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. I believe this design is equal to the Fonitronic version but Doepfer also has a Eurorack LowPass Gate for sale for around €100. It's the A-101-2. That's also the same one as in this project. I also have a Eurorack sized layout further down this article.

There are PCBs available for the Resonant LPG. See 'PCB Service'

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 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 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. The 'Both' title of this setting is somewhat misleading because it's not the case that the filter and VCA are simply switched in series. It's a setting of its own and I urge you to watch the last video of the ones I listed below to see the full explanation. 

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.

SCHEMATIC:

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 the slower VTL5C4's will work fine too, maybe even better. It's a matter of taste and experimenting. I used home made ones myself. Somehow, slower working Vactrols make this Lopass Gate sound twice as good as with fast reacting ones. With slower LDR's in your Vactrols this module sounds really amazing. You get that snidy 'ripping the fabric of the universe' synthesizer sound from it.

C7 and C8 should be good quality, none ceramic,  capacitors. The rest can be ceramic although I myself always use film capacitors throughout the LPG. You know those green oblong ones.

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 made my own Vactrols from 5mm red LEDs and LDR's that had an 'off' resistance of over 200MOhm and with a bright red 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.

I now understand the function of the Vactrols a bit better. The characteristic filter sweep sound that we normally get from filters by applying an envelope signal to the filter cutoff is created in the LPG by the slowness of the LDR's inside the Vactrols. The LPG filter sweeps through as the Vactrols lower in resistance. So using super fast LDR's in your Vactrols would be counter productive. It sounds better if they're a bit slow reacting so you get a distinctive filter sweep.

Here's a chart of LEDs you can use in your DIY Vactrols and how much voltage drop each LED normally has. As you can see the blue and white ones have a much too high voltage drop which will seriously reduce the possibility to dim these LEDs with lowering CV voltages. I highly recommend using red LEDs with slow LDRs.

LAYOUTS:

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 built this module using two TL074 chips, not the TL084 as mentioned in the layout. It doesn't really matter which quad opamp you use as long as they're low noise types. 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. As you can see the components are quite spread out over the stripboard because I designed this for my Kosmo sized DIY synth but further below is a more compact Eurorack friendly layout and I also have PCB's which I designed myself, that will easily fit a Eurorack setup. (see Menu: PCB Service):

The trimmers are listed as multiturn but you might aswell put in single turn (normal) trimmers because that makes tuning the circuit so much easier. There's no real need for precision here. If you put in normal ones you need to take care with the 500K trimmer because the legs won't be in line on one copper strip so for TP1 it won't matter but for the 500K 'deep' trimmer you need to cut at H-10 and I-10 and put the trimmer in at H-5, I-3 and J-5 and put in a wire bridge from I-7 to J-7. 

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 multiturn 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:

EURORACK LAYOUT:

The layouts above are quite spread out so here is a more compact layout fit for Eurorack. I did not wire up the 3 pole switch to make it easier to view the layout. All connection points are numbered and colour coded. Refer to the other layout above if you can't work it out. 

Wiring diagram:

Stripboard only:

Cuts and Wirebridges seen from component side:

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. Just do it on ear and set for what sounds best to you.

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. It extends the range of the offset knob and thereby the tonal range of the module.

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:

Remember, the LED circuit above only applies to my own self built module. It's not something common to the Lopass Gate. It's not necessary for you to replicate this. It's just something I did to fill two holes in the faceplate. It turned out to be very useful though.

DEMO VIDEOS:

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 more recent video of me playing around with the PCB version of the LPG behind a self designed faceplate. The LPG is connected to the Klavis Twinwaves mkII digital oscillator, using 8 sawtooth waves with phase shifting. (The Klavis twinwaves II is my alltime favourite digital oscillator.)

I have the feeling it sounds better than the stripboard version but that could just be me. It sounds amazing though. Listen with headphones if you can, to hear the deep bass it has:

Before I list some more demo videos I found on YouTube I want to show you a video by Eurorack Youtuber 'Red Means Recording' about the Lowpass Gate. He made a great video about LPGs explaining all about them. It even has one of my LPG layout in it ^____^:

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! Slower reacting LDRs in the Vactrols are definitely the way to go with this module. Decide for yourself. Here's the video:

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

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.

Below is one final video that I posted here for people interested in the inner workings of the Lopass Gate. The video goes into all the electronics and their specific functions in the module. It's very interesting especially for electronics students:

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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EDDY BERGMAN DIY Projects Help and Discussion Facebook Group.

