Sunday, 26 January 2020

Synthesizer Build part-19: CHEAP OSCILLOSCOPE for the synthesizer.

A cheap but good functioning little oscilloscope that you can buy on eBay and use for your synth.


As a vital addition to my synthesizer I bought this little oscilloscope so I can easily check the signals my synth produces, so I can keep an eye on things and easily check if something is broken. I couldn't include it by giving it its own panel because I had no more room for that in the synthesizer so I decided to make a special holder from aluminium that goes over the case and holds the scope and has a mono 3,5mm input jack to which I can connect a cable from any output I choose.

The scope I bought is the little DSO138 that you can find on eBay or AliExpress or many other sites for around $20 dollars. If you order it, make sure you get it with an acrylic case because the buttons are on a lower PCB than the screen so it's not easy to make your own case for it. You can order acrylic cases for this scope for about $5 dollar.

If you are new to DIY synthesizer building I strongly advise to get one of these!! You are going to need a scope for trouble shooting so get one! If you can, buy a good multi channel digital oscilloscope, a really good one, like the Rigol DS1054Z 4 channel oscilloscope that I use, or a Siglent for instance. But if you can not afford to invest that much, just get this cheap scope. It'll do for 90% of the testing you're going to have to do. For tuning filters and testing VCO's you can't do without one.

Here are some pictures of how I use the scope:




As you can see I used a Banana plug to BNC adapter on the input and then some thick copper wires to the 3,5mm mono jack input. The holder is made from 1.5mm thick aluminium which I had left over and I decorated it with that circle pattern using a Dremel tool because it was scratched. Looks really cool I think. This is an ideal set-up because I can move the scope over almost the full length of the synth so it's never in the way. All the buttons are within easy reach and it is powered from the power supply from the synthesizer itself. There's a little wire coming out of the back of the synth that plugs straight into the scope.
[Edit: Room to move it around? Ha, the next article will take care of that. No more room for anything after that, LOL]


The oscilloscope menu is not very clear and user friendly. The documentation doesn't explain it in depth at all so here is a little explanation of the scope-menu:

Use the  [SEL] (select) button to go between the different options. There's no need to confirm settings with the OK button.
The option you selected will have a square around them, at the bottom of the screen, but there are a few options that are hidden from normal view. I'll explain those here:
With the 'trigger slope' selected, press [SEL] one more time to change the trigger level. You can see it rise or fall by the little arrow on the right side of the screen. (Use + or - to move it up or down.)
With the 'trigger slope' selected, press [SEL] two more times to change the zero level. This changes the position of the waveform on the screen up or down. (Use + or - to move it up or down.)
To display all the parameters like frequency, voltages, duty cycle, etc. long press the [OK] button (the top button) and the text display will turn on. At the same time the 'Hold' function will engage so you'll have to short press the 'OK' button again to get the scope running again. Long press again to turn the text display off and then short press 'OK' again to turn the waveform display on again.

The scope is delivered with lots of documentation to show you the different modes for triggering and what all the switches are for so I'm not going to go into that here. If you have any questions about this scope you can always put them in the comments below and I will answer them for you as far as I can.

So that's how you can add a little scope to your modular setup for a minimum amount of dosh. I hope this was of use to you and you enjoyed this article. Please leave me a comment below and I'll see you on the next one.

Tuesday, 14 January 2020

Synthesizer Build part-18: A REALLY GOOD AS3340 VCO DESIGN!!

This is the Digisound-80 VCO. The answer to my DIY VCO prayers. Easy to build, easy to tune and all the extra's like Synchronization, Frequency-Modulation and PWM.

Not only is this VCO easy to build, it can actually be tuned easily too. Before I found this, I used the datasheet VCO schematic for the AS3340 using the stripboard design from the LookMumNoComputer website. I could never get that VCO in tune over a wide range of octaves and I couldn't get it to play really deep tones either. I think the fact that he left out the HF tracking had something to do with that. This 'new' design however changed all that! After looking through all sorts of VCO schematics, like the ones on the Electric Druid site, I decided to go for the Digisound 80 design but I left out the triangle- to sinewave converter because I already have one of those build in. (Click here for the Converter article) I can tell you, these VCO's (I built five so far) sound soooo much better than the Datasheet VCO. Of course it's the same waveforms but the range is so much bigger (0.1Hz to 50kHz!) and tuning this VCO is a breeze! And, having at least two of them side by side, I can slightly detune one and get that frequency beating effect. As a first time synth builder and having been into modular synths for only 6 months (at the time of writing this article) this was a real revelation for me. You can even use this VCO as an LFO, a Low Frequency Oscillator, because it goes down to 0.1Hz. If you're looking for a good AS3340 VCO to build, I think this is it. It certainly is perfect for my synthesizer DIY project.

