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 the most popular project on my website. Lately it pushed the 'Really Good VCO' from the number one spot but I was 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 (VCO) and that 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. But you could use a +/-5V pulse and run it through the Dual Voltage Processor and give it some positive offset. That would work fine. I tried clocking it with the Behringer RD-8 but it wouldn't work. But if 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!)
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. I leave that up to your imagination.

SPEED CONTROL and STOP/RUN.
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 experience cases where the sequencer would still take a few steps even after being stopped by the switch so I experimented with moving the position of the switch to after the 2 hex-inverters but it doesn't make any difference. It is quite possible that my CD40106 chip is a Chinese fake so that might be the cause but moving the switch didn't help so just know that this sequencer can have this little quirck.

MOMENTARY SWITCH
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.

SCHEMATIC:
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%.

LAYOUTS:
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 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!


Print only. 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.:



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.


Here's an example of a CV output sequence with potmeters set to various notes.  As you can see the maximum output voltage is a beautiful exact 8V 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! A lot of performers use an SQ-1 in their live sets. 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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