Sunday 26 January 2020

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

A cheap but good functioning little oscilloscope that you can buy for cheap 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.

Recently I also took delivery of an other small scope, the FNIRSI-138 Pro. This is a little square scope, very much like the one we discussed earlier but this one has a battery on board. The accompanying documentation says it will last you about 4 hours on a full charge. It comes with a BNC to Alligator clips probe and a short USB-C cable and a printed quick start manual in English and Chinese. 
It works very well and this has an 'Automatic' function. You press it and it automatically sets the scope to the best settings to display the waveform. This works very well although I find the knobs on this scope even more difficult to figure out than the DSO138. Their functions are again not very intuitive or obvious. The scope goes for about €30,- and measures 6 x 6½ Centimeters and 2,2 CM deep.
Here's a picture of mine. It has the protective screen cover still in place.

This scope has the advantage that you don't need to buy an extra case for it. It already has a very practical shape with all the knobs located underneath the screen. The ON/OFF switch is a minuscule little thing located on the righthand side. Above it is a tiny little reset button which comes in handy if you fiddle around with the knobs and the waveform disappears from the screen, which is what happened to me. Not even pressing the 'Auto' button helped, but the reset button did the trick. I think you could easily make a panel and mount this scope inside it and figure out a way to override the battery and feed it from the synth's own powersupply. This should not be too difficult. The scope is held together with 4 little nuts and bolts with standoffs for the circuitboards so it can easily be disassembled to make alterations. The battery is connected with a standard mini Molex connector. But if you decide to alter it you do it at your own risk! I haven't tried it myself so I'm not giving any guarantees.

You can have the display show some measuring values like frequency and voltages (Peak-to-Peak, Vrms, Vmin, Vmax etc). To display this info you long press the right button. Long press it again to clear the screen. The second button from the left will select different settings like V/division, Speed, Trigger Threshold, etc. You click it and the item that is selected is displayed in blue. Then you can use the third and second button from the right as up and down button to set the parameter to your liking. The left button is the Auto function. If you press it the scope will automatically select the best settings to display the waveform.

Beware of the advert for this scope on eBay. It shows a full size digital desk oscilloscope with a price of about € 30,- but you have to click a few boxes and then it shows that this page advertises 3 models of scope and you have to choose which one. If you go straight to 'buy now' and you think you're getting a full size scope for 30 euro's forget it. The picture shows the most expensive model with the price of the least expensive model. Beware of that.

If you are really serious about your DIY synthesizer hobby then I strongly advise to fork out some cash for a digital oscilloscope. I myself use a Rigol DS1054Z 4 channel oscilloscope which cost me €400,-- but you can get a reasonably good 2 channel one for under 200 US dollars these days. Here's a link to a Hantek scope that'll do the job nicely and will serve you well for a long time.

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. And now there's also a Sinewave output. (There's a demonstration video further down the article.)

More than 3 years of experience and feedback from people who built this VCO have gone into this article to bring you what I think is the best DIY VCO stripboard project on the internet. Many hundreds of people have now built this for Kosmo aswell as Eurorack systems and in the beginning people were having problems with using this circuit on a dual 12V powersupply but they came up with solutions and all of those solution have been weaved into this article. I have not had many people reporting issues in the last few years and if they did it was due to some mistake or other made in the building proces.
So I hope you enjoy building this awesome sounding VCO.
There's now also a link to get PCB's for this VCO at the bottom of the article.

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 I decided to go for the Digisound 80 design and I added the triangle- to sinewave converter later on as a separate stripboard. 
But we are concentrating first on the main VCO and the Tri- to Sinewave converter is discussed at the bottom of this 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 this Digisound design isn't even that different from the Datasheet design. Except for the extra trimmer, the Hard Sync options and a few resistor value changes, but this makes all the difference in the world. As a first time synth builder and having been into modular synths for only 6 months (at the time of first writing this article) this VCO 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.

Below here is 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. (PWM = Pulse Width Modulation for the squarewave) So a total of 4 buffer stages are used here, all housed in one IC, the TL074. Each output opamp is wired in such a way that it outputs the waveform at 0 to +10 Volt (when using +/-15V powersupply). For the quad opamp chip you can also use a TL084 or an LM324 or any other low noise quad opamp with the same pinout. (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.