I have set up a Help and Discussion Facebook group for the projects on this website. If you have any questions or encounter problems while building one of the projects, or even if you just want to know how something works, then you can post your question(s) on the Facebook group page. If you're located in the United States, Canada, Australia or in the Azian part of the world, you might even get an answer quicker than if you asked me directly because I'm in the Central European Timezone (The Netherlands) and the members of the EB Facebook Group span the whole planet. And within a few days of setting up the group, it already had 50+ members and one and a half months later it passed the 100 members point, and now we're over the 700 mark as of Sept 2024, of which I'm very proud and thankful.

I've made the group 'public' so anyone can read it, even without having joined first, but I hope of course that you will join. I have appointed some moderators for the group in different timezones all over the globe because we got some spam posts at a certain point but the moderators make short work of that and this way I can keep the group public.

I don't think you'll be overloaded with posts from this group so don't let that stop you. I will of course remain available personally for questions as I always have been, but because the website is gaining somewhat in popularity I was advised by Jonathan, the Admin of the LMNC group, to start this Facebook Group and I thought it a very good idea. So here we are. I hope you'll benefit from joining this group and I hope your enjoyment of building the projects available here will be greatly enhanced by this group. People also started publishing their own stripboard projects in the group which I highly encourage. Plus there's a files section that holds some real gems of schematics. So what are you waiting for? Join up. The link is down below here. THANK YOU!  ^_____^

Here's the link: https://www.facebook.com/groups/325860521842129

Synthesizer Build part-34: TRIPLE WAVEFOLDER.

A wavefolder with three folding stages which produces amazing sounds and it's very easy to build too!

I came across this project on YouTube when I watched a video by YouTuber Adamski A. called "DIY analog synth project part 18 - The Wavefolder".

So I set about building it and it came out very well so I asked Adam for permission to write an article for my website, based on his project, to which he very enthusiastically replied in the affirmative so here it is; The Triple Wavefolder.
Like I said, it's a very simple design and in my experience those work the best. This wavefolder produces sounds that I would describe as sharp or hard and accurate. In some settings it almost resembles a Harpsichord or an electric piano. They can be real speaker rippers too. If you watch Adam's video (see link above) you can listen to the wavefolder in action. The latter part of the video is full of sound samples in different settings. I also made a little demo video myself which is at the bottom of this article. The sound is very different from that of the filters we've become so used to, with their resonance and cut-off frequency. It sometimes almost sounds like an FM synthesizer. That's why this is a very useful addition to any modular set-up because diversity in sound is what we all want don't we?
Now, I built the Yusynth Wavefolder after first building this because I thought that this triple wavefolder was more of an experimental thing and the Yusynth one would be the official implementation to go into my synthesizer. But the Yusynth one only has a single folding stage and eventhough that sounds amazing too, I found that this one actually sounded even better. So I made a panel for it and mounted it in my synthesizer. The voltage it runs on has an influence on the number of folds you can get so changing the value of the 22K resistors going to the emitters of the transistors influences the behaviour of this circuit.

This wavefolder works best with Triangle or Sawtooth waves or even Sinewaves but Squarewaves pass through almost unchanged. That's convenient because squarewaves are best used for conventional filters because of their harmonic content.

Adding CV control to the parameters:
This wavefolder has in it's original form only three controls; the input level or 'Amount', the Dry/Wet control and a Saturation control. As an experiment I added voltage control to two of those, the Amount and Saturation by means of two self-made Vactrols connected between pins 2 and 3 of the respective potmeters.
These vactrols are made up of a bright white LED, a Light Dependent Resistor (LDR) and a 2K2 current limiting resistor connected together with some heat-shrink tubing that seals it off from any light from the outside.
These Vactrols both have their own level potmeter too so you can dial in the effect it has very accurately. I've had questions about what LDR's you should order for these and I really can't tell you. I had mine in stock for ages. A while back I ordered a batch of 5 x 10 LDR's from China. 5 different values with 10 of each value. They all have a dark-resistance of at least 100 Mega Ohm and a full light resistance around 1 Kilo Ohm or lower. That's all I can say. You can also buy Vactrols ready made like the VTL5C3 which should work fine here.

