Synthesizer Build part-5: THE CABINET & TIPS.

How I made my synthesizer cabinets and tips concerning building the projects on my website and Glossary explaining terms you hear a lot in the synthesizer world.

My first synth cabinet:

I'm not going to get too deep into this because every individual will make their own cabinet or case to their own taste I think. This cabinet represents the vintage 70's look that I wanted for my synth and I'm very happy with it. 

If you are going to build all, or almost all, of the projects on my website you're going to end up with an amazing synthesizer (that I call the "Bergman-Berlin") that can rival the big Moog systems of back in the day and will cost you far less money. You can build as many oscillators as you wish (my system has five) and a variety of filters and other sound manipulators and noise generators and mixers that will make this an amazing sounding synth. There are examples enough in the demo videos on this site. Here is a picture of my synthesizer as it now is, at the end of October 2020:

Here are some pictures from different stages of building the first cabinet:

Almost finished, just one more panel to fit but already working very well. You can see I incorporated a 'Keyboard Garage' in the case so I can push the keyboard underneath the synth if I'm not using it, to free up space for other things.

I made no drawings and I measured everything as I was building it. That's the way I usually approach woodwork. The drawing is in my head. I did make two cardboard templates for the side panels to make sure I got those exactly the same. I measured the current draw with all panels that I have build so far switched on. All together it drew 250mA. That's less than an old fashioned bicycle lamp. :) I also installed a temperature sensor that is directly in contact with the heatsink of the LM317 that regulates the 15 volt output and it runs up to about 60°C. That's perfectly fine and normal. It can handle double that and the temperature stays at 60° and doesn't climb.

The panels I use are made from aluminium (or aluminum if you're in the States ^^ ). They are 20 centimeters high so you could say I use the LookMumNoComputer Kosmo format. I bought 2 sheets of 1 meter long and 1.5mm thick and they are powder coated in gray/black. This powder coating is something I can really recommend because it's hard to scratch. If you just spray-paint your panels they will scratch very easily. You can write on the powder coating with a white acrylic pen. The one pen I bought had too wide a tip and I sharpened the tip with a razorblade but in the end it was un-useable. I ordered a pen online with a 0.7mm tip and that works far better. But if you laser-engrave your panels then you don't need all this anyway.

One IMPORTANT TIP I want to give you is the following: When you make your front panels for the modules set up a standard for their measurements right from the beginning. What I mean by that is decide on a fixed width for all of them. Choose for instance, 10CM for the bigger projects like the VCO's and 5CM for the smaller ones. Use those widths throughout the synth and don't do what I did and make them just the smallest size they can be. 

Here is why: You're going to build more panels/modules (eventually) than will fit in your synth cabinet and if they have a standardized width you can easily exchange them. For instance you might need more LFO's so you can take out a VCA and put in an LFO panel because they are the same width. I can not do that as easily because all my panels are designed with different widths.  

The thickness of the panels I use is 1.5mm That is thick enough and won't bend or flex when connecting patch cables. It also leaves enough room for the thread of the sockets and potmeters.

IF YOU BUILD THE MODULES ON THIS WEBSITE YOU WILL END UP WITH A MODULAR SYNTHESIZER THAT CAN RIVAL THE SOUND OF A BIG MOOG SYSTEM FOR JUST A FRACTION OF THE PRICE PLUS THE SATISFACTION OF HAVING BUILT IT YOURSELF!

Make sure the panels you are going to use are at least 1.5mm thick aluminium!! If they are any thinner than that they will bend or flex if you put a cable into a socket on the panel. So keep that in mind!
This cabinet is 1 meter and 11 millimeters wide, 38 centimeters high and deep. 
If you are building your own synthesizer I would be very curious to see some pictures so if you can link to that please leave the link in the comments. That'd be awesome!

POTMETER PIN NUMBERING:

To avoid confusion here is the way we number the pins of a potmeter:

Pin 1 is the Counter Clock Wise part (the left pin if you look at the potmeter with shaft facing you)

Pin 2 is the middle pin.

Pin 3 is the Clock Wise part (the righthand pin if you look at the potmeter with shaft facing you).

Here's an illustration of this:

WHAT DOES "U" AND "HP" MEAN IN SYNTHESIZER MEASUREMENTS?

