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

One of the best sounding analog VCO's you can build, with 4 waveforms including Sine. It has excellent 1V/Oct. tracking. With verified layout. I also made a layout specifically for EuroRack with a stripboard that is cut in half and folded over. You can find that further down the article. This VCO project has very rapidly become one of the most popular on my website together with the 3340 VCO.

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 if you listen to just the basic wave outputs. 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 Hard 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. Plus it's very stable with the PTC installed for temperature compensation. 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. (You should really have at least two of these in your setup.)

This is a medium difficulty project. Not one I would recommend for beginners and certainly not as a first project. You need to have a reasonable knowledge of electronics for this one and you can not do without an oscilloscope. Please read the entire article before you start building so you are aware of things you need to look out for. Be extremely accurate in copying the layout. Check and double check and mark the components you soldered in on a paper printout of the layout.

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. In the lower part there are three resistors named R26. I think Thomas made a little mistake there:

The VCO uses no exotic chips. There's only two TL074's, an LM13700 and a TLC555. You can also use the ICM7555 (which is what I used) or the LMC555. 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 in this circuit.

I didn't put in the bypass 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. 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. 

Here is an amazing Falstad simulation of this circuit made by Fabian Kempe: -- CLICK HERE --

ABOUT THE TRANSISTORS:

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) and that is correct 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. For the first VCO I built I covered them in thermal paste and bent some thin copper sheet around the bodies to keep them together. For the second VCO I just glued 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. You can then bend the thermistor legs so that the body of the PTC rests on top of the transistors and then glue it in place. Once the glue is dry you can cover the PTC with a little bit of Heatsink Compound if you wish.

 

SOME OF THE FEATURES OF THE VCO:

The VCO has four waveforms: Sine-, Ramp-, Triangle- and Square/Pulsewave. 

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. 

Frequency range is roughly from 0,1Hz to 28.000Hz (28kHz).

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. The linear FM input has its own 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 are trimpotmeters for one Volt per Octave tuning (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 correctly, 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 and really only to save space on the stripboard.

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. So I guess that is called a Sawtooth wave officially. I always get Saw and Ramp mixed up anyway so we'll keep caling it a Ramp wave ^___^

PULSEWIDTH MODULATION:

There's a potmeter for the Pulse Width Modulation which goes from 21% to 75% if you use the 330K resistor (R47) as seen on the schematic. I changed that resistor to 190K 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 so you can set the range you want manually. If you don't have a 190K resistor, use a 180K or 200K, just the closest you have or combine two resistors in series to make up the right value. The layout below uses the 190K resistor instead of the 330K of the original schematic.

Make sure the potmeter for External Pulsewidth Modulation has a ground wire that connects straight to the Ext. PWM input socket. I had problems with it, in so far that I couldn't turn the external signal off completely by closing the potmeter. There was still some external modulation going on. It turned out that the cause was that I had the input socket just grounded through the metal of the front panel. When I soldered in a wire from the socket ground to pin 1 of the potmeter the problem was solved. So you can see that good grounding is very important!

Btw, you can still use the internal PWM potmeter when you're using external Pulse Width Modulation. If you want to change that you'll need to install a switch in the PWM connection to the print. You can't use the Ext. PWM socket switch because that connection goes through a different value resistor.

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 the decal) to one stripe before that (between 40 and 50 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. I measured the frequency range and it goes from 18Hz to 28kHz. So I decided to tame it a little by increasing the value of R29 from 100K to 300K. This worked out pretty neat at first but I noticed that my VCO would sometimes not work because of this change so I have now changed it back. You can put in a value that's a bit higher than 100K but not much higher. 180K would be the max I would go for but better still, don't do it at all. I tested it so you don't have to. Certainly don't do this when you first build the VCO. If you want to try this, do it after you build and tested the VCO to avoid you having to do any troubleshooting.

To get in the right octave, compare the sound with a VCO you already tuned and then turn the Coarse potmeter so it's roughly the same pitch. Mark that position of the Coarse pot with a little pen stripe. Keep the potmeter there when you start tuning the VCO and don't touch it.

TUNING:

Before I started tuning, I set the 'Frequency Coarse' potmeter in the 11 o'clock position (the pen stripe I mentioned in the text above) to get in the right octave, and the 'Frequency Fine' adjust was set to the 12 o'clock position. Let the VCO warm up for about 15 minutes before you proceed.

