Synthesizer Build part-47: DUAL LFO for EURORACK.

A simple LFO with pulsewave (with variable pulse width) and a seamless transition between a Ramp wave, Triangle wave and Sawtooth wave using one potmeter. With LED rate indicators and Speed and Shape controls.

Well what more is there to say about this LFO. It's such a simple design that I could easily fit two of these on a small piece of stripboard and still have it small enough to fit a normal Eurorack case. The circuit is derived from the 'Utility LFO' by Ken Stone which is a larger version of this LFO. 

I now also have a project for the complete Utility LFO and it's even smaller than this one and a panel width which is only 1HP wider at 9HP.  Go to project 50 for that.  

This LFO is still useful on its own though because it is so small. It can easily be incorporated into other projects as an on-board LFO for instance.

The depth of this module is 55mm. I made the panel 4CM wide, that's 8hp, and I put the potmeters to one side leaving enough room to glue the print straight to the back of the panel at a 90° angle using hot glue. All the output sockets fitted nicely next to eachother at the bottom.

Naturally you can just as easy build this module in the Kosmo size and run it on 15V. If you do, you need to keep to the resistor values as they are in the schematic, not the layout because as I mention further down, I changed the 1K output resistors to 1K8 to get a nice +/-5V output signal. If you power this with 3 more volts you probably don't have to do that. Do some testing first to make sure though.

I tried my hand at using Falstad recently and tried to make a simulation of the complete Utility LFO circuit and it was surprisingly easy to do. 

So here is my very first ever Falstad simulation: --- CLICK HERE ---

Here's the schematic drawing of the dual LFO circuit:

The module consists of two of these circuits on a single piece of stripboard. I placed the LEDs on a separate piece of stripboard with a dual opamp, the good old TL072, and I used bi-coloured 3mm LEDs in red and blue. I drilled two 3mm holes to the left and in the middle of the first two- and last two potmeters for the LEDs and glued them in place with hot glue so the little print sits over the potmeters. See pictures below for illustration. Btw, you can use any dual opamp chip for this circuit as long as the pinout is the same; like the TL082, NE5532, LM358 etc.

LAYOUT:

Here is the layout I made for this Dual LFO. As always, the layout is verified. I used it for my build. I placed the Eurorack powerconnector on the left side for better access. In my build it's on the other side and very near the panel. Not a good place for a power connector but you only find these things out when you start building it. See, I make the mistakes so you don't have to LOL! (I hot-glued the print to the back of the panel with the righthand side closest to the panel.)

Stripboard only:

After doing the first tests I found the output voltages a bit on the low side. They were just +/-3,24V so I decided to experiment with the 1K resistors between the outputs and ground. I tried several values and I ended up using 1K8 resistors. That brought the output voltages to a nice +/-4,8V. Almost 5V so that's perfect for eurorack. If you want that voltage to be even higher in your LFO then experiment further with making the resistor(s) between the output socket and ground even higher in value.

I wanted to make one of the LFO's a bit slower than the other to give me a wider overall range so I used a larger capacitor for LFO number one. I used a 147nF and that made it perfect for my needs, between 0,2Hz and 10Hz. In the layout both timing caps are 47nF though.

 

TECHNICAL DATA:

Here are some measurement results for this Dual LFO:

Duty cycle of squarewave is 5% to 95% this varies a bit with the frequency but not more then 2%.

Lowest frequency: LFO-1 = 0,219Hz  LFO-2 = 0,653Hz (changed timing cap of LFO-1 from 47nF to 147nF)

Highest frequency: LFO-1 = 9,82Hz   LFO-2 = 34,2Hz

Output voltage is +4,8V or 9,6Vpeak-to-peak. That's after changing the 1K resistors in the schematic for 1,8K ones. Otherwise the voltage was just 3,2V and 6,4Vpp.

Current draw: positive: average 12mA max.: 18mA

                       negative: average -13mA max.: -18mA

Here's the Bill of Materials:

Here are some screenshots from the oscilloscope with some measuring data underneath the images. Some images may still show the lower output voltage but that's been fixed:

The following are screenshots from the oscilloscope showing two signals, one from each LFO, being combined in a simple passive multiple. A squarewave and a triangle wave each at different frequencies. The results are pretty cool looking:

In the top picture you see more of the waveform in the positive voltage region and very little below zero Volts. You can set that with the shape potmeters to your own liking or best sounding result. As you can see this makes the Dual LFO module much more versatile as a modulation source. Plenty to experiment with.