When you're just getting into the modular synthesizer hobby you will hear/read terms like 'this module is so and so U high and so and so HP wide. What does that mean?

Well U stands for RU which means 'Rack Units'. It comes from the 19 Inch rack system. 1U equals one Rack Unit which is 1.75 Inch or 4.445 centimeters high. Eurorack modules for instance are practically always 3U high which is 5.05 Inches or 12.85 centimeters. There are also horizontal modules in Eurorack that are 1U high. Some rigs have an extra 1U row to accomodate them.

HP stands for Horizontal Pitch and is a unit for width. 1HP is 0.2 Inch or 5.08 millimeters. 

So the Kosmo panels I use are 4.5U high and a 10 centimeter wide panel would be 20HP.

Now there must be some rounding off of numbers going on in these standards because if I multiply 4.445 centimeters with 3 (for 3U) I get 13.335 not 12.85 centimeters. So I'm not sure what's going on there.  Maybe there are some differences between USA and UK standards. I don't know.

DUCKING and SIDECHAINING:

What does Ducking and Side Chaining mean? Ducking is a term for lowering the volume of some sound source so an other sound can better be heard over top. So the sound is ducking underneath the louder sound. This is a technique often used in percussion setups where, for instance, you need a kick drum to be heard over a bassline so the trigger signal that triggers the kick-drum also goes in an envelope generator which produces an inverted envelope that closes a VCA with the Bassline signal going through it. So as soon as the Kick drum is triggered, the Bassline is silenced somewhat so you can hear the Kick better. That technique with the VCA being triggered by the Kick drum is called side-chaining.

LEGATO: 

This is a term you hear a lot when talking about synthesizer or sequencer playing. Legato means that notes are played without the Envelope Generator being triggered by a Gate signal. The new notes you play will blend in with previous notes until the Envelope Generator's Release phase has died out and the synth falls silent or until a new Gate pulse is fired. There is usually a special setting on a synth or sequencer that makes this possible. Legato is usually achieved on a keyboard by not lifting the fingers from the keys completely before playing a new note. The sequencer in the Keystep by Arturia for instance can be programmed to play Legato. It will then play the notes you programmed in but without giving off Gate pulses for each note.

March 2020 the second stage:
Here are some pictures of the second stage of the synthesizer. This is a much simpler case and it is 20 by 20 by 100 Centimeters so it sticks out at the back a bit. This was necessary to accommodate the power buss system. I made some trunk locks on the sides so I can clamp the top section to the main synthesizer. This works just perfectly. I did have to solder these locks though because the locks themselves were connected to the main plate with the screwholes just by three flimsy bits of folded-over metal. So if you put any force on that they would bend and let loose over time. So I heated them with a blowtorch and soldered them from the inside. This worked really well because I used a bit of flux on the metal and this made the solder flow into all the little seems so it is very neatly soldered.

I made the width of the second stage too short by 1 centimeter so I had to use extra pieces of wood to connect the locks to.

EXTRA INFO: ABOUT THE STRIPBOARD I USE:

For everything I build and publish on this website I use standard stripboards of 24 strips high and 56 holes wide. The layouts I make are 24 by 55 so you have one hole extra room in case you make a mistake. You can order those stripboards from AliExpress for a very reasonable price. However, sometimes the pre-drilled holes are not quite in the middle of the copper strips which makes it difficult to solder components but this doesn't happen often and is the compromise you have to be willing to make. I have built my entire synthesizer with these 24x55 stripboards and all the layouts I publish use this size as a starting point. That's 6,5 by 14,5 Centimeter. Here's a link to where you can order them:

LINK ONE

LINK TWO

SOME EXTRA TIPS:

Here are some extra tips about general topics, not necessarily relevant to my projects but just things I want you to keep in mind.

All the projects here require wire bridges to be soldered in. Don't make those wire bridges out of electrically insulated multistrand wire, because it will get messy very fast on your stripboard if you use that. They take up a lot of room and create big solder blobs on the copper side. Avoid doing that if you can. Instead use single core copper wire, like transformer wire. Use sandpaper to clean off the insulating lacquer layer and reveal the bare copper and then you can easily solder them in place. I always neatly bend them to the right size too with a pair of neadle nose pliers. The picture below shows the stripboard for the Moog ladder filter (chapter 39) and imagine doing these wire bridges with normal electrical wire. It would be a mess.

