 ADSR and one in the Steiner-Parker filter.

About the Vactrols:
The layout I made for this module worked rightaway but I did some experimenting with the Vactrols. I ordered a batch of VTL5C4 vactrols and they have now arrived but the Vactrols I made myself seem to work so well that I was at first hesitant to replace them. I have now tried them and they worked well but a bit too slow for my taste so I put my home made ones back in. I recommend you use VTL5C3's in this circuit or make your own like I did. However this is really a matter of taste. One of the videos I posted below uses VTL5C4's and it just sounds awesome so don't take my word as gospel please.
I made mine from 5mm red LEDs and LDR's that had an 'off' resistance of over 200MOhm and with a bright white LED shining on them the resistance was about 200 Ohm. I later soldered a 3mm red LED in parallel over the vactrol LED on the left to dim it a little, because I found out that sounded better. Later I mounted that LED on the front panel to have a visual indication of the working of the Vactrols. I only put a LED over one of the Vactrols, the top one going by the layout below.
I made some Vactrols earlier and used bright white LEDs in them but although they did work, the LEDs hardly came on because the maximum voltage over them was about 2,7 Volt which was too close to the threshold voltage of those LEDs. The red LEDs will shine full on with that voltage which works much better. (NOTE: because the LEDs in the Vactrols are part of the circuit and not connected directly to a powersupply they don't require their own current limiting resistors.) 
If you want to build your own vactrols using LDR's from the GL55** series then I refer you to a comment below posted by Tim who tried several LDR's from that type. He had the best results with GL5528's and GL5537's but read the comment below for his full review.

The picture below is the wiring diagram. The module is meant to work on a dual 15V powersupply but it will work fine on a dual 12V powersupply (Eurorack) . I did notice a bit less self-oscillation in the resonance when I tested it, but it still sounded amazing and you can still get that cool sharp synthesizer sound out of it. You might be able to get the resonance back up with the 20K trimmer but I didn't try that. One other thing, I built this module using two TL074 chips, not the TL084 as mentioned in the layout. It doesn't really matter which of the two you use where. It's up to you. As always the layout is verified. I used it to build my module and I already had confirmation from others who built this successfully. All potmeters in this layout are viewed from the back side.

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Below are the cuts and wirebridges seen from component side. I marked the spot where you need to cut the copper strip between holes J3 and J4 with a vertical line, for the 500K trimpot to work properly.
As always, mark the holes on the component side with a Sharpie or equivalent and then stick a pin through the marked holes and mark them again on the copper side where the pin pokes through. Then cut the copper strips at the marked holes with a sharp, hand held, 6 or 7mm dril bit.

Bill of Materials:

How to calibrate this module:
There are two trimmers on the board, the 20K trimmer directly influences the voltage that the Vactrols get so it plays a part in determining the sound. So you need to set it for best resonance, at least that's what I did. The influence it has is not that obvious though. 
The second one is for the 'Deep' switch and determins the 'deepness' or the low frequency emphasis of the circuit. It's a sort of tone control and the effect it gives is like turning the Offset knob down. You can set it to whatever you like best.

Here's a video demonstrating the sounds you can get from this module (listen with headphones to get the best effect). When I say "In 'Both-Mode' you don't get Resonance" what I mean is that you don't get self-oscillation in 'Both-Mode'. Resonance still works. When watching this video please keep in mind that I didn't yet know how to properly use this module. I'm simply turning knobs to see what happens, nothing more. Imagine what a skilled synth user could get out of this module when it already sounds so cool in the hands of a noob like me. ^____^

TIP: Try altering the pulse width of the squarewave going into the Lopass Gate. You'll get some really cool sounds that way.

The video below was made in answer to a question about running this module on a dual 12V powersupply. Here I am testing that (I have an adapter that I can put between the powercables of my modules which changes the power from +/-15V to +/-12V).
As you can hear it makes absolutely no difference what so ever.