PICTURES:

Below are some pictures of the print. I took these before I changed the 1K resistors to 1K8 ones. In the top picture and the 3rd one you can see how I mounted the little print with the bi-colour LEDs. The print rests above the middle two potmeters and the LED's are bent backwards over the sides of the stripboard and go straight into the holes in the panel and are secured with hot-glue. The little print itself is not mounted in any way. It just relies on the LEDs to keep it in place.

This time, instead of spray-painting the panel I decided to keep it blank aluminium and I used an engraving tool to put the text on. That didn't work too well and it didn't look good at all so I printed some labels I made in Photoshop, laminated them with Scotch Tape and put some double sided sticky tape on the back and I put those on the panel. That looks much better. 

TRIGGER OUTPUT

A few days after completing this build I added a trigger output to this module. I connected it to the squarewave output of the second LFO (the faster one). I thought it might come in handy to have a trigger source. You can see in the picture below how I did that. It gives of both positive and negative trigger pulses of 5V and a length of about 4mSec.  If you're thinking of putting in a diode to only get positive pulses forget it. That won't work. It'll kill off the pulses completely. If you turn the Shape potmeter the positive and negative pulses will move further away or closer to eachother. Just like the rising and falling edges of the squarewave with different pulsewidths.

(The above drawing actually translates to a high pass filter with a cut-off frequency of 268Hz. So it filters out the actual square- or pulsewave and only lets through the initial harmonics of that wave, creating this spike pulse trigger response, but you can forget about this theory. This is not important.)

Here's a look at the final panel with trigger output. I just made some labels with text to put on the panel. Looks better than the engravings.

One other thing worth noting is that because we have two LFO's on one board, they will very slightly influence eachother. What I mean is, if you have one LFO running at almost twice the speed of the other, the faster one will adopt some multiple of the rythm of the slower one if you set the speed to some value close to that. That's a form of resonance and I won't get into the technicalities of that but it's quite easy to set an LFO at twice or 4 times the speed of the other because they share the same circuitboard. It's the same idea as when you have a group of people walking together and they all start to walk at the same pace. That's also a form of resonance. Don't think this will be an obvious thing to observe. The occurrence is very subtile.

Okay, that's number 47 done! A very useful little module and I saved a few bob by building it myself instead of buying a dual LFO module. Okay it doesn't have any fancy extra's like synchronization but that's okay by me. I think I'll mostly be using this as a clock source and some random modulation. That's why I made both LFO's run at different frequency ranges.

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

This is just two Korg MS-20 filters behind one panel with switches to go between using them as individual filters or switching them in series.  This is one of my early projects so the implementation is a bit clumsy with how I use the switches etc. but I'm keeping the article up for archival reasons plus this dual configuration actually sounds pretty awesome. But I wouldn't be surprised if you found a better way to make a dual MS20 VCF than this one. 

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

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

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

In the first filter I used the LM13600 chip and in the second the LM13700 chip. And having them side by side is a good opportunity to compare them and the LM13600 is a bit tamer than the 13700. So if you have both chips in stock you can decide whether you want your filters to sound aggressive or a bit less aggressive. It's not a big difference though.

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

Here is the wiring diagram for those two switches. By all means try and think of a better way to do this. This was one of my early projects so not everything is perfect. I still get confused by these switches when I use this module after 1,5 years of using it LOL

It would be better to use one double pole switch and connect the output of filter 1 to the wiper of one pole and the input socket of filter 2 the wiper of the second pole. With the switch set to the left the output of filter 1 goes to the output socket 1 and the input of filter 2 goes into filter 2. Set to the right the output from filter 1 goes into the input of filter 2 and the input socket of filter 2 is left disconnected.

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

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

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

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

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

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

Synthesizer Build part-10: THE VCA or VOLTAGE CONTROLLED AMPLIFIER.

A very simple, old school, transistor based VCA circuit that's tried and tested. Easy to build and small enough for Eurorack. You do need an oscilloscope to calibrate it though.  This module runs on +/-12V as standard so a very Eurorack friendly design.

EDIT April 2024: There's now also a new VCA project available that's also a 3 channel mixer and has a line out option. Please go to Project 58 to check that out. 

Please read the whole text before building this project. 

WHAT'S A VCA FOR?

