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

It's a mixer and a passive attenuator in one with added 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. Then I streamlined the layout and made it much more compact so if you want the latest version then scroll down and check the last layout in 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 split in two and also lead 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 toggle switches for the signal to be added to the mixer.

LAYOUTS:

There are four different variations on the mixer theme represented in this article:

- Right beneath here we have just the mixer/attenuator stripboard.

- Below that we have just the clipping indicator stripboard.

- Below that is a spread out version of the mixer with clipping indicator which will work for the Kosmo sized modules.

- And finally we have a Eurorack sized stripboard project which includes the clipping indicator but is only 24 strips by 26 holes, so very compact. That's the version I would advise you to build. All the previous ones are older projects, made very early on in my synthesizer building career.

PROJECT 1: THE MIXER/PASSIVE ATTENUATOR.

Here's the first stripboard layout that I made of just 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:

You can use other dual opamp chips for the mixer. It doesn't have to be TL072. As long as the pinout is the same (which it almost always is) and make sure they are not fakes, otherwise it won't mix the audio correctly.

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.

PROJECT 2: THE AUDIO CLIPPING INDICATOR ADDITION.
When I first made this mixer, the blue LED was just there so I knew the mixer had power but it served no real function other than that it looked cool. (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 board with hot-glue, using a spacer in between so the boards 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) and make sure they are not fakes from AliExpress or other dubious sources. 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.

PROJECT 3: FOUR CHANNEL MIXER w. CLIPPING INDICATOR for KOSMO SIZE.
Wiring diagram:

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.)

ADDING A GAIN POTMETER:

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.)

PROJ. 4: MAKING IT SMALLER AND EURORACK FRIENDLY. (BUILD THIS VERSION)

Below is a new layout which I made in Dec. 2024 which puts the whole mixer with all the extra's on a much smaller piece of stripboard. It's only 24 strips by 26 holes wide. 

This is the version I advise you to build if you need a mixer. I added a Eurorack connector because there was room left to put one in. You can build this mixer in an afternoon. It took me two hours to build one; re-using the old panel of course. The layout is verified, I used it to update one of my mixers with this new design and it worked like a charm. A day later I also updated the second mixer in my synth and again it was done in two to three hours and it worked rightaway.

Here's the new layout:

(Last revised: 19 March 2025. Updated layout and removed jumpwire.)

If you don't want to include the gain potmeter then you can leave it out and also the 47K resistor. Instead you need to connect a 100K resistor between pins 1 and 2 of the mixer IC (IC-1).

Stripboard only:

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

Bill of Materials:

The end result; a neat and tidy looking mixer module.

There is one other function you can add to this mixer and that is an Offset function or Bias control. This will shift the whole audio signal up or down in voltage without changing the dynamic range of the signal. In other words the audio won't change in amplitude or volume only a positive or negative offset voltage will be added to the signal to counteract any DC voltage coming in at the audio inputs..

This is very easy to do as you can see in the layout below. I just summed the voltage from the wiper of the Bias potmeter through a 100K resistor to pin 6 of IC-1 and the other potmeter lugs go to positive and negative voltage as shown in the layout below.

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 just under the top potmeter:

DISTORTION:

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.

Synthesizer Build part-16: SAMPLE and HOLD.

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

NOTE: THERE IS NOW A NEW REVISED VERSION OF THIS PROJECT WHICH IS THE BETTER OPTION TO GO FOR IF YOU'RE PLANNING ON BUILDING THIS PROJECT.

CLICK HERE TO GO TO PROJECT 51, THE SAMPLE & HOLD VERSION 2.

This doesn't mean the design in this article doesn't work well. On the contrary. It works very well but the new one is  better designed and has a better output impedance so it's more stable when connected to other modules. So for a S&H you really should build project 51. It uses the same components as this version. I'm leaving this old version online as an archived article so people who built it can refer back to it.

Original text:

Every synth needs a sample and hold circuit in my opinion to have an extra source of control voltages. The S&H samples a voltage when triggered and holds that voltage until it is triggered again. If you feed it a white noise signal it will give you random voltages on the output which can create random tones if you input that signal into a VCO. If you feed it a signal from the LFO it will turn that signal into a stepped signal. The LF398 chip samples the input signal in 4 to 20 millionth of a second (!) and is used in many more applications that just synthesizers.
For this build I used the schematics from Rene Schmitz called 'Yet Another Sample and Hold'. (<-- click to have a look at the schematic)
I had ordered the LF398 chips a while ago and had a try earlier at building this circuit but I couldn't get it to work, but this time everything went fine and the circuit works very well. I added some extra's to this circuit in the form of a DC offset feature so I can control the voltage range of the output signals a bit better and I installed two input sockets between which you can choose with a SPDT switch. I also installed a switch that gives me the normal output voltage range (0 - 10Vpp) or half the normal output voltage range (0-5Vpp) which is better as input for the VCO's. The DC Offset in particular has proven to be a very useful addition. If you turn it into the negative the random notes get very deep and if you then put that through, say, the Steiner-Parker filter, you get the most amazing sounding low notes that sound really deep and sharp and in some cases can resemble the sound of drops of water if you put reverb on it. I can experiment for hours with this module.