Even better than a Vactrol, at least for the Amount parameter, is to use a VCA on the Wavefolder input, That way you can control the level and thus the Amount by sending a Control Voltage into the VCA. This works much better and it's what Adam also demonstrates in his video. (See the 'Metalizer' project elsewhere on this website. That is in principle a quadruple wavefolder with a VCA on the input.)

In the video demonstration the vactrols didn't have that much effect but I had forgotten that the potmeters for Amount and Saturation need to be set a good way counterclockwise for the Vactrols to take full effect. And in that sense you could in principle do away with the CV Level potmeters because the Amount and Saturation potmeters have that function too for Control Voltages. But you must not forget that this module was built first and foremost as an experiment that I didn't think I would publish. 

Anyway, you could decided to leave the CV inputs out alltogether, it's up to you.

[EDIT May 2026] I designed and built a eurorack version of this wavefolder in KiCad, using the VCA from the Metalizer (proj. 41) in place of the audio level potmeter. It works pretty good. There's some problems with the throw of the shape potmeter because when I close it fully (ccw) the wavefolder flatlines, but other than that it works as I expected it to.

THE GAIN CONTROL:
The other thing I addressed was the fact that the Amount control is also the Amplitude or Volume control so turning it up increases the volume and turning it down decreases it. For that reason I added a Gain potmeter to the output opamp which increases the volume by a factor of 2 to 22 times!! This option is a game changer for this wavefolder especially with this much Gain! This gives you the possibility to really boost the sound output and it sounds awesome I can tell you. You can really boost the lower and mid ranges of the Amount potmeter to match the high output level when Amount is fully open. I even found that opening up the gain helps to level out the output amplitude across most of the Amount potmeter throw without really clipping the output. But even if it does clip, it adds a very musical sort of distortion to the sound. It's never unpleasant to listen to.

Here's the schematic drawing of the Triple Wavefolder by Adamski A. I have re-drawn it and added the Vactrols to it. I mention on the schematic that you can also use BC transistors instead of the 2N transistors I used, but that will influence the sound or the number of folds you get because BC transistors have a greater multiplication factor. You can of course experiment with that by setting up the circuit on a breadboard first. (Btw, the BC548 and BC558 can also be BC547 and BC557 types.) The CV-Level control potmeters are not included in the schematic. This is because I added the CV control as a bit of an after-thought to see how it would work out. Like I mentioned before you don't have to include the Vactrols and CV level controls. I leave that up to you.

(Last revised: 11-July-2020: Changed GAIN potmeter from 20K to 100K.)

Here's the stripboard layout. It's verified because I used this for my own build. All potmeters are viewed from the front side with shaft facing you!
(Wiring diagram):

(Last revised: 11-July-2020: Changed GAIN potmeter from 20K to 100K.)

Print only. Note that pins 5 and 10 of the TL084 are connected underneath the chip!!:

Bill of Materials.

15V vs 12V and de-coupling:
As you can see a very simple and easy to build project and it sounds amazing so I can really recommend trying this one out. The circuit is meant to work on a dual 15V powersupply but I tested it on 12V and it works just fine but changing the voltage does influence the number of folds you get so it sounds a bit different but it still works fine, trust me =)
I did not include any de-coupling capacitors in the schematic because I didn't use any but if you need to have those included just put a 100nF from plus to ground and a 100nF from ground to minus and place them as near to the TL chip as possible. Should you have problems with hum, you can also add a few electrolithic caps (22µF or 33µF) on the plus and minus rails like the other caps and you can also try putting Ferrite Beads in series with the plus and minus power supply input, or if you don't have them, a few 10 Ohm resistors. There's plenty of room on the stripboard for that. But you only need to do that if you're having problems with hum or noise in the audio output.

Here's a picture of the finished panel:

Here's a look at the stripboard: I see from these pictures I added an extra opamp to the output but because I actually built this module a while ago I can't remember why I did that. Probably to have more room to experiment with the output and add the Gain potmeter. Anyway, it works the same so you don't have to include that.

And, to close off this article, I made a video demonstrating the different settings and the sounds they produce. As you can read in the article, I recently changed the GAIN potmeter from a 20K into a 100K one giving a total gain of up to 22 times. This gives the option of boosting the middle range of the Wavefolder which sounds really awesome!!

Here's a video that's also posted in the "Sample and Hold" article. It's a triangle wave going through the Triple Wavefolder and then through the Steiner-Parker filter, fed by random notes from the Sample and Hold connected to the CV-2 input of the VCO.