Here's the new version of the schematic with all the opamp buffer stages drawn in. All the outputs are buffered and the same with the PWM input. So a total of 4 buffer stages are used here, all housed in one IC, the TL074. (There's a link at the bottom of this article to the original text and schematic.) I did not include any de-coupling caps in this schematic because I don't use them. If you have a normal linear dual power supply there should be no need for de-coupling but I have included them in the stripboard layout.


(Last revised: 14-Feb.-2020 Added numbering to opamps and extra text. 23-March-2020 Emphasized connections with black dots and added remark about R4.)

The picture below shows the stripboard layout that I made and used for my build(s). This layout is verified and the placing of the buffer stages follows the numbering on the schematic. For the Octaves control I recommend you use a 100K potmeter with a center detent. You know the ones they use for balance control on amplifiers with a little click you can feel when you reach the center point. This is very useful to easily re-tune the VCO after you've been using the Octaves control knob. I put in a 47K resistor for R21 which is the pull down resistor for the squarewave output. It originally was a 10K resistor, because the CEM3340 was used, and that will work fine too. (Use 10K if you're using a CEM chip) It's stated in the datasheet for the AS3340 that it needs to be 51K but in practice it doesn't matter at all. For the current limiting resistor (R23) I put in a 1K. This is necessary because we will connect it to negative 15 Volt. It says to use a 910 Ohm in the schematics but I always play it safe and use a 1K resistor. I know it's overkill but meh... it works just fine.

Running this VCO on a dual 12V powersupply:
If you're going to use this VCO with a dual 12 Volt power supply then use a 680 Ohm resistor for R23. (On the stripboard layout R23 is the 1K resistor going from pin 3 of the AS3340 to the negative 15 Volt rail.) These resistors are needed for both the CEM3340 and the AS3340 ICs.
Further down this article, in the 'Tuning' section, I mention that if you experience problems with tuning while running this VCO at 12V, you can make R4 bigger. Use a 270K or even a 300K resistor instead of the 200K. Some people experienced problems because trimpot-A was at its end before the VCO was in tune. Making R4 bigger will prevent that.

The potmeter for High Frequency Tracking or Linearity can be a normal trimpot, not a multi-turn one. The influence it has is minimal. But you must use multi-turn trimpots for A and B on the layout, otherwise tuning will become very difficult. I used metal film resistors with 1% tolerance in those places where it matters and this is good enough. The 100K CV input resistors should all be measured and matched so they all have the same resistance value. This makes it easier when you connect different CV sources to those inputs, they will be in tune straight away. I was surprised that the two 100K potmeters I used in the panel for Octaves and for Fine Tune give exactly the range that is stated in the original description although the Octaves control is not linear, at least mine wasn't but maybe that is due to the potmeters with center dent I used. Octaves is plus and minus 5 octaves and Fine is plus and minus half an octave. I'm not used to things actually working out as originally described in DIY projects. It's usually either a bit off or way off but the Digisound 80 designs are really good and spot on.
What's not really spot on is the range of the Pulse Width Modulation potmeter. There's a bit of throw left over at the beginning and the end. In other words 0% PWM starts at about one sixth in, counter-clockwise and 100% ends about a fourth before the end of the clockwise throw. This differs a bit with each build, due to potmeter tolerances. The latest VCO I built only had a bit of throw left on the clockwise side, the rest was okay. You could solve this problem and spread the PWM over the entire potmeter throw by using resistors at pins 1 and 3. I tried to do this and experimented with different resistors with as a result that my TL074 blew up. Literally with a loud bang and magic smoke. I must have created a short circuit somewhere but I couldn't find it. Luckily I had a reserve VCO print all ready so I wired that one up and now everything is back to normal. This was a nice opportunity to install a jack socket with internal switch for the PWM so I could do away with the switch. This works much better. The AS3340 was fine after this incident. They are quite robust chips. Even putting them in the socket the wrong way around doesn't destroy this chip. (They call that 'burning in the chip', LOL) I actually did this once, by accident, and had it powered on for 20 seconds before I noticed. The chip had gotten really warm but it suffered no damage. The through hole package can be deceptive because it has a little circle on it which can be mistaken for the indicator for pin 1 but it isn't.
Don't place this VCO directly over the power supply in your modular set-up. If it gets influenced by the heat from the voltage regulators too much it can de-tune a bit but I think this is true of almost all VCO's. The AS3340 has internal temperature compensation but this only really works for changes in room temperature. If you put it over a heat source like a power supply it will most definitely de-tune.

