Synthesizer Build part-37: THOMAS HENRY VCO-555.

One of the best sounding VCO's you can build, with 4 waveforms including Sinewave. It has excellent 1V/Oct. tracking. With a newly designed stripboard layout.

This VCO is a brilliant design by Thomas Henry and he worked on it for a long time. He calls it his best design to date and it sure is that! VCO's of different designs, can sound different from eachother despite them producing the same basic waveforms. This difference is not really that noticeable with just the basic waves although there's a noticeable difference in the sound of the squarewave from this VCO compared to the Digisound 80 VCO. It starts to become more noticeable when you start playing with the Synchronization and the Frequency Modulation inputs. That's where this VCO really shines and I think that's why it is so popular in the modular synthesizer world. With the sync and FM functions the VCO-555 produces a very full sound, rich in harmonic content and very musical sounding. And it tracks very well over the octaves. You'll find out once you build this module and start experimenting with it. It's the VCO of choise for many hardened Modular Synthesizer aficionados.

As I mentioned, there is now a new layout for this VCO. The old layout worked fine too but the component placement wasn't that practical. This one is much better and it's verified. The old layout has been deleted. 

This is a medium difficulty project. Not one I would recommend for beginners.

ABOUT THE CIRCUIT:

I wasn't going to post the schematic here and instead linked to it on the Electro Music forum but then the forum had occasional accessibility problems so here's the circuit. (The link to Electro Music also has the original parts list if you scroll down.) 

For the schematic below, just click on the image to enlarge it. Then right-click and 'save as...' Then you can zoom in on it:

The VCO uses no exotic chips. There's only two TL074's, a LM13700 and a TLC555. You can also use the ICM7555 (which is what I used). Do not use a normal NE555 for this, you need the CMOS version. Although I'm told it will work with the normal NE555 these chips consume a lot more power and they can have oscillation problems under certain conditions (see my 81 LED chaser article.)

Instead of the LM13700 you can also use the LM13600. I tested this myself and there's absolutely no difference between them.

I didn't put in the de-coupling capacitors for the chips. I almost never do because I have a good powersupply and no noise problems in the circuit. If you want to include them solder them in over the powerrails on the stripboard as near to the chips as possible. The connections are shown at the bottom of the schematic. You can use small ceramic 100nF caps for this. You can solder in the two 10µF electrolytic caps over the power rails on the stripboard too. One from plus to ground (- to gnd) and one from ground to minus (+ to ground). The exact position of the electrolytic caps on the stripboard doesn't matter. Put them where you have room. Note these caps are not included in the Bill of Materials below.

For temperature compensation the circuit uses a PTC Thermistor which I guess is the only exotic component in this VCO. PTC stands for Positive Temperature Coefficient, meaning that when the temperature goes up the resistance also goes up. This temperature dependent resistor has a nominal value of 2K. However, it's not necessary to use a Thermistor. You can get away with just using a 2K resistor, especially if the temperature doesn't change much anyway like if your synthesizer stays in the same location all the time. It'll just mean that it isn't as stable as it can be, but many people use this VCO without the Thermistor. If it does go out of tune you can easily adjust the Frequency Fine Control and set it right. It is handy to have a hacked tuner like the JOYO tuner attached to the VCO to keep an eye on the tuning.

Capacitor C4 (2200pF or 2.2nF)  is the timing capacitor for the oscillator and therefore it should be a non-ceramic type like a Polystyrene or a Polyester or Silver-Mica type capacitor for temperature stability.

The VCO doesn't have an extra CV input because actually the Exponential FM input has that function. If you look at the schematic you'll see it is connected to the same pin as the 1V/Oct input and it has an attenuator too. The Frequency Coarse and Fine tune are on that same pin too. 

The two PNP transistors Q2 and Q3 need to be matched. I matched them using the Hfe transistor tester on my multimeter and this is good enough. When measuring Hfe, give the transistors time to cool off after you touched them because the Hfe value will change with temperature. Some will tell you that the transistors need to be matched on the Base Emitter Voltage (Vbe) but I noticed that if you match them on Hfe, the other parameters will be pretty close too. Anyway it works fine this way. 

The transistors need to be thermally connected to eachother on the print. Look at the pictures below to see how I did this. I covered them in thermal paste and bent some thin copper sheet around the bodies to keep them together. But you can just as well glue them together with some super-glue. That'll work fine too. On the layout below, the transistors are mounted in such a way that you can bend them towards eachother so the flat surfaces connect to eachother. If you do decide to use a Thermistor in this circuit you will need to find a way to also connect that Thermistor to these two transistors. It shouldn't be too difficult if you use some thin copper sheet or heat-shrink tubing. I advise to get one or more SMD Thermistors and solder legs to one and use that. I think it'll be easier to place with the transistors. 