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 also the 1V/Oct. trimmer. In the tuning proces you mainly use the 1V/Oct. and the HF Tracking trimmers but you can also use the Fine Tune potmeter on the panel if you're just off frequency. Changing the 1V/Oct. potmeter also influences the tracking so it's a delicate balancing act. 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 at first, but it was a lot more difficult than tuning the Digisound-80 VCO. I must admit though that I had trouble getting the lowest octave in tune. Octaves 2 upto 5 would be tracking beautifully but octave 1 was a bit high. Anyway I left it like that because it's not bothering me.

After I installed the Thermistor and re-tuned, the VCO was rock solid with temperature changes.

I had my VCO in tune over 4 octaves in a timespan of about 10 to 15 minutes. Before I installed the Thermistor the VCO would go out of tune after a while because of temperature changes but after I put the PTC in it stayed in tune beautifully. It can still be off a little when you first switch on but a slight re-adjustment with the fine tune potmeter and it's all back in track.

What can be really helpful with tuning is to use a sequencer to play a string of C notes from low to high in a slow tempo. That way you can easily hear how the tracking tunes or de-tunes the VCO over the Octaves as you turn the trimmers. (Just an idea.)

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 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.

Edit: There is now a Eurorack compatible layout down below which has the resistor changes already implemented. 

SYNCHRONIZATION:

The Hard Sync function works like a treat. If I connect a sawtooth signal from VCO-1 to the Hard Sync input of VCO-2 the second VCO will follow the tuning of the first VCO perfectly, even if that second VCO isn't tuned very well of itself. If you then force the second VCO out of tune by adjusting the Coarse Frequency control on VCO-2 you'll get some awesome distortion-like sounds that sound very musical. It totally blew my mind when I fed that through the Steiner-Parker filter. The VCO tries to stay in tune with VCO-1 and you can almost hear it struggling to do that. On the oscilloscope you can see the wave jumping in frequency as it tries to stay in tune and it does stay mostly within the main note played on VCO-1. But hearing the VCO struggle to do that is so awesome sounding. Unfortunately I didn't film this in the demo video below because when I filmed it I only had one VCO built, but you're going to get some great results if you build more then one Thomas Henry VCO-555.

I might make a new demo video soon, but in the mean time you can get an idea of the Hard Sync function by listening to the Fonitronik video I posted below my own demo video. 

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. It has also been verified by at least 10 people who gave me feedback that they built it successfully. 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. 25-jan.-2021: Slight cosmetic changes. 24-Aug-2021: Slight cosmetic changes, removed color coding from resistors to make values more ledgible).

Here's the print only view. Note the stripboard used is 56 holes wide (not 55) with 24 strips:

And here's an overview of the cuts that need to be made and the wirebridges that need to be put in. I'm giving you a component side view for the wirebridges. Also mark the cuts on the component side with a black felt pen. I always mark the cuts on the component side first and then stick a needle through the marked hole and mark and cut it on the copper side where the needle comes through. That's my procedure and it guarantees that all the cuts are made accurately.

Here's the 'cuts only' view from the copper side:

ABOUT THE PTC THERMISTOR:

Here is a link to UK retailer Thonk. who has the PTC Thermistors listed. These are the ones I use. They are 3300ppm/°C instead of the desired 3500ppm/°C but it's close enough and will work fine. My VCO's stay in tune rock solid with these. Choose the 2K version: https://www.thonk.co.uk/shop/tempco-resistor-akaneohm/  

Here's an other link for the same item as the first link, this time from the United States. Last time I looked however, the website was offline. (404 error) : - CLICK HERE -

And here's a link to a supplier in Germany, twice as expensive (€2 per PTC) but no VAT. These were sold out but I believe they are now back in stock:. - CLICK HERE - 

There's also a 3500ppm/°C version from the UK Thonk retailer, (they are back in stock) but it is a lot bigger in size. (I would recommend the 3300ppm/°C model above.): - CLICK HERE -

I bought eight of the Akaneohm 2K PTC thermistors from the first link (Thonk) in the UK and they work like a charm. They are also the ideal small size. The VCO is now rock solid on it's frequency. I ordered 8 of them to build up some stock while keeping the total cost below €22 because I'm in The Netherlands. With Brexit, I was afraid I might have to pay extra when the parcel was delivered at the door but that was not the case. But there are some new changes to the EU regulations that go into effect around this time (summer 2021) that make the €22 limit obsolete. So you'll just have to order them and see what happens if you're outside the UK.

Bill of Materials. Please note the decoupling caps and electrolytic caps you can see on the schematic bottom right, are not included in this BOM. 

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. Again, there are two R26's in the BOM but they are both valid resistors, they just got the same number by mistake.

--- EURORACK LAYOUT ---

I made a second layout for stripboard that is 59 holes wide. That can be cut in two halves and folded over to get a smaller footprint and it requires 14 jump wires to connect the necessary copper strips together. It has the resistor changes for operating this VCO on dual 12 Volt already done. I have not built or tested this VCO on 12 Volt but I'm assured it will work fine. As you can read in the comments below I already had confirmation that this layout works like it should but if you have any feedback you think could benefit others then please do share it in the comments below.

Again, this stripboard is 59 holes wide so the standard 24 by 56 hole stripboards will be too small.

Here it is:

Wiring Diagram:

Print only. Once you finished the build and tested the VCO fold the print over with the copper sides facing eachother and glue a little plastic spacer between them with hot glue so they can never touch. (Hot glue works well because it can be removed should you need to solder something.) 

Then you can use the L-Bracket to mount the print onto your panel. One bracket will be strong enough, the wiring will help keep everything in place. Do note that this VCO will have a considerable depth and won't fit into some Eurorack cases if you mount the print at a 90° angle to the panel. It's better to have it parallel with the panel in such a way that you can easily remove the print so you can get at all the trimmers for tuning. Or maybe have some holes in the panel to stick a little screwdriver through. I leave that up to you.

The cuts that need to be made seen from the Copper Side. Naturally, the six cuts that are directly next to the edge of the stripboard cutting line don't need to be made, but I left them in to make it clear where the jumpwires need to be placed.

Of course, instead of jump wires you can also use Pin-Headers like in the Wavetable Oscillator project. You can get extra long ones or push the male pins further through the plastic holders to make them stick out more so they make good contact with the  female strips.

This is the layout for the copper side. So use this as a guide and cut where indicated.

The next layout shows the cuts as seen from the component side!!! (always a good idea to mark the cuts on the component side). So only use this one for marking purposes. 

Bill of Materials. Like in the other BOM there are two resistors designated R26. This is a little numbering mistake but both resistors are needed:

DEMO VIDEO:

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%.

For some odd reason my YouTube embedded videos don't show up on mobile devices so here's the link to this demo video in case it's not visible underneath. - CLICK HERE - 

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 (confirming that it is the exact same VCO). There is some reverb on the signal in this video so it sounds a bit fuller than the clean audio you get from this VCO. This video also demonstrates the awesome Hard Sync function:

https://www.youtube.com/watch?v=wnXtOu3ljA8

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. They differ a bit from the layout because since the first build I made some cosmetic changes to the layout:

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 compound and some thin copper. I left some extra copper on there which I intended to use to mount the thermistor to but I decided to put that on top of the transistors, so I later cut the extra copper off.

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 the Thermistor installed on top of the transistors, covered with heatsink compound:

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 (the second and third TH VCO-555 I built.). Note the fine tune buttons. It's the first time I used knobs with a number scale on them, like the ones LMNC uses only smaller, and they fit very well here. I used big knobs for the Frequency Coarse potmeters and they leave only the numbers of the decal visible. I somehow like that, but that is just a personal consideration and may change in time:

ADDING LEDs:

Lately I installed some 3mm LEDs in the front panel to have a visual indication that the VCO gets proper power. I had some trouble earlier with dodgy contacts on my powerbus system so I thought this would be a good idea to detect any trouble. The LEDs each have a 15K current limiting resistor and they are connected straight to the +15V and the -15V on the stripboard. I made the current limiting resistors a high value to make sure they wouldn't pull much current and with a 15K resistor each LED pulls 0,882mA. So less than 1 milli amp at 15 Volt. The Cathode of the positive LED is connected to the Anode of the negative LED and from there goes a wire to ground on the print. The plus and minus are connected through the resistors to the power rails on the print.

