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.

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.

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, so I'm sure you could design a smaller stripboard layout but I didn't bother with that because I designed this for my Kosmo sized DIY synth and I also have PCB's which I designed myself, that will easily fit a Eurorack setup. (see Menu: PCB Service):

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:

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.

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. 

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. 

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 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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Synthesizer Build part-15: DUAL KORG MS-20 HP/LP FILTER.

This is just two Korg MS-20 filters behind one panel with switches to go between using them as individual filters or switching them in series.  This is one of my early projects so the implementation is a bit clumsy with how I use the switches etc. but it'll be easier to use socket switches instead of toggle switches but in my early building days I didn't know these things. 

I'm keeping the article up for archival reasons plus this dual configuration actually sounds pretty awesome but in hindsight, I wouldn't build it like it's presented below. Just so you know. 

I wasn't too pleased with the performance of the Prophet 5 lowpass filter so I decided to remove it and put a new filter in its place. I've seen lots of videos about the Korg MS-20 and really like the sound of it. I noticed that synthesizer has two nearly identical filters next to eachother; the highpass- and the lowpass-filter, so I wanted to emulate that in my own synth. So I set out to build two of the 'Late MS-20 filters' by Rene Schmitz, and fit them behind a single panel that was the size of the old Prophet 5 filter that I took out. It was a tight fit to put all the knobs and switches on but it worked out beautifully in the end.
The schematics and layout I used are just the same as the ones I used in article 12 of this blog, so if you want to build your own dual filter arrangement you can go to the Korg MS20 filter page and build TWO of those. Build both filters with the HP/LP switch but do not include the bandpass switch. You build two MS-20 HP/LP filters and put them behind one panel. Then, as extra, you add switches to the inputs so you're able to put them in series or use them independently of eachother. The wiring diagram for those switches is further down the article.

Here's a picture of what that looks like. You can see I have filter one on the left side and filter two on the right. Each has its own Cut-Off Frequency and Resonance controls and each has audio and CV-in level controls and each has it's own Highpass/Lowpass switch. Beneath those switches you see two more switches which enable me to switch both filters in series without using patch cables to connect them to eachother.:

The wiring of these two stripboards was a bit of a nightmare but I got it done in the end. I made some initial mistakes and had to re-wire some potmeters so that's why the wires look like such a mess. Luckily it doesn't affect the working of the filter.

In the first filter I used the LM13600 chip and in the second the LM13700 chip. And having them side by side is a good opportunity to compare them and the LM13600 is a bit tamer than the 13700. So if you have both chips in stock you can decide whether you want your filters to sound aggressive or a bit less aggressive. It's not a big difference though.

You can see in the pictures above that it's a tight fit but I did managed to include two volume or level potmeters for the audio inputs, which are not included in the original build but are very useful to have. I'm going to make sure that every filter I build in the future has input level control.
Beneath the HP/LP switches are the two switches that give you the option of using these filters as two stand-alone filters, so not internally connected, or if you want to put them in series so the output of filter 1 goes into the input of filter 2.
One other weird option would be to switch the output of filter 1 to the output jack but leave the input switch for filter 2 as is, so filter 2 has no input. Because these filters are self-oscillating you can now use filter 2 as an oscillator. Put the output into the 4 channel mixer described in article 17 and put the resulting wave through filter 1 or wherever you like. Connect a 1V/Oct voltage to the CV input. Just an idea, but you see there are endless possibilities. That is the beauty of modular synthesizers. :)
However, it would be better to use a single DPDT switch here and use it just to switch between two filters in series or both separate. That makes it easier to switch but that way you can not use one of the filters as stand-alone oscillator, but you won't use that function much anyway I'll bet.

Here is the wiring diagram for those two switches. By all means try and think of a better way to do this. Like I mentioned in the beginning, this was one of my early projects so not everything is perfect. I still get confused by these switches when I use this module after 4 years of using it LOL (I don't use it any more)

Use the switches inside the sockets instead of toggle switches. Much less confusing. But I used the wiring diagram below. 

