Synthesizer Build part-27: QUADRATURE LFO Bergman edition.

A very quirky sinewave LFO with 4 outputs with a 90° phase shift to eachother. With extra waveform and an input for Frequency Modulation.

The quadrature LFO is something you won't find in many modular set-ups from the early days but it's a real little gem of a module to have in your set-up. It produces 4 sinewaves that are shifted in phase by 90° each. This can be used to create the 'Barber Pole' or 'Shepard Tone.' effect. "What's that?" I hear you ask. In short, it's an effect whereby it seems a tone is continously rising (or falling) without actually seeming to get any higher (or lower). See this Wikipedia Article for more on this effect. So that's one use. You could also feed four Voltage Controlled Panners with this LFO and have the sound go three-dimensionally around the room. Your imagination is the limit with this module which is why I was keen to experiment with this.

Eurorack 12V vs 15V.

I tested this circuit on a dual 12V powersupply and it works the same as on 15V. The maximum amplitude of the sinewaves stays the same too (+/- 7,5 Vpp). I think this is due to the zener diodes. They keep the amplitude constant. The maximum amplitude of the Trapezoïdal wave however will be lower at +/-10Vpp maximum. You can of course turn the amplitude of the waves down with the Output Level knob to, for instance, the Eurorack standard of +/- 5Vpp. No problem. So this is, in the way it works, a very Eurorack friendly module. For Eurorack you can cut the stripboard in half along the cuts in the copper strips, in the middle and fold it over. Then connect the top 3, strip J and the bottom ground copper strip together again with wire and you have a print small enough to fit behind a Eurorack panel.

How I came across the Trapezoïdal function:

I found this module with the schematic on the Yusynth website and set out to make a layout. Once again it turned out I made a faultless layout but it was with building it up that I made a mistake that turned out be an asset later on. This design uses two 5V Zener Diodes and somehow I had managed to stick one of them in the wrong hole on the stripboard so it was not connected correctly. This resulted in the output being Trapezoïdal instead of Sinusoïdal. Not knowing I had made this mistake, I posted about this on the Synth DIY Facebook page and people allerted me that it had to be the Zener Diodes that were at fault and they were right. After correcting the mistake everything worked fine but I then got remarks that I should include this waveform option in the final design of the module. So I did. I adapted the stripboard layout and added a switch to go between the two waveforms. If you want to build it as originally intended, without the switch, then just don't cut the copper between the two connection points for the switch on the stripboard, so the zener diode is connected again.

One more very important thing to note: YOU MUST USE POLYSTYRENE OR POLYESTER CAPS FOR THE TWO 10nF CAPACITORS!!  I've had several people build this module and then it didn't work because ceramic caps were used. They need to be high quality non-ceramic ones or it simply won't work.

Bill of Materials:

LAYOUTS:

Here are the new layouts. I have deleted the old ones I made because I don't want anyone using them. As always the layouts are verified. I added more cuts to the right of the stripboard to make the copper strips a bit shorter and to prevent short circuits with anything touching the side of the board. 

Wiring diagram:

The current limiting resistor for LED D4 (bottom right) is a 12K and not a 1K2 like the other three. I did this because otherwise the white LED would be much brighter than the other ones. So it's not a mistake, it should be 12K for any white LED you use.

Stripboard only:

Red wirebridges are connections to +15V, Pink ones are connections to -15V, Green ones are connections to ground and the blue ones are internal connections between components.

Cuts and wirebridges as seen from the Component Side!!

As always, mark the cuts with a Sharpie or Edding pen and then put a pin through the marked holes and mark them again on the copper side. Then cut the strips at the marked holes with a 6 or 7mm sharp hand-held drill bit.