The VCA is nothing more than a voltage controlled volume knob. It lets audio pass through when you press a note on the keyboard and it shuts it down after you let go of the key. (So actually the term Voltage Controlled Amplifier is a bit misleading because it does not amplify the signal, it attenuates it.) The opening and closing of the VCA is done using signals coming from the Envelope Generator or ADSR or from an LFO depending on what you use it for. Don't mistake a VCA for an Audio Amplifier. You can not hang speakers on the end of this circuit. A VCA is used to make sure your synthesizer only produces sound when you press a key on the keyboard. The keyboard produces a Gate signal that is high as long as the key is pressed down and a 1 Volt per Octave signal that tells the VCO which note to play. That Gate signal then triggers the Envelope Generator and the output from the Envelope Generator goes into the VCA together with the audio output from the VCO and as the VCA detects the Envelope signal it opens up and lets the audio pass through with the volume or amplitude depending on how high the envelope signal is in voltage. The output of the VCA must later be attenuated to audio line level if you want to feed it into a HiFi amplifier. So the VCA is just a link in the synthesizer chain.

In the picture below you see a block diagram to show you the position and function of a VCA in a synthesizer. There seems to be a lot of confusion about this with people new to (modular) synthesizers. The VCA is the white triangle marked 'Amplifier'. I does say "Output to amp/speaker" but they mean that the VCA outputs a line level signal going into a normal HiFi power amplifier. There is no line level output in my VCA layout but it's easily added with a voltage divider.

(Image taken from 'The Complete Synthesizer' PDF Book)

I used a very simple design for the VCA which I again found on the Yusynth website.
Here's the schematic:

If you want to experiment with this circuit then here is a link to a Falstad simulation I made:  --- CLICK HERE ---

This is an old design and there are some updated versions out there but this is tried and tested and has served me well for over 3 years now. It's very simple and it works very well except that in my build the signal came out inverted. This isn't really an issue because it's an audio signal and they sound the same whether inverted or not but my Obsessive Compulsive Disorderly mind wants it coming out the same way it came in so I added a little opamp inverter to the output to set this straight. Afterall a VCA can also be used for Control Voltages like signals from an LFO and in that case polarity is important. 

I put a 10µF Electrolytic Capacitor on the output (plus connected to output VCA) because I noticed a 240mV DC offset voltage on the output which I couldn't trim away with the potmeters. This may be due to a slight difference in resistance between the two 10K resistors connected to opamp U1b. Make sure you measure those and use two that have the same value resistance. In fact all resistors with the same value should be matched because this circuit depends on symmetry!! The transistor pair must be matched too.

Use an oscilloscope to set the trimmer potmeters. You should be able to measure a DC voltage (before the 10µF cap I mentioned earlier) and, with trimmer R18, trim away as much DC voltage on the output as you can and with the other trimmer R14 you can trim the balance of the signal. You set it so the positive part of the wave has the same amplitude as the negative part of the wave, with the zero volt line being the dividing line. If you want to use this VCA for Control Voltages then you can't put a capacitor on the output because that will block low frequency signals, which it sees as DC.
The output level may be a bit lower than the input level, even if the ADSR potmeter is fully opened up. If that's the case and you want to correct that then you can change the gain of the output buffer opamp. This is something I added myself and is not included in the schematic but only in the layout. If you change the 150K resistor between pins 1 and 2 of IC-2 for a 470K resistor, you should get 3 times gain! That should bring the level back to input level. You can experiment with this yourself. If you put a 500 KOhm potmeter between pins 1 and 2 you can control the gain of the opamp with a knob on the panel. Just a thought ;) Make sure you give it a different name though otherwise you'll have two gain potmeters on your panel :D 

If you are interested in the functions of the transistor pair and the other components in this circuit then here's a link to a blog post that explains it in great detail:  -- CLICK HERE --

LAYOUTS:

Here's the layout I made for standard 24x55 hole stripboard using only 24x39 holes. Like I said before, I added a signal inverter in the shape of a second TL072. Only one of the two opamps in the chip is used, the other one is properly connected to ground. Make sure the Gain potmeter is linear. In fact use linear types for both pots. You can also use other values like 47K or 100K because one is an input level control and the other a voltage devider switched between 12V and ground so the values have no influence on the working of this circuit.
The layout below is verified and absolutely faultless. I guarantee it. It has been used successfully by many people now. All potmeters seen from the back.