Finally I want to show you a little video I made which demonstrates the sound of these two filters in series with eachother. I filmed this just after I had finished the build and I was still figuring out what the filter could do but it shows the added benefit of having two of these in series. It can do really deep and full sounding bass tones and it can also scream and distort and sound really weird. I am glad I fitted these and they are certainly a big improvement over the Prophet 5 filter although I will use the AS3320 chip inside it for a future build.
Plus all those knobs and switches so close to eachother look really cool I think ^_____^

Here's a look at the different sounds this filter can produce. (The phaser effects come from the special effects unit and not from this filter):

Okay, that's it for this one. If you enjoyed this article please check out the rest of my synth build and leave me a comment if you have any questions, or even to just say hi. Please also subscribe to my YouTube channel. That would be a great help. THANK YOU!!

Other websites that deal with the DUAL MS-20 configuration:

https://www.modulargrid.net/e/befaco-sallen-key-filter-bf-22

https://www.perfectcircuit.com/signal/korg-ms-20

Synthesizer Build part-14: AD/AR Envelope Generator.

An updated/slightly improved version of the LookMumNoComputer simple AD/AR.  Improvements suggested by Sam Battle himself.

This Envelope Generator is a fantastic little extra to put in your synthesizer. It's always handy to have a few extra envelope generators in your synth to trigger filter responses or other parameters. I built a 'proper' ADSR a few pages back and this simple version is just perfect to have as extra. I found this on the LookMumNoComputer website and Sam has also done a video about this on YouTube which you can watch here:

Because the LFO from the last blog post didn't have a synchronization input I needed something that could trigger a filter response when I pressed a key on the keyboard so I decided to build this. I had just enough room left on the panel for the LFO to include this and it only needs a small bit of stripboard to build it up on.

Here is the layout that I made, which is just a copy of the one on the LMNC website but with a few changes (see text below. All potmeters viewed from the front.):

AD stands for Attack and Decay, this is when the switch S2 is in the Trigger position. That means there is no sound after you let go of the key. AR stands for Attack and Release and this is when S2 is in the Gate position and now the tone will fade out after you let go of the key.
It's fun to build it like Sam does in the video with a big arcade button with an internal LED light.
It's pretty straight forward build. In fact, it's so simple that I didn't even test it before building it in and luckily it worked straight away. It didn't work perfectly though. It needed pretty high voltage Gate signals and Triggering didn't work at all. There was a discussion about this on the LMNC Circuit Discussion Group Page on FaceBook and it turned out that Sam had advised to lower 3 of the 100K resistors to 10K and an other suggestion was to remove the diode from the input to the switch and make it a normal wire connection. I implemented these changes in the stripboard layout but I left the diode in place. I also advise to put in a bigger capacitor than mentioned on the original layout.  I was lucky enough to have kept some bi-polar capacitors that I took out of some circuitboards years ago because they came in very handy in this build. On top of the 1µF cap I put an extra 2,2µF bi-polar cap to get 3,2µF in total. (put in 5µF if you can) That gives a bit more time for the release to fade out. With just the 1µF it fades in just a few seconds.
Because I mounted this on the same panel as the LFO I was able to just connect the power leads to those of the LFO stripboard because they both use + and - 12 volt. So no need for an extra power cord and connector. This circuit can also be powered by +/- 15V.

Now, if you want something that is just as small but works a lot better then I can refer you to my 'Synthesizer Extra's No:01 SIMPLE AD/AR using the 7555'

This article is about the Thomas Henry designed AD/AR from 2014. He used the 7555 and his design works very well.

Okay, that's all for this one. If you have any questions about this or other builds on this website then please put them in the comments and I'll answer them asap. And while you're here, leave me a comment anyway!
Until the next one!

Synthesizer Build part-4: THE ENVELOPE GENERATOR or ADSR

This was the first Envelope Generator I built but I no longer use this design myself since I discovered the Digisound 80 ADSR and the Yusynth 7555 ADSR both of which are much better designs with the Digisound design also using the AS3310 chip (or the CEM3310). So my strong advise is not to build this particular design. "Why is it posted here then", I hear you ask. Well, because this website is an archive of the synthesizer I built and that includes all the lesser designs too. But you get ample warning if I advise against building. It's mostly the early projects that can be a bit problematic. The design below is simply the datasheet circuit of the AS3310 chip, which is not the best design by a long shot.