Here's the layout. I didn't put in the input jacks for the sync inputs or the output jacks for the wave forms and CV-OUT. It's already spaghetti junction and that would make it even worse. I assume you know how to hook up jack sockets. All potmeters are frontal view with shaft facing you. I have recently added 100nF decoupling capacitors directly between the IC's and 22µF electrolytic capacitors on the power rails, because this came up on Facebook. These are not included in the schematic drawing but they are in the original schematic in the PDF linked below. (You can use any value for the electrolytic caps between 10µF and a 100µF as long as they are 25V or over.) This is the version that uses a switch to go between internal Pulse Width Modulation or external PWM.



(Last revised: 06-March-2020: Added 100nF decoupling caps near IC's and 22µF electrolytics on the power rails.)

To make things easier, here's a close-up of just the stripboard:



Here's the wiring diagram for the VCO using an internally switched jack-socket to go between external- and internal Pulse Width Modulation. As you can see the 47K resistor has moved from the print to the wiper of the PWM potmeter or the switch connection of the PWM jack-socket, which ever you prefer to use. This is the version I built for VCO's two and three in my synthesizer.:


And here's a close-up of the print for this version:


Bill of Materials.


(07-June-2020 Revised version. Numbering now follows numbering on schematic.)

Here's a look at the finished stripboard. I soldered on a little copper eye to make mounting the print on the particular panel I made easier, but there's room enough left on the print to drill a few holes to mount it however you like. Make sure the copper traces are cut so no contact is made with the bolt and nut etc. In this picture you can also see the annoying little circle at the bottom of the AS3340 chip. Do NOT mistake this for the pin-1 indicator, and put the chip in the wrong way as I once did!! (Although the chip survived, they don't like it.):



Here are the two identical VCOs side by side in my synth:



About the CV-OUT connector:
You can see in the picture that I have CV-OUT jacks on the VCO's. These are simply in parallel over the 1V/Oct. input jack so I can daisy-chain more VCO's to one 1V/Oct. signal so that all VCO's receive the 1V/Oct signal from the Doepfer A-190-3 MIDI to CV converter. This is not included in the stripboard layouts but you can see it in the schematic drawing. Do not use the CV-OUT as a CV-IN because it shares a 100K resistor with the 1V/Oct in and will detune your VCO. If you use the CV/Gate Expander described on this website, then you don't have to include this CV OUT and you can spread the 1V/Oct. signal over all VCO's with the expander.

I recently added two more outputs. One is parallel over the squarewave output socket and is used to connect the VCO to the negative Hard Sync of one of the other VCO's, so I can keep the Squarewave output free for normal use. The other output is in parallel over the triangle wave output and is used to connect the hacked Joyo tuner to the VCO, also to keep the Triangle output free for normal use. These outputs are not on any of the pictures or on any of the layouts but you can easily add them if you feel you need them. I find them very useful.

Btw, the Doepfer A-190-3 is the one module that I bought because I didn't trust myself to build one of these and I wanted the interface between the keyboard and the synthesizer to be absolutely fool-proof and reliable and it was certainly worth the €130 I paid for it. It adds a Portamento (or Glide) function to the synth and it has extra outputs for the modulation and pitch-bend wheels on the keyboard that you can connect to CV-2 for instance to get pitch-bend. I'm waiting for more patch cables that I ordered so I can hook these functions up. It also has a USB input so I can connect the synth to my computer.

And finally a look at the waves this oscillator puts out. All nice clean waves as may be expected from the AS3340 chip but the ringing issue in the downward slope of the squarewave, which I mention in article 2 of this build series, is still there even in this design. Although it is significantly less prominent. This ringing must be common to this chip or something. Anyway, it's not audible so no real problem. I thought the zener diode over the squarewave output resistor might help to eliminate this problem but it has no influence but you can see from the pictures below that there are only a few spikes and only on the lowest notes. The picture below shows the ringing at note C1. Only 3 spikes! They only occur on the downward slope of the squarewave and they have a frequency of 28kHz so well above human hearing capabilities.


