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.

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. 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 sequence of control voltages that can be routed into the CV input of a Voltage Controlled Oscillator and the VCO then produces the actual notes you hear.

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.
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 around 8V. 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. An other thing I noticed is that in my particular case the sound got duller/less crisp in high tones when using external clocking. I have no idea what causes that and if it's only the case in my synthesizer (probably) but for honesty's sake I felt I had to mention it.
NB: There's enough space left on the stripboard to put in an other dual opamp and make a little amplifier maybe with a Gain potmeter so you can set the external clockpulse to any height in voltage. I might try that in the near future.
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 but just be warned. It might be a good idea to change the 120K resistor that goes from the negative 12V line to the offset potmeter to a 150K resistor. That would restrict the upwards transpositioning to +2 Octaves and give you -4 Octaves down. That way you won't go too high. I actually made that change with my own sequencer recently. You must remember 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 also why the wiring of the offset potmeter is the other way around from normal.
Number three, this sequencer has a "End of Cycle trigger output". This means the reset pulse that goes to the wiper of the rotary step switch is available as external trigger, so 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 pulse is an extremely fast 6 Volt spike, about 280 nanosecondes long. You could also use it to trigger an envelope generator and have that signal go to other destinations but you might need to amplify the pulse for that because in itself it's not really a strong pulse. That's up to you.

The sequencer has a speed control that will go from 1 second to about 100Hz. I would recommend to 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 stop/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. 

I did an experiment with a momentary push switch to advance the steps one place with each push of the switch and although it worked it was problematic. You need a good quality switch for this and you also need to use a double pole toggle switch for the 'Stop/Run' switch and route the momentary switch through it in such a way that the push switch will only work when the toggle switch is set to Stop. Otherwise you can put 8 volt on the clock input or internal oscillator and that's not good. I got it working but it was very problematic and didn't work well at all. You would need to use the two left-over hex-inverters to filter the signal of the push switch. Believe me the method with stopping the sequence with the 'Stop/Run' switch works much better. Don't bother with momentary switches.

Here is the schematic for this sequencer. I drew it up in Photoshop and the proportions are a bit 'how yer doin' but okay. It's readable LOL. 

In the schematic I use normal 1N4148 diodes but in the layout they are 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.
I did not go for the option of including 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.
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%.

Here's the layout I made for this project. If you built the first version before, then it's really easy to replace just the stripboard with this new one. You only have to make one extra hole in the front panel for the external clock input socket (or two 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 circuitboard :)
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.
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 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!

(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.)

Print only view. The voltage regulator and the big 470µF electrolytic capacitor plus the other two 10µF caps are quite crammed in together on 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.

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,

Bill of materials. 

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.:

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 if you really need to make use of it it might be a good idea to include a little opamp amplifier to get a stronger pulse.

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. 

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:  https://youtu.be/lcsw4txNoj0

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 it 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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Thanks for your support!!

Friday, 4 February 2022


 The simplest modules you can build but they will be the ones you use most often because each modular synth needs at least a few of these.

This will probably be the shortes article on this website and the simplest of projects but I just thought I'd mention these multiples here because recently I built a few for my Eurorack case and some newcomers to this hobby might not be aware of them. They are so useful to have in your setup.

Multiples are passive devices because they do not require any power source. They just split up signals so you can have 'multiple' sources of the same signal (hence the name). You put the output of an audio or CV source into one of the sockets and then you can tap that signal from the other sockets to turn one output into multiple outputs.
There are two versions of these that you can build.
The straight forward passive multiple that consists of just a number of 3,5mm female sockets connected together. Usually it's between 8 to 10 of these underneath eachother. You can add a switch in the middle so you can split it into two rows of half the number of sockets. That way you can use if for more than one signal source.
Then there's the switched multiple. That's the same idea only now each socket has a 2-way switch next to it, with a middle off position. Each switch has their left pins connected together underneath eachother and the same for the right pins and the middle pin of each switch goes to a female jack socket. (see schematic image below)
This will allow you to put one of two signals on each of the outputs or switch that output off. This can be very handy in live settings where you want to switch between two sequencers for instance. So you need toggle switches with a middle off setting for these. At least, that's the best way of doing it. ON-OFF-ON switches. You can use ON-ON switches but then you can't switch anything off.
There are also buffered multiples like the one in project number 25 on this website but they require opamps so they are not passive in any way.