Original text of this article:
The Envelope Generator is generally better known as the ADSR which stands for Attack, Decay, Sustain and Release. These are the four phases a note goes through when you press a key on the keyboard. The attack is the speed of the initial rise of the note, once you press the key. Decay is the time it takes for the note to go from the peak attack level to the sustain level. Sustain is the level of the note as you keep the key pressed down. It is usually a bit less loud than the first instance of the note being played. Then we have Release and that is the amount of time it takes for the note to fade out once you let go of the key. So the envelope generator produces a signal that determines the volume of the note over time and this signal is being used by the Voltage Controlled Amplifier (VCA) which interprets it as an output level. In some Minimoog synths it is also called the Loudness Contour.

Now of course the envelope output is a control voltage so it doesn't mean that you need to use it for the above mentioned purpose. You can connect it to anything that can be controlled with a control voltage like the filter cut-off or the resonance or the pulse width of a squarewave. This opens up a miriad of options but let's not get ahead of ourselves here. If you're just starting out with synth building, you need the ADSR to open the VCA and the fancy stuff will come later.

I decided to build this ADSR using the chip series that I plan to use for the most important components of my DIY synth, the AS33xx series of chips. The AS3310 is the ADSR chip and it costs way less then its CEM counterpart. It's about €6,- 

So I looked up the datasheet and used the circuit that was presented there. I made the following stripboard layout for it. This layout is verified, I used it for my build. (All potmeters viewed from the front.):

(Last revised: 16-March-2020: Removed direct potmeter connection to 5Volt. Revised potmeter wiring. Trigger when not used shorted to Gate via internal switch in trigger input socket.)

Print only:

This design works very well and does the job it needs to do. It has a few little quircks though. The potmeters for instance. I used normal linear type potmeter and that works but it would be better to have reversed logarithmic or anti-logarithmic potmeters because the difference between 1 second and 10 seconds on the Release for instance is only a few degrees of turning the knob. But once you're used to this it's not really a problem. The Sustain potmeter is at its maximum at about 2/5th of its maximum throw. If you turn it further the Sustain level rises but the attack won't be able to reach it. So if you have Attack set to, for instance, one second, it will rise normally and after one second it will suddenly jump to the Sustain level. I've got some oscilloscope pictures below to illustrate this.
The external trigger input is normally shorted out through a switch in the Trigger input socket. So if there's no trigger cable attached, the trigger for the chip is provided by the Gate signal through the 3nF capacitor. The AS3310 needs a simultaneous gate and trigger signal to function. So if you plug a cable into the trigger input but you don't provide a trigger signal, the Attack parameter of the ADSR will not work! So this is not a malfunction, this is how it's supposed to work.

I added a few extra's to this design. First there is the option to output a signal that is twice the voltage of the normal output (10Vpp instead of 5Vpp). You can use this, together with a passive attenuator in the mixer panel, to drive a filter's resonance or other parameters of the synth. Then there's also an inverted signal output, to add to the options of driving parameters of the synth. This goes from 0V to -10V.
All this takes place at the opamp on the lower left of the circuitboard. You can use the old favourite TL072 for this or the TL082. Pins 1,2 and 3 together with the two 100K resistors form the 2x amplification. You can use other resistor values as long as you use 2 resistors of the same value. Then the input signal is split at the non inverting input and goes, via a 100K resistor to the other side of the chip where the opamp is set up as an inverting buffer. Both opamp outputs have their own output jack socket. The normal 5V ADSR output is a separate socket (of course).

And finally I added a manual trigger option, at first I added it so I could put a gate signal on the gate input for test purposes, but then I thought this would be handy to have anyway so I added the switch to the final panel. I decoupled the manual trigger signal from the gate input socket with a Schottky Diode so no voltage goes into the circuitry that delivers the normal gate signals so as not to damage it (although this is probably not necessary). I used a Schottky diode because their voltage drop is only 0.2 Volts so it doesn't detract too much from the usual 5 Volt gate signal.

Here's the E.G. mounted in my synth. You can see that I doubled the output sockets. There's 2 outputs for normal 5Vpp ADSR and 2 for either 10Vpp or Inverted -10Vpp I also installed a Gate output and a Trigger output. The Trigger output is connected to the Gate output via a 3nF capacitor and the Gate output is simply switched in parallel over the Gate input. I will however install a opamp buffer for the gate output in the near future.:

Here are some oscilloscope screenshots showing the function of each variable:
This is the normal envelope CV at 10Vpp.