At note C3 there's only one spike left and after that it is completely clean. Maybe it adds to the character of the sound though. Who knows ^___^


Squarewave. You can see that the ringing is not even visible once you zoom out on the oscilloscope image:



Here's the ramp wave:


And this is the triangle wave:


Just for fun, here's a Triangle and a Ramp wave after being mixed together and after it's been through the dual Korg MS20 filter. You can see the high frequency resonance, produced by the filter, on parts of the wave form:



You can see that the output voltages are all around the 10 Volt except for the squarewave which is 13.4 Volt. I recently received a batch of 10 Volt Zener Diodes from China and I have soldered those in over the squarewave output and now all signal output are at the same 10Vpp level. Perfect!

TUNING THE VCO:
This VCO has 3 trimpots for tuning but we're only going to use 2 because the High Frequency Tracking or Linearity potmeter is not really effective for the lower octaves. So we leave that in the middle position. I have developed a tuning procedure of my own that is very simple and will get this VCO in tune over many octaves in less than 15 minutes.
If you don't have a useful tuner for calibration purposes but you do have a smart-phone then I recommend you download the 'Universal Tuner' app by Dmitry Pogrebnyak. It's available in the Google Play store for free. Of course any tuning app that displays frequency and notes will do. There's plenty to choose from.
Before we start tuning, turn the Coarse Tune or Octaves potmeter off with the switch on the panel and set the Fine-Tune potmeter in the middle position. In the original text the wire connected to the wiper of the Fine Tune potmeter is de-soldered, but I recommend just leaving it in the middle position. Take a little screwdriver and turn trimpot A up about 3 quarters of the way.  Now go to trimpot B and turn that down about 3 quarters of the way (it's not necessary to be accurate with this and it also doesn't matter which way you turn them. It's just for setting a start position.)

- Launch the Universal Tuner app. or the tuner of your preference.
- Now press key C5 on your keyboard and turn Trimpot A until note C5 is in tune on your tuner.
- Now press key C2 on your keyboard and turn Trimpot B until C2  is in tune.
- C5 will now be out of tune again so press key C5 and retune it with Trimpot A.
- Now C2 will be out of tune again so press C2 and retune it with Trimpot B.
- Repeat these steps over and over until the VCO is in tune.
- You'll notice that you will need to turn the potmeters less and less to reach the C notes. After a few cycles of tuning they will be spot on their respective C notes.
- If you find that you need to turn the trimmers more and more to reach the C notes then switch potmeters and use A for C2 and B for C5

We tune with the Octaves control switched off to prevent variations in resistance from de-tuning our VCO. This keeps the VCO nice and stable but if you want it to be in tune with both control potmeters in the middle position then you can re-tune the oscillator after the first tuning session but now with the Octaves control switched on. It's up to you. :)

Be precise with the final tuning. Check the exact frequencies of the C notes. The app I mentioned will display the note graphically and it shows the frequency. You can get it in tune to at least 1/10th of a Hertz although in my experience you don't have to go more than one figure behind the comma.

Extra info for tuning on +/-12 Volt and using the V3340 chip:
As mentioned earlier, if you're running this VCO on +/-12V and you have trouble tuning it, change R4 from 200K to 270K or 300K. Someone kindly reported having trouble tuning this VCO on 12V and reported this as the solution in the comments below, so I thought I'd include it here. That's why the comments are so useful. If you come across problems like this please report it down below and we'll find a solution.
I've also had a comment on Facebook about the V3340 chip not holding tracking when used in this circuit. I have no further details on that, but just so you know. It's recommended you use either the AS3340 or the CEM3340 chip.

It will usually be the case, when we start tuning, that the notes are too far apart rather than too close together and if you repeat the steps above and keep switching between C2 and C5 and using trimmer A for the high note and B for the low note, you will notice, as mentioned before,  that the notes get closer together and you'll have to turn the trimpots less and less to hit the right note. Eventually you will be spot on and the VCO will be in tune over at least 4 octaves. Be careful that you don't overshoot but you'll notice that soon enough if you have to turn the trimpots more and more to hit the right C note. In that case switch trimpots and use trimpot A for the low note and B for the high note.
You can of course use even higher octaves and other notes, like tune it between A2 and A7 for instance. I leave that up to you. I don't use C1 for tuning because it is so low my phone with the app has trouble tuning into it.
You'll get the hang of this tuning proces soon enough. It's really simple. It took me just 15 minutes after turning the VCO on to have it perfectly in tune, and when I say perfectly I mean perfectly! I was really chuffed about this ^____^