Here's a schematic overview of the multiples. The length of these is dependent on the space you have available on your panel. For Eurorack it's usually 10 in a vertical line for a passive mult or 8 if there's a switch in the middle. And for a switched mult it's usually also 10. For Eurorack that is. For a Kosmo sized panel there's room for much more sockets.

Here is a picture of the mults I made and built into my Eurorack case. This was rather hastily done on a sunday afternoon so it looks a bit rubbish (so what else is new) but that doesn't matter to me as long as it works. My switched mult (on top) uses dual pole switches because that's all I had so there's some redundancy built in because single pole switches are really all you need.

The bottom one has a switch in the middle so I can use it as one 8 socket multiple or two 4 socket multiples.

If you want to build Buffered Multiples with opamp buffers then you can go to this article here.
Buffered Multiples are often used to connect more then one VCO to a 1V/Octave signal, because with those signals it is important that the voltage on the input is reproduced accurately on the outputs, in other words it's not dragged down however many VCO's you connect to the multiple. Passive Mults with many things connected to them can draw the voltage down a bit wich would de-tune a VCO.

Okay that's all. Just a simple little item but one you will have to build at least once. The good thing about them is they need no circuitboards and you can use off-cuts of panel material to build a few of them.
Okay, until the next one!

Saturday, 15 January 2022

Synthesizer Build part-47: DUAL LFO for EURORACK.

 A simple LFO with pulsewave (with variable pulse width) and a seamless transition between a Ramp wave, Triangle wave and Sawtooth wave using one potmeter. With LED rate indicators and Speed and Shape controls.

Well what more is there to say about this LFO. It's such a simple design that I could easily fit two of these on a small piece of stripboard and still have it small enough to fit a normal Eurorack case. The circuit is derived from the 'Utility LFO' by Ken Stone which is a larger version of this LFO. The depth of this module is 55mm. I made the panel 4CM wide, that's 8hp, and I put the potmeters to one side leaving enough room to glue the print straight to the back of the panel at a 90° angle using hot glue. All the output sockets fitted nicely next to eachother at the bottom.
Naturally you can just as easy build this module in the Kosmo size and run it on 15V. If you do, you need to keep to the resistor values as they are in the schematic, not the layout because as I mention further down, I changed the 1K output resistors to 1K8 to get a nice +/-5V output signal. If you power this with 3 more volts you probably don't have to do that. Do some testing first to make sure though.
Some people use this design, because it is so compact, as an on-board LFO in other projects. For instance if you build a VCO you could include this in the same module so you have an on-board source of modulation. 

I tried my hand at using Falstad recently and tried to make a simulation of the complete Utility LFO circuit and it was surprisingly easy to do. 
So here is my very first ever Falstad simulation: --- CLICK HERE ---

Here's the schematic drawing of the dual LFO circuit:

The module consists of two of these circuits on a single piece of stripboard. I placed the LEDs on a separate piece of stripboard with a dual opamp, the good old TL072, and I used bi-coloured 3mm LEDs in red and blue. I drilled two 3mm holes to the left and in the middle of the first two- and last two potmeters for the LEDs and glued them in place with hot glue so the little print sits over the potmeters. See pictures below for illustration. Btw, you can use any dual opamp chip for this circuit as long as the pinout is the same; like the TL082, NE5532, LM358 etc.

Here is the layout I made for this Dual LFO. As always, the layout is verified. I used it for my build. I placed the Eurorack powerconnector on the left side for better access. In my build it's on the other side and very near the panel. Not a good place for a power connector but you only find these things out when you start building it. See, I make the mistakes so you don't have to LOL! (I hot-glued the print to the back of the panel with the righthand side closest to the panel.)


