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.

-- 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, and using the knowledge I gained in the three years I've been building modules, I wanted to offer people something better to build. Something that I was sure wouldn't disappoint anyone building it. I'm really happy with how it turned out and I love using it. This is one of the few sequencers that can actually deliver negative voltages if you want to. Because a sequencer doesn't have to be used to make melodies or bass-lines, it can be a modulation source aswell. It really is a step above the basic baby 8 sequencers without using Arduino's. 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. 
01 External Clocking. The sequencer 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: One option 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. 
An other option would be to use an opamp as a comparator so that if the trigger rises above a certain threshold the opamp goes high and low again when the signal trigger goes below the threshold. I've tried a comparator and it worked, only it triggered the sequencer on the down- aswel as the up-slope of the trigger signal so I need to do more experimenting.
02 CV OffSet. 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 0.0833 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 or melodies 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!) On the other hand, without a quantizer we can tune by ear and to what sounds right to us, without being bound by set scales. So there are advantages too to not having a quantizer. I always tune my sequencer with the Joyo tuner which I hacked for modular use (click here for that project.)
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. 
For those of you interested, 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. The output opamp has a 470pF capacitor over the feedback resistor to suppress little voltage spikes which I could see on my oscilloscope, just like in the Sample and Hold v2.0 project. The 470pF gets rid of that very effectively.
03 External Speed Control. 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 1K 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. Also make sure you test the Vactrol before putting it in. My own Vactrol died on me a few days later but I used a 470 Ohm resistor instead of a 1K and that is too small as a current limiting resistor. Use 1K and a bright white LED. That should work fine for CV voltages upto +10V.
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 a Light Emitting Diode (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.
An other way to vary the sequencer speed is to vary the external clock speed when using an external clock. This would be a more exact option if, for instance, you are clocking the sequencer with an LFO with an external Frequency Modulation input.
Please only add the external speed control if you think you'll need it, otherwise save yourself the trouble.

SPEED CONTROL and the TUNE/RUN switch.
The sequencer has a speed control potmeter that will go from 1.34 seconds (746mHz) to about 185Hz. 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. The old procedure was to just set the speed very slow and then stop the sequencer at each step so you could tune it. Then you would switch to Run to go to the next step and stop it again to tune that one. 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 (which I hadn't done). 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 easily be done with a little patience and accuracy. 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 and in tune. As I mentioned earlier, you could buy a cheap eurorack quantizer which would help a lot with accurate tuning but that's up to you. Or, an other really helpful tool to have is the Joyo Tuner, hacked for use with modular synthesizers. It's what I personally use to tune my sequencer.

MOMENTARY SWITCH
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. 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. 

SCHEMATIC:
Here is the schematic for this sequencer. I made it using Photoshop. 
I have only drawn two 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. The CV Rails is just a bare copper wire where the outputs of each of the 8 steps are soldered to. From there a wire then connects the CV Rails to the offset opamp on the stripboard.


The connections of the rotary switch follow one step behind the connections of the potmeters. So step 1 is reset by the pulse from step 2 so be very accurate in that. I got that wrong in the previous version. 
There are two diagrams on the layout below explaining where all connections go for the potmeters and for the rotary switch.
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 current draw of this module is incredibly low. The +12V part only draws 6 mA in normal use and with the internal clock at full speed it draws 10 mA. The negative side, -12V, only draws 0.73 mA! That's 730 µA! Regardless of the clock-speed. That's nothing! So you can easily run this module on two 9V batteries and use it as a stand alone sequencer should you wish to do so, and your batteries will last for months. (use 2 batteries to create a dual voltage supply)
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 IS A FALSTAD SIMULATION OF THIS CIRCUIT: --- CLICK HERE ---

LAYOUTS:
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.  

Wiring diagram:
If you don't want to include the External Speed control, just leave it out. 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. This layout also contains an LED pinout diagram.
All potmeters are viewed from the front!


(The connections of the rotary switch follow one step behind the connections of the potmeters to the CD4017. That's why there are two columns of connections on the lower right.)

THE LAYOUT BELOW IS THE SAME LAYOUT AS ABOVE ONLY THE POTMETERS ARE NOW SEEN FROM THE BACKSIDE! I thought this might be helpful for some people when wiring up the panel.


Wiring procedure:
When you are ready with the stripboard and ready for wiring, make the face plate first and put all the potmeters, sockets. LEDs and switches in. Then wire up the potmeters with the diodes to the CV rail (a bare copper wire) that connects with a wire to the stripboard. Wire up all the LEDs too. Now loosely connect the stripboard to the faceplate and then you can wire everything up to the stripboard very easily.
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. In my build the stripboard is mounted parallel to the faceplate with one M3 threaded stand-off. The wiring also helps to keep it in place nicely.

TIP: You could connect a socket to each of the 8 stages (before the potmeter) and so use this as a trigger sequencer too.

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 so it doesn't need a heatsink. 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. In fact, I myself used a 2200µF/16V cap in that position to be on the safe side. (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.
In my own sequencer I actually have two Gate Out sockets, just connected together. I found it useful in some patches to have two so order 5 sockets it you want to include that option.



Pictures:
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. (Holes N-41 and O-41 in the picture below) You can see it's added in the picture below it:




OSCILLOSCOPE SCREENSHOTS:
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:



DEMO VIDEOS:
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/lcsw4txNoj 

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

Btw, these videos were filmed with a Galaxy S22 and audio recorded in front of my speakers with a Zoom H8 audio recorder. (Someone asked me about that so that's why I mentioned it)

CLOCKING AT AUDIO RATES or the SEQUENCER as WAVEFORM GENERATOR:
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 9:25:

KORG SQ-1
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.

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 4 February 2022

Synthesizer Build part-48: PASSIVE MULTIPLES and SWITCHED PASSIVE MULTIPLES.

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. 
I now also have a project for the complete Utility LFO and it's even smaller than this one and a panel width which is only 1HP wider at 9HP.  Go to project 50 for that.  
This LFO is still useful on its own though because it is so small. It can easily be incorporated into other projects as an on-board LFO for instance.

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.

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.

LAYOUT:
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.)


Print(s):


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.
 
TECHNICAL DATA:
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.


PICTURES:
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. 

TRIGGER OUTPUT
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.

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