For those of you who want to go the scientific route and calibrate the VCO with a scope and an accurate voltage source and meter, I posted the original calibration procedure below here and it's also in the PDF I link to at the bottom of this article.
(click on text to enlarge)



Here's a little overview of features and technical data about this VCO:

Frequency range:                    0.1Hz - 50kHz
Most accurate freq range:          5Hz - 10kHz
Waveform amplitude:                0V to +10Vpp
Octave adjust control range:     +/- 5 Octaves
Frequency Fine control range: +/- 0.5 Octaves
+ and - Hard Sync
Soft Sync
FM input with level control
CV-2 input with level control
Pulse Width Modulation both internally controlled and externally controllable.
Extra CV inputs can easily be added by using 100K resistors connected to pin 15 of the VCO chip. Measure the resistance of the 100K CV input resistors and make sure they are all the same, that way anything you connect to the inputs will be in tune right away.
All outputs are protected and can be short-circuited continuously without damage to any components.

The positive Hard Sync doesn't obviously do anything but I think it needs a pulsed signal. I'm getting excellent results with the other inputs. Negative Hard Sync and Soft Sync all work fine and FM also gives great results. I can't describe how it sounds but if you build two of these and input one into the other and you turn the Octaves control back half an octave on the oscillator connected to the inputs, you're gonna get some great results. I have all three VCO's synchronized by having a squarewave from VCO-1 divided into two signals in the mixer and inputting them into the Soft Sync inputs of VCO's 2 and 3.

Finally and by request, here's a list of individual notes and their corresponding voltages, should you want to tune the VCO without a keyboard, using an accurate voltmeter. Ignore the 'Expo Output' column. It is not relevant to this VCO:



Here's a picture of VCO number three installed in the second case of the Bergman-Berlin synthesizer :)


Btw, I recently built a version with the sinewave section included but can't get the sinewave to work using the original Digisound schematic with the CA3080 chip. Maybe my chips are Chinese fakes though, I don't know but I resorted to using a discrete transistor based triangle to sinewave converter. This VCO gives you a choise in output voltage for the Triangle and the Sinewaves between +/-5Vpp or 0 to 10Vpeak-to-peak. Any future Digisound VCO's I might make will be made using this panel design and using the triangle to sinewave converter I've built into this one. I'll put up schematics for that in the near future because it is a slightly different design than the one in the Triangle to Sinewave Converter I describe on this website in chapter 3.

Here's a picture of VCO number four with the sinewave output. I have some more experimenting to do but there will be extra info on this website soon on adding the sinewave option:



VCO number four is 2 centimeter less wide than the other 3 which are 10 CM wide. I included an output labelled 'Tuner' to which you can connect the JOYO Tuner after you hacked it. That is connected in parallel over the 10V Triangle output, soldered to the switch directly. (This is not included in any layout or schematic because it was a last minute addition to keep the normal outputs free.) As I mentioned earlier I later also added an extra output in parallel over the Squarewave output to connect the VCO to the negative Hard Sync of an other VCO so as to keep the regular output free for normal use.

Okay, that's it for this one. I hope this is useful to you. After searching for a good VCO design using the AS3340 chip it was a real relief to see that this VCO performed so well and was so easy to build and tune too. I wish I found an article like this one when I first set out to build my first VCO but now I've written one of my own I really hope it will help out all those of you who are building their own synthesizer, maybe for the first time like me, and are looking for a good VCO design.
If you found this article useful or if you have any questions, please leave your feedback in the comments below. I would really love to hear from you. Share this article with your friends and follow this blog to keep informed of new posts. There are buttons for sharing on social media right below here. It would help to maintain this website. If you want to support my work you can buy me a coffee. There's a button for that if you're on a PC or MAC under the menu in the sidebar.
Thank you for checking my website out and see you on the next one! :)

Here is a link to the original PDF file with all the text and schematics and tuning procedures for the Digisound 80 VCO: <click here to download or read the PDF file>

Saturday, 11 January 2020

Synthesizer Build part-17: MIXER and PASSIVE ATTENUATOR in one.

It's a mixer and a passive attenuator in one and I recently also added a Clipping Indicator LED. Very useful module for the DIY Synthesizer.

This is a very simple project to build yourself and it will be a module you will use a lot in your synthesizer. You can use this mixer for both Control Voltages (CV) and audio signals. I wanted to have a mixer in my synth but also a passive attenuator that I could use for signals that have no level control. So I decided to put both functions into one panel. I recently also added a clipping indicator. More about that at the bottom of this article.