THINGS TO KNOW AND SCHEMATIC:

The LFO needs a few seconds to start up, when you first switch it on. If the 'Rate' potmeter is turned all the way counter clockwise it won't start up at all. With Rate set to 1Hz it takes a good 20 seconds to start-up and before the LED's light up. So make sure it's not on minimum when you switch on, or you could have a long wait on your hands. Turn Rate all the way up and it'll start up almost immediately.
The normal sinewave output goes from -7.2V to +7.2V with a frequency of about one cycle per 30 seconds to about 140 Hz. You can set the lowest frequency you want available with the trimmer on the stripboard (T1). I had a warning from someone on YouTube not to set the trim pot all the way down because his went up in smoke. I had no such problems when testing this print but I just thought I'd mention it here. Anyway, he must have done something wrong because that shouldn't even be possible.
The Trapezoïdal wave has a higher amplitude and lower frequency so beware of that when you switch between them. It's maximum amplitude is -14,4 to +14,4 Volt and maximum frequency is 52Hz compared to the Sinewave's 140Hz. So you can roughly say the Trapezoïdal wave is double the amplitude and half the frequency of the Sinewave. That means both waves go high enough in frequency to be in the audio range so you can hear them. That opens up a wide range of uses for this LFO. One of my favourites is to have the CV control the Cut-Off frequencies of multiple filters and then feed the Quad LFO FM input with a ramp wave from an other LFO so you get a sweep. Sounds very cool!
The amplitude of the waveform(s) can be set by the dual gang 100K potmeter so you can set it at any level between zero and the maximum I just mentioned. The direction of the phase shifting between the 4 outputs can be turned around by the DPDT switch. You can see the direction it has by four 3mm LEDs on the panel. I didn't have an Orange LED so I used Red, Yellow, Green and White LEDs and the white one is quite a bit brighter than the rest so that's why it has a 12K resistor as current limiter and all the others have 1K2 resistors. This keeps the brightness perfectly in balance with the other three LEDs. One little drawback is that the LEDs only come on when the dual-gang output level potmeter is at the ten o'clock position or higher. At the lowest levels the LEDs don't light up. That's just how the circuit works.
Instead of the LM13700 you can also use the LM13600 and the TL074 can also be a TL084. From testing I also came to the conclusion that the 10nF caps don't necessarily need to be matched so closely. But remember they can NOT be ceramic capacitors. They don't work in this circuit. The two PNP transistors do need to be matched but you can use your multimeter's transistor tester for that and matched them to within 2 points of their hfe or amplification factor.

Here's the schematic I made the layout from:

I made a Falstad simulation of this circuit and it shows the start-up time really well. When you drag the 'RATE' potmeter to the left (faster rate) it will start up quicker. If you drag it the other way it will take quite a while.

---- CLICK HERE for SIMULATION ---

Here's a picture of the finished panel:

Here are some screenshots from the oscilloscope:

Here you can see the beautiful and absolutely perfect sinewaves this LFO produces:

Here's the feature I added myself. Trapezoïdal waves. You can see the amplitudes differ a tiny bit from eachother but that's not a problem in normal use. It's the same with the Sinewaves btw.

Fast Fourier Transform (FFT) image of the sinewave at 11Hz. As you can see, it's a near perfect sinewave at this frequency. Almost no harmonic spikes to the right of the main wave. When the frequency gets higher the sinewave becomes a little less perfect but that's only visible on a oscilloscope's FFT readout, not with the naked eye:

Here's what happens when you add a Ramp wave from an LFO to the FM Input. You get a frequency sweep:

Finally a little video showing all the different aspects of this module.

Here's a video of the first test I did with the Quadrature LFO. The patch is set up as follows:
3 VCO's are feeding 3 filters with a squarewave. The Steiner, Korg and the ARP filters. The Steiner-Parker receives two sinewaves from the Quad-LFO, one on CV-1 and one on CV-2 which are 90° separated in phase. The FM input of the Quad-LFO is being fed by a 0 to 10V ramp wave from the other LFO. Each filter output goes in a different channel of the mixer where they are summed and the output then goes through the CaraOK effects unit set to preset 61 which is a chorus effect. From there it goes to the speakers. The "da-daa-dang da-daa-dang" drone you hear is produced by the waves from the Quad LFO. :)

Here's an other short sample of the Quad LFO. It's being fed by a slowly rising ramp wave and two of the outputs are going into the Steiner Parker filter. The ARP and the Wavefolder are also in the mix:

Just for my own record keeping, here is an image of the state of readiness my synthesizer is in now. I have 8 free power-buss connections left so plenty of room to build more modules in the near future.