Here is a link to the Datasheet of the AS3340:  ---CLICK HERE---

The stripboard layout I made from this schematic (further down the article) is verified and the placing of the buffer stages follows the numbering on the schematic drawing. For the Octaves control I used to recommend you use a 100K potmeter with a center detent, but I think after due consideration that it's better overall to use normal linear potmeters. I thought it would be useful to easily re-tune the VCO after you've been using the Octaves control knob and it is, but those potmeters often are not linear. Often they are logarithmic towards the center point. Anyway, decide for yourself what you want but whatever you choose use a normal 100K potmeter for the Frequency Fine control  not one with a center detent because you need accuracy around the center settings.
I put in a 47K resistor for R21 which is the pull down resistor for the squarewave output. It originally was a 10K resistor in the Digisound-80 circuit, because the CEM3340 chip 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, so I use 47K. 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. Use a good quality polystyrene or polyester or silver mica type capacitor for C7 (1nF). This is the frequency determining capacitor (a.k.a. timing cap) and must be stable with temperature changes. So do not use a ceramic capacitor for C7. When soldering in a polystyrene capacitor, make sure you don't heat it up too much! These types of capacitors can change their value if they get too hot from soldering and when they cool down the value will stay changed. But don't worry, with normal soldering they will be fine and I never had problems with them myself. Some polystyrene capacitors have a black line on one side. This indicates the leg that is connected to the outer layer of aluminium that makes up the capacitor. This leg should be connected to the lowest voltage potential (usually ground), that way it will act as shielding against hum. If it doesn't have a stripe, just put it in anyway you want. It'll work fine.
This is what a Polystyrene capacitor looks like. You can see the right side has a dark band, indicating the right leg is connected to the outer layer of aluminium:

Running this VCO on a dual 12V powersupply:
If you're going to use this VCO with a dual 12 Volt power supply (Eurorack) 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.) It doesn't matter if you are using the CEM3340 or 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. It's not always necessary to make this change but if you do, you may have to experiment to find a value that works best for you. It seems many people have different experiences with this but it's just a matter of finding the right value.
   When running this on dual 12V you might also need to change C7 from 1nF to 0.5nF or 500pF. Otherwise you might only get very low frequencies out of this VCO. This is not in the Datasheet but it has been established by feedback from many readers who built this VCO for Eurorack. Again, this change is also not always necessary but I leave it up to you. Many people commented that they needed to do this change to get it working right on 12V. Instead of 500pF which may be hard to find you can also use a 470pF capacitor as long as it's not a ceramic one.
   I also had feedback that mentioned changing R7 from 300K to 150K. This is the resistor in series with the wiper of the Octaves control potmeter. This will increase the range of the Octaves potmeter. Again, I'll leave it up to you whether you need this change or not. I can see that the lower voltage can mean less Octave range on the potmeter but halving the resistor value might be a little too much? You'll have to experiment to get the range to your liking.
   Next thing to do is to change R18, the resistor connected to the PWM potmeter to 18K or 20K or 21K (which ever value you have to hand) to make full use of the throw of the pulse width modulation potmeter. You could even put in a 33K trimpot and then set it so that the voltage over the PWM potmeter is exactly +10V but it's not necessary to go that far. A resistor is fine as long as the value is around the 20K, plus or minus 2K.
   Finally, some useful feedback I got via the LookMumNoComputer forum is to change R11 from 1M5 to 1M2. This should really help with tuning especially if changing R4 doesn't improve the tuning situation much. With all these changes you should be able to get it working fine on +/-12V.

So to sum up the changes you need to make for +/-12V operation:
- Change R23 to 680 Ohm.
- Change the Timing Capacitor C7 from 1nF to 500pF (0.5nF). 470pF will work too. (Not always necessary, see text.)
- Change resistor R4 for a 270K or 300K resistor to assist tuning (again, only if necessary. You may have to experiment to find the right value that works for you.).
- Change R18 to 18K, 20K or 21K.
- If necessary, change R7 (resistor in series with wiper of Octaves control potmeter) from 300K to 150K to get more range from the Octaves control. Use your own judgement if you need this change or not.
- Change R11 from 1M5 to 1M2. This will help with tuning and if you're having problems to get the VCO to track right over the octaves

When running this VCO on dual 12V the waveform levels won't be as high as 0 to +10 Volt. The levels will be a bit lower. Because all my VCO's run on +/-15V I can't tell you exactly how high each output will be but the levels can be adjusted by changing the value of the feedback resistors of the buffer opamps or alter the unity gain buffers so that they give positive gain. This will only be an option if you know your opamps of course.
If you're a beginner and this is all gibberish to you don't worry. Just continue building. The outputs will be high enough to make the VCO work fine.