Okay that's an other project done. I hope you enjoyed it. Check out Adamski A. 's youtube channel. It is full of awesome synthesizer projects and electronics tutorials. It's an enormous source of inspiration for anyone interested in building synthesizers.

As always, any questions or remarks, please put them in the comments below or post them on the new EB Projects Facebook Group.

Synthesizer Build part-33: DIGISOUND-80 ENVELOPE GENERATOR with AS3310.

A great ADSR with 3 different types of envelopes and extra outputs including an inverted one. Warning: This was a temperamental build because it didn't work perfectly when I first built it. However the problems have been identified and solved. See text below for a more in depth explanation. 

NB: Please don't attempt to build this if you're a beginner in need of a simple reliable workhorse ADSR. This can be a bit of a temperamental build because of the many options this ADSR offers. I recommend the Precision ADSR (proj.: 67) if you want an easy to build, good, reliable ADSR. Regard this one as an experimental or advanced project. 

This Envelope Generator or ADSR is a very luxurious one because it produces three different types of envelopes. The following description is from the original text for this module:
First there's the 'Damped' mode. The object of this mode is to more closely simulate the piano envelope which has a sharp attack, a brief initial decay, a long release and finally a very short release as the damper is applied to the string. So it's an ADRR response and in this mode the end of the gate pulse causes the final short release to occur. In other words releasing the note has the same action as applying the damper on a piano.
In 'Normal' mode the ADSR functions as any ADSR would with the duration of the Sustain period being equal to the duration of gate signal being present and the key being pressed down.
The 'Automatic' mode is particularly beneficial when envelopes are being initiated from non-keyboard sources like an LFO or from a clock signal. A short pulse will now generate a complete ADR envelope and, by adjustment of the time constants, this type of envelope can be made to approximate the ADSR type envelope. Usually these external sources would only generate a limited AD type of envelope.
    When I first built this ADSR I had my problems with it and so did many others so please treat this project as experimental. However the layouts are 100% verified. Mine is working fine in the normal and damp settings, and for a long time I thought automatic mode was faulty but that is meant for external trigger sources so it's behaviour is normal although useless for normal use. Read the comments below to see what problems people run into and the solutions they came up with. If you want a reliable ADSR without any bells and whistles then build the Kassutronics Precision ADSR.  That's my favourite ADSR on this website.

Further features of this envelope generator are:
- Independent trigger input for re-triggering and generating multiple peak envelopes in the Damped and Auto modes.
- Gate and Trigger pulses within a range of +3V to +15V are acceptable.
- Wide range of time constants. Typically 2 milliseconds to 20 seconds. If longer times are needed you can increase the value of C9.
- 0 to +10V peak attack output
- 0 to 100% Sustain level.
- Low control voltage feedthrough which means low residual voltage when the envelope cycle is completed thus ensuring that the VCA is off.
- Manual gating facility.

Features I added:
- Extra buffered envelope output.
- Extra inverted envelope output (0V to -10V).

Dual 12 Volt operation:
This envelope generator is designed to run on a dual 15V powersupply but I tested it on a dual 12V supply and it works just as well with only a very small loss in envelope voltage. On 12V the envelope is about +9V so no problem running this on +/-12V. One change you might consider is the current limiting resistor R25. It should be changed from a 750 Ohm to a 470 Ohm according to the datasheet of the AS3310. However I test ran it on dual 12 Volt without changing the resistor and it worked perfectly fine.

PROBLEMS I ENCOUNTERED:

I had build this envelope generator some time ago and I've been using it in my synthesizer for all that time but I didn't write an article about it until now because there was something wrong with it. In the 'Normal' mode, which is the one you'll be using most I think, the Decay was oscillating. It kept on being triggered for as long as a gate signal was present. The only way to stop it was to turn up the Sustain level so it matched the Attack level and then you wouldn't hear the constant up and down oscillation of the volume level. This is mentioned above in the features, that it has an option for multiple peaks in the Damped and Auto mode but that's not supposed to happen in 'Normal' mode.