Passive attenuator is nothing more than a fancy word for a volume knob. It's just a potmeter inbetween the in- and output of the signal. It's called 'passive' because it doesn't require any power source. But by flipping the switch from passive to mix, the signal is now also being led into the mixer and becomes part of the signal coming out of the 'Mix' output while still being available at the original output too. I used the super simple mixer circuit that Sam from LookMumNoComputer also uses and I added switches for the signal to be added to the mixer.

Here's the stripboard layout that I made of the complete Mixer/Attenuator. All potmeters viewed from the front. (There's a layout of the mixer with the clipping indicator, further down the article):


(Last revised: 05-Feb.-2020)

Here's the schematic for this module:


In the panel wiring diagram, on the drawing, you can see how I combined the mixer with the passive attenuator function by simply adding switches that lead the output signal coming from the wiper of the potentiometer into the mixer. Only the top output jack carries the mixed signal out and the other three always carry an attenuated version of their respective input signal. That line is never interrupted. Those other three outputs are all for passive attenuation. So you could get a mixed signal out of the top output jack, and 1 to 3 original signals on the other outputs. You can also split a signal into two parts by putting it on, for instance, input 2 and sending it into the mixer. Then you can tap the signal from the mixer output and from the output of channel 2. So very versatile! The mixer takes + and - 12 Volt but will work just as well on +/- 15 Volt. I glued the circuit board straight to the back of two of the potmeters in the panel with hot glue. Works great! :)
You can add as many channels to this mixer as you like by simply adding more potmeters with input jacks and 100K resistors to the input of the opamp. You can even add an inverted output by tapping off the signal from pin 1 of the chip (I've marked the place in the schematic on the drawing and the layout) with a 1K resistor going to an output jack. I didn't include that in my build because inversion doesn't do much for audio signals and my LFO already has an inversion option so there's really no need for it. There is also the possibility of adding a 'Gain Control' potmeter by changing the 100K resistor over pins 1 and 2 for a 500K potmeter in series with a 50K resistor. It is drawn in the schematic in dotted lines. This will give a gain of x 0.5 to x 5.5
The inputs of the 4 channels can never be shorted out because they are connected to pin 3 of the potmeters so the input impedance stays the same as the value of the potmeters you used. If one or more of the potmeters switched to the mixer is turned to zero, there still is a 100K resistor in series with the wiper(s) so there's never a short circuit possible. You won't even hear the slightest drop in volume, this is a very simple but very good working mixer. I've built two of them so far and they are used all the time!

Here's a picture of the finished panel (before I added the clipping indicator, so here the blue LED is always on.):



THE AUDIO CLIPPING INDICATOR ADDITION:
When I first made this mixer, the blue LED was just there so I knew the mixer had power but it served no real function other than that it looked cool. But later I decided to give it a useful function and to use that LED as a clipping indicator. So I started looking for schematics of clipping indicators and I found some low resolution circuit images. I used one of those to draw my own schematic:



I made this stripboard layout for it which is verified, I used this for my build:


I built the clipping indicator on a separate piece of stripboard and glued it to the mixer print with hot-glue, using a spacer in between so the prints wouldn't touch each others copper strips. Since then I have made a new layout combining the mixer with the indicator which is of course much more convenient. The layout below is verified, I recently built a 2nd mixer using this layout and it works fine.



Print only:


(Last revised: 27-April-2020: Corrected mistake with 10K resistor to pin 6 of IC2, it was connected to ground when it should be connected to V+.)

Calibrating the clipping circuit:
This clipping circuit works on anything from 9 to 15 Volt. It happily takes 10V peak-to-peak audio input signals (if powered from 12-15V). You can set the sensitivity or the 'clipping threshold' with the 5K trimmer potmeter. I just fed it the normal signals from the two VCO's on channels 1 and 2 and slightly mistuned one VCO so you get that frequency beating effect where the two signals amplify eachother when they are in phase and subtract when they are in opposite phase. With the mixer-level pots turned to maximum I set the trimmer in such a way that the LED would just come on when the combined signals from the two VCO's would be at their highest amplitude. That way any signal louder than the VCO's will trigger the clipping light and you also get a visual indication of the 'frequency beating effect' because the LED will blink in time with this effect. So at the level I set it, the audio isn't actually clipping yet but the volume is louder than the normal 10Vpp. You can of course choose any threshold level you like and calibrate the circuit with whatever method you wish.

Here's the mixer panel with the newly designated clipping LED:



Here you can see the two prints behind the Mixer panel. You can also see how I glued the two prints together with a plastic spacer inbetween to prevent them touching. It looks a bit messy with that big blob of glue but that doesn't matter to me. It needs to work and work it does! And it saved me from having to rebuild the whole mixer. (I might do that anyway though on a later date, using the layout above.)
If you decide to build this mixer you should of course use the layout with the clipping indicator included. It's still small enough to hot-glue it straight to the back of two or three potmeters in the panel, so no need for screws or brackets. I'm really happy that I was able to include this little visual aid to this mixer. Makes it just that little more appealing and useful.






Here's a few pictures of how the complete synthesizer, described in these 17 blog-posts, now looks. (These pictures were taken before I changed out the two VCO's for the Digisound 80 VCO's):




It may not look very professional, especially the hand written labeling of the controls, but I love it and I must say I'm really proud of it too. I even made the local newspaper with this synthesizer. A journalist came and interviewed me and wrote a cool little article about it. Let me know in the comments what you think of it please! I would love to hear your input and feedback, ideas or even criticism.

There's at least one more item that will be added and that's a little oscilloscope. I have the scope here but I'm still waiting for the acrylic case.  I also want to change the VCO's because I can not get them in tune over the whole 4 octaves. So I'm looking with great interest at the modules Sam Battle is putting out at the moment especially the VCO with build in tuner. In the mean time I'm building two VCO's using the Digisound 80 design from 1980. More on that in the next article.

Okay, that's an other one done. Stay tuned (pun intended) there's more to come!  :)

Thursday, 9 January 2020

Synthesizer Build part-16: SAMPLE and HOLD.

Makes random R2D2 bleeps from noise or turns an LFO signal into a stepped signal which you can use to control a filter. Lots of options.

Every synth needs a sample and hold circuit in my opinion to have an extra source of control voltages. The S&H samples whatever you put on the input at a given rate, which you can control with a potmeter and delivers the voltage it sampled at the CV output. If you feed it a white noise signal it will give you random tones. If you feed it a signal from the LFO it will turn that signal into a stepped signal. The LF398 chip samples the input signal in 4 to 20 millionth of a second (!) and is used in many more applications that just synthesizers
For this build I used the schematics from Rene Schmitz called 'Yet Another Sample and Hold'.
I had ordered the LF398 chips a while ago and had a try earlier at building this circuit but I couldn't get it to work, but this time everything went fine and the circuit works very well. I added some extra's to this circuit in the form of a DC offset feature so I can control the voltage range of the output signals a bit better and I installed two input sockets between which you can choose with a SPDT switch. I also installed a switch that gives me the normal output voltage range (0 - 10Vpp) or half the normal output voltage range (0-5Vpp) which is better as input for the VCO's. The DC Offset in particular has proven to be a very useful addition.

Here is the layout I made and used for this build. It's a revised version from those published before the 12th of Februari 2020. The DC Offset feature is newly designed and there also was a wire bridge missing in the previous version. (All potmeters viewed from the front):


(Last revised: 12-Feb-2020)

At the bottom right on the layout you can see the circuit for the DC Offset feature. I re-designed this from the previous version. This is a better way to add DC offset and it makes use of both opamps in the TL072 chip. (You can also use a TL082).

Here's a close-up of just the stripboard:



This S&H has an internal clock pulse generator based around one Schmitt Trigger NAND gate of the CD4093. You can also choose to trigger it externally by selecting the external input with switch S1.

Here's the schematic drawing of the extra features I added myself; the DC-Offset and the output range switch:



The CV output goes through a resistor voltage devider that halfs the output voltage. This puts the different random tones closer together which sounds better. It's something I added after testing and seeing the output signals on the oscilloscope. Later on I added a switch that bridges that voltage devider and gives the original output voltages. I labeled it "Output x 1 and x 0,5". I did this because I wanted the full voltage available in case I want to use the output of the S&H to control the resonance of a filter (among other things). The resistor voltage devider however is something I strongly advise to include in your circuit if you're building one of these. The range switch is a good feature to have.

I didn't have any more space in the synthesizer I build to put this S&H in as a separate module so I cut a hole in the wood above the panels and mounted it there. This works very well and adds yet more buttons and switches and a flashing light. That always looks cool ^___^

Here's a picture of the finished panel and one that shows the placement within the synthesizer:




Here's a little video to demonstrate the sound you get when you put white noise on the input:




This sound was going through the Dual Korg MS-20 filter described in the previous article.
Okay that's another one done. Hope you enjoyed it and if you did please consider following this blog to get notified of new uploads and while you're here, leave me a comment please!