This module will work fine on both +/-15V or +/-12V.

Here's the layout. All green wirebridges refer to connections to ground. All potmeters viewed from the front:

(Last revised: 19-Aug.-2021: Cosmetic changes to layout)

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

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

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

Here's the schematic drawing of the extra features I added myself; the DC-Offset and the output range switch. A very observant reader noted that my output range switch does alter the low impedance that the normal opamp output would provide and this might be problematic in some cases. He suggests to put the range option in between the two opamps (see comments below). My reasoning is that the signal from this S&H usually goes back into an opamp like the CV input of a VCO or of a filter and most of the times these are opamp buffered and those inputs have an infinitely high input impedance so in those cases it really doesn't matter, but if the signal goes into an opamp inverter with resistors than that resistor balance can be upset. That's nothing serious but it would mean the amplitude of the S&H signal can be influenced in a way not anticipated. 

If that's all gibberish to you just ignore it and proceed building ^___^

[EDIT: This is one of the reason I re-designed this project in early 2023 and wrote a new article about it (project 51).  I'm keeping this article online because this is an archive of the modules I built and the progress I made over the years and as reference for people who already built this project in case they need to refer back to the schematics or layout.)

At first I used a potmeter with center detent for the OffSet control but I later decided to change it back to a normal one because it was difficult to set the offset accurately with the center detent spring pulling on the potmeter around the middle setting. 

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

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

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

Here's a little video to demonstrate the sound you get when you put white noise on the input. This sound is going through the Dual Korg MS-20 filter described in the previous article.:

Here's a cool demonstration of the S&H with the Triple Wavefolder and the Steiner-Parker filter:

Okay that's another one done. Hope you enjoyed it and if you did please consider following this blog to get notified of new uploads and while you're here, leave me a comment please!

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

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

The DIY Modular Sessions YouTube channel made some videos about building this sample and hold module which you can watch by clicking the links below:

PART-1 Preparing the stripboard

PART-2 Soldering the components in.

PART-3 Making Noise with the new S&H. Wow it's amazing!

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 it'll be easier to use socket switches instead of toggle switches but in my early building days I didn't know these things. 

I'm keeping the article up for archival reasons plus this dual configuration actually sounds pretty awesome but in hindsight, I wouldn't build it like it's presented below. Just so you know. 

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. Like I mentioned in the beginning, 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 4 years of using it LOL (I don't use it any more)

Use the switches inside the sockets instead of toggle switches. Much less confusing. But I used the wiring diagram below. 

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-14: AD/AR Envelope Generator.

An updated/slightly improved version of the LookMumNoComputer simple AD/AR.  Improvements suggested by Sam Battle himself.

This Envelope Generator is a fantastic little extra to put in your synthesizer. It's always handy to have a few extra envelope generators in your synth to trigger filter responses or other parameters. I built a 'proper' ADSR a few pages back and this simple version is just perfect to have as extra. I found this on the LookMumNoComputer website and Sam has also done a video about this on YouTube which you can watch here:

Because the LFO from the last blog post didn't have a synchronization input I needed something that could trigger a filter response when I pressed a key on the keyboard so I decided to build this. I had just enough room left on the panel for the LFO to include this and it only needs a small bit of stripboard to build it up on.

Here is the layout that I made, which is just a copy of the one on the LMNC website but with a few changes (see text below. All potmeters viewed from the front.):

AD stands for Attack and Decay, this is when the switch S2 is in the Trigger position. That means there is no sound after you let go of the key. AR stands for Attack and Release and this is when S2 is in the Gate position and now the tone will fade out after you let go of the key.
It's fun to build it like Sam does in the video with a big arcade button with an internal LED light.
It's pretty straight forward build. In fact, it's so simple that I didn't even test it before building it in and luckily it worked straight away. It didn't work perfectly though. It needed pretty high voltage Gate signals and Triggering didn't work at all. There was a discussion about this on the LMNC Circuit Discussion Group Page on FaceBook and it turned out that Sam had advised to lower 3 of the 100K resistors to 10K and an other suggestion was to remove the diode from the input to the switch and make it a normal wire connection. I implemented these changes in the stripboard layout but I left the diode in place. I also advise to put in a bigger capacitor than mentioned on the original layout.  I was lucky enough to have kept some bi-polar capacitors that I took out of some circuitboards years ago because they came in very handy in this build. On top of the 1µF cap I put an extra 2,2µF bi-polar cap to get 3,2µF in total. (put in 5µF if you can) That gives a bit more time for the release to fade out. With just the 1µF it fades in just a few seconds.
Because I mounted this on the same panel as the LFO I was able to just connect the power leads to those of the LFO stripboard because they both use + and - 12 volt. So no need for an extra power cord and connector. This circuit can also be powered by +/- 15V.