Finally a look at some scope images of the VCO. The picture below shows the basic waves and the Duty Cycle of the squarewave with the Pulse Width 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. 

In the next picture we see the Sinewave FFT or Fast Fourier Transform at the top left. This shows the main peak in the middle at approx. 332Hz and then the harmonic frequencies as the peaks to the right of the middle. As you can see the harmonics are at least 30dB attenuated compared with the main wave so well suppressed.

The rest of the pictures show the waveforms being 'Hard Synced' by an other Thomas Henry VCO. Here you can see how it influences the different waveforms. (The picture at the bottom right shows the output of my VCA after the Hard Synced squarewaves have gone through the Steiner-Parker filter (not important)).

TROUBLESHOOTING TIPS:

I've had a few people who built this VCO and then it didn't work. Most of them eventually got it working though. Here's a little summation of the most common causes of trouble that I came across. This list can be used for any of the projects on this website:

- Forgetting a cut or a wire bridge. I think this must be the number one cause of the VCO not working.

- Grounding problems. This is something I experienced myself. If you rely on grounding the sockets through the metal front panel you are asking for trouble. It can cause the VCO to just not work. This design is particularly sensitive to grounding errors I found. So make sure everything is grounded with wires. You can connect all the grounds of the sockets together by weaving a copper wire through them, soldering the connections and then connect it to the print.

- Short circuits between strips. Solder whiskers. This is also a leading cause of modules not working. (This has actually happened to me and also a lot of other people posting problems on the Facebook group) A tiny bit of solder can be the cause of the short circuit or sometimes even bad etching of the stripboard itself so two strips are connected by remaining copper. Take a sharp iron pin or small screwdriver and scrape the area between the copper strips to make sure there are no connections. Measure strips that are next to eachother for continuity before you start building.

- Chips not working. Seems obvious but it has occurred. Make sure your chips are good and from a reputable source (in other words not fakes from China.)

I've had feedback where someone had static noise in the audio outputs. This turned out to be caused by a bad 7555 chip. You can also see in the comments below a comment about a bad TL074 chip being the cause of the rampwave not working properly. 

- Small bits of wire getting into potmeters and causing a short circuit. This is also something that happened to me and it is almost impossible to find. I discovered this once when I couldn't find a short circuit which I knew was there and decided to put the full voltage of the powersupply on the short to see where it would start smoking (after first taking out the chips). A big flash came from one of my potmeters as the wire evaporated and that solved the problem. The potmeter wasn't even damaged.

- Powersupply issues. The VCO is not getting power or intermittent power on one or both of the power rails. Check your powerrails with an oscilloscope while in use and while you're at it, check the powercord or ribbon-cable to the VCO. 

- Bad solder joints. Bad soldering can be a difficult to spot problem. A wire or component might seem to be connected but that isn't always the case. When in doubt, re-flow your solder joints to make sure. Also make an effort to solder neat and tidy. Use thin solder (0.5 or 0.6mm) with rosin core and don't use too much. A good solder joint looks from the side like a christmas tree not like a snowball. (I also prefer to use lead solder 60% tin/40% lead). Make sure your soldering iron is at the right temperature and that is HOT! (about 370°C) Better too hot than too cool because with a hot iron you can solder more quickly and expose the component to heat for less time.

- Faulty patch cable. A no brainer right? It happens though. Use good patch-cables!

< this list will be updated as more causes and solutions come in >

Okay, that's another one done. 

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

If you like what you see and want to help to keep this website up and running and to support new projects you can buy me a coffee. There's a button for that underneath the main menu if you're on a PC or Mac. Otherwise you can use this PayPal Donation Link (that's a bit easier too) if you can spare a few bob. It would be a great help and all donations go towards the purchase of new components. Thank you!!

DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.

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 five different functions!  Offset, Attenuation, Inverter, Glide or Lag control and, if you follow the tip below the schematic image, it can also be a CV splitter. 

Now also with Eurorack compatible layout.

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 more specialized specifically for the ARP2600 so 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 an 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. 

If you want that control to behave more like a Glide control to smoothly go between different notes with a 1V/Octave signal then use a 1µF electrolithic capacitor. Try it and experiment. Maybe use a 100K potmeter instead of a 1M one.