Further hints and tips:
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. In fact, I used cheap 1% resistors from China and they are not 1% but more like 3% but this is still good enough. But 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 detent 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.
If I can give you one important tip, and this goes for all the projects on this website: Measure every component before you solder it in place. This can save you an enormous amount of work in troubleshooting
Please beware the outputs are 0-10Vpp and not the usual +/-5Vpp. This is because I built this from the original Digisound 80 documentation. If you want the outputs to conform to +/-5Vpp then you need to add a negative 5V offset voltage to the output buffers or have the outputs go through an extra opamp that is wired to provide this negative offset. Ask in the Facebook group if you don't know how to do this. I will re-address this point later this year (2024) and add a solution to this article. 
SET  YOUR OSCILLOSCOPE TO DC WHEN MEASURING OUTPUT SIGNALS!! Otherwise you'll get wrong readings and it won't measure DC offset voltage.

About Pulse Width Modulation:
Pulse Width Modulation is now also spot on. Before the 18th of October 2020 I had the PWM connected as is shown in the original schematics in the PDF file linked below (in series with the wiper of the PWM potmeter) but that didn't work perfectly. There was a significant amount of throw left on the potmeter when you reached the 100% mark. However, I got a suggestion in the comments below to move R18 from the wiper of the PWM potmeter to pin 1, the +15V connection to the potmeter, and that did the trick.
I should have realized this myself it's so obvious. The 47K resistor R18 forms a voltage devider with the 100K potmeter that takes off 5V from the +15V supply and leaves the potmeter with +10V on pin 1. This is then halved by the voltage devider made up of R19 and R20 (both 47K) to feed the chip with 0 to +5V, which is exactly what it needs for the correct pulse width modulation.
This Pulse Width problem was really buggin' me because it was the only thing that was not working right in this design but now that is solved too.
The results I get are as follows: With the PWM potmeter fully counter clockwise I get 0% pulse width, meaning that there is no signal, just a flat line. Then as I turn it clockwise the pulse appears and goes through the percentages to stop at 99% pulse width when the potmeter is turned fully clockwise. So fully clockwise there's a very thin pulse left over. This is absolutely perfect. Of course your results can differ a tiny bit because of resistor tolerances but I got the same results with all 4 of my VCO's. 
So if you are using this VCO with a Eurorack powersupply of +/-12V you need resistor R18 to be near to 20K. (21K or 18K will work fine I think. The schematic and layouts have all been updated with the new R18 position.)
For external Pulse Width Modulation you need a signal that goes from 0 to +10V on the PWM input jack. This can be a problem if you use this VCO in a Eurorack setting where the signals are usually -5/+5V.  Just so you know. But there are LFO designs on my website that will give you the 0V to +10V output option you need. You can also use a module like the Dual Voltage Processor to give a +5V DC-Offset voltage to the control voltage and then use it for Pulse Width Modulation.
NB: If you have built this VCO and all the waves work except for the squarewave then check the voltage on pin 5 and see if it is between 0 and +5V. If not, the output will be a flat line.

Temperature Compensation:
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. Of course other components around the chip will also warm up and add to the de-tuning of the VCO when influenced by the heat from the powersupply.