Now, if you want something that is just as small but works a lot better then I can refer you to my 'Synthesizer Extra's No:01 SIMPLE AD/AR using the 7555'

This article is about the Thomas Henry designed AD/AR from 2014. He used the 7555 and his design works very well.

Okay, that's all for this one. If you have any questions about this or other builds on this website then please put them in the comments and I'll answer them asap. And while you're here, leave me a comment anyway!
Until the next one!

Synthesizer Build part-13: THE LFO (MusicFromOuterSpace version).

A very useful, good working and simple to build LFO for square-, sine- and triangle-waves plus a stepless transition between ramp- triangle- and sawtooth waves. A good LFO for beginners to build too. I still use this as my main LFO.

This is the Variable Skew LFO from MusicFromOuterSpace. It doesn't have a sync option but nevertheless it's a very useful LFO and it has been the main LFO in my synthesizer for a long time. It's ideal for all the modulation duties in your modular synthesizer. I was allerted to an alteration that you can make to give this LFO a synchronization mode! That didn't really work for this LFO but more on that later further down the article.
This LFO has the following features: Stepless transition between Sawtooth to Triangle to Rampwave with one potentiometer. Sinewave. Pulsewave with changeable pulsewidth. Frequency control and a switch to go from High to Low frequency setting. 

Frequency Range with switch in 'HI' position = 1 wave every 2,39 seconds to 84 waves per second (239mHz to 84Hz)

Frequency Range with switch in 'LO' position = 1 wave every 7 minutes and 46 seconds to 1,43 waves per second (1,43Hz). The readings you will get will differ a bit from mine due to tolerance fluctuations in capacitor and resistor values.  

Squarewave pulsewidth (or dutycycle) goes from 1% to 99%. The pulse width of the squarewave is set with the same potmeter that controls the shape of the other waves. It also influences the shape of the sinewave. So it can be a bit fiddly to calibrate.
A very feature rich design and a design with very few components so not much can go wrong. It uses a TL084 quad opamp chip and a LM13700 OTA chip.
I even managed to add a little extra of my own design: normally this is a bi-polar LFO meaning all the outputs go from -5 to +5 volt but I added a uni-polar feature with two extra outputs for the saw-triangle-ramp wave and the sinewave that go from 0 to +10 volt. There was room on the circuitboard to put a little TL082 on and make the two inverting buffers with DC offset potmeters. I'm sorry there's no schematic for these additions, I did it from memory, but this feature is included in the stripboard layout. You can take a look at the 8 step sequencer V2.0 schematic which also has an offset feature of my own design and it's the same design as used here. Remember these 0 to +10V signals are inverted, so the waveshape potmeter works the other way around for these waves.

Unipolar LFO's are particularly useful for modulating the pitch of a VCO when you want to set the tuning very accurately.

This LFO is meant to be used with a -12V/0V/+12V powersupply but it works equally well on a -15V/0/+15V powersupply without any changes needed. The overall frequency range will go up a bit with a dual 15V powersupply of course.

LAYOUTS:

Here's the layout, wiring diagram (All potmeters viewed from the front). The layout is verified. I recently built a second one of these LFO's to use as a standalone signal generator and it all worked first time. There's an explanation of the colour-coding of the wirebridges on the layout. If you're wondering why C4 is 10pF instead of 100pF as it is on the schematic, it's a change that Ray Wilson himself made. You can read it in the original text.

(Last revised: 21-Jan.2021 Updated the old layout with some components re-arranged and got rid of a jump wire.  28-Aug.-2021: Cosmetic changes, got rid of resistor colour coding lines. 

Stripboard only:

Cuts and wirebridges seen from COMPONENT SIDE!!

Here's the schematic for the Music From Outer Space LFO. I put in a 100K potmeter for the Wave shape function instead of a 50K as is shown in the schematic. This doesn't make any difference. It'll work the same but put in a 50K if you have one. 

The timing capacitors are C1 and C2 (two 10µF electrolytic caps) switched in series with their negative poles connected together thus forming a 5µF bi-polar cap. This is used for the low frequency setting. The high frequency setting uses just C3, a 100nF capacitor.