Here's a video (not by me) from 2008 showcasing the Resonant Lopass Gate using the VTL5C4 Vactrols which are slower than the VTL5C3's. This gives a more vintage sound (according to some people). People nickname this version the Slowpass Gate. It sounds very TB-303 Acid House to me. I really love it! Decide for yourself. Here's the video:

Here's an other one I found from 2015 demonstrating a dual lopass gate:

Here are some pictures from the build proces. The two black thingies at the bottom left of the stripboard are my home made Vactrols. Everything is in place only nothing has been wired up yet in these first two pictures:

I used a vintage 6-pole 3-way switch but unfortunately I drilled the holes for the screws in the wrong place but since they were 3mm holes I put some 3mm LEDs in them and connected them to a free pole of the switch so that the yellow LED goes on when the switch is set to VCA mode and the red one goes on when switched to VCF mode and both go on when in 'Both' mode. =)
Here's a sketch of how I connected the LEDs to achieve that. In 'Both' mode they are a bit dimmer because of the 0,6V voltage drop of the extra diodes but you hardly notice that. I could have used Schottky diodes to prevent that but anyway. It works perfectly fine:

It would be very cool to have three or four of these Resonant Lopass Gates in a modular synthesizer set-up and to use them partly as VCA's with a twist. You can do some really cool things with this module, I know that. But I myself haven't figured out yet in how many ways you can use this.

There are now Gerber files available made by Daniel Karlsson, one of our cool Facebook members:


Okay, that's it for now. As always, put any questions you might have in the comments below or on the facebook group.

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  1. Hi Eddy, thanks for your excellent work!
    I have some vtl5c9 vactrols on the way from china and I'm wondering if they would do the trick (providing they are what they say they are!)?
    I've had a look at the datasheet but I don't really understand it haha.

    1. Yes I think that will work although it says in the datasheet they have a fast response time and for this circuit a slower response time seems to be favourited by most users but it should work at least.

    2. Thank you very much for the reply.
      I will report back with results as they were £5 for 5 so would be a good option for budget builders :)

    3. Thank you, I will be interested in the results :)

  2. Hi Eddy,
    "I later soldered a 3mm red LED over the vactrol on the right to dim it a little, because I found out that sounded better."
    What do you mean with "over the vactrol"?
    In parallel with the vactrol LED ?

    1. Yes, it's in parallel over the Vactrol LED. That dimmed the LED in the Vactrol a bit wich worked beneficial for the sound.

  3. Only on the right one ? (the one to the right on the schematics ?)
    Well, I guess I'll see when breadboarding this, maybe this week-end...
    Thanks !

    1. It's on the left one on the schematic, that's the top one on the layout. You can see the LED in action at the end of the demo video I made.

  4. Ah, OK. Because you said "..the vactrol on the *right* to dim it...".
    I'll watch that video again, paying more attention to the images than the sound :)

    Thanks again !

    1. Yes, you are right. That was my mistake. I should have written, on the left. I've corrected that in the text now. You can see it's the Vactrol who's LDR is connected straight to pin 1 of the IC.

    2. You can experiment with putting extra LEDs on both Vactrols. I've had feedback from one person who did that and mounted the LEDs on the panel so he could see how much voltage the Vactrols were getting. That was a clever idea I thought. =)

  5. Hello! Fantastic site you have and thanks for sharing!
    I think I might have some issues With this.. i be made my oen stripboard design bit dillpesto your schematics.

    In vca mode I have quite prounanced boink when connecting lfo to cv in.
    But in both mode and vcf mode its mostly ringing and the cv effect is VERY subtle.
    Filtering is quite weak as well.
    If I turn down the deep trimpot to be darker. The sound just dies evetually and I must ”kickstart” it with the normal switch again. But resonace work very well!

    Is this dull filtering per design? Or is this my homemade vactrols? Red led/ldr. Vca mode workshop well though.

    1. Well, I think this must be due to your Vactrols. I can't think what else would cause it. This is certainly not how my Lopass Gate reacts. Also check if your capacitor values are in spec because that could also affect the working of the LPG. I hope you can fix it. Glad to hear you like the website!

    2. Thanks! i have ordered some ”real” vactrols from china to try with. Tried to change the 220p and 4.7n before but it sound the same. Is it any other cap that might be important just for the filtermodes? Everything kinda works, but not that very good..