Some of the features of the VCO:

The VCO has four waveforms: Sine-, Ramp-, Triangle- and Squarewave. All the waves have an amplitude of +/-5V peak-to-peak.

It's got a Linear and an Exponential FM input, one Hard Sync input and a 1V/Octave input naturally for the keyboard. The FM inputs have attenuators. If you connect a signal to the Exponential FM input the pitch of the oscillator will change, with it being connected to the same input that also drives the 1 Volt per Octave control voltage. With the linear FM input this isn't as big a problem because it goes through special circuitry and it has a capacitor on the input, blocking any DC voltages but with Exponential FM the VCO pitch will change the moment you open the attenuator. Obviously you have to have some external way of influencing the pitch otherwise you couldn't, for instance, connect a sequencer to it.

There's also a potmeter for the Pulse Width which goes from 21% to 75% if you use the 330K resistor (R47) as seen on the schematic. I changed that resistor to a 190K one and now the Pulse Width goes all the way from 1.7% to 95%. You can also put in a 200K trimmer with a 47K resistor in series for safety so you can set the range you want. The layout below uses the 190K resistor instead of the 330K.

There are trimpotmeters for 1 Volt per Octave Tracking (100 Ohm), High Frequency Tracking, Ramp Wave connection (this makes sure the ramp wave has a smooth slope. If it is set wrong the ramp wave will have a step in it at the zero Volt level. There are two trimmers for the Sinewave. One for roundness and one for symmetry. The roundness trimmer will also change the amplitude of the Sinewave a little.

You will need an oscilloscope to set these parameters, but a cheap 20 dollar one from eBay will do fine. Make sure you set it to DC when measuring.

At first I used multiturn trimmer potmeters for all but the 1V/Octave trimmer but I have changed that because I found it very tedious to tune the VCO with a multiturn trimmer for HF tracking. It's not necessary. I only use multiturns for the Sawtooth step and Sinewave symmetry.

When I built this module I had set all the trim-pots in the middle position before I soldered them in and when I started the module up, everything was perfect except for the tuning. Even the Sinewave was perfectly symmetrical right from the get go. So was the Rampwave :) Btw, the Rampwave is the reverse from what the 3340 VCO produces. It goes straight up and then slopes down.

CONCERING THE COARSE FREQUENCY:

When I wanted to measure the range of the Coarse Frequency potmeter I discovered that its range was enormous. When I turned it from the middle position (50 on de decal) to one stripe before that (40 on the decal) I was already 4 octaves down. And turning it up it wasn't long before the frequency was so high I couldn't hear it anymore. So I decided to tame it a little by increasing the value of R29 from 100K to 300K. This worked out pretty neat. Now every stripe on the decal is one octave up or down. I also tried a 500K resistor but that was too much because the VCO fell silent, so you can't increase it to any value you like. 300K would be the maximum I would advise to use.

I haven't implemented this change in the layout or the schematic because I want to leave it to you, the builder of this project, whether you want to change it or not. I myself did change both my VCO's but as a consequence the Coarse potmeter's normal position is now about 9 o'clock. That's the rest position as opposed to the normal 11 o'clock position.

Btw, a nice side effect of this change was that the tracking of the VCO improved a little also.

The Frequency Fine Control has a range of 1 Octave. From the 12 o'clock position half an octave down and half an octave up.

TUNING:

Before I started tuning I set the 'Frequency Coarse' potmeter in the 11 o'clock position to get in the right octave range, (this was before I changed R29) and the 'Frequency Fine' adjust was set to the 12 o'clock position.

Tuning the VCO is just a matter of playing a low C note like C2 and a high one like C5 and turning the trimmer for 1V/Octave and checking it against a good tuner or tuning app on your smartphone. The trimmer is just a 100 Ohm one and I used a normal type for this, not a multiturn trimmer, and it works fine. It's a matter of tuning the C notes and seeing if the higher note is a bit lower or higher than it should be and compare it with the low note. If the one is too high and the other too low and the middle note is spot on then you have to turn the HF Tracking trimmer a tiny bit and try to get them all in tune over a wide range of octaves. You should use all three tuning potmeters in the tuning process; the 1V/Oct., the HF Tracking and the Fine Tune potmeter on the panel. Changing the 1V/Oct. potmeter also influences the tracking. Once you get it right it'll track marvellously over a wide range of octaves. I was impressed. It tracked even better than the Digisound-80 VCO and I didn't even had the Thermistor installed, but it was a lot more difficult than tuning the Digisound-80 VCO.