Varying the Decay time:

The picture below shows the quirck I mentioned earlier where the Sustain level is set higher than the Attack can reach and so after the Attack cycle has finished the Decay is skipped all together (because there is no Decay if the Sustain level is higher than the Attack level) and the envelope jumps to the Sustain level. You can clearly hear the jump in volume in the audio. You can use this to your benefit though because it sorta has a percussive quality to it. Anyway, if you don't want this, just turn the Sustain down a bit. Problem solved. You can also limit the maximum resistor value of the Sustain potmeter by adding a resistor or trimmer to pin 3, but you'll have to experiment to find which value works best.

Lowering the Sustain level:

Increasing the Release time:

And finally switching between the inverse envelope (which was set to 0 to -5V in my ADSR but to 0 to -10V in the stripboard layout) and the 10Vpp envelope.

Okay, that's it for this one.
I hope you enjoyed this article and leave a comment please if you found this helpful! Much appreciated! Also, any questions? Put them in the comments or contact me on Facebook. I'm a member of the 'Synth DIY' Facebook group and the LMNC Discussions FBgroup and also the "Synth DIY for non engineers" Facebook Group all under my own name Eddy Bergman.
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Synthesizer Build part-3: TRIANGLE TO SINEWAVE CONVERTER

This article has been re-written at 11-11-2020 and updated in Dec. 2025.

After I had finished the VCO I wanted to add a Sinewave option to it. The first design I had posted here was a bit sketchy so I now present a new layout here. This layout has been made using the schematic of the Thomas Henry CEM3340 Deluxe VCO.

This circuit needs the Triangle input wave to be +/-5 Volt peak-to-peak. You can input a Trianglewave of 0V to +10Vpp but then the input must first go through the 1µF electrolytic capacitor to take away the offset voltage. The Triangle to Sinewave converter will not work properly if you input a 0/+10Vpp Trianglewave without first filtering out the DC offset voltage.

Here is the new stripboard layout. This converter offers a +/-5Vpp output and a 0/+10Vpp output. 

Here is the schematic drawing. I did not include any de-coupling capacitors but if you want to include them then just add two 100nF ceramic capacitors to the voltage rails as close to the chip as possible. One going from +15V to ground and the other from ground to -15V.

If you only need a bipolar output (+/-5V) then you can leave out the two opamps on the right and just tap the sinewave off at pin 1 of IC1-A as shown below, and leave out the 100K resistors and the opamps D and C.

Updated schematic 2025:

(There's also a KiCad version of this schematic further down this article.)

Here are two pictures from the oscilloscope. One without offset from the +/-5Vpp output and one with offset from the 0/+10Vpp output. If you look closely at the pictures you see that the scope is set to 2V per division and therefore that the amplitude of the sinewave is 8V. But now that I changed the feedback resistor Rf, that has changed to 10V (even a tiny bit over):

As you can see they are beautiful sinewaves and you can set the symmetry and distortion very accurately with the trimpots on the stripboard. 

It will be easy enough to mount this little stripboard on one of the M3 bolts used to mount the print of the 'Really Good VCO' and thus add a Sinewave output to that VCO. You can tap the Trianglewave straight from pin 10 of the AS3340 (or CEM3340) chip or from pin 12 of the TL074 quad opamp chip. I think that will be even easier. On those pins the Trianglewave is not yet given a +5V offset voltage so it is still +/-5Vpp and therefore doesn't need to go through the 1µF electrolytic capacitor on the layout of the Triangle- to Sinewave converter. 

Okay that's the new version of this article done. If you have any questions please put them in the comments below or on the EddyBergman Facebook Group page.

UPDATED VERSION OF THE SCHEMATIC Oct. 2025:

Here's the KICad version of this circuit which I used in the Eurorack PCB version of the 3340 VCO of project 18. It works like a charm and it has an extra 1K trimmer at the top of the transistor pair which lets you trim away any offset voltages. (I updated the stripboard layout with this feature too). It has a small range though so you need to make sure the 10K resistors are well matched to eachother. The transistors themselves must also be matched to get the best results. 

Here's a screenshot of the sinewave it produces. A really beautiful symmetric sinewave:

Btw, all the comments below upto August 2020 refer to the original Tri- to Sinewave converter article (which is now deleted) and not to this new updated one. So please disregard those comments as they are not relevant to this article.