If you want to know more about sample and hold circuits I refer you to this Wikipedia page.

Here's a link to the LF398 sample and hold chip datasheet in PDF form:  (Click here)

Sunday, 5 January 2020

Synthesizer Build part-15: DUAL KORG MS-20 HP/LP FILTER.

Now here's a DIY project that will instantly make your synthesizer sound amazing! Two awesome Korg MS-20 filters in one module.

     I wasn't too pleased with the performance of the Prophet 5 lowpass filter so I decided to remove it and put a new filter in its place. I've seen lots of videos about the Korg MS-20 and really like the sound of it. I noticed that synthesizer has two nearly identical filters next to eachother; the highpass- and the lowpass-filter, so I wanted to emulate that in my own synth. So I set out to build two of the 'Late MS-20 filters' by Rene Schmitz, and fit them behind a single panel that was the size of the old Prophet 5 filter that I took out. It was a tight fit to put all the knobs and switches on but it worked out beautifully in the end.
The schematics and layout I used are just the same as the ones I used in article 12 of this blog, so if you want to build your own dual filter arrangement you can go to the Korg MS20 filter page and build TWO of those. Build both filters with the HP/LP switch but do not include the bandpass switch. You build two MS-20 HP/LP filters and put them behind one panel. Then, as extra, you add switches to the inputs so you're able to put them in series or use them independently of eachother. The wiring diagram for those switches is further down the article.

Here's a picture of what that looks like. You can see I have filter one on the left side and filter two on the right. Each has its own Cut-Off Frequency and Resonance controls and each has audio and CV-in level controls and each has it's own Highpass/Lowpass switch. Beneath those switches you see two more switches which enable me to switch both filters in series without using patch cables to connect them to eachother.:



The wiring of these two stripboards was a bit of a nightmare but I got it done in the end. I made some initial mistakes and had to re-wire some potmeters so that's why the wires look like such a mess. Luckily it doesn't affect the working of the filter.


In the first filter I used the LM13600 chip and in the second the LM13700 chip. And having them side by side is a good opportunity to compare them and the LM13600 is definitely a bit tamer than the 13700. So if you have both chips in stock you can decide whether you want your filters to sound aggressive or a bit less aggressive.

You can see in the pictures above that it's a tight fit but I did managed to include two volume or level potmeters for the audio inputs, which are not included in the original build but are very useful to have. I'm going to make sure that every filter I build in the future has input level control.
Beneath the HP/LP switches are the two switches that give you the option of using these filters as two stand-alone filters, so not internally connected, or if you want to put them in series so the output of filter 1 goes into the input of filter 2.
One other weird option would be to switch the output of filter 1 to the output jack but leave the input switch for filter 2 as is so filter 2 has no input. Because these filters are self-oscillating you can now use filter 2 as an oscillator. Put the output into the 4 channel mixer described in article 17 and put the resulting wave through filter 1 or wherever you like. Connect a 1V/Oct voltage to the CV input. Just an idea, but you see there are endless possibilities. That is the beauty of modular synthesizers. :)
However, it would be better to use a single DPDT switch here and use it just to switch between two filters in series or both separate. That makes it easier to switch but that way you can not use one of the filters as stand-alone oscillator, but you won't use that function much anyway I'll bet.

Here is the wiring diagram for those two switches:


Finally I want to show you a little video I made which demonstrates the sound of these two filters in series with eachother. I filmed this just after I had finished the build and I was still figuring out what the filter could do but it shows the added benefit of having two of these in series. It can do really deep and full sounding bass tones and it can also scream and distort and sound really weird. I am glad I fitted these and they are certainly a big improvement over the Prophet 5 filter although I will use the AS3320 chip inside it for a future build.
Plus all those knobs and switches so close to eachother look really cool I think ^_____^

Here's a look at the different sounds this filter can produce. (The phaser effects come from the special effects unit and not from this filter):



Okay, that's it for this one. If you enjoyed this article please check out the rest of my synth build and leave me a comment if you have any questions, or even to just say hi. Please also subscribe to my YouTube channel. That would be a great help. THANK YOU!!


Other websites that deal with the DUAL MS-20 configuration:

https://www.modulargrid.net/e/befaco-sallen-key-filter-bf-22

https://www.perfectcircuit.com/signal/korg-ms-20