Below here is 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 rated for 25V or over.) There's an extra CV input marked on the layout. This is just incase you want to permanently connect something, like a sequencer, to the VCO and don't want to sacrifice an existing CV input for that. (If you don't need it, there's no need to include it.) 
Again I want to repeat what I said earlier: measure every components value before you solder it in. I always do this myself too because resistors and especially capacitors can be way out of spec sometimes and it is always best to be sure, especially when using cheap Chinese components. (I always use cheap resistors and they work just fine.)
I added and extra Hard Sync input to this layout recently. You can install a 3 way rotary switch for the Hard Sync and have some options this way. You can also leave it out, it's up to you. (The middle hard sync input comes from the Digisound 80 VCO Deluxe schematic.)
Here's the wiring diagram:

(Last revised: 18-Oct.-2020: Changed position of R18 from the wiper of the PWM potmeter to between the +15V and pin 1 of the PWM potmeter. I also changed the colour of the capacitors to be in line with other layouts and I made the wirebridges that connect to ground a green colour for clarity. 26-Nov.-2021: Removed striping from resistors added extra Hard Sync input. 18-Oct-2023: Colourcoded wirebridges.)

And here's a close-up of the stripboard. Don't forget to cut the copper strip underneath the 1M resistor above trimpot A. (Position A-31) The cut is difficult to see on the layout but it's there of course, otherwise the resistor wouldn't work. Also don't forget the jumpwire from that resistor to the wiper of trimpot C! (Position B-16 or 17). I made an extra layout with just the cuts and wirebridges, below.
Beware that some stripboards are sold with 56 instead of 55 holes horizontally. This layout is 55 holes wide. 
Stripboard only view:

Below an overview of the cuts and wirebridges seen from COMPONENT SIDE! As always, mark the cuts on the component side with a waterproof Sharpie and then stick a pin through the marked holes and mark them again on the copper side. Then you can make the cuts with a sharp hand held 6 or 7mm drill bit. Then lightly rub the copper strips with a fine grain sandpaper.
Cuts and wirebridges:

Bill of Materials.

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

Please note there's an extra Triangle to Sinewave converter print you can add to the VCO, to give it a Sinewave output, at the bottom of this article!

I've had a request for an alternative solution for the pulsewidth modulation inputs. This person wanted to be able to have the pulse width connected to an LFO and still be able to control the pulse width on the panel as well. So I designed a little CV-mixer stage as an alternative for the standard PWM inputs. It requires an extra opamp but there is just enough room for that on the stripboard:

Here the total view layout for this alternative version:

Stripboard only:

I haven't tested this yet myself but I had confirmation that it works perfectly. Make sure you make the extra cuts and wirebridges etc. I made an extra layout to help you with this:

Bill of materials for the above version with Pulse Width Modulation CV-mixer:

Schematic for this version:

That concludes the bit about the alternative Pulse Width Modulation arrangement. We now continue discussing the normal VCO version:
Below is a look at the finished stripboard. I soldered on a little copper eye to make mounting the stripboard on the particular panel I made easier, but there's room enough left on the stripboard 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!! I had the chip mounted in the socket the wrong way around and had it switched on for about 20 seconds. It got so hot that I could smell it, that's what allerted me, and I switched it off immediately thinking the chip would be waisted but no, it survived! (They call that 'burning in the chip', LOL.  DON'T TRY IT!)

Here are the two identical VCOs side by side in my synth. You can see a 'Tuner' and 'Sync Out' output, which I added later on. More on that in the 'Synchronizing' section below:

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. 
If you use the Dual Buffered Multiple 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 Multiple. But I do advise to include it. If you daisy-chain your oscillators like this you keep the Buffered Multiple free for other functions and you can daisy-chain upto 8 oscillators of this design before you'll get a slight drop in voltage in the 1V/Octave signal.

I made a little expansion board with 4 inputs and 4 outputs that gives you the option of giving a -5V offset voltage to the waveforms this VCO outputs and so turn them into bi-polar signals. This will make sure you can use this VCO with all the other projects on this website without the problem that the signals are 0-10Vpp. Most modules require signals that are -5/+5V peak-to-peak. This expansion board will provide that for you. You will have to find a way to implement this stripboard in your module design but if you're handy with the hot-glue gun you can connect it to the VCO stripboard copper side as long as the two don't touch eachother. Anyway I leave that up to you. Here is the link to the expansion board project, Synthesizer Extra's No.4:

Synchronizing multiple VCO's:
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. If you want to synchronize two VCO's then just take a square- or sawtooth-wave out from the main VCO into the Negative Hard Sync input of the second VCO. 
The main function of the sync options on this VCO is actually not to have them track together but to create more interesting sounds. If you input a VCO signal into an other VCO's Hard or Soft Sync input you can get some really cool results if you change the frequency of the secondary VCO with the Octaves control potmeter. It will produce higher tones but the waveforms will be cut to the frequency of the synced VCO giving you all sorts of overtones and harmonic frequencies. If you never tried this I strongly recommend experimenting with this. (Also see the article about the Thomas Henry VCO-555 about synchronization). You will get that famous FM synth sound that can be really phat and rich in harmonics. 
The difference between Hard Sync and Soft Sync is that with hard sync de oscillator always resets the waveform when it receives a sync pulse. With soft sync it only resets the waveform if the slope of the incoming signal is within a percentage distance to the slope of the original signal. So it will ignore the sync signal unless the two oscillators (master and slave) are tuned close to some octave interval. With hard sync it doesn't matter, it will always force a reset and that's why it sounds harder and harsher. Negative, Both or Positive hard sync just determins if the waveform is reset on the negative going slope, both slopes or the positive going slope of the incoming signal. Funnily enough I never had much success with Positive Hardsync with this VCO design for some reason. Maybe I'm doing something wrong.

Doepfer A-190-3V MIDI to CV converter:
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. You can connect any keyboard, that has a 5 pin  MIDI output, to it and it will output a 1V/Octave Control Voltage. It adds a Portamento (or Glide) function to the synth and besides the normal CV out it has 3 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. It also has a Velocity output and a 'Learn' option on CV-4. It will assign CV4 to any mod wheel or knob that you touch on the keyboard. And it also has a USB input so I can connect the synth to my computer. Naturally it also produces a Gate signal for the Envelope Generator. The voltage of all the outputs can be set with jumpers on the circuit-board. I got the A-190-3V which is the Vintage edition which means the panel is black with white lettering to stay in keeping with the other panels in my synth. It's only 5 more euro's than the normal silver edition. I just made a 20cm high panel and cut a Eurorack sized hole in it. I first made it from cardboard so I could easily adjust the size of the hole and when it was ready I transferred it to an Aluminium panel and mounted the Doepfer in there. Then I made a special adapter cable to go from the Eurorack power system to the one I invented for my own synth. (See powersupply article).

With VCO number four I changed the Octave potmeter for an octave switch, as an experiment. I used a 10 step rotary switch and I measured out a bunch of 10K resistors so I had 10 with the same resistance upto 10 Ohm accuracy. They were all 9K99. I soldered on the resistors in the way shown in the drawing below:

So you get 5 Octaves up and 4 Octaves down with a 10 step switch. If you want -5/+5 you'll need a 11 step switch, which you can easily find on eBay. This is more than enough though.
Now, this works but it is not the case that when you turn the switch you land on the exact same note as the previous Octave. To try and address that problem I exchanged the 10K resistor connected to -15V for a 10K multiturn potmeter, with a 2K resistor in series with it, going to -15V. Now it's not possible to tune it so it is spot on but I did manage to tune it so that each octave I go up, I can turn the Fine Tune one stripe up on the dial (decal) and I'm in tune. So you switch one Octave higher, you turn Finetune to the next stripe on the dial and you're bang on. And the same but backwards for switching down the octaves. This works well enough for me :) This will really only work well if you also have a hacked Joyo tuner connected to the VCO so you can see what you're doing.
To get this bang on the right note, you would need to experiment with the 3M3 resistor and try to buffer this potmeter and use really accurate resistors. So I wouldn't advise this switch solution, as it is presented here, for any serious project where everything needs to work perfectly. I'm just documenting it here because it is something I personally tried and want to keep a record of.

Here's a picture of the switch in the panel:

The output waves:
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. BTW, I just found out that to get rid of this ringing you can connect a 1M resistor between pins 4 and 5 of the 3340 chip. But this is not implemented in the layout.

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 of 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 outputs are at the same 10Vpp level. Perfect!  You might have wondered why there is a 2K resistor (R22) in series with the squarewave output. Well, it's there to make the Zener-diode work. Zener-diodes always need a resistor in series with them for them to function as voltage regulators. The zener brings down the voltage from 13.4V to 10V so there's 3.4 Volt that has to be shaven off. That voltage is dropped over the 2K resistor and that's why it's there.