The frequency of this oscillating Decay could be changed by changing the Decay time. Short time equals fast oscillations, long time equals slow oscillations so you could almost think this was meant to be but I can not believe it was meant to work like this in the 'Normal' mode.
So I was using this ADSR with Sustain turned up but it annoyed me that is wasn't functioning quite right because this is an awesome ADSR and I wanted to do an article on it. So I asked on the Synth DIY Facebook group what could be causing this. I was told it was due to capacitor C7 and that I should remove it. They were absolutely right. Removing C7 did the trick, at least in the 'Normal' mode but when I switched to 'Damp' or 'Auto' mode the ADSR was hanging. It wouldn't go into the  Release state. So for these two modes capacitor C7 needed to be in place.
By happy coïncidence I used vintage double pole 3-way switch to switch between the different modes and on that switch I had one pole left unused. So I connected the capacitor to that unused part of the  switch in such a way that it was connected in 'Damped' and 'Auto' mode and disconnected in 'Normal' mode. This worked fantastically and now it behaves just as it should do. Should you want an oscillating Decay in 'Normal' mode you could easily add a switch to connect C7 again. Now you have the choice between the two. (This option I leave to you. It is not documented anywhere in this article).
All these changes have been drawn into the layout and into the new schematic that I made.
I used a SPDT toggle switch to go between manual triggering (with a momentary switch) and inputting gate signals. You could use the internal socket switch of the Gate input socket for this too, that's up to you but then you can not press trigger when a cable is connected to the Gate input.

NB: In 'Dampened Mode' the Decay control determins the length of your envelope. 

EDIT 30th of JULY 2023: I was made aware of an article addressing the decay oscillation issue and it offers a solution for the multiple triggering in Normal Mode: It advises to use a schmitt trigger on the trigger input so the trigger level is always at the highest possible voltage. The cause of this retriggering namely, is an impedance issue and the fact that the trigger pulse isn't high enough in voltage. I'm posting the original article below here, so you can read it yourself. I'm not going to try this because it says with this modification it will trigger fine with Gate's higher than +9 Volt. With signals lower than 9V the schmitt trigger doesn't trigger. Not much use then.

I think my own solution is a much better one.

 
NOTE ABOUT AUTO-MODE:

One little thing you need to be aware of with this ADSR is that you need to switch to Auto mode whilst holding down a key on the keyboard. If you don't do that, then the ADSR only gets triggered (in Auto mode) if you push the manual trigger button but not by the keyboard. I think that's meant to be though because Auto mode is for external sources so that would make sense. If however you switch to Auto mode whilst holding down a key then it will work with the keyboard. Any key you press after switching it on will keep sounding until you press an other key and it will keep sounding until you switch back to Normal mode. Once you get used to this it's actually not a problem at all. Just something to be aware of.

IMPORTANT CONSIDERATION:

If you plan on building this ADSR you might just build it first like it was intended with C7 connected to pin 7 of IC1-B and without connecting C7 to the second pole of the 3 Way switch. In the stripboard layout it's simply a matter of connecting the 10nF cap between pin 2 of the LM358 and the strip directly underneath the LM358 which connects it to pin 7 via a wire bridge. Then it's back to how it was originally. Should you encounter the same problems I had then you can make the same alterations I did and have it function perfectly that way. Instead of a double pole 3-way (rotary) switch you can use a single pole one and if you need C7 to be disconnected in Normal mode, just use a little toggle switch for that. Double pole 3-way switches can be expensive unless, like me, you have some lying about in your junk box.

SCHEMATICS:

Here's the new schematic drawing that I made and used for my build with C7 connected to switch S1-B. (That's the only difference to the original schematic) :

This is a re-drawn version of the original Digisound-80 schematic, without any changes. You can click on the picture and then use the "J" and "K" keys on your keyboard to quickly switch from one picture to the other so you can easily see the changes (only on a Mac or PC).:

THE LAYOUTS:

Here's the verified stripboard layout. The changes I made are implemented in the layout but if you connect the lower pin of C7 one strip higher, you can do away with switch S1-B and everything is back to how it originally was, so the changes (if needed) are very easy to make.
BEWARE! All IC's are mounted with pin 1 to the lower right!
The layouts were rivised to make them easier to read in Nov. 2023.

Wiring diagram:

Stripboard only. Don't forget to cut the copper strips at holes H-32, K-32 and P-42 (under the capacitors):

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

Cuts and wirebridges as seen from the COMPONENT SIDE!

As always, mark the cuts on the component side first with a Sharpie or Edding pen and then stick a pin through the marked holes and mark them again on the copper side. Then cut the strips using a sharp 6 or 7mm hand held drill bit. Then solder in all the wirebridges before you get on with soldering in the components.

Bill of Materials:

CALIBRATION:

There are two trimmers in this circuit, RV2 and RV6.