Okay, that's it for this one. With special thanks to the folks over at the Synth DIY FaceBook Group for all their help in the initial troubleshoot.
As always, if you have any questions or comments please put them in the comments below. 

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]

MENU:
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.

ANOTHER SMALL SCOPE:

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.

MENU:

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.

2 CHANNEL OSCILLOSCOPE:

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.

--CLICK HERE--

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.

Synthesizer Build part-7: THE MOOG LADDER FILTER.

The iconic Moog Ladder Filter. This version is built with transistors only, not transistor array IC's. This is an early project of mine so please bear that in mind when reading the article.

A note before we begin: I have PCBs available for the Moog Ladder Filter using the CA3046 transistor array chips. Just go to the 'PCB Service' option at the top of the menu for more details.

Since this article was written I have made a new version of this filter, this time including the CA3046 transistor arrays and it works very well and no need to match transistors with that version. If you want to build this filter I would really advise you to use that layout instead of this one (although this one works fine too of course). You can find it in Chapter 39 (click here).

I used the schematic from Yusynth's website.

Before we start. Most people always want to know if it works on 12V. I tested the filter on dual 12V and it works just fine.
In this schematic the top and bottom transistors are applied in the form of a transistor array chip, the CA3046, but I couldn't get hold of that quickly enough and this early in the build I didn't really trust myself to design a layout including those arrays, so I decided to use all transistors and that works just as well. It makes the layout a lot easier. It is always mentioned that you must use matched pairs of transistors for this filter but really, that's a throw-back to the early seventies when transistors were not as consistent and reliable as they are now so if you have transistors from the same batch they will probably be matched well enough but put them through the transistor tester on your multimeter and match them on hfe value. The only place where the transistors must be matched well is on the place in the schematic where they use the CA3046; the top and bottom of the filter and the output on the side. I personally matched all my transistors by using the Transistor Curve Tracer I described in an earlier article on this website.
I built a second ladder filter as a test for the layout below and I used all unmatched transistors. The layout works fine but using unmatched transistors did not turn out well. I could not get the resonance trimmed correctly and there were enormous differences in volume when using the resonance potmeter. I used a squarewave for testing and the top of the squarewave had an angle to it instead of being horizontal. So you must used matched transistors!

This filter has a few quircks that you need to know about but which are normal for this design.

- The Resonance potmeter has only a small area of influence. For most of the throw of the potmeter you will hardly notice anything. This is normal for this design. That's why we need a reversed logarithmic potmeter for Resonance. To stretch out that last bit of the potmeter.

- When the Resonance is fully open, the output volume drops. This too is normal for this filter and even the original Moog ones have this. Yusynth also talks about this on his website.

- If the audio input level is too low you can loose the self-resonance on the bottom parts of a squarewave. (If you use a squarewave as input wave of course). Again, this is a known quirck of this filter type. It needs decent level of audio input.

The build is quite straight forward but you need to be very accurate. The 50K anti-logarithmic potmeter for the Emphasis or Resonance control was an other thing I couldn't get a hold of so I made my own by using a linear potmeter with a 5K resistor between pins 1 and 2. This works very well, In the layout I used a reversed logarithmic potmeter and I show the alternative that I myself used, next to it.

The input level potmeters are 50K logarithmic ones but if you don't have those just use linear ones. They don't even have to be 50K. You can use 100K or 1M or even 10K if that's what you have available. They're just audio input level pots so they act as attenuators or voltage dividers and the value has no impact on the working of the circuit. This goes for all the level potmeters in all the projects on this website unless it is mentioned otherwise on the layout.

The Frequency Cutoff potmeter however must be a 10K!

I made a layout for stripboard including the wiring. I used this layout to build a second filter and it worked straight away so this layout is verified. (All potmeters viewed from the front):

(Last revised: 24-June-2020: Corrected polarity of C3. 15-July-2020 added alternative for reversed potmeter.)

Stripboard only. Beware that some stripboards are sold with 56 instead of 55 holes horizontally. The layout is 55 holes wide:

Here are a few pictures of the finished circuitboard:

As you can see in the pictures, I added two trimpotmeters which are not on the stripboard layout above. These are two 200K trim pots and they go over pins 1 and 2 of each opamp, to make the gain adjustable. It says in the schematic to 'adjust the value of the feedback resistors according to audio level'. These trimpots make that possible without having to use the soldering iron. It's a bit awkward with the wires but I had to put the potmeters on the print where there was room enough to accommodate them and the wires. Plus I added them as an after thought, so after I made the layout. At least they are all neatly in a row. :-)
For clarity I made a second layout which includes these alterations. If you decide to replicate this then don't forget to remove (or not solder in) the original resistors over pins 1 and 2; the 56K on IC-1 and the 120K on IC-2 because these are replaced by the trimmers, as the layout below shows. Lay-out is verified not only by me but I heard from many people who built this filter successfully using this layout.:

I lowered the Control Voltage input resistors from 100K to 5K6. In the schematic they are 100K but after installing the filter in my synth set-up I noticed that the CV hardly had any effect at all when I connected a LFO to it, so I lowered these to 5K6.

After finishing the build I tested the filter on the oscilloscope first and set all the trimmers to the right positions. It's easy enough to do, you just watch the scope for the best response. Then, after installing the filter you can adjust the trimmers to get the best sound out of it. This filter is self-oscillating, meaning that if you have nothing connected to the inputs and you turn Frequency Cut-off and Resonance all the way up the filter will oscillate of its own. There's no 1V/Oct. input though so resonance won't keep track with the notes on the keyboard. An other frequently occurring problem is that the self resonance only occurs at the top of the squarewave and not the bottom part. When you start it up it will self oscillate on both the top and the bottom but as soon as the transistors warm up, about 20 seconds, the bottom oscillations disappear. This happens when the transistors are not perfectly matched. Using the CA3046 transistor arrays will solve this and that's why I made a second Moog Ladder Filter project using the CA3046 chips. You can find that in chapter 39

The first all transistor filter I built didn't have this problem so if you're careful with matching the transistors you should be fine. Please note that the input volume is also an influence on this. If the level is too low you can also have the bottom oscillations disappear.

Here are some oscilloscope images to illustrate what I mean:

Self oscillation when filter is just switched on:

And here's the situation after about 20 seconds. The bottom oscillations are gone. The filter still sounds pretty cool though:

This is something I only discovered a short while ago but well matched transistors or using the CA3046 chip should solve this issue. The first Ladder Filter I built using all transistors didn't suffer from this problem because I was careful to match the transistors accurately.

MATCHING TRANSISTORS:

 I used to advise people that matching transistors on their Hfe value was good enough but I have found an easy way to do it properly which I advise you to use. This is the Ian Fritz method. We set up a small differential amplifier on a little breadboard and measure the voltage between the two emitters. If the transistors are matched that voltage should be zero. Here's a schematic of the test setup:

The diode is a 1N4148 or any silicon diode will do really. It's voltage drop ensures both transistors get the same base-collector voltage. Make sure the 100K resistors have exactly the same resistance value!

Beware this setup uses a dual 12V powersupply.

VIDEO DEMO:
Here's a video showing the test results on the oscilloscope so you have an idea of what the waveforms should look like. In this first filter I built and which I demo in this video the transistors seem to be reasonably well matched because I do retain some self oscillation on the lower parts of the squarewave:

       

A demonstration of the sound of the filter:

I recently made a Falstad simulation of the ladder filter and it works reasonably well. It shows the resonance and the trimmers work as expected. It even shows the amplitude getting smaller with resonance and self oscillation. You can view the simulation by --- CLICKING HERE --

Here's a look at my PCB for the filter. This is what is actually in my synthesizer and I build it from a different layout that I made earlier especially for the Eurocard format of stripboard:

Here's the panel I made for it:

As you can see in the picture, I've installed a bypass switch for this filter. It's great to have this filter in series with the Korg MS-20 or the ARP2600 filter but sometimes you want to be able to easily switch to one filter. With a bypass switch there's no need to constantly connect and disconnect patch cables so this is really helpful. The switch only works with audio input 1 and it sends the signal straight to the output jack and disconnects the in- and output from the in- and output jacks at the same time. Here's the wiring diagram for the bypass switch:

Okay that's it for this project. Way more synthesizer build articles to find on this website and while you're here leave a comment please!