Make sure all potmeters are linear types. You can see that only one half of the LM13700 is actually used so it would be easy enough to turn this into a dual LFO. All you need to do is duplicate the LFO circuit and connect it to the pins that lay directly on the opposite side of the 13700 chip. You'll need to make a new layout for that yourself though. A nice exercise in layout making ^___^ 

Bill of Materials. As mentioned earlier, C4 has been changed from 100pF to 10pF by Ray Wilson himself on the MFOS website, so that's why it's 10pF in the B.O.M.:

SYNC OPTION:

There is a circuit design available on the internet that will add a synchronization option to LFO's with a triangle core. I have tried that circuit on this LFO but the timing capacitors in this LFO design are too big for this to work. However it will work on other LFO designs from MFOS. I have linked to the schematics for the sync circuit below so you can check it out. There's also a link to a video by Rich Holmes from Analog Output who shows some changes he made to the circuit to make it work better with his LFO. Very useful to watch if you want to use this circuit with other LFO's.

--- SCHEMATIC ----

--- GITHUB ---

--- ANALOG OUTPUT YT VIDEO ---

CALIBRATING the LFO:

Calibrating the circuit should be very straight forward. Connect an oscilloscope to the sinewave output and manipulate the Sine shape trimpot until you get a symmetrical sinewave. Make sure the wave shape potmeter on the face plate is set half way. Turn the symmetry trimmer until the waves look the way they should.

Set the DC offset potmeter so the output reads 0 to 10V peak to peak on those two outputs. That's the bit I added on myself so it's not in the schematic.

Here's a high resolution picture showing oscilloscope screenshots of the different waves.

Here are some pictures of the stripboard with wirebridges and with components:

This is not the board I ended up using. If you look closely you can see the 10pF cap is over pins 6 and 7 instead of 5 and 7 on the left TL084. There may have been more mistakes on it, I can't remember but the layouts are absolutely 100% verified so don't worry about it.

Here's a picture of the panel I made for it. Like I mentioned earlier, it is combined with an AD/AR, the version that uses the 7555 chip. I used multi-coloured LEDs on the outputs to indicate positive and negative cycles of the outputs. There's no practical reason why I did that, I just thought it looked cool. I think every synthesizer module needs at least one LED :)

Please, share and follow this blog and see you on the next one. :)
If you have any questions and/or comments please post them below in the comment section or post them on the  EddyBergman Discussion and Help Facebook group.

Synthesizer Build part-12: THE KORG MS20 FILTER.

A good working version of the famous Korg MS-20 filter by Rene Schmitz, with updated stripboard layout and wiring diagram with LP & HP .

As popular as this filter is, I would not recommend it as your first filter build. Just because of the audio input level behaviour. For your first filter, build the Steiner-Parker diode filter. Instant gratification. 
Now on with our regular program:

There are PCBs available for this VCF. See 'PCB Service'

This is a filter I absolutely had to include in my DIY synthesizer project, for one because Sam Battle from LookMumNoComputer raves about it and it sounds amazing in his videos and the other reason is that it has the option to go between Low-Pass and High-Pass and I didn't have a High-Pass option in the synthesizer yet. I also added the Band-Pass option in the layout drawing however I tested it and it adds no extra benefit to this filter. 
There was a discussion on the Synth DIY Facebook group a while back about the possibility of switching this filter between 6dB per Octave and 12dB per Octave. I've tried it and it works but only in LowPass mode. More about this further down the article.
Btw, I tested this filter on dual 12 Volt and it works just as well as on 15 Volt so no need to change anything if you are feeding it from a +/- 12V powersupply. Of course you need to open Resonance a little more than on 15V but it's all within the normal throw of the potmeters so no problem there.

Btw, eventhough this is THE going design for the OTA version of the MS-20 filter, in my humble opinion it doesn't sound much like a vintage MS-20. But that's just me. It does sound good though so don't get me wrong, you're not waisting your time.

SCHEMATIC:

For this filter I used the 'Late MS20 Filter' schematic from Rene Schmitz which you can find by clicking here. I think he called it the 'late' version because the earlier versions used the Korg MS35 module which was sealed off and what was in it was only revealed in 2000. It was transistor based. The later versions used the OTA chip the LM13600.

This filter is definitely different from other filters. It doesn't sound like the Moog Filter but it does have that 'ripping the fabric of the universe' synth sound and it's a real Speaker Ripper!. It's a 'Sallen-Key' type filter and it produces really divers sounds. (I always think of this filter as the 'Heavy Metal Guitar' pedal of the synthesizer world.) You can get deep bass tones out of it and if you connect an Envelope Generator like the little AD/AR to the CV1 input you'll get a squarewave changing into a really bassy sinewave as the note progresses. It's awesome to experiment with this filter.
In HighPass mode the Cut-Off Frequency potmeter doesn't work over its complete throw. I found it usually only works over the first 50%. This is also true of the other MS-20 filters I built for the Dual version so this is normal behaviour for this design.
This filter is self oscillating in both the Low and High-Pass configuration. The more Resonance you give it, the more the two yellow LED's light up. I found that by using an LM13600 instead of a LM13700 you can  tame the filter a little. The 13600 seems to be a bit less aggressive although the difference is really small. Btw, instead of the TL074 you can also use the TL084 or the LM324. They are pin for pin compatible and work just fine. I personally tested them all successfully.