RV2 is used to set the maximum Sustain voltage to the same value as the peak Attack voltage so no sudden voltage change occurs when the attack cycle is finished or so that the Sustain voltage can never be higher than the peak Attack voltage. The best way to set this is to use an oscilloscope but you can do it with a voltmeter too. I advise to check out the original text (second link below) and read the calibration instructions there. They are on page 4.
RV6 is more for polyphonic systems and for normal use it can be left in the middle position.

So, that's all the calibration you need to do ^__^

Here's a screenshot of the oscilloscope that illustrates the oscillating Decay problem I had in the beginning:

Here are some screenshots of the different modes of this ADSR:
This is the Damped mode with short and continuous key pressing You can see that every time you let go of a key an almost instantaneous release kicks in and kills off the note:

Here's the 'Automatic' mode with the same quick key presses.

Here you can see that letting go of the key will not stop the envelope. It will go through its complete cycle even if no gate signal is present. If you press a key before the cycle is finished it will start at the beginning again as you can see at the right side of the waveform in the screenshot above. This way you can create multiple peaked envelopes by re-triggering the ADSR.:

Finally here's a shot of the normal ADSR mode:

Here's a look at the response time of this ADSR. It's not the fastest response but still, 1.36mSec is pretty fast I suppose. The yellow line is the Gate signal and the blue is the ADSR output with Attack set to zero:

I'm really glad I was able, with the help of the Synth DIY group, to get this envelope generator working like it should at least in Normal and Damped mode. I do have one little quirck with mine. I can only use Auto mode if I switch from Normal to Auto while holding down a key on the keyboard and then the envelope is constantly retriggered so it functions as an LFO. Personally I find this very useful so I'm keeping it like this but let me know in the comments if yours does the same and/or if you found a solution for this. Or maybe this is just how it should be. I really don't know.

Here are some pictures of the module and print. The first one was taken after I installed it in the synth and the second one after I just finished the build. You can see that I put in a lot of output jacks for the envelope. It's always useful to have a few extra I think. The top two outputs are switched in parallel over the ADSR output and the bottom two are switched in parallel over the extra output on the stripboard. Below the inputs for Gate and Trigger there are two more sockets. They are Gate and Trigger outputs. They are each switched in parallel over their respective input sockets. I later added a yellow LED to have a visual indication of the envelope. The LED is soldered over one of the extra ADSR output sockets using a 15K resistor as current limiter so as to not influence the envelope voltage and to make sure the LED doesn't shine too bright:

Here's a link to the Electro-Music Engineer PDF article by Charles Blakey about this module:
http://www.digisound80.co.uk/digisound/other_documents/doc_files/1981-12_EM_Eng_CEM3310.pdf

Here's the original Digisound article in PDF form, about this ADSR:
http://www.digisound80.co.uk/digisound/modules/80-18_files/80-18.pdf

In the original Digisound modular synthesizer this is actually a dual ADSR:
http://www.digisound80.co.uk/digisound/modules/80-18.htm

Okay, that's number 33 done. If you have any questions please post them on the Eddy Bergman Projects Discussion and help Facebook Group, or the comments below or contact me directly.

See you on the next one!

Synthesizer Build part-32: ELECTRIC DRUID VCLFO-10 with extras.

A truly awesome LFO with 16 waveforms and 8 different controls. I added 2 extra outputs for 10Vpp and a frequency indicator LED.

This Low Frequency Oscillator was one I had on my wish list for a long time. Last month I decided to buy the chip, it was only 5 Pounds anyway, and it arrived precisely a week later from Tom Whiltshire of Electric Druid in Portugal.
I downloaded the Datasheet PDF with all the schematics etc on it but I found parts of the schematic a bit confusing. The Frequency and the Level controls have their own opamps and they both have two potmeters connected which looked a bit weird to me. So I asked on the Synth DIY Facebook Group what the deal was with those two potmeters. It turns out both the Level and the Frequency controls can be connected to an external control voltage so one potmeter is connected to an input jack and serves as the input level control or attenuator and the other potmeter is for manual setting of the Frequency and the same for Level. So after I had that straight I set about making a stripboard layout. Now, I noticed there was no rate indication LED in the schematic. I always find it handy to have a frequency indicator LED on the panel, so I designed a second board and included a rate indicator LED on it, together with two extra outputs that have a DC offset of +5V so the outputs are 0V to +10V. That is the level I use most on my synthesizer so I needed to have that included. This does mean that the Noise level through these outputs has a +5V DC offset so use the +/-5V output for noise.
I made a second stripboard layout and made a mounting hole in it, on the upper right corner, so the second board can be stacked on top of the main circuit board, using a 3 cm M-3 bolt  and a bit of plastic tubing as a spacer to keep the two boards from touching eachother. The layouts worked like a charm and everything worked fine when I tested it.