    3. C9, the 1nF would also be important. C7,8 and 9 are the main ones to look out for. The Vactrols from China are usually fakes but they should work fine non the less. I used them too.

    4. Yes ok! If I attenuate my input from vco. I works much better! Probably me shitty vactrols😁 cant handle the current.

    5. Ah yes, that's a good sign. At least there's no fault in the circuit itself. :)

  6. Eddy - thanks so much for your work. I have made several of your projects and they work well and I enjoy them. As I prepare to build this one I'm really confused with how does the 3pdt switch work on the VCF side. Since there is only one lead across the three posts it seems as if there would be no difference when switching to it. Am I missing something? Thanks so much!

    1. Hi Mark. Glad you're enjoying the website! The VCF part of the switch connects the filter to the resonance section via cap C8 as far as I can see. It puts feedback on the Vactrol resisitors. I'm not sure how it all works as I, of course, didn't design this circuit but I assure you it makes a difference and it works very well. This is a very cool module to experiment with.

    2. Thanks so much for the quick reply Eddy! I think my brain issue :) is how the 3pdt works. It's on order, and when I get it I think I will spend some time getting to know how the switch and it's leads work. Also, from watching the inspiring videos I think I'm going to build two, then mix into one combo out. Really looking forward to this module, sounds amazing in your video. Thanks again!

    3. Well the switch as shown in the schematic works like this: if the switch is in the upper position the middle pins are connected to the upper pins. In the middle position the middle pins are not connected to any of the other pins (I see how this can be confusing and I now understand your question :) and in the lower position the middle pins are connected to the lower pins. Have a look at the original schematic by clicking on the 'modularsynthesis' link above the schematic. I copied mine from that one.

      I think it's a great idea to make two of these or make a Dual LPG. This gives you even more options for experimentation. Good luck!

  7. Hi Eddy, thank you for publishing all your projects. I have built quite many of them by now and am really happy with the results :) While going through the schematic for the LPG, I think I spotted a mistake: between R11 and S1 there should probably be a 15k resistance. In the layout, there is one, and it would also make perfect sense. Kind regards, Johan

    1. Yes well spotted. I don't know how I came to put in that 15K resistor because it's not in the schematic but everything works like it should. I must have thought the same as you and put it in.

  8. Hi there, Ive got some good news for people who want to build this module using ldr's from the 'GL55' series. I've tested this circuit with 4 different types so you dont have to. At first I used GL5539's. With these, the offset throw will be on point but I didn't get any resonance in VCF mode, probably due to their 100K light resistance. GL5506's will give you loads of resonance but their dark resistance of just 0,15M is way too low for the cutoff to work properly. sound will still blast through when you turn the cutoff pot all the way counter-clockwise. GL5516's are way better but they still bleed lots of signal with a closed cutoff, especially in VCA mode. I recommend GL5528's. In my build these still dont fully mute the signal with a closed cutoff pot but I was happy enough with them so I didn't even try GL5537's. Long story short, if you want to build this module with homemade vactrols from the GL55 series, get GL5528's (light 10-20K, dark 2M) or try GL5537's (light 20-50K, dark 5M)

    1. Thanks for the information Tim. I will make a reference to your comment in the article so people can read your results. Excellent stuff!

  9. ...thanks to Tim! A useful post, something like that is always helpful! Maybe he'll come back here and read my thanks :)

    1. That was some really useful feedback he gave. Always very cool when people participate like that :)

  10. You just cannot have too many modules, can you... This type has definitely sailed under my radar and now that I've accidently found it I have but one question. I will be running my modular machine on +/-12V, so some hints what has to be made to operate this one on the lower voltage?

    1. Haha you can never have too many modules. I don't think running this on a dual 12 Volt supply will present any problems. When you look at the circuit the first potmeter is for an Offset control so with that you should be able to compensate for the 3 Volts lower powersupply.

    2. I did a little test for you with my own LoPass Gate and I ran it on 12V and it makes absolutely no difference what so ever. I filmed the test so I'll put that video in the article here as soon as I finish editing it. So I hope that puts your mind at rest as to running on dual 12V. :)

  11. Thanks for your prompt answer. Now I've just to get me a pcb ... (I very seldom can get my constructions on veroboard to work).


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