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 =).  In fact, there's a Eurorack friendly layout further down the article. One thing though, with a dual 12V supply the maximum offset will be 4 Volt instead of 5 Volt but I addressed that issue and fixed it by changing some resistor values.

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. You can also use other quad-opamp chips instead of the TL084. You can use TL074, LM324 or any other, as long as it's a low noise opamp (good for use with audio, which most of them are these days) and they have the same pin-out as the TL084 (and most quad opamps also have that these days).

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 in my case. 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 but test it and check to see it works like you want it to. Use an oscilloscope set to DC mode for measuring the output.

If you don't want a Lag control but a 'Glide' control you can use a 1µF electrolithic cap. My advise again is to experiment and use whatever suits your needs.

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 -15V) 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. This layout was made for the Kosmo format modules but now there's also a more compact layout below for Eurorack size modules.

TIP: Solder a wire from the input-socket of stage one to the socket switch of the input socket of stage two. That way the signal on input one will be present on both inputs and can be processed by both stages and split in two. If you connect a patch cable to the input of stage two, that first connection will be broken by the socket switch and it's back to normal. Very useful I think! Of course you need input sockets with built in switches for this but most types have that as standard.

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

Bill of Materials:

EURORACK LAYOUTS:

As of December 2021 there is now also a new layout for the Eurorack format. The stripboard is 24 by 41 holes. Just like in the Kosmo format layout above, the right part of the dual voltage processor is a mirror image of the left part so I could place all the connections to the potmeters on the edge of the stripboard on both sides. The TL084 has 4 identical opamps in it so it doesn't matter which opamp is used for which part of the circuit. As I mentioned earlier, you can use other quad opamps like the TL074 or LM324 for this without problem.

Here is the wiring diagram:

Print only view:

I built this version on Dec 10th 2021 and everything worked fine except that I had to use a lower value capacitor for the Lag control. The layout has a 470nF cap in it and that works fine in my Kosmo format panel but for this one I had to use a 10nF cap. Not sure why this one is different, maybe it's the fact that this runs on +/-12V instead of +/-15V or it's the potmeters I used I don't know. Anyway, it's not important because if you find, when testing, that the Lag potmeter doesn't work over the full throw then you need to lower the capacitance. Just a matter of experimenting. The cap can easily be de-soldered and changed for another one.

The trimpotmeters are for setting the Attenuverter midpoint and as with the previous layout they don't have much influence but you need to set the Attenuverter so that at the midpoint, when the waveform is a flat line, that line is at the zero volt mark. Measure this with a scope and make sure all the other potmeters are turned fully counter clockwise and the offset switch is set to 0/+5V.

Btw, because this module is running on +/-12V the actual offset voltage is plus or minus 4 Volts, not 5 Volts but I only discovered that after I made the panel so I kept the labeling as is.

EDIT: To get a higher offset voltage in this Eurorack version you must change the resistor R6 from a 47K to a 68K. This will actually give you +/-6V offset voltage. R6 on the lefthand side of the stripboard is the 47K between pins 13 and 14 of IC-1 (positions I and J-16). On the righthand side it is the 47K between pins 1 and 2 of IC-2 (positions I and J-27). Change those for 68K's and your offset voltage will be upto +/-6V.

The 470 Ohm resistor(s) are not in the original circuit schematic. I put those in myself when I built the first version because the Lag control produced some noise when the potmeter was fully counter-clockwise. This resistor sort of prevents that the Lag pot can be fully closed. It has no negative influence on the amplitude or sharpness of the signal so it works fine.

Here's a picture of the finished Eurorack module:

VIDEO DEMO:

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 module 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!

DISCLAIMER: The author of this article does not accept any responsability for the correct functioning of this, and any other, module/project on this website. What you build, you build at your own risk. All project layouts are thoroughly tested before publication, it's up to you to replicate them and the author can not be held responsable for any mistakes made.

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

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

This module is not like your conventional Lowpass Filter. It's a combination of a VCA and a VCF. It helps if you're trained a bit in your modular synthesizer knowledge to get the best out of this module. As a beginner you might be better of building some normal filters first and leave this one for later. But then again, if you're feeling adventurous, then hop to it. You will certainly learn a thing or two as I did. Plus it's quite easy to build. 