 

If you find that you wired up the potmeters the wrong way around, you can easily correct that by connecting pin 2 of the VCLFO chip to ground. That reverses the working of the potmeters. Make sure the two level potmeters are wired like on the diagram though. They are not influenced by pin 2 of the chip.
The main stripboard has its own plus and minus 5V power supply included, so everything can be powered from a single dual 12 Volt power supply. I used the big 7805 and 7905 in TO-220 package because that's the only ones I had available but you can use the smaller L versions. This circuit hardly draws any current at all so they won't run warm and don't need heatsinks. I did not include any de-coupling capacitors or electrolytic caps on the power rails (except for the -5V because that was indicated in the schematic). You can put those in though, if you think you need it. Use two 100nF ceramic caps, one from +12V to ground and one from ground to -12V.

SYNCHRONIZATION POTMETER:

There is an extra 100K trimmer in the layout with which you can set the Synchronization mode between Sync Off, sync-ing the LFO, the Sample and Hold or both. Instead of a trimmer, as seen on the layout,  you can also make this a feature on the front panel and connect a potmeter to the same points as where the trimmer now sits, and of course you then leave out the trimmer. That way you can change the sync setting on the panel itself. This is what I later did. Beware these extra's are not listed in the Bill of Materials. I used a 100K potmeter but you can use any value potmeter or trimmer for this function because it is just a voltage divider connected between +5V and ground. (Use a value of 10K or higher.)

Looking at the panel-potmeter front-on with pins pointing downwards, connect the left pin to ground, the middle pin (wiper) to pin 8 of the chip and the right pin to +5 Volt.

It took me 5 and a half hours to solder the stripboard components in place and to wire it all up. The whole proces of designing the layout, designing and making a panel etc. took a whole weekend so it was a nice project to do because at the time I, and everyone else, was stuck at home in Covid Lockdown anyway.

Here is the (verified) layout. Wiring diagram:

The +5V and -5V points on the left side of the stripboard are simply indicators, so you know that voltage is available at that point. You could say they are test points you can check if you are troubleshooting this board (which of course I hope won't be necessary. ;) But nothing needs to be connected to those points. Sorry if that's a bit confusing.

This circuit will run fine on +/-15V too. The voltage only feeds the IC's and the regulators and they can all take it without problem or without any changes needed.

Here's the layout for the main stripboard. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide!!

In this layout the synchronization mode is set with a 100K trimmer at the top of the stripboard. As I mentioned before I myself changed that for a panel mounted potmeter later. The sync mode depends on the voltage on pin 8 and the trimmer or potmeter sets that voltage.

There is no capacitor on the input of the 7905 voltage regulator, it doesn't need one to work but if you want you can put a 1µF electrolytic cap over the input to ground. Easiest way to do that is to connect it to holes B-11 and C-11 with the negative pole in position C-11 (-12V)

The Zero Adjust 100K trimpot is used to set the symmetry of the output signal. You must use an oscilloscope to adjust this. Set it so the LFO output signal's positive amplitude is the same as the negative amplitude.

Make sure you get the cuts right in the stripboard. Especially those in the power rails at the top otherwise you'll have a direct short circuit between ground and -12V. Always measure continuïty over the power connections to rule out short circuits before you connect it to power for testing.

Here's the Bill of Materials for the main stripboard. Note: component numbering does NOT follow the numbering in the datasheet schematic. 

OFFSET AND LED INDICATOR BOARD:

Here's a close-up of the second board with the extra outputs and rate indicator LED. You can use this board for other projects too, if you need to add a DC offset voltage to a certain output. I chose 150K resistors for around the opamps (R3,4,5 and 6) because I have a lot of them but you can use any value from 47K to 500K instead of the 150K's as long as you use the same value for all four resistors:

The DC Offset trimpot must be set to a +5VDC Offset to get a 0V to 10Vpp output signal. Use an oscilloscope connected to the output socket to set this. I don't have a schematic drawing for this part but it's a really straightforward opamp offset circuit with the offset trimpot connected to the non-inverting (positive) input of the opamp. The signal goes in at the inverting input and then it goes through a second opamp stage to invert it back to normal again. Then the second output (at the bottom) is simply fed the signal of the first output via an opamp buffer. The second opamp of the chip on the right is not used and properly connected to ground to 'park' it, so to speak.