I had my VCO in tune over 4 octaves in a timespan of about 10 to 15 minutes. After I installed the panel back into the synth, because I didn't use a PTC, the change in temperature did alter the tuning a bit but it was not dramatic. 

12V vs 15V:

I have not tried this circuit on a dual 12 Volt powersupply yet. However there are some notes about this on the Electro-Music forum stating that for 12V you need to change these resistors:  

R13 = 2K This is the 3K resistor in series with the Square- or Pulsewave output from pin 14 of IC4. 

R27 = 22K This is the 39K resistor in series with the Sinewave Roundness trimpot to pin 16 of the LM13700.

R33 = 137K This is the 100K resistor over pins 6 and 7 of IC4. 

That last one is a bit of a weird value for a resistor but you can put some in series to get that precise value if you want. However, the resistor values don't have to be spot on so you can also just put in a resistor closest to that value. It determins the gain of that opamp so a few K's more or less won't be a big deal. The circuit is quite forgiving anyway.

As for the Pulse Width Modulation resistor (R47). I already changed it from 330K to 190K and for 12V operation I guess it'll have to be changed to a lower value still. You'll have to do some experimenting with that to get it to your own liking. My advise would be to use a 200K trimmer with a 47K resistor in series and solder that in temporarily, set it so the pulse duty cycle goes from 1% to 100% or closest to that, de-solder it again carefully and measure the resistance and then put in a resistor of the measured value to replace the trimmer.

LAYOUTS:

Okay, below here is the layout I made for this VCO. The first VCO I built was made with a different layout. That layout was published in this article before and is still visible on the LookMumNoComputer Forum, but it had the transistors and the thermistor quite far away from each-other and it also had some jump-wires. I have since made a new layout and built a second VCO with the new layout to verify it and luckily it all worked first time. So here is the new and verified layout. Don't forget the 220nF capacitor between the linear FM input socket and its potmeter. 

Wiring diagram:

(Last revised at: 17-Jan.-2021: Made cosmetic changes to layout and changed two trimmers from multi-turn to single turn (also updated in BOM).

Here's the print only view:

And here's an overview of the cuts that need to be made, seen from the copper side:

Bill of Materials. There's an extra 2K resistor included if you want to put a 2K in, instead of the 2K PTC Thermistor. I advise to order a batch of 100 2N3906 transistors so you can easily find a matched pair.

Here is a link to a UK retailer who has the PTC Thermistors listed. They are 3300ppm instead of the desired 3500ppm but I think it's close enough and will work fine. Choose the 2K version:

https://www.thonk.co.uk/shop/tempco-resistor-akaneohm/  

Here's a second link for the same items:

- CLICK HERE -

I have not tried these Thermistors myself (yet) but I plan on ordering some and testing them out. Until then I can give no personal guarantees that PTCs from this link will actually work. Order and use at your own risk.

Here's a demonstration video, demo-ing the waveforms and especially the Exponential FM option. I put it through the Steiner-Parker filter and I compare it with the Digisound-80 VCO. That comparison is not entirely fair because the DS-80 has no Exponential FM input, only a Linear one. Although, I suppose you could use the normal CV input as am FM input. That should be the equivalent of exponential FM but I haven't tried that. Btw, I forgot to mention the Steiner-Parker filter has a slow Triangle wave on the CV input which accounts for the 'Wah' sound you can hear. This video was made before I altered the Pulse Width Modulation so here you only hear it going from 25% to 75%..

Here's an other video (by Fonitronik) with a very cool demonstration of this VCO. If you look closely you can see that the Coarse potmeter on this VCO is also set to the 11 o'clock position to hit the right octave. There is some reverb on the signal in this video so it sounds a bit fuller than the real audio you get from this VCO:

Here are some pictures from the build proces. I always start by making the cuts and then I put in all the wire bridges. You can see the cuts marked in black on the component side. There are 33 wire bridges to put in:

Here you can see how I bent the two transistors Q2 and Q3 towards eachother and then thermally connected them together with some thermal heat-sink paste and some thin copper. I left some extra copper on there which I can use to connect the Thermistor to. I have ordered a few of them.

As you can see from the pictures below it is quite an easy build. Just over 40 resistors, 4 IC's and some other components. If you work methodically you should be able to easily copy this design and have yourself a fantastic VCO for a fraction of the price they cost new.

In the picture below you can see progress of the third VCO-555 I'm building. Here I used super glue to connect the two matched transistors together and I neatly bent the legs so it all fits in place nicely.

Here's a look at the finished product:

Here's a look at the panel I made for it. On the right you can see a 1V/Oct. output socket. It is connected in parallel over the 1V/Oct. input without any buffering. It's just a wire connection. I use that to 'daisy-chain' all my VCO's together and so keep the Dual Buffered Multiple free for other things. This feature is also included in the Digisound-80 VCO in article 18. Do not use this output as a CV input because it has no resistor in series. So that wouldn't work and could even damage your MIDI to CV converter.

Here's how it's installed in my synth. A Digisound-80 VCO flanked on both sides by a Thomas Henry VCO:

Finally a look at some scope images of the VCO. At the top we have the Duty Cycle of the squarewave with the potmeter fully counter clock-wise and then fully clock-wise. The somewhat limited range of the PWM was the only drawback of this VCO and it was naging me so I changed resistor R47 from a 330K to a 190K (after experimenting with a trimmer) and now the PWM has a nice range all the way from 1% to 95%. You can see the exact values in the image below. At the bottom we see the Sinewave and the FFT readout of that Sinewave. The Sinewave has a bit lower amplitude than the rest of the waveforms if you want it to be a pure Sinewave. You can turn the amplitude up by turning the 'Sinewave Roundness' trimmer but then the Sinewaves gets a slightly pointy curve at the top and bottom. On the picture below you can see the best compromise between the two. The bottom right picture shows the Fast Fourier Transform (FFT) of the Sinewave. This shows the main peak in the middle at approx. 100Hz and then the harmonic frequencies as the peaks to the right of the middle. As you can see it's not a perfect Sinewave but that's not really important. What's important is how it sounds and it sounds great! Little imperfections can actually make the Sinewave sound better. It's not like we're dealing with an FM Broadcast transmitter where the Sinewave needs to be as clean as possible.

Okay, that's another one done. 

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For questions and other help you can use the comments below but I also advise to check out the EddyBergman Discussion and Help FaceBook group. You can also find the schematic of the VCO in the Files section of that group

Synthesizer Build part-36: DUAL VOLTAGE PROCESSOR.

 This is the Fonik Buchla Style Dual Voltage Processor. A very useful module for altering Control Voltages with four different functions.  Offset, Attenuation, Inverter and Lag control.

I wanted a Voltage Processor module in my synth for a long time and I was thinking of copying the ARP2600 VP, but that one is fairly limited in its options and when I saw this design I thought it would fit much better in my system. This module lets you alter the offset of a control voltage by 0 to +5V or -5 to +5V. It lets you attenuate and invert a control voltage by means of a Attenuverter and it has a Lag control that is a direct copy of the Lag control from the ARP2600, with a 1 MOhm potmeter and a 470nF capacitor (The ARP used a 100nF cap). This alters the slew rate of, for instance, a Squarewave and rounds off the corners turning it into a Sharkfin Wave. In fact it adds a 90° phase shift to the signal. Besides control voltages this module can also handle audio signals.

This module will work fine on either a dual 12V or a dual 15V powersupply so no problem for you Eurorack fanatics =). 

The circuit is primarily meant for control voltages but it can handle audio signals just as well. Even at very high frequencies it won't distort the signal. With audio you can use the Lag control to turn a Triangle wave into a Sinewave although with less amplitude. It won't be perfect but it's possible. It can also turn a 0V to +10Vpp signal into a +5/-5V signal by adding a -5V DC Offset voltage to it. The other way around works too of course, turning +5/-5V into 0V to +10Vpp signal. Very useful.

The circuit consists mostly of 47K resistors but you can actually alter the value of those and use for instance all 91K resistors. I actually did this as a test with the second part of this dual module and it didn't change the working of the circuit in any way. Just make sure you use the same value for all 7 resistors. Don't make them lower than 47K though. 

This circuit was designed by Chris MacDonald and modified by Peter Grenader and then further improved by Matthias Herrmann who added the Lag (Glide) control function. The only thing I did was adding the Offset switch, changing the potmeter values from 50K to 100K, changing the value of the Lag Capacitor from 1µF to 470nF and adding the 470 Ohm resistor before the Lag potmeter to eliminate noise issues.

The original schematic and a PCB design can be found in this original PDF and I made a new drawing from that schematic which is posted below. Like I just mentioned, they use 50K panel potmeters in the schematic but I didn't have those so I used 100K potmeters. Again, this made no difference what so ever. You must however use a 1 MegaOhm potmeter for the Lag control because this, together with the capacitor, forms a simple lowpass filter and these values are important to get the correct frequency response. The original schematic uses a 1µF capacitor for the Lag control but with testing I found out that this is way too much. So I changed it for a 150nF in the layout but that turned out to be not quite enough. (The original ARP2600 Lag control uses a 100nF capacitor.) In my own build I experimented with different values and I ended up using a 270nF and a 180nF in parallel to make a total of 450nF and that works fine. So I set the capacitor value on the layout to 470nF. I found that this gives the best Lag control response. Of course, if you don't have a cap of that value available, you can use an other one with a value close by. Anything between 300nF and 700nF will work fine and you can put two (or more) in parallel to create the value you want.