Here's the original schematic with the pinout numbers for the LM13700:

Here's a link to an image I made showing the switch connections for HP and LP mode: --- click here --

About Resonance and Self-Oscillation:

On the oscilloscope you won't see much Resonance ringing on top of a squarewave in Lowpass mode. I've noticed this with all 3 Korg filters I built so far. Maybe one little sinewave bump on the top left corner of a squarewave but it's not like the ARP or Steiner filters where the whole top and bottom of the squarewave is full of Resonance or self-oscillation. (It gets better with lower input levels though.) But in HiPass mode you get much more. However, this is normal for this filter and it still sounds pretty amazing so this doesn't matter.

Beware this filter does not like high input levels! When I feed it 0-10Vpp squarewaves directly from the Digisound-80 VCO it doesn't work right. It is much happier with the -5/+5Vpp levels of the Thomas Henry 555-VCO. It is always a good idea to put an input level potmeter on the audio input of this filter so you can regulate the input volume. You can try to put a capacitor between 470nF and 1µF on the audio input to block the DC offset voltage of a 0 to 10V signal and turn it into a -5/+5V signal. That should work.

This is a well known characteristic of the Korg MS20 filter. If you increase the level of the audio input you will drown out the resonance of the filter. So you must find a balance between input level and resonance.

6dB vs 12dB.
Although this is a 12dB per Octave filter (2 pole) it is possible to get an output with 6dB per Octave roll-off (1 pole) if you tap the signal from pin 7 of the TL074 (A1 in the schematic drawing) with a 470nF capacitor, just like the normal output. The 6dB however won't work in Highpass mode because the signal is output from the stage before the one where we input the Highpass audio signal.
I have experimented with 6dB and at first I dismissed the option but having recently rebuilt my filter using (vintage) Polystyrene capacitors (and Polyester box caps for the 470nF output caps) it now sounds much better and the 6dB function sounds better too. So I made new layouts which includes the 6dB/Octave function with a switch to give you the choice between 6dB or 12dB. In the rebuilt filter I also used two BC558 transistors that were roughly matched. I'm not sure if this is necessary but it can't hurt. The 6dB/Oct setting sounds more distorted an rough around the edges if you know what I mean. It's a heavier sound. 

LAYOUT:

Below here's the stripboard layout. I made my own version from the ones that are circulating on the internet to which I added the potmeter connections and the audio in and out, CV in, plus the switch connections for Low-, Band- and Highpass with the altered position of the 1nF cap at pin 12 of the LM13700. So with this and the schematics you should have all the info you need to build it right the first time. BTW, the 2 transistors used on the layout below are BC558 PNP's but you can also use the 2N3906 but those have to be put in the other way around. In subsequent builds I used the BC557.

ABOUT THE LED's: 

Make sure you use LED's with a voltage drop of around 1,8 Volts. That means Yellow, Green or Red LED's are okay but do NOT USE Bright white LED's or Blue LED's. They have a much higher voltage drop and won't work well in this filter. Remember the LED's replace the original 3 diodes in series. These diodes have a voltage drop of 0,6 Volt so 3 in series means 3 x 0,6Volt = 1,8 Volt.

(Last revised 7-Oct.-2023 Cosmetic changes to make layout clearer.)

Please note: I did add the Band-Pass option to the layout but if I were you I would just leave it out. But do some tests and decide for yourself.

About the DPDT switch wiring for HP/LP mode: 

Connect the top two pins and the right middle pin of the High Pass/ Low Pass switch together and connect that to the High-Pass input. You could even just forget about the top right pin and bypass it but I thought it was neater to include it. The lower right pin goes to ground. The audio signal goes into the middle left pin, and the lower left pin goes to the Low-Pass input. The Band-Pass switch is simply connected to the Low-pass input and the audio input. If the filter is in Low-Pass mode the BP switch won't have any effect but in High-Pass mode the switch will connect the audio to the Low-Pass input aswell so both inputs get the audio signal thus creating the bandpass characteristic.

A little sidenote about the writing next to the switch in the layout: the HighPass and LowPass marking next to the switch on the wiring diagram reflect the function of the actual wires connected to that part of the switch. A Toggle switch works the other way around. If you flip the switch upwards, the middle and bottom pins are connected to eachother. If you flip it downwards the top and middle pins are connected so on the eventual panel, the switch should have LowPass marked at the top and HighPass at the bottom.  This way of marking the switch functions goes for all the layouts on this website that have switches in them.

