Friday, 30 October 2020

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

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

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. 


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 modularsynthesis.com. 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 the same VCA function but unlike a pure VCA not all frequencies are attenuated equally. 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 CV 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 input, the module produces more of a percussive, pulse like sound. At least, that's where its strenght lies. (See demo video lower down the article for sound samples)  The CV inputs can be anything from a Gate signal to an Envelope signal or an LFO. 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, 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.

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.) :


I did not use any 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, than 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 represents the bottom pins of the switch. 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'm hesitant to replace them. 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. 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. (So 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.)

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 are viewed from the front with the shaft facing you.



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



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.

Here's a video 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. Anyway, decide for yourself. Here's the video:


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



Below is a video demo-ing the Erica Synths Lopass Gate (LPG) which is based on the same schematic as the one we are using. However, the Erica Synths LPG has a built in Envelope Generator which our module doesn't have. No problem though because we have E.G.'s in our modular synths anyway. Go to 1m22 to skip the intro nonsense:



Here are some pictures from the build proces. The two black thingies at the bottom left of the stripboard are my home made Vactrols:




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. 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.

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

If you would like to support my work and this website you can do so by buying me coffee. There's a button for that under the menu if you're on a PC or Mac. Or you could donate a few bob by clicking here  Even a small donation will be a great help and all donations go towards buying components. Thank you so much! See you on the next one!



17 comments:

  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.
    Cheers

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    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.

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    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 :)

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

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  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 ?
    Thanks.

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    Replies
    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.

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  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 !

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    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.

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  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 !

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    Replies
    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.

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    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. =)

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  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.

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    Replies
    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!

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    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..

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    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.

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

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

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