A little bit of history:

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

Here's the link to the NatualRythmMusic website which features the same project.

(I'm not associated with any of those websites.)

Resonant Lopass Gate:

To be honest with you, I had never heard of Resonant Lopass Gates before I held a poll on Facebook to see what people would like me to build for future projects. This was one of the options that was mentioned. It instantly intrigued me  because I didn't know what it was. So I asked for schematics, did some research and started building one. 

This module consists of three parts and there's a mode switch to switch between them. There's a voltage controlled amplifier or VCA and a lowpass filter (12dB) and the option to have both on at the same time. The VCA is nothing more than a voltage controlled attenuator and with the switch in VCA mode that is what you get. Now if you set the switch to 'Both' mode, you get the same VCA function but unlike a pure VCA not all frequencies are attenuated equally. Depending on the height of the Control Voltage, the filter cuts off parts of the high frequency content of the input signal. If we now switch to VCF mode we have the full function of the lowpass filter including resonance (and it can self-oscillate) and the CV voltage determins the cut-off frequency of the filter. The VCA part is no longer working in this mode but we still get a mixture of changing cutoff frequencies and changes in amplitude driven by 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 input, the module produces more of a percussive, pulse like sound. At least, that's where its strenght lies. (See demo video lower down the article for sound samples)  The CV inputs can be anything from a Gate signal to an Envelope signal or an LFO. You can experiment with what sounds best. I think it's better to have signals going into both CV inputs at the same time. The CV 2 input has an inverter connected to it in the form of opamp U2-A, The more you turn it clockwise the more the CV signal gets inverted. This is one of the changes that has been made (by Thomas White) from the original design as described in the 'modularsysthesis' article in the link below here, which I incorporated into the redrawn schematic. It works very well. The CV-2 control contributes a lot to the funky sound of this module.

Here's the schematic drawing that I re-made from the schematic on 'modularsynthesis. It has all the changes that are suggested in the linked article implemented. (Click on the image to enlarge it and then right-click and 'Save as' to save it to your computer. Then you can zoom in on it.).

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

About the Vactrols:
The layout I made for this module worked rightaway but I did some experimenting with the Vactrols. I ordered a batch of VTL5C4 vactrols and they have now arrived but the Vactrols I made myself seem to work so well that I'm hesitant to replace them. I made mine from 5mm red LEDs and LDR's that had an 'off' resistance of over 200MOhm and with a bright white LED shining on them the resistance was about 200 Ohm. I later soldered a 3mm red LED in parallel over the vactrol LED on the left to dim it a little, because I found out that sounded better. I made some Vactrols earlier and used bright white LEDs in them but although they did work, the LEDs hardly came on because the maximum voltage over them was about 2,7 Volt which was too close to the threshold voltage of those LEDs. The red LEDs will shine full on with that voltage which works much better. (So because the LEDs in the Vactrols are part of the circuit and not connected directly to a powersupply they don't require their own current limiting resistors.)

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

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

Bill of Materials:

How to calibrate this module:

There are two trimmers on the board, the 20K trimmer directly influences the voltage that the Vactrols get so it plays a part in determining the sound. So you need to set it for best resonance, at least that's what I did. The influence it has is not that obvious though. 

The second one is for the 'Deep' switch and determins the 'deepness' or the low frequency emphasis of the circuit. It's a sort of tone control and the effect it gives is like turning the Offset knob down. You can set it to whatever you like best.

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

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

Here's a video from 2008 showcasing the Resonant Lopass Gate using the VTL5C4 Vactrols which are slower than the VTL5C3's. This gives a more vintage sound (according to some people). People nickname this version the Slowpass Gate. Anyway, decide for yourself. Here's the video:

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

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

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

I used a vintage 6-pole 3-way switch but unfortunately I drilled the holes for the screws in the wrong place but since they were 3mm holes I put some 3mm LEDs in them and connected them to a free pole of the switch so that the yellow LED goes on when the switch is set to VCA mode and the red one goes on when switched to VCF mode and both go on when in 'Both' mode. =)

Here's a sketch of how I connected the LEDs to achieve that. In 'Both' mode they are a bit dimmer because of the 0,6V voltage drop of the extra diodes but you hardly notice that. It works perfectly fine:

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

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

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