After doing the first tests I found the output voltages a bit on the low side. They were just +/-3,24V so I decided to experiment with the 1K resistors between the outputs and ground. I tried several values and I ended up using 1K8 resistors. That brought the output voltages to a nice +/-4,8V. Almost 5V so that's perfect for eurorack. If you want that voltage to be even higher in your LFO then experiment further with making the resistor in parallel over the output socket even higher in value.
I wanted to make one of the LFO's a bit slower than the other to give me a wider overall range so I used a larger capacitor for LFO number one. I used a 147nF and that made it perfect for my needs, between 0,2Hz and 10Hz. In the layout both timing caps are 47nF though.
Here are some measurement results for this Dual LFO:
Duty cycle of squarewave is 5% to 95% this varies a bit with the frequency but not more then 2%.
Lowest frequency: LFO-1 = 0,219Hz  LFO-2 = 0,653Hz (changed timing cap of LFO-1 from 47nF to 147nF)
Highest frequency: LFO-1 = 9,82Hz   LFO-2 = 34,2Hz
Output voltage is +4,8V or 9,6Vpeak-to-peak. That's after changing the 1K resistors in the schematic for 1,8K ones. Otherwise the voltage was just 3,2V and 6,4Vpp.
Current draw: positive: average 12mA max.: 18mA
                       negative: average -13mA max.: -18mA

Here's the Bill of Materials:

Here are some screenshots from the oscilloscope with some measuring data underneath the images. Some images may still show the lower output voltage but that's been fixed:

The following are screenshots from the oscilloscope showing two signals, one from each LFO, being combined in a simple passive multiple. A squarewave and a triangle wave each at different frequencies. The results are pretty cool looking:

In the top picture you see more of the waveform in the positive voltage region and very little below zero Volts. You can set that with the shape potmeters to your own liking or best sounding result. As you can see this makes the Dual LFO module much more versatile as a modulation source. Plenty to experiment with.

Below are some pictures of the print. I took these before I changed the 1K resistors to 1K8 ones. In the top picture and the 3rd one you can see how I mounted the little print with the bi-colour LEDs. The print rests above the middle two potmeters and the LED's are bent backwards over the sides of the stripboard and go straight into the holes in the panel and are secured with hot-glue. The little print itself is not mounted in any way. It just relies on the LEDs to keep it in place.

This time, instead of spray-painting the panel I decided to keep it blank aluminium and I used an engraving tool to put the text on. That didn't work too well and it didn't look good at all so I printed some labels I made in Photoshop, laminated them with Scotch Tape and put some double sided sticky tape on the back and I put those on the panel. That looks much better. 

A few days after completing this build I added a trigger output to this module. I connected it to the squarewave output of the second LFO (the faster one). I thought it might come in handy to have a trigger source. You can see in the picture below how I did that. It gives of both positive and negative trigger pulses of 5V and a length of about 4mSec.  If you're thinking of putting in a diode to only get positive pulses forget it. That won't work. It'll kill off the pulses completely. If you turn the Shape potmeter the positive and negative pulses will move further away or closer to eachother. Just like the rising and falling edges of the squarewave with different pulsewidths.

(The above drawing actually translates to a high pass filter with a cut-off frequency of 268Hz. So it filters out the actual square- or pulsewave and only lets through the initial harmonics of that wave, creating this spike pulse trigger response, but you can forget about this theory. This is not important.)

Here's a look at the final panel with trigger output. I just made some labels with text to put on the panel. Looks better than the engravings.

One other thing worth noting is that because we have two LFO's on one print, they will very slightly influence eachother. What I mean is, if you have one LFO running at almost twice the speed of the other, the faster one will adopt some multiple of the rythm of the slower one if you set the speed to some value close to that. That's a form of resonance and I won't get into the technicalities of that but it's quite easy to set an LFO at twice or 4 times the speed of the other because they share the same circuitboard. It's the same idea as when you have a group of people walking together and they all start to walk at the same pace. That's a form of resonance. Don't think this will be an obvious thing to observe. The occurrence is very subtile

Okay, that's number 47 done! A very useful little module and I saved a few bob by building it myself instead of buying a dual LFO module. Okay it doesn't have any fancy extra's like synchronization but that's okay by me. I think I'll mostly be using this as a clock source and some random modulation. That's why I made both LFO's run at different frequency ranges.

If you have any questions or remarks about this or any other project on my site please comment below or post in the FACEBOOK GROUP for this website.

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