Close up of just the stripboard layout (Print this one and use it for your project. The lay-out is guaranteed, tested and verified faultless. It has been used by hundreds of people by now). Don't forget to cut all the copper strips under the IC's.

(Last revised 7-Oct.-2023 Cosmetic changes. Removed colour codes from resistors, added colour coding to wirebridges.)

Cuts and wirebridges as seen from the COMPONENT SIDE! As always, mark the cuts to be made with a black waterproof Sharpie on the component side. Then put a pin through the marked holes and mark them again on the copper side. Then cut the copper at the marked positions with a sharp, hand held, 6 or 7mm drill bit. Check your cuts with a continuïty meter or a powerful magnifying glass before you proceed.

Bill of Materials:

About the components:
I used fairly cheap LM13700 chips I got out of China via eBay and I think I got lucky and got real ones instead of fakes. But play it safe and get your chips from a reputable source.
I used a DPDT toggle switch (Double Pole Double Throw) to switch between High-Pass and Low-Pass but you could also use a jack socket for High-Pass input with a build-in switch that connects C4 to ground when nothing is plugged in there.
The Cut-Off Frequency potmeter in this build is a 100K one, but you can use any type you wish because pins 1 and 3 are connected to the + and - of the power supply so it is nothing more then a voltage devider. I saw that Sam Battle uses a 4K7 pots for this in his layout so use what ever value you want. (Beware that the voltage difference over that potmeter is 24 Volts so don't go too low with the value or you'll fry your potmeter. Remember Ohm's law!)
I don't think the Resonance potmeter is that critical either but you better stick to the schematics for that one. Keep it a 100K potmeter. I used a logarithmic one but linear will work fine too. As mentioned earlier, you can use an LM13600 instead of the LM13700 and instead of the TL074 you can use the TL084 or the LM324. They all work just fine.
I rebuilt the filter recently and used polystyrene caps instead of ceramics. It did make a difference in my filter but the first version I built of this filter was a mess to look at. Building something neat and tidy always makes it work better I found out. So don't expect miracles by changing the caps from ceramic to polystyrene.
One thing that might be worth experimenting with is the 10K feedback resistor over opamp A3 in the schematic. The one by the LEDs. You could put in a 15K trimpot and see what it does if you change the feedback resistance. I haven't tried this yet myself but it might be worth experimenting with.

Here's a picture of the panel I made for it. Because I recently re-built the filter I had one unused mounting hole left in the panel so I put a white 3mm LED in there. It's a bit bright =)

Here's a picture of the insides. Those big brown Mullard 'tropical fish' caps (so called for their colourful stripes) at the top and bottom are just 100nF caps for de-coupling. They're spread out a bit because they're so big. I advise to use polystyrene caps for all the other capacitors although that's not necessary for the filter to work. I just found polystyrene caps to sound better but you mustn't attach too much importance to the use of polystyrene caps. They are difficult to find. Any type will do.:

DEMO VIDEOS:

Here's a new demo video I recently made. Just trying out the different functions. I had it connected to the 8 step sequencer. It sounds really awesome but it needs a snappy ADSR input and attenuation on the audio input to make it sound like this:.

This is an old test video, made right at the beginning of when I started building modules. It has the Korg filter (in Highpass mode) and the Moog Ladder filter (in Lowpass) in series and using my 8 step sequencer and reverb from the CaraOK effects module:

Sounds quite good doesn't it? Especially with added echo or phase-shift effects. 

WARNING: Beware your speakers!! This filter can oscillate at below audible frequencies and the cones of your bass speakers will take a hell of a beating if you've got some serious amplification going. If ever a filter could be called a 'Speaker Ripper' this one is it. Quite literally. (I added this warning because tonight I almost blew my speakers up with this filter.)

EXTRA DOCUMENTATION:

I want to direct your attention to a very useful page from Scott Stites' website. He talks about all the different aspects of this filter, using two of these filters in tandem and his approach to adding a Band-Pass mode to it. If you want to build this filter, you have to read this text I think.: Click here for Scott Stites website.

Here is a link to a PDF file called: "A Study of the Korg MS10 and MS20 filters by Timothy E. Stinchcombe --- click here for that PDF ---
It's an in depth study with formulas and schematics, excellent if you're an electronics student and want to get into the details. I must admit it goes way over my head ^__^

Lastly, here's an other very interesting document I found by Sound Semiconductor entitled "Designing Voltage Controlled Filters for Synthesizers with the SSI-2164."
It goes into great detail into how filters work and how to design them and places specific emphasis on the Korg MS-20 filter.

Okay, that's it for this one. If you have any questions or comments then please put them in the comment section below of post them in the special Facebook Discussion and Help Group for this website.