Measure the value of every component before you solder it in place. Most multimeters these days have transistor, resistance and capacitance modes so this should be no problem. It might save you a lot of time in trouble shooting later on.

Get an oscilloscope if you don't have one already! You are going to need one very soon if you go on building modules. Get a cheap one from eBay like a DSO138 for about 20 dollars. You can also look at the second hand market. Very good analog cathode ray oscilloscopes can be had on eBay for very little money. I myself decided to invest in a good digital scope and at that time the Rigol DS1054Z was just coming on the market and was praised to the hilt by Dave from the EEVblog on YouTube so I got one of those. I've never regretted that because it's a tremendous help to me and it has 4 channels.

Don't use lead free solder. I know, environment etc etc. The stuff is CRAP! Get the good old 40/60 or 37/63 Tin/Lead solder at a thickness of 0.6 or 0.5 mm. You can thank me later ;)

Why is output impedance important? Output impedance is a combination of the normal DC resistance and the AC resistance of any module that outputs audio signals. This is usually determined by a resistor in series with the output socket. Why is the value important? If you have a high output impedance and you use long cables, the capacitance of this cable combined with the output resistance (impedance) forms a lowpass filter that can cut off some of your high frequency audio. Don't ask me for details, you will have to Google that but this is one reason why output impedance is a thing. Normally we have a HIGH INPUT impedance because the inputs go into opamps which have an infinitely high resistance and we have a LOW OUTPUT impedance around the 1K Ohm mark among other reasons because of what I mentioned above. Now long cables are not usually used between modular synthesizer modules but they are used in amplifiers. I just want you to know about this, as part of you electronics knowledge.

Don't put those cheap Chinese Volt and Ampere meters into your power supply. I know these digital displays look cool and it's handy to know how much current your system is drawing but these meters introduce a shit-load of noise onto you powerrails. If you do want a measuring system in your powerrails, use analog meters with pointing needles instead. They look even cooler, especially when back-lit, and don't have internal circuitry that can introduce noise into your system.

Any questions or remarks? Put them in the comments below please. Comments containing links will be deleted!

Synthesizer Build part-4: THE ENVELOPE GENERATOR or ADSR

This was the first Envelope Generator I built but I no longer use this design myself since I discovered the Digisound 80 ADSR and the Yusynth 7555 ADSR both of which are much better designs with the Digisound design also using the AS3310 chip (or the CEM3310). So my strong advise is not to build this particular design. "Why is it posted here then", I hear you ask. Well, because this website is an archive of the synthesizer I built and that includes all the lesser designs too. But you get ample warning if I advise against building. It's mostly the early projects that can be a bit problematic. The design below is simply the datasheet circuit of the AS3310 chip, which is not the best design by a long shot.

Original text of this article:
The Envelope Generator is generally better known as the ADSR which stands for Attack, Decay, Sustain and Release. These are the four phases a note goes through when you press a key on the keyboard. The attack is the speed of the initial rise of the note, once you press the key. Decay is the time it takes for the note to go from the peak attack level to the sustain level. Sustain is the level of the note as you keep the key pressed down. It is usually a bit less loud than the first instance of the note being played. Then we have Release and that is the amount of time it takes for the note to fade out once you let go of the key. So the envelope generator produces a signal that determines the volume of the note over time and this signal is being used by the Voltage Controlled Amplifier (VCA) which interprets it as an output level. In some Minimoog synths it is also called the Loudness Contour.

Now of course the envelope output is a control voltage so it doesn't mean that you need to use it for the above mentioned purpose. You can connect it to anything that can be controlled with a control voltage like the filter cut-off or the resonance or the pulse width of a squarewave. This opens up a miriad of options but let's not get ahead of ourselves here. If you're just starting out with synth building, you need the ADSR to open the VCA and the fancy stuff will come later.

I decided to build this ADSR using the chip series that I plan to use for the most important components of my DIY synth, the AS33xx series of chips. The AS3310 is the ADSR chip and it costs way less then its CEM counterpart. It's about €6,- 

So I looked up the datasheet and used the circuit that was presented there. I made the following stripboard layout for it. This layout is verified, I used it for my build. (All potmeters viewed from the front.):

(Last revised: 16-March-2020: Removed direct potmeter connection to 5Volt. