Sunday, 26 March 2023

Synthesizer Build part-52: 4 CHANNEL FEEDBACK EQ/DISTORTION (Monotropa) Eurorack.

A 4 channel feedback equalizer / distortion module that will fit a Eurorack system. 

I came across this circuit in a post on the LookMumNoComputer forum. Bpbby posted a Falstad simulation of this circuit and it intrigued me because I never heard of it before. He found the circuit on this website:

It's a pretty cool circuit. Simple too. We have 4 filters, each covering a part of the audio range, and then there's a feedback loop that connects the output back to the input. The circuit is called the Monotropa, which is the name of a plant. Don't ask me why. I don't see any logic in that. ^___^
Here's the schematic of this circuit:

Here is the Falstad simulation of this circuit:

Here are the layouts I made for this project. They are verified as always. I built it for my Eurorack case but you can just as easy make this for a Kosmo sized synthesizer. I that case you could even build the 7 channel version because you'd have more space on the panel for the extra potmeters. Yes there is a 7 channel version of this circuit but you'd have to Google that. This article deals with the 4 channel version. This circuit is designed for +/-12V but I can't see why it wouldn't run equally well on a +/-15V powersupply.

Here's the wiring diagram. For the first time in the history of this website I show the potmeters from the back side! I should have done that all along because it's easier with wiring up the panel but there it is. I started out showing potmeters from the front in my layouts and for the sake of consistency I stuck with that, upto now. I had to connect some components straight to the potmeters and audio jacks to save space on the stripboard:

Below is the stripboard only view. The stripboard is small enough to mount parallel with the front panel behind the potmeters and sockets. You could drill a hole through the lower two strips which are not in use and use a standoff to mount it to the front panel. The wiring will also act as a stabilizing feature. I just used some plastic tube and hot-glued them to the back of the potmeters and to the copper side of the stripboard. That's secure enough. I soldered the powerconnector straight to the stripboard without using pinheaders and sockets. That way you only have 3 thin wires coming from the board with a Eurorack connector (female) on the other side to plug it in. If you want to use bypass/de-coupling caps there's room enough to solder those in over the powerrails and add some 10µF electrolytic caps if you want extra stabilization of the power supply voltage. These components are not in the layouts and are not listed in the Bill of Materials!

The Cuts and wirebridges as seen from the COMPONENT SIDE!!

Here's the Bill of Materials:

Here's a look at the finished product:

Here are some screenshots from the oscilloscope showing the influence of the feedback on the output signal: 

And finally a little DEMO video I made. I built my version with 100K potmeters because that's all I had and consequently it doesn't sound as good as it could be with 10K pots. I assure you though, it is worth building but keep to the component values in the layout and schematics. Some potmeters are more effective than others depending on the frequencies that are put through this circuit because this is of course an equalizer. So a Low Frequency potmeter isn't going to have much effect on a high frequency bit of audio that's put through it. In the video I have it connected to a 555 VCO that is fed by the Sample and Hold of the previous project.

Okay that's it for now. Not much of a write up I admit but real life issues got in the way. I might revisit this article later and expand on it. I hope you understand and don't mind. For now I just wanted to give you all the necessary layouts etc. to build this Feedback Equalizer. I already heard from one person who built it and he's very happy with it. If you have any questions please put them in the comments below or on the special Facebook Group for this website.

If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards the website and projects. Thank you!

Friday, 10 March 2023

Synthesizer Build part-51: SAMPLE & HOLD new version.

A revised version of the earlier 'Yet Another Sample and Hold' (YASH by Rene Schmitz) project.

This is the same S&H circuit as the earlier one I published. That was one of the first projects I built and that was just over 3 years ago so it was time to update it. The things I added on myself were put together rather clumsily, especially the x1/x0.5 CV range switch that I put in. (The previous version works fine, don't get me wrong.) I have now replaced that range switch with a range potmeter so you can set the range to any level you want. The toggle switch for external or internal input has also been removed and instead I used the internal switch inside the External Clock input socket.
If you built the previous version then it should be quite easy to just replace the stripboard with this new version. You will have to make room for the extra Range potmeter but there will be a hole left over from the range switch so maybe you can drill out that hole to 7mm and use that for the potmeter.

Below here is the schematic drawing I made in Photoshop of this sample and hold circuit. It has the offset opamps and attenuation I designed added on, connected to the output of the LF398 Sample & Hold chip. The offset potmeter is a 100K linear type. You could use other values but that will alter the range a bit so I'd stick with the 100K. The Attenuation potmeter however must be a 100K one because it determins the gain factor of the opamp. If you put in a 1M for instance it won't only attenuate but also amplify which you absolutely don't want because the CV voltage will get way too high and it'll sound really bad connected to a VCO, if you get sound at all. There's a 470 Ohm resistor in series with the attenuation potmeter to make sure it doesn't go all the way down to zero and you'll always get a little bit of a signal.  (I suppose you could get away with using an other value for the Range potmeter as long as the resistor between pins 2 and 7 is the same value as the potmeter. Then the balance between the two stays the same.)
There's an Offset control included in the circuit so you can transpose the whole CV output up or down by as much as you like. This is handy if some of the notes produced are below the 0V line. In the normal case you would just hear a C0 note for those but if you give the CV an overall higher offset then all those notes will be audible.
I put a 470pF capacitor over the output opamp because with testing I noticed a lot of little voltage spikes on the signal when I viewed it on the oscilloscope. The capacitor turned out to be a very good solution to suppress those little voltage spikes.
The Sample Rate potmeter needs to be a 1M Ohm linear type. I didn't have one so I used a 500K potmeter and that also works perfectly fine. Don't use any value lower than that though otherwise your frequency range will be very limited.
If you build this project and you find you can hear the pulse train in other modules, coming in over the power rails, then try putting a big electrolytic capacitor over the powerrails of this S&H module. Something like a 470µF or 680µF over the plus and the ground should do the trick. That should be enough but if the problem persists then also put one over the negative rail.
This circuit works equally well on +/-12V as on +/-15V.

Here are the layouts I made for this project. I used these layouts for my own build so they are verified as always. Make sure you copy them accurately and it'll work first time.
This is the wiring diagram:

The Offset and Output Range options are of my own design. The Output Range is particularly useful. It determins the range between the lowest and the highest possible notes you will hear. You can set it so all the random CV Voltages (or notes) fall in the same octave upto a range of over 8 octaves.

Here is the stripboard only view:

Here's an overview of the wirebridges and the cuts that need to be made in the stripboard as seen from the COMPONENT SIDE!
As always, mark the cuts on the component side and then stick a pin through the marked holes and mark them again on the copper side and then cut with a sharp, hand held, 6 or 7mm dril bit.

Here's the Bill of Materials:

Here are some screenshots from the scope:

In the shot below you can see the little voltage spikes I was getting when I first tested this circuit. They are the thin overshoots on the rising and falling edges and they also appeared in between in some cases.

Here's the result after I put in the 470pF capacitor over the output opamp. Nice clean CV output:

With all these images I used a sawtooth wave as 'Signal to be sampled', not noise. You can still get sort of random notes even without using noise on the input if the sample rate differs enough from the frequency of the wave you're using. However the paterns will be repeating, they won't be totally random, which can be good for creating melody or bass lines.
Here's a sawtooth wave being sampled at a very high rate:

In the picture below you can see that the original wave being sampled was a sawtooth wave. Fast rise and slow decline. The CV voltages all lie in one octave (0 to 1 Volt) because the range potmeter was set almost fully counterclockwise. This picture was also taken before I put in the 470pF capacitor so it shows the voltage spikes too.

Here are some pictures I took while I was building this project:
The cuts and wirebridges ready:

Below you can see the components mounted except for the IC's. Note the vintage Polystyrene 1nF timing cap I used at the top right. It has a red stripe on it. That doesn't mean it's polarized. The stripe indicates which of the legs is connected to the outer layer of aluminium in the capacitor. That leg should always go to ground (or the lowest voltage potential) That way it acts as shielding to prevent humm. Now, if you have a cap that isn't marked but you want to find out which leg is connected to the outer layer and you have an oscilloscope then connect the probe to the capacitor; ground to one leg, probe tip to the other leg. Set the scope so it's quite sensitive and touch the capacitor with your fingers. If you get a pronounced waveform on the scope then your probe tip is connected to the outer layer and the ground clip is connected to the inner layer. If you reverse them and touch the cap you should get little to no deflection. Now you know which leg should go to ground.

The finished print. I used the old Sample and Hold panel and soldered the new print in. This particular panel I made is a bit of a weird shape because it actually sits in a little wooden plank above the other modules. You could say it's sort of a 1U module but for the Kosmo size :)

I made a short video demo of the sample and hold in action. The S&H is connected to a Thomas Henry VCO. The input is white noise from the 5 sorts of noise module. The audio goes through the Steiner Parker filter and boy does it sound good!!

I was thinking that you could also use this Sample and Hold as an external clock source if you need a clock signal to synchronize different modules with. It would be easy to add this feature to this circuit. I placed the schematics below and in red I added what you would need to add a clock signal output. It just requires one opamp buffer and a 1K resistor and an extra output socket.

There would just be enough room for this at the bottom right of the stripboard, where the mounting bracket is now positioned but I'll leave this op to you. It was just a thought and if you have a squarewave LFO in your modular already then that can serve as a clock signal too of course so then you wouldn't need to add this feature anyway.

Finally I want to leave you with an excellent video by the 'Monotrail Tech Talk' YouTube channel which explains all the different things you can do with a Sample and Hold and discusses some awesome patches. Subscribe to him while you're there. It's an excellent channel for anyone into modular synthesis.   

That's it for another one. If you have any questions please put them in the comments below of post them on the special Facebook Group for this website.

If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards the website and projects. Thank you!

Friday, 10 February 2023

Synthesizer Build part-50: UTILITY LFO for EURORACK

This is the Ken Stone Utility LFO, the bigger version of project 47 with an extra feature that I added. I managed to make the stripboard even smaller than the 'Simple Dual LFO' so it fits even the 'Nifty Case' Eurorack skiff. The LFO has the following waveforms: Pulse, Square, Saw to Triangle to Rampwave and a Variable output which has a mix between Square and Triangle/Saw/Ramp waves. I later added a mini-mixer that adds the two variable outputs together to get even weirder waveforms.

An other LFO might not be the most exciting of projects but I think this LFO will be worth it because of the weird waveforms it can make and because I realized, now that I have my own Eurorack system, that modulation sources are important to have. To quote a popular YouTuber: Modulation is what makes Eurorack interesting. I even added a feature of my own later on. It's the ability to mix both variable outputs together to get a variable A+B output. That's at the bottom of this article.
The Dual LFO from project 47 is just the first two stages of the Utility LFO without the mixing stage. Because I actually use the Dual mixer in my Eurorack setup and I loved the idea of having the full Utility LFO available, I thought I'd build the whole LFO this time and try and make it as small as possible. It is 49mm deep and the panel is 9HP wide (4.5 centimeter). This is a bi-polar LFO meaning the waveforms go both positive and negative in voltage.
This LFO does not have a sync option but this is a really useful LFO for a Eurorack system because it has not only the normal waveforms you'd expect but the option to merge waveforms together which makes for some very weird modulation possibilities. And because this is a Dual LFO you could mix the 2 variable waveform outputs together in a Multiple and make even weirder shaped waveforms. That mixing needs to be done outside of this module though. I couldn't fit that inside this design. This LFO would pair really well with the Dual Voltage Processor for that reason.
I managed to get everything on an 18 by 24 hole wide piece of stripboard. Six potmeters and eight output sockets on the panel, and an extra little print for the two bi-colour rate indicator LEDs which I made separate from the main print just like on the Dual LFO and as mentioned before a second little print to mix the two variable outputs together. The panel I made for it is 9 HP wide (4.5 centimeter). You might be able to make it even smaller if you use smaller potmeters in the panel. Even though I included an L-Bracket in the layout, I didn't use one. The stripboard is actually hot-glued in place in between the two columns of potmeters straight to the back of the panel making sure no copper strips make contact with the metal of the panel. I did this to keep the overall depth of the module as low as possible.
I did not include a Eurorack power connector on the print although there is room enough left to put one in. Instead I made a powercord with a Eurorack connector at the end so it always stays connected to the stripboard. This is handier because eurorack ribbon cables take up space too and this is a smaller footprint solution with only three thin wires.
This is mainly a Eurorack project but naturally this circuit will work just as well in a Kosmo sized synthesizer. I optimized the circuit to run on +/-12V but it was originally intended to be run on +/-15V. You will get higher amplitude waveforms when you run this on a dual 15V powersupply so if that's a problem for you, you can change the 1K8 resistors I used on the waveform outputs back to the 1K's you see in the schematic. The original schematic uses +/-15V.

Below is the schematic I used for my layout. I put in bigger timing capacitors because I wanted one LFO running really slow and the other to about 20Hz. That is more useful for my modulation needs but you might want different values so I strongly advise you to do some testing with different capacitor values to get the LFO in the frequency range you desire.
The types of quad opamps and the one dual opamp used in this project are not critical. The schematic calls for TL074 and TL072 opamps but you can use anything with the same pinout. I used two LM324's and an NE5532 dual opamp. Btw, I did not include the transistor with the rate LED as seen on the schematic but I designed my own circuit for that so I could use bi-coloured LED's. It's just two opamp voltage followers (or buffers) feeding the LED's with the signal from the Pulse output. You can connect them to any output you wish but the pulse is the clearest for the LED's.
Here's the schematic drawing. Beware the opamp numbering does not follow the opamp order I used in the layout.

Below are the layouts I made for this project. As ever they are verified. I used them for my build. I changed the order of the opamps used from the order on the schematic. I setup one LFO using all the opamps on the left hand side and the other LFO using all the opamps on the righthand side of the IC's on the stripboard. That made it more compact plus easier to keep the overview. Luckily I did not need to make any changes or do any troubleshooting. I built it and tested it and everything worked first go.
You need four 100K and two 500K linear panel potmeters to this project. Measure and test them before you use them. That can save you some troubleshooting later. If you plan on mounting the stripboard to the panel with hot-glue like I did then try to solder the hook-up wires of the righthand side (the side that's glued to the front panel) as far to the middle as possible so you can still get to them once the stripboard is glued in place. This won't be possible with all wires but try. You could also solder them straight to the back/copper side.
Wiring diagram:

Print only view:

Here are the cuts and wirebridges as seen from the component side! As always, I advise to mark the cuts on the component side first with a black waterproof marker pen and then put a pin through the marked holes and mark them again on the copper side. Then you can make the cuts accurately and you can see where the cuts are when you're soldering in the components.

Bill of materials. 
The timing capacitors listed were chosen for my specific needs. I advise you to go by the schematic (47nF) or else test and determine the best values for your needs. You can use the Falstad simulation linked at the bottom of the article to test different values before building.
No bypass caps are included in this BOM because I didn't use any. They are on the schematic and you can put 100nF caps over the power connections on the IC's to ground if you want to. To do all IC's you'd need six 100nF ceramic caps (you don't have to do the LED driver IC).
This BOM does not include the components for the little mixer I added on further down the article. For that you will need 4 x 100K resistors, a 1K resistor and a 200K trimpot and a TL072 dual opamp.

Here are some pictures of the build proces and the panel I made for it. I had some fill-in panels left from when I bought my 'Nifty Case' Eurorack case and I used one of them to make my front panel. It was already cut to the right height so ideal for this project ^___^  All I had to do was cut off a piece that was 4.5cm wide (9HP) and spray paint it. I labeled everything with an Edding 400 marker pen.

Wirebridges and cuts. I made some minor changes to the layout after I built this so there are a few discrepancies between this picture and the current layout.

Stripboard ready for testing side A:

Panel, not yet labelled:

The finished module with the little 10 by 6 hole stripboard for the rate indicator LED's glued to the back of one of the potmeters. If you look closely at the sockets you can see I soldered all ground connections together with one copperwire going round them all. A wire goes from there to the ground connection on the stripboard.

The finished product:

Here's a little demo video I made showing the LFO in action in a little Eurorack setup. You can hear how the 'Pico Voice' VCO changes in sound as I connect the LFO to it. But then I get distracted by the Pico DSP effects module and the 2hp Freez, LOL. Oh well. You can hear the difference it makes anyway :)

(As you may have noticed, I really suck at making demo videos, LOL)

Here are some screenshots from the oscilloscope showing some of the waveforms. The one below shows the sharpest a sawtooth wave you can get. Pretty fast rise!

Cursor readings at LFO-B squarewave output:

Here is a collection of variable output readings from Variable output B:

To give you an idea of what can be achieved by mixing the two variable outputs in a simple passive multiple, here are eight images I put together to give you an impression. This should sound awesome with very slow running LFO's which is why I soldered a few more caps in parallel over the timing cap of LFO-B.

You can imagine that some of these waveforms, when put through a quantizer, would generate awesome melody- or bass-lines that can be easily manipulated with the LFO parameters. I tried to capture a little of that in the demo video I posted below.

Here's some technical data from the LFO:
LFO A: freq. range: 270mHz to 24Hz. 
LFO B: freq. range.: 41mHz (one full cycle every 24 seconds using a 650nF cap) to 2Hz.
Duty cycle of the pulsewave output goes between 2% and 98%. I made a mistake and used a 100K potmeter for the Shape but it needs a 500K potmeter. I only had one 500K pot so I put it in LFO B and the screenschots above are from that LFO. You can use a 100K for shape but that will significantly lower the range of the duty cycle (20 to 80%) also the slope of the Saw-/Rampwaves will be slower rising. It will also speed up the overall frequency of the LFO.
Maximum current draw = 20mA for both the positive and negative side.
The maximum amplitude of the waveforms is about +/-5V except for the Variable waveform output. When that potmeter is set to the mid point the output amplitude drops to about +/-2.5V because the potmeter acts as a voltage divider so in the mid position the amplitude is half of what it is when it is fully clockwise or counter clockwise. (We effectively have two 50K resistors on either side of the wiper of the 100K potmeter.) These may seem like low voltages but keep in mind that an LFO is usually attenuated anyway because if the changes are too big it just doesn't sound good on a VCO or in most patches. If you really need higher voltages you can make the 1K8 resistors on the outputs even bigger.

Here's the link to the Falstad simulation of this circuit. This is the Dual version with one speed potmeter at 100K and the other at 500K as it is in my LFO. You can change these variables by right clicking on them and choosing 'Edit':

Okay as I write this it is four days since I posted this article and the cool looking results from the mixed variable outputs kept going through my mind. I really wanted to incorporate that into this design and now I found a way. I used an other small piece of stripboard on which I soldered a dual opamp and wired it up like the mixer in article 17 with 2 inputs. I knew there was no way to put an extra output socket on my panel so I sacrificed the squarewave output of LFO-B. That was the least useful of the outputs because I can get a squarewave anyway from the pulse output if I need one and I still have the squarewave of LFO-A. So I carefully soldered in the little mixer print which was just as big as the little LED driver and wired it all up and I re-labelled Square output B into 'Vari A+B'. I used a 200K trimmer to go over the inverting input of the second opamp and the output so I could adjust the gain and make it a little higher than the Variable outputs. I placed the little print above the sockets and used hot-glue to stick it in place. It is actually glued to the little LED driver board. This works perfectly! Now I have an output with two variable waveforms mixed together coming out of it.
Here is a picture showing how I added the mixer. I used two rigid copper wires to tap the signals from the sockets and lead them into the mixer. The non-inverting inputs of the opamps are connected to ground on the copper side:

Here's the layout of this little mixer. Mine is even smaller than this layout, but this will work fine. All resistors are 100K:

Here's the schematic for this little mixer:

Finally I want to show you a little experiment I did using the new output. I put the Variable A+B signal through a quantizer (the 2hp Tune) and from there into the Pico Voice Wavetable Oscillator. The audio then went through the Pico DSP for some added reverb. You can really generate the weirdest melodies with this although this setup would benefit from the Voltage Processor because the negative phase of the signal does not produce any notes so it needs a positive offset voltage and the signal can do with some attenuation to get the notes closer together but I think you get the idea watching this short demo:

Okay, that's it for now. Article 50, wow I can hardly believe it. This journey started for me in October 2019 and I knew nothing about synthesizers then, but I was determined to get to grips with it and learn as much as possible. And what better way to learn then to build your own modular. So here we are more then 3 years and 50 projects later with a cool collection of builds helping hundreds of people to do the same. I'm really proud of what, not only I created but also of all the people who helped so much along the way with comments and directions. I'm not gonna name names but you know who you are, all of you. Thank you!! So many people told me they find the site a great help in their hobby and that's the biggest reward I could wish for. I am however going to wind down the DIY aspect of the hobby because having now built my own system and also haven gotten into Eurorack, I need to spend more time actually using it and figuring out how to use it all together. But I will remain available on Facebook and here to answer questions and help as much as I can to ensure you have a good experience using this website and get as much enjoyment out of it as I did.

If you have any questions please put them in the comments below or post them on the special Facebook Group for this website where there are some awesome people willing to answer your questions.

If you find these projects helpful and would like to support the website and its upkeep then you can buy me a Coffee. There's a button for that underneath the menu if you're on a PC or Mac. Or you can use this PayPal.Me link to donate directly. All donations go towards the website and projects. Thank you!

Tuesday, 12 April 2022

Synthesizer Build part-49: 8 STEP SEQUENCER version 2.0

A redesigned version of the 8 step sequencer (project number 8) with external clock input option and offset control that allows you to transpose the sequence upto 3.5 octaves up or down and external speed control.

This article has had a little update on April 24th 2023 concerning the 'Tune/Run' switch and about adding a momentary switch. Both these options have now been optimized and work very well. On May 22nd 2023 I also added an External Speed control option.

-- Please read the entire article before you start building!! --
The sequencer project I posted when I was just starting out on this modular journey has rapidly become a very popular project on my website. I was however never really happy with that design. It was a bit clumsy and it had no extra features and although it worked like it should, it was never as good as I wanted it to be. It always nagged me.
So because it is such a popular project now, I wanted to offer people something better to build. Something that I was sure wouldn't disappoint anyone building it. Now, I'm not saying it's perfect but it sure is an improvement over the previous one and just as easy to build. So here it is; the 8 Step Sequencer version 2.0 

A NOTE FOR BEGINNERS: A sequencer does not actually produce any sound itself. It produces a stepped control voltage that can be patched into the CV input of a Voltage Controlled Oscillator and the VCO then produces the actual notes you hear. In a sequencer you can set each of these (8) steps manually to any voltage (and therefore any note) you want within 1 to 7 octaves.

This sequencer has a few advantages over the first one. 
Number one, it can be clocked by an external source. In order to make sure this external clocking would work as well as possible I decided to use some of the hex inverters with Schmitt-Trigger inputs in the CD40106 chip (that we use to provide the internal clock signal) as buffers. A Schmitt-Trigger input has hysteresis. This means there's a voltage difference between when the hex inverter flips from on to off. For instance it might jump to off when the input goes over 6 Volt and turn on when the input gets below 4 Volt (remember it's an inverter). That's a 2 Volt hysteresis and this means it will be less susceptible to noise on the external clock signal. This turned out to work just like I hoped it did. I used two of these inverters switched in series so the (external) clock pulse itself is not inverted. I did a test and installed a bypass switch to feed the signal past the two hex inverters and immediately the external clocking stopped working for some signals, proving it's a good solution.
There is one little downside. The external clockpulse needs to be quite high in voltage. A 5 Volt pulse won't be enough. It should preferably be 6V or higher. I tried clocking it with the Behringer RD-8 but it wouldn't work. But when the pulse is high enough in voltage it won't matter what wave shape you use, it'll work.  The Yusynth LFO with sync and FM input was able to trigger it if I used the triangle wave as external trigger. 
NB: Some options to solve this problem: You can lead the trigger pulse through the Voltage Processor first and give it a little positive offset voltage. That way it will breach the 6V minimum threshold. Or an other method to solve the clockpulse issue goes as follows: you can use an NPN transistor (like a 2N3904 or BC547) and have the external pulse come in to the Base of the transistor via a 10K resistor and connect the Collector to +8V and have a 100K resistor from the Emitter to ground. Now tap the clock pulse off above the resistor at the emitter of the transistor. This will make sure any signal coming in will produce a useable clock pulse. There's room on the lower left side of the stripboard to put this in. I'd advise to use a toggle switch to go between internal and external triggering if you implement this change so that only an external pulse goes through the transistor. Please note I have not tested this so I'm not giving any guarantees it'll work perfectly.
Number two, this sequencer has an Offset feature. That means you can transpose the whole CV output chain up or down by as much as 3.5 octaves without compromising the Volt per Octave tracking. With the previous sequencer we were limited in the lowest notes by the voltage drop of the diodes on the potmeters. So it was never possible to get to note C0 which is 0 Volt. But now we can trim the offset down and that will drag the whole sequence down in voltage making it possible to get to the lowest possible notes without screwing up the volt/oct. tracking and without need for special Schottky Diodes. I did use Schottky diodes (which have a voltage drop of only 0.2V) but only because I re-used the old sequencer for this build and the diodes were already soldered to the potmeters.
The offset feature is a game changer for this sequencer in my opinion because we can make really cool basslines and transpose it up or down without any problems. Naturally the output is not quantized so you won't land on true notes every time when tuning or transposing the sequencer. It's always a good idea to have a quantizer in your rig but they are too complicated for me to build as a DIY project. However, you can buy a eurorack quantizer like the Doepfer A-156 QNT (which I have in my Eurorack system) for €119,- (and that's a dual quantizer with extra options!)
The offset feature can also be very useful if you use the sequencer as a modulation source for instance to modulate the filter. You can drag the sequence down so some steps have a negative voltage and others a positive voltage and so create weird VCF responses. 
You must be careful with the offset because it will be able to push the total output CV voltage to over +10V. Usually that's not a problem for a VCO and of course it can never be pushed higher than the power-supply voltage, but just be warned. 
Also keep in mind that the CV voltage (the sequence) goes into the inverting input of the offset opamp and the offset voltage goes into the non inverting input. Then in the second opamp the CV voltage is inverted back to normal again while the offset voltage now gets inverted. That's why the wiring of the offset potmeter is the other way around from normal.
Number three, I recently added an external speed control. So you can influence the speed of the sequencer with an LFO or ADSR or any other CV source while still being able to set the overall speed with the speed control on the panel. I did this by simply putting a Vactrol over the Speed potmeter. It's really easy to do. Just add an extra input socket and make a Vactrol and connect the LED of the Vactrol to the socket using a 470 Ohm current limiting resistor to protect the LED in the Vactrol. Make sure you connect the plus to the audio lug of the socket and not the ground one. I did not add an attenuator for this function because I ran out of space on the face plate but you could of course do that if you wanted to. Vactrols consist of an LED and a Light Dependent Resistor (LDR) wrapped together in some Heat-Shrink Tubing. They are really easy to make. There are enough videos on YouTube that show you how to make them. You can of course also buy them new. Now Vactrols are not very accurate in how they work so you can't set the speed exactly to a certain value with a certain voltage. This feature is meant to influence the sequencer in a musical way, like speed it up every x number of beats, to create some variation.
Number four, this sequencer has a "End of Cycle trigger output". This means the reset pulse that goes from the wiper of the rotary step switch to pin 15 of the CD4017 is available as external trigger. You could, for example, lead that into a filter to add an accent at the end of each sequence. You could also use it to trigger an envelope generator and have that signal go to other destinations. Or, if you build two of these, you can make it so that when the first sequencer has gone through one sequence, it will trigger the second one to go to the next step. Alas it can not trigger the sequencer to go through an entire sequence at the start of a trigger pulse. That would make this build much more complicated. I myself did not include the end of cycle trigger output in my build because I don't need it. It's up to you whether you want to include it or not. The original pulse is an extremely fast 6 Volt spike, about 280 nanosecondes long, so I decided to amplify the pulse a little by leading it through two left over hex inverters of the CD40106 chip. 

SPEED CONTROL and the TUNE/RUN switch.
The sequencer has a speed control that will go from 1 second to about 100Hz. If you want that speed to be lower you can add a 4,7µF cap in parallel over the 10µF cap to bring that speed down to about 2 seconds which is slow enough and it will still be able to go very fast too. 
It also has the tune/run feature of the previous sequencer to make tuning each step easier. You just set the speed very low and then stop the sequencer at each step so you can tune it. Then you run it to the next step and stop it again to tune that one. At least that's how it worked on the old sequencer. With this new version you can use the push switch to manually go through the steps to tune them.
I had some problems with stopping the sequencer in the beginning. The sequencer would continue running even after switching it to Tune but the solution was simple. When switched to Tune the clock input must be grounded. I used a 1K resistor for that, connected to the switch and to ground. This is why it is important to connect the wire that is in direct contact with pin 14 of the CD4017 to the middle pin of the Run/Tune switch. Otherwise this doesn't work. 

A word about tuning: it's not the easiest thing to tune this sequencer to an exact note due to the fact we have 7 Volt to deal with and a 100K potmeter to set it to an exact note. In the 1V/Octave standard the difference between two notes is only 0,1666 Volt. And between two half notes only 83,3 thousandth of a Volt. That sort of resolution is difficult to achieve with a normal potmeter. However it is not impossible. It takes little touches of the potmeter but, as you can see in the demo videos below, it can be done. This problem goes for all Baby 8 sequencers, not just this one and I use mine a lot and I've always managed to get it sounding really good. Of course you could buy a cheap eurorack quantizer which would help a lot with accurate tuning but that's up to you. A really helpful tool to have is the Joyo Tuner, hacked for use with modular synthesizers. I always use one of those to tune my sequencer.

I had a lot of problems in the beginning, implementing a momentary switch into this design. With that switch you can advance one step each time you push it to make tuning easier. The problem was that I forgot to ground the clock input of the CD4017 when the sequencer is in Tune mode. I made some changes and I connected a 1K resistor to the 'Run/Tune' switch that grounds the clock input. After I had done that I thought I'd try to get the momentary switch working. I connected a momentary switch between +8V and the pin of the 'Run/Tune' switch that has the 1K resistor connected to it. But when I switched to Tune mode it still kept jumping several steps when I pushed the switch. I figured this must be due to contact bounce in the switch. To counter that I soldered a 100nF capacitor in parallel over the momentary switch and that did the trick. Now it works like it should, at least most of the time. It's not perfect. Occasionally it will still skip a step randomly but that happens very rarely. Overall it works really well and it's perfectly useable. 

Here is the schematic for this sequencer. I made it using Photoshop. 
I have only drawn one of the output steps coming out of the CD4017, with the LED, the potmeter and the diode connecting to the CV Rails. You need 8 of those to complete the 8 steps of the sequencer of course.

In the schematic I use normal 1N4148 diodes but in the layout they are named as 1N5819 Schottky diodes. Go with the 1N4148 diodes. I only have Schottky's in the layout because that's what was already in there from the previous version of this sequencer. Schottky diodes have a lower voltage drop than normal silicone diodes but now that we have the Offset feature the voltage drop is no longer an issue.
It is totally unnecessary to include transistors in the output steps to feed the LEDs. I just put 10K resistors in series with the LEDs to keep their current draw to a minimum and because opamps don't draw any current at all the LEDs do not pull down the voltage when they are on. The LEDs are fed with 8V from the outputs of the CD4017 and with the 10K resistors they are still bright enough. It is however important that you do NOT use any LEDs that draw a lot of current like blue LEDs or bright white LEDs. Those may draw the voltage of the CV output down. Not only that, in the long run they can heat up the CD4017 so much that it fails. The outputs from the chip can't deliver more than 6.8mA maximum with a 15V power supply. (see datasheet)
The Gate output produces gate signals that are exactly half the length of a single CV step. In other words they have a duty cycle of 50%.

Below is the layout I made for this project. If you built the first version of this sequencer earlier, then it's really easy to replace just the stripboard with this new one. You only have to make two extra holes in the face plate for the external clock input socket and for the External Speed CV input (or three if you want to include the 'end of cycle' trigger output) and all you have to do is reconnect the old wiring to the new stripboard,
In this layout I connected only the first 3 steps to show you how it should be done. You can easily repeat these connections for the other 5 steps. I did this to prevent the layout becoming a mess of hook-up wires. Follow the numbering in the text at the bottom right to connect the potmeters and rotary switch pins to the correct pins of the CD4017. Of course you could include the left over two pins from the chip to make it a 10 step sequencer but 10 steps don't sound right. Normally you got 4 steps to a beat so 8 steps sounds better. If you do want to make it a 10 step sequencer you will need a rotary switch with 11 positions! Otherwise, if you use a 10 step switch, the CD4017 will be reset at step 9 instead of 10. In case of a 10 step sequencer you don't need a reset pulse because the CD4017 only goes to 10 steps so it will reset itself.
This sequencer is equipped with an ON/OFF switch so you can easily introduce it into a jam session by flicking the switch. That way you can leave it patched up and have it instantly available when you need it. The ON/OFF switch for this module is located behind the voltage regulator and that is done on purpose. If you put it before the regulator the sequencer would shut down very slowly because the 470µF capacitor discharges very slowly. So you would hear the sequence getting slower and slower and the notes going out of tune and fading out if you switched off. With the switch behind the voltage regulator the switch-off (or on) is instantly done. The regulator itself doesn't use any current when not in use and this has the added bonus that the sequencer starts up immediately too if you switch it on.
If you want to include 'Mute' switches for each step then I indicated on the layout the best place to put them (after the diode of each step). If you mute a step there will still be a gate signal for that step present on the Gate output.  
All potmeters are viewed from the front!

Wiring diagram:
If there any any features you don't want to include, like the End of Cycle pulse output or the External Speed control for example, just leave them out. I added as many features as I could think of and I leave it up to you to choose the ones you want to include. I show only 3 of the 8 steps in this layout to avoid it becoming too cluttered with hook-up wires. It's already a mess of wires. Just repeat the LED, Diode and Potmeter combinations five more times. The list at the bottom right tells you where to connect each step.

The mysterious grey component at the bottom middle is an L-Bracket to mount the stripboard to the faceplate. But you can use any method you want to connect the stripboard to the faceplate.
The explanation of the colour coding of the wirebridges on the layout only goes for the wirebridges on the stripboard, not the hook-up wires connecting the pots and sockets to the stripboard.

Stripboard only view:
Below is a close-up of the stripboard. The voltage regulator and the big 470µF electrolytic capacitor plus the other two 10µF caps are quite crammed in together on the top left of the board but it shouldn't be a problem. The voltage regulator doesn't even get warm in normal use. You could also use the smaller 78L08 types that look like a TO-92 package transistor. 
You do need the big 470µF cap in there otherwise the pulsetrain could be audible on the voltage rail. (The light blue edges of the caps indicate the negative pole.)

Cuts and Wirebridges:
Below are the cuts and wirebridges seen from the component side. As always, mark the cuts on the component side first using this layout and then stick a pin through the marked holes and mark them again on the copper side and cut the strips by hand with a sharp 6 or 7mm drill bit. That way you have the least chance of making mistakes. Check the cuts afterwards with the continuïty mode of your digital multimeter or a strong magnifying glass.

Bill of materials.  This is a new version including a Vactrol for the external speed option.

Here are some pictures of the stripboard during the building proces. My stripboard had one little error in it which was spotted by one of our awesome Facebook members and that was that the copper strip connecting pin 8 of the CD4017 to ground had a cut in it. I built it with that error included and strangely enough it worked normally but I have now corrected it. I made an other error in the top picture, the connection to +12V for the TL072 chip has been forgotten. You can see it's added in the picture below it:

Here's a picture of the 'End of Cycle' trigger pulse that resets the CD4017. You can see how enormously fast this pulse rises and falls, within 280 nanoSeconds. (Ignore the yellow line above it.) 
In itself it's not too strong a signal so the signal is routed through two of the left over Schmitt Trigger inverters of the CD40106. This makes sure we'll get a strong 8V end of cycle pulse output. 

Here's an example of a CV output sequence with potmeters set to various notes.  As you can see the maximum output voltage is around 7 Volt (or 7 octaves) when the potmeter(s) are fully set clockwise and goes all the way to 0V with the potmeter(s) fully closed and a little negative offset applied. You can easily set or adjust this just by ear. 

And this is a scope image of the clock pulse generated by the CD40106 as it enters the CD4017:

Finally, here's a little demo video showing the sequencer in action connected to a Thomas Henry VCO-555 and the Steiner Parker filter.

If you can't see the video above, here's the link: 

And here's a video of me trying to replicate the intro from Vangelis' Spiral. I always loved that sequence. The sequencer is again connected to a Thomas Henry 555-VCO and the Steiner Parker filter. Plus a little echo from the FX Unit:

To get this sound connect the sequencer CV out to the Exp. FM Input of the Thomas Henry VCO-555 (attenuator fully clockwise). Take the Sawtooth output and patch it to the input of the Steiner-Parker filter set to Lowpass. From there into the VCA and add some echo with preset 59 of the CaraOK FX Unit. Just add a little of the echo. Play with the VCF Cut-off to get the variation in the sound.
To program that pattern, tune your steps as follows:  F2, C2, F3, C3, F4, C4, F5, C5

And finally a test with Vangelis' Dervish-D intro. Make sure you use a fast envelope on the VCA because the notes need to sound punchy.

The note progression for this sequence is: C#3, C#3, C#4, G#3, B3, G#3, F#3, E3

I was watching one of Sam Battle's (LMNC) videos and he talked about using a Baby 8 sequencer as a waveform generator. If you clock this sequencer at audio rates you will in fact get an other oscillator with a waveform that is cut into 8 steps (or less) and adjusting each step will change that part of the waveform and with it the overall sound of the oscillator/sequencer. The sequencer is clocked by a VCO so the clock frequency will change with the note you play so it will track over the octaves and the CV out from the sequencer can go straight into a filter.
It's a really fun thing to experiment with. I'll link to the video in question below here, where Sam explains how this works. The video should start at 9:49:

TIP: Finally here's a little tip for all you who want to take sequencing a little further. Buy a Korg SQ-1 sequencer. It costs less than €100,- and will give you 16 steps or 2x8 steps and a miriad of options. This is of course not a module but a stand alone sequencer but it is very small and runs on 2 AA batteries. The case is made of metal and the thing weighs over a kilo! It outputs quantized CV voltages and you can set the voltage range to upto 8V. A lot of performers use an SQ-1 in their live sets. It also works very well with the modular synthesizer I've built from all these projects.
If you want to know more about this easy to use sequencer then just Google "Korg SQ1" or even better do a search on YouTube and you will find a sea of videos on this machine that will tell you all you want to know. 

Okay that's it for the Sequencer V2.0. As always, if you have any questions or remarks about this or other projects please comment below or post on the Facebook Group for this website.

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