Here's the Bill of Materials for the extra board:

You don't have to use this extra board for 0 to 10 Volts outputs of course. If you find you need more Bi-Polar outputs of +/-5V then set the trimmer to 0V offset and it's done. 

WAVEFORMS:

This VCLFO produces 16 waveforms in 2 sets of 8. I have made a little compilation image of oscilloscope images I took of the waveforms and some sample and hold results. The blue line is the original +/- 5V output and the purple line is the one I put in myself with 0 to 10Vpp. You can see that the noise has a +5V DC Offset on the purple line. When you start testing this circuit after completing the build, it's possible you don't see a waveform but just a flat line. That means your offset voltage is too high or too low, so all you need to do then is set the offset voltage with the trimmer on the main stripboard. Then check the 10V outputs and set that offset with the trimmer on the small print. I advise to use multiturn trimmers for those, but you don't have to. Make sure your oscilloscope is set to DC mode for measuring these waveforms.

Each of the waveforms produced can be sent through a sample and hold unit which is built into the chip and as the chip can also produce noise you can also get random tones produced by this LFO if you connect it to the CV-2 input of one of your VCO's. The sample rate of the S&H can be set with a 10K panel potmeter and if you turn it to zero the S&H switches off automatically.
The VCLFO has a synchronization input and it can be frequency modulated by means of a Frequency CV input with attenuation potmeter. There's even a separate input for the Level control which is a volume control changing the amplitude of the waves.
There's also a control on the panel for 'Distortion' which bends the bottom or top part of the wave with the middle setting being the clean, undistorted wave.
The LFO has 4 frequency ranges and they are:
8 seconds per wave to 12,5 Hz
4,6 sec/wave to 25 Hz
2,6 sec/wave to 50 Hz
1,2 sec/wave to 100 Hz
You set the frequency range with the LFO Range potmeter and then you can set the Frequency within that range with the Frequency potmeter. There's a smoothing switch included in the circuit which rounds off the corners of the waves and makes them smoother (obviously, LOL). This is to prevent the sharp edges of some waveforms from causing clicking sounds when you're using the LFO as a Tremolo.
The possibilities are endless with this LFO and with the chip only costing 5 UK Pounds, like I mentioned, you should really get this one.

Here are some pictures of the finished panel and of the stripboard and wiring. I admit the panel is a mess but it works for me:

In the picture below you can see I made a change by adding an extra potmeter (the one with the yellow knob) with which you can set the Synchronization mode between synchronizing the LFO, the Sample and Hold or both.

And here's a little video I shot using the LFO in a reasonably complicated patch. I've got 3 VCO's feeding squarewaves into 3 filters and a triangle wave into the wave folder. Each filter receives an LFO signal from a different LFO. The Electric Druid VCLFO-10 is feeding a quad pulse into the Steiner Parker filter. All LFO's are synced from the main LFO which is the Music From Outer Space LFO. You can also see the Mixer/Passive Attenuator in action with the bright blue clipping LED coming on occasionally and the Digisound 80.6 LPF sounding really good!

If you're interested in recreating this patch then here is the basic set-up I made. The eventual sound is, of course, dependant on the settings of all the potmeters and little changes can make a big difference but this at least is the foundation of this patch:

ORDER THE CHIP:

Here is a link to the product page of the Electric Druid VCLFO-10 from where you can order the chip:  https://electricdruid.net/product/vclfo-10/

If you have any questions about the chip or simply want to say thanks to Tom Wiltshire, drop him a line on his website. He's a really nice guy and he'll appreciate your feedback.

Okay, that's it for now. I have now finished the second stage of my synthesizer and so I have no more room to put new modules unless I build a third case. That will no doubt happen but not right away, what with summer coming it's going to be too hot in the attic to spend all day in there wood-working or soldering. I also built up a Eurorack system in early 2022 which took a big chunk out of my budget, which wasn't/isn't too big anyway, but that's all in the game right?

As always, if you have any questions please post them on the EB Projects Discussion and Help Facebook group, or in the comments below or contact me directly via Facebook.