Stripboard only:

Here's the Bill of Materials. It says to use logarithmic potmeters but you can also use linear types. In fact for the Gain pot it's best to use a linear one:

EDIT 24-April-2025

I made a new smaller version of the VCA without any extra's. It's just what you see on the schematic drawing. This will be handy if you need a few of these behind one panel or if you're building for Eurorack. I haven't built this layout so I can't say it's verified but I double checked the connections with the schematic and I can't see any mistakes. If you do please contact me immediately.

I did switch the 10K connections to the opamp U1b in the schematic. The left BC547 (Q1) is connected to the inverting input (-) and the right BC547 (Q2) is connected to the non inverting input (+). I can not imagine that will make any difference. It doesn't in the Falstad simulator anyway.

Here's that layout, wiring:

Stripboard only:

Connect the 3 ground positions of the eurorack powerheader together by flowing extra solder to bridge the gaps and short them together. 

Here's a picture of the finished VCA installed in my synthesizer:

The Level control is for the ADSR input signal and determins the volume of the audio signal. The Gain controls the quietness of the VCA when no keys are pressed on the keyboard and it should normally be set to zero. If you turn it up, the last note you played will become audible.
You must use an oscilloscope to test the signals and trim the offset and signal levels. Put it in DC mode when testing. 

In the picture above you see a yellow patch cable connected to the audio output. That is actually my oscilloscope probe, so I can see the output signal on my scope screen, and the audio output is connected internally, behind the panels, to the Line-Out and Effects Unit to the left. From there the audio signal goes to a stereo RCA output on the back of my synthesizer and from there to the Line-In of my HiFi Audio Amplifier.

Below is a picture of the double VCA that I built on April 25th and 26th of 2020 from the same schematic and layout. These VCA's work like a charm! They are so handy to have, I use them a lot in patches as a sort of Gate to let audio through when there's a signal on the ADSR input. When the ADSR potmeter is turned fully open (clockwise) the output signal will have the same strength as the input signal. Of course this is also influenced by the type of control voltage you feed it. 

With the double VCA I wired up the inputs in such a way that when there is no connection made to input 2, that input gets the same signal as is present on input 1. (Input 2 is normalled to input 1 as it is officially called.) So with one input you get two outputs. As soon as a patch cable is connected to input 2 that connection with input 1 is broken (by the socket switch of input 2) and the VCA's work as two independent VCA's.

A look behind the panel:

Eurorack module:

EDIT 25-Januari-2022: I'm now building a Eurorack system and I needed a VCA that I can use until I buy some ready made VCA's so I took one board out of the Double VCA I built earlier and made a Eurorack sized panel for it. This worked perfectly and I just transplanted everything to the new panel and it worked fine. Here's a look at the new module. I sawed off the extra copper strips that weren't in use (where the M3 bolt went through) which left me with a 52mm deep print which I glued directly to the back of the panel next to the potmeters and sockets which are all on the right side of the panel.

Here are some pictures of the Eurorack panel:

I made an extra output on this VCA that has 1/10th the amplitude of the normal output. I though that might be handy to use as a line level output but I'm not sure if I'll need it or use it. It just consists of a 100K resistor going from the output to the 1/10th output and a 10K resistor from the 1/10th output to ground to complete the voltage divider.

Troubleshooting:
If, after building this VCA, you have trouble with noise, especially at low volume levels, then you most probably need to replace the opamp you're using. I've had people contact me about this and it turned out the opamp was the cause. An other cause of excessive noise has been bad soldering joints. So make sure your solder connections are good. 

Like I said, I built 3 of these and they all work fine and are absolutely quiet.

Here's an oscilloscope picture of how the VCA lets through audio (the yellow waveform) only when it gets the Control Voltage from the ADSR (the blue line). You can see how the amplitude of the audio closely follows the amplitude of the CV from the Envelope Generator:

I had some eurorack layouts here before but I'm having problems with them so I took them out again. I won't have layouts here that are not verified.

Here's a link to a Falstad circuit simulation of this VCA showing how it works. 

-- CLICK HERE --

--- OR CLICK HERE ---

If you want to know more about the theory behind this VCA or in effect, how this VCA works then I have a link here to a great blog post that explains the workings of this very circuit.

https://kassu2000.blogspot.com/2015/10/vca.html

There's also a fantastic video by Moritz Klein which explains the technical details about the workings of a circuit like this and he also explains why the transistors need to be matched.

https://www.youtube.com/watch?v=yMrCCx6uqcE

Okay, that's it for now. Any questions? Put them in the comments below and I will answer them asap. You can also post questions in the Facebook Group for this website.