The trim potmeters are for setting the attenuverter mid point, but they don't have too much of an impact so you don't have to use multiturn potmeters for those. The normal ones will do fine.  I added a switch to the offset control so you now have a choise to offset a control voltage from 0 to +5V or from -5 to +5V. 

A little quirck I found, at least in my build, is that there can be a lot of noise on the output if the Lag potmeters are set fully closed (counter clockwise). Because this was the case with both sides of the Dual Processor I figured this was a fault in the circuit design so I added a 470 Ohm resistor between the Lag potmeter and R6. The value is low enough not to influence the Lag filter and it gets rid of all noise issues that I had.

The schematic drawing doesn't include any de-coupling capacitors but they are included in the layout. Just four 100nF ceramic caps on the power rails as close to the chips as possible. If you experience hum on the audio output you could even put some 10µF to 47µF electrolytic capacitors on the power rails. There's room enough left for that. Make sure they are rated 25V or higher and put one on the +15V to ground (negative pole to ground) and one on the ground to -15V (negative pole to -15) rails. I leave that up to you but for my module it wasn't necessary to include them. (The electrolytic capacitors are not included in the layout, only the de-coupling caps.)

Here's the schematic drawing which I re-made from the original, from the above linked PDF file. The Dual Voltage Processor consists of two of these circuits side by side with only the Ground as a common link:

Here is the verified stripboard layout I made for it. It's the same layout once repeated and mirrored to make it a dual module. You can easily cut the stripboard in half and fold it over, connecting the traces that need to be connected, together with some copper-wire to make it a Eurorack size.

Print only. 

Bill of Materials:

Here's a video with a quick overview of the different functions. 

I watched a demonstration video about the ARP Odyssey and in it they showed the effect that the Odyssey's Lag control had on the filter cut-off control voltage. It made the filter make these 'Wah' sounds. And I'm very chuffed to see that the Lag control in this module has the precise same effect on a filter.

Here are some pictures from the build process:

In the picture of the panel (below) the 'Lag' control is still called 'Glide'. That's what it's called on the schematic but I chose to use the same term that ARP uses in the 2600.  I think it's a more accurate description because it actually creates a phase shift of about 90 degrees (see also the article about the ARP Envelope Follower). So that makes the signal lag behind the original in a small way. 

The picture below shows one side of the dual module wired up and the other side has not yet been wired up. The LEDs of that side are still mounted on the print (which was necessary for testing) instead of in the panel.

When the panel is in 'rest' position so to speak, all potmeters should be set fully counter clockwise and the switch set to 0/+5V. That way, any signal you put in will come out unchanged. You can then alter it by turning the controls.

Okay that's an other one done. If you have any questions please put them in the comments below or on the EddyBergman Facebook group. Please read the whole article before asking questions.

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Thank you very much!

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

 A combination of a Voltage Controlled Amplifier and a LoPass Filter using Vactrols. It has three modes and sounds amazing! 

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. 

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:

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.:

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 demo-ing the Erica Synths Lopass Gate (LPG) which is based on the same schematic as this one. 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.

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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, 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 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.

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. Btw, you can add as many wavefolding stages as you wish to this circuit. You can easily build this on a breadboard and experiment with the number of stages. The voltage it runs on also influences the number of folds you can get so changing the value of the 22K resistors going to the emitters of the transistors also 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 they have the most 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. 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.

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.

Please consider supporting this website. There's a "Buy me a Coffee" button below the menu if you watch this page on a PC or MAC. And there's also PayPal. 

You can request the original DIYLC layout file for any of the projects on this website from me if you need it but you'll have to PM me on Facebook for that. Then I will send you the file in the Facebook Chat.

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

A fantastic ADSR with 3 different types of envelopes and extra outputs including an inverted one.

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.

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 should be made however; the current limiting resistor R25 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.

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.

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 had used a vintage double pole 3-way switch to switch between the different modes and I had one pole left unused. So I connected the capacitor to that 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 choise 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 gating and inputting gate signals. This is marked in the schematic as a gate input socket with internal switch, which you can of course also use instead of an external switch.

One little thing you need to be aware of with this ADSR is that I 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.

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.

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 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).:

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!
Wiring diagram:

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

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 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:

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.

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.

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See you on the next one!