Fun fact: this article is on a solid number two listing of most popular builds on this website, number one being the AS3340 (Digisound-80) VCO. (you know, the really good one.. ^___^  )

Synthesizer Build part-11: ECHO and SOUND FX UNIT and LINE OUT.

100 different sound effects combined with a line-out and head-phones connection for the DIY synthesizer.

Here's a little item I found on eBay and thought it would work great in the synthesizer; and luckily I was right. It is a fantastic asset and makes the synth sound really professional and full. 

A word of warning though, this is not a beginners project. You need to know your electronics to follow my plans. This unit also needs two voltages to run on. The effects unit runs on +5V and the stripboard runs on +/-15V or +/-12V. So you must make sure you have those voltages available in your modular set-up or add a +5V regulator to the print. I recently added a Dry/Wet control so you can dial in as much of the effect as you want in your sound. This makes this unit really useable and perfect for modular synths. (see further down the article)

The unit goes under different names but mostly as the Cara OK ("Karaoke" get it?) or DSP 5V Red Digital Stereo Mixer Reverberation Karaoke Reverberation Board Module (if you copy and paste that into the search bar on eBay, you'll find it). The prices vary from $15 to about $30 us. (DSP stands for Digital Sound Processor)
This is the cheapest listing I could find on eBay: click here.
It offers 100 presets with reverb, echo and even chorus, phaser, flanger, phase shift and reversal effects and combinations of them together. There's a rotary encoder with which you choose the preset of your choise and then you just press to confirm and engage the effect. It's a favourite with many synth builders I noticed.
I made a special panel for it and combined a 'Line-Out' control and bypass switch option with the module. Here is the schematic or wiring diagram to make this unit part of the synthesizer. This was one of my early projects and at first I didn't have dry/wet control in there so this is the version just with a switch between normal line out or line out with effects:

I had to put extra attenuation on the bypassed (normal) signal because it was much louder than the output from the Echo Module, but that was easily fixed as you can see in the diagram above. The panel that I built into my synthesizer also includes a headphones out stereo jack which is not included in this article but it's just an output jack soldered straight to the negative poles of the Left and Right output Electrolytic Capacitors. Weirdly enough, if you listen through the headphones, the echo module is much louder than the normal line out. This is probably some impedance matching issue but it doesn't bother me. It's easy enough to turn a volume knob so I'm not bothered.

The Stripboard can work on +/-12V also. It doesn't matter if you use 15 or 12V. The CaraOK effects unit must have it's own +5V powersupply! You can use a 7805 voltage regulator and connect it to the 15 or 12V of the stripboard to get 5V for the effects unit. This is not further described in this article. I'm assuming you have the knowledge to make powersupplies. If not, check out my article on this subject here or see the datasheet for the 7805 voltage regulator.

Here's the stripboard layout (older version). This one has a 3 pole switch but that's not necessary. Just connect the wires on the left hand side of the switch together; the red line coming from the 'normal level pot', the blue line from pin 1 of the chip and the effects unit input. They can all be connected together and then you just need a 2 pole dual throw switch (DPDT).
(All potmeters viewed from the front)

Please note: The GROUND connection of the Effects Unit must be connected to the GND of the stripboard. Everything must share the same ground. I myself made a central grounding point with a solder eye connected to one of the M-3 bolts holding the Effects Unit to the panel. To put it in a simple way: the ground of the 5V powersupply for the Effects Unit must be connected to the ground of the +/-15 or 12V powersupply of the stripboard. The Grounds of the Line Out cables are also connected to this common ground. You can tap that off from the 4th strip above the Left Line Out connection. (2nd strip below the chip).

Print only:

(Last revised: 14-Aug-2020: Corrected mistake with negative voltage supply to the TL074.)

I used a 3 pole double throw toggle switch (ON-ON type) to be able to switch the synthesizer between normal output and output through the effects unit. This is a bit of over-kill because you can just as well connect the inputs together (part S1-A of the switch) and then use a 2 pole switch to switch between the outputs. Better still. I describe down below how you can put in a dual gang potmeter instead of a switch and so have a DRY / WET control. 

I'm going to adapt this article soon and make a new layout to put in this Dry/Wet control permanently. 

[EDIT] March 2021 DRY/WET CONTROL:

I wanted a Dry/Wet control on this effects unit for a while now and I wanted to install it without having to rebuild the whole module.

I changed the 3 pole toggle switch for a dual gang potmeter and I connected the inputs, normally connected to part S1A of the switch, all together. If you do this make sure you keep to the right order with the wires. Best to make a few pictures of the switch connections first before you solder in the dual potmeter, that is, if you built this module already.

Below here is a schematic. You can see the switch has been replaced by a stereo potmeter. This MUST be a linear type potmeter!!

There is a half drop in volume at the half way stage of the potmeter because we have 50K of resistance in our signal path there but other than that it functions fine! It's just a matter of turning up the input or output levels to get it where you want it. You must make sure the output audio goes into a very high impedance input, like a HiFi amplifier because if you pull even a tiny bit of current from this circuit it won't behave normally anymore. But it works fine on audio amplifiers, I guarantee it.

Here is Juanito Moore (Modular for the Masses) schematic for this unit with a properly implemented dry/wet control

The Dry/Wet control really makes a world of difference! Now you can set it to reverb and then precisely dial in the effect to where you like it. It's fantastic sounding!

Here is a picture of the panel with the Dry/Wet control where the switch used to be:

I made a little demo video. You can hear the drop in volume at the midway point of the Dry/Wet control. (I know it's wired backwards LOL :)  But being able to dial in the effect makes a world of difference and the volume can easily be crancked up by the level controls.

Continuing the original text:

I've put in 4 buffer stages, using the TL074, for the input, normal output and FX Unit outputs Left and Right channels and I gave the latter two adjustable gain by means of two 50K potmeters in the feedback loops of the opamps; one for each channel. The gain is adjustable from 2 to 5.3 times. You can increase that by using 100K potmeters instead of 50K ones. That would give a maximum gain of 8.6 times.

When I first tested this unit I noticed I was receiving an FM broadcasting signal through the effects unit. (There's an FM Broadcast transmitter and antenna on a flat 100 meters from my location). So I took a ferrite ring and wound the audio input wire around the ferrite ring about ten times. I also put ferrite beads in the 5V power-supply line to the effects unit and to the print with the opamps on it. This solved the problem. One more little thing: beware of the little crystal X1 near one of the screw holes on the circuit board. It is rather flimsy and fragile. Take care not to damage it.

The Cara OK is a really versatile unit with lots of really cool sounding effects. Here's an overview of the possibilities it offers. I myself printed a small version of this list out, laminated it and stuck it at the bottom of the panel I made for it. Handy to have around I thought :) :

This picture shows all the connections to the circuitboard:

It's small so it won't take up too much space. Beware that it needs just +5V for power supply. Luckily in my synth build I made a power-supply that delivers dual 5, 12 and 15 Volts so I can feed it right from there. I can really recommend you picking this up and trying it in your build project. It will add a lot of options and is a very useful addition to the filters and its output is in stereo! The sound quality is just great so no problems there. The only thing is the difference in volume I mentioned earlier but that is easily fixed. You can use opamps buffers with it, like I did, but it's not absolutely necessary. I did without them at first but then installed output buffers with variable gain as I mentioned before.
The audio response of this module is so good that it even lets through the ultra low frequencies the Korg MS20 filter produces (see next article) and that can go as low as 10Hz. You can really see the speaker cones move bigtime!
Before I installed the 3-pole toggle switch I had a single pole and a double pole switch side by side to switch between FX-unit and normal line out. So after installing that 3-pole switch I had a hole in the panel left over. I mounted a 3,5mm stereo output jack in that hole as a connection for head-phones. The output jack is connected straight to the audio output on the stripboard. One thing I noticed with this arragement is that the normal line-out through the head-phones, sounds a lot quieter than when the effects unit is switched on. That's probably due to a difference in output impedance because we're effectively putting an 8 Ohm resistance between Line-Out and Ground in the form of the head-phone speakers. This doesn't occur when I listen to it on the normal audio amplifier, at least not if the head-phones are not plugged in. It would be a good idea to build a little head-phone amplifier for this purpose.
Here is a picture of the finished module in my synthesizer:

Here's a picture of what's behind the panel. Now you understand why this is not a beginners project ^_____^  You can see the yellow Ferrite ring with the black 'audio in' wire wound around it above the blue circuitboard and there's also one on the stripboard. I advise you to include these in the power supply line and audio in line. In red you can see the 3 pole toggle switch. This panel works really well like this.

Here's the Line Out Panel I made on the back of my synth, with two gold plated RCA outputs and a 6,3mm (1/4") Stereo Output Jack, which is connected straight to the RCA left and right outputs.

ONE LAST REMARK:

I have had feedback that some chips on the CaraOK board can run hot when run for a long time and even fail after a few years so it is advisable to add an ON/OFF switch to the module so you can turn it off when it is not needed thereby preventing the chips from being on needlessly for a long time.

(see comments below)

Okay, that's the 11th module I put in the synthesizer. We're nearly there. I have just room enough left for one more module and that has got to be the Korg MS20 filter. But I'm waiting for some supplies from China before I can build it. (Circuit boards for one, coz I'm fresh out at the mo.)

Okay here's an excellent video by Juanito Moore that shows you how you can circuit-bend this device and make it voltage controllable. Click here

Right, that concludes this article. Thanks for stopping by and while you're here, why not leave me a comment or if you have any questions put those in the comments too and I'll get back to you asap.