I made a little demo video showing the main features of this VCO. Sorry it's not very good, speech is not loud enough. Don't be fooled by the scope image, the signal really is 0 to +10Vpp but the scope measures the VCA output and that is -5/+5V. The VCA is also the cause for the slight rounding in the saw- and triangle waves. You can not hear that in the sound though. The VCA works fine. Btw, this video was made before I added the Sinewave option so that is not demonstrated here.
One other thing: Please keep in mind that this project and this demo video were made in the early stages of my synthesizer building career and thus I was not yet fully aware of everything you could do with a VCO in terms of using Synchronization and FM Modulation so therefor this demo is a bit basic.

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. This app can tune anything. Set it to the Chromatic Scale for tuning VCO's. 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. Now turn the two trimpots all the way to one side until you hear it clicking. That means it's at the end. It doesn't matter which way you turn. Take a little screwdriver and turn trimpot A up about 3 quarters of the way (that's 22 turns). Now go to trimpot B and turn that up about 1 quarter of the way (That's about 7 turns. 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.)
If you want to be extra precise then let the VCO warm up for 15 minutes after turning it on so it can reach its normal operating temperature, before starting to tune it.

- Launch the Universal Tuner app. on your smartphone or use the tuner of your preference.
- Open up the 'Gain' potmeter on your VCA so you get continuous sound.
- 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. Having the wiper of the potmeter exactly in the middle results in 0V on the 1V/Oct. input so that would be the same as having the wiper disconnected by the switch, but not all potmeters are perfect and that's why we use the switch. Also because a little movement of this potmeter has a huge influence on the frequency. This is less important with the Frequency Fine control although you must make sure not to touch the fine control during tuning and make sure it stays in the 12 o'clock position during tuning.

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.

Here's a PDF about tuning 3340 VCO's that maybe of help to some of you if you have trouble tuning this VCO:  -- CLICK HERE --

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 in the Eddy Bergman Facebook group or in the comment of an other article. (Comments for this article are closed because it got too long.)
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 ^____^

Here is a very interesting article that one of my readers sent to me. It deals with a tuning process for 12 Volt, using four trimmers which only need to be adjusted once, instead of going between two trimmers for ten or more times. I have not tested this methode but it's a very interesting approach so here's the link to the article:

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
Linear Freq. Modulation input with level control.
The other (normal) CV inputs are in fact Exponential Freq. Modulation inputs.
CV-2 input with level control. (This is an Exp. Freq. Mod. input.)
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.

Synchronization and FM input:
The positive Hard Sync synchronizes on the rising edge of a squarewave and negative H.S. on the falling edge. I'm getting excellent results with the Negative Hard Sync and Soft Sync. They all work fine and FM also gives great results. I personally use the negative Hard Sync input for syncing up two or more VCO's. I input a square- or ramp-wave from an other VCO into Neg H.S. and then they both react to pitch changes of the main VCO.  The FM input is also very cool to use. 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. You can also input a Control Voltage from an LFO to get Vibrato or Tremelo effects. I demo this in the video.
Like I mentioned before, the normal CV inputs are equivalent to Exponential FM inputs. Try connecting the output of an other VCO to a CV input and change the octave of the input VCO upwards. Sounds pretty cool!

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's one, two and three. The third one is installed at the top in the second case of the Bergman-Berlin synthesizer. I installed a Sinewave output too in VCO-3, but that was after this picture was taken. :)

The picture below shows the latest VCO, number four, and it now also has a Sinewave output!
Something many people have been asking me about for a long time, but now it's here. The design is too big to be included on the original VCO stripboard layout so I made it on a small piece of stripboard that can easily be added to the original print with a M3 bolt and a little stand-off tube. Read the paragraph below here for more on the Sinewave option.

Here's a picture of VCO number four with the sinewave output and two switches for Triangle and Sinewave with or without a +5V DC Offset. (No offset = +/-5Vpp, with offset = 0-10Vpp):


I added the Sinewave option to the VCO waveforms by adding an extra bit of stripboard with a Triangle- to Sinewave converter, the design of which I took from the schematic of the Thomas Henry VCO Deluxe which you can find in the 'Files' section of the 'Synth DIY for non engineers Facebook Group'.  It's a very simple design so only needs a small piece of stripboard. I think you can easily figure out yourself the best way to add it to your specific VCO panel. I did not use the original Digisound 80 sinewave part of the VCO because it uses a CA3080 chip and there are a lot of fakes of that chip being sold. Anyway I tested that design and could not get it to work.
The Triangle- to Sinewave converter needs a Triangle input wave of +/-5Vpp and you can tap that straight from pin 12 of the TL074 Quad OpAmp chip on the main VCO stripboard. I have drawn an input socket on the layout but if you're building the converter into the VCO panel then just solder a wire straight from pin 12 of the TL074 on the VCO to the input of the sine converter. You can have two outputs for the Sinewave: a +/-5Vpp and a 0/+10Vpp one and you can use a switch like I did (see picture above) to choose which type you want or you can simply use two output sockets. A simple SPDT ON-ON Toggle switch will do fine. Solder each of the outputs to one of the top or bottom pins of the switch and then the output socket to the middle pin, making sure the outputs are nicely grounded the way it should be. You can tap the power supply straight from the VCO stripboard, that insures all the grounds are connected together like they should be.

I altered the feedback resistor (Rf on the layout) from 10K to 15K to get the amplitude correct with the other waveforms of the VCO. This had the effect that the +/-5V output got a negative DC-offset voltage which is why I put a 1µF electrolytic capacitor in series with the output of the +/-5Vpp sinewave with the minus pole towards the source of the sinewave. That resistor should really remain a 10K, but I found the higher amplitude more important. What I could have done, and may still do is change the other 10K that goes from the non-inverting input of the opamp to ground into a 15K also. Then everything is in balance again. Anyway, you should make sure the other two 10K's are matched and the transistors should also be matched. (Match them on hfe, that's good enough).
The output amplitude on a dual 12V powersupply is +/-4.2Vpp or 0 to 9.4Vpp. For a dual 15V power supply it is +/-5Vpp or 0 to 10Vpp.

Here's the layout of the Tri to Sine converter I installed in my VCO:

Triangle to Sinewave converter built into the VCO. This was my experimental stripboard so there's two TL072 opamps on there instead of the single TL074. As you can see, the VCO trimpots are still accessible:

Here's the schematic drawing. You can see I added a 1µF electrolithic cap on the +/-5V Sinewave output because I had a little negative offset voltage present there. See the text above the layout for the reason. If that solution doesn't help then you can use a cap on the output like I did.

Chapter three way back at the beginning of this synthesizer build series, deals with the Triangle to Sinewave converter and I have deleted the original article text and replaced the layout with this one because this one is much better and simpler. It works like a charm. That article also has pictures of the sinewave outputs.

NOTE: The VCO-Deluxe version with the tri- to sinewave converter and sub-oscillator all on the same print as the VCO, did not come through testing and I actually managed to blow up an AS3340 chip. So until I get new components for that project it's been put on ice for now. I'm actually concentrating on an other VCO design, without the 3340 chip (with sinewave option).

More extra's I added to VCO number four:
The panel I made for 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 5V/10V 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 have any questions or suggestions you can go to the special Facebook Group for this website. I had to close the comment section for this article because it became too long but it is full of useful information so check through it and maybe the answer to your question is already in there. Otherwise, like mentioned before, you can post your question on the Facebook Group.
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.
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>

There are now PCB's available made by our Facebook member Brian Harper. Click the link below. This will take you to the schematic and PCB design which you can open in EasyEDA. From there you can generate the Gerber files and order the PCB's.

COMMENTS CLOSED FOR THIS ARTICLE.  Please comment in an other article or on the Eddy Bergman DIY Projects Discussion and Help Facebook group. The comments under this article have become so numerous that it was becoming impractical so I decided to close it. Sorry for the inconvenience. But they are a good source of information so read through them if you have any problems with the build. One other thing that came up recently: people trying to friend me on Facebook with a profile that is only a few minutes old with no pictures or info on it, get blocked by me for security reasons. If you have no FB, please use the comments of an other article to ask me questions.

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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 and a Gain potmeter. 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. I built two of them so far and they are in constant use. 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 later added a clipping indicator and a potmeter to add more Gain to the output signal. 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. This can also be useful for side-chaining for instance if you lead the original signal into the Envelope Follower and then use that to trigger the Lopass Gate. Just a thought :)
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.  You can easily add a passive output to the first channel too, like with the other three channels, by adding a switch and extra socket, but I didn't do that to save space on my panel. The switches also function as Mute switches for the mixer. A very versatile and useful circuit.  
(There's a layout of the mixer with the clipping indicator, further down the article):

(Last revised: 05-Feb.-2020)

Stripboard only:

Here's the schematic for this module:

Like it says in the schematic, it doesn't really matter what value potmeters you use as attenuators but it's probably advisable not to go above 1M Ohm and not below 47K. 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. (Of course no other signal must go into the mixer otherwise the signal at the mix output will contain the mix of the multiple inputs but you can use the other passive attenuators at the same time.) 
A very good idea was posted in the comments below and that is to use the socket switches of the attenuator outputs instead of using toggle switches. If no cable is attached to the output the signal will go into the mixer and if you attach a cable the signal to the mixer will be cut and you just have attenuation. The downside of this approach is of course that it is not possible to split the signal in two by having it go into the mixer and still be available at one of the other outputs but it will save space on your panel not having to include toggle switches. It's up to you as to how you want to use this mixer.
So a very versatile mixer design! The mixer takes + and - 12 Volt but will work just as well on +/- 15 Volt. 
Because it's a tiny little circuitboard and weighs next to nothing, 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 replacing 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. Like I mentioned earlier, I've built two of them so far and they are used all the time!
Sam Battle of LookMumNoComputer fame has a similar setup with the switches choosing between 'Mix' and 'Individual Output' on his quad VCA module, which you can see in this video (5m33 into the video). I noticed this only recently when I watched the linked video and when he talked about the VCAs it reminded me of this mixer.
You might wonder why the input signal is connected to the negative (inverting) input of the opamp. Why not just on the positive (non-inverting) input and do away with the second opamp? Well that's because the opamp's summing function only works on the negative (inverting) input. The way it's shown in the schematic is the right (and only) way to do it.
You can also ad a 'Mute' function by simply putting an extra single pole double throw switch into the mixer output. Some people find that a handy function to have but it's not included in this project.

Here's a picture of the finished panel with the blue clipping indicator LED.

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. (I had actually drilled the 3mm hole by accident so I put a LED in it to fill it up.) 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 detectors and I found some low resolution circuit images. I used one of those to draw my own schematic in a better readable higher resolution:

Here is a Falstad simulation of the clipping circuit:  --- CLICK HERE ---

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

I had already built the mixer previously so 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. 
You can use a variety of (dual) opamp chips for this circuit. The TL072, TL082, NE5532 or even a 4558 or an LM358 will work for both the clipping indicator and the mixer. It doesn't matter which you use where, they all work fine. As long as the dual opamps are suitable for audio circuits and are pin for pin compatible with the TL072 (see datasheets). Note the opamp in the clipping indicator circuit has it's minus pole (pin 4) connected to ground. It only needs a positive voltage source not a dual one. 
Btw, you can use other value potmeters for the attenuator/mixing pots. I used 1M but 100K or 50K will work too. I'd recommend not to go lower than 47K though.
All potmeters are viewed from the front with shaft facing you.


Stripboard 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+. 30-March-2021: Cosmetic changes to make layout clearer.)

Here's the layout for if you want to add a 'Gain' potmeter. With a 500K potmeter the gain will be between 0.5 and 5.5 times and with a 1M pot it'll be between 0.5 and 10.5 times. 500K is really enough because with gain set to above 3 times the audio will start to clip anyway but it's up to you.
Note the 100K resistor over pins 1 and 2 of the mixer IC (to the left) is now changed for a 47K in series with the Gain potmeter. The 47K resistor is there to make sure the feedback resistance can't go all the way down to zero. I've highlighted the potmeter connection in the square on the layout. The rest is the same as the previous layout (although this is a somewhat older version but it's all correct.)

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 stripboards behind the Mixer panel. You can also see how I glued the two 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.

If you want to add some grit to this mixer in the form of variable distortion, then you can. It's a matter of adding two diodes and a potmeter. This video by Moritz Klein explains how you can do that and how it works.

Okay, that's an other one done. If you have any questions or remarks please put them in the comments below or post them in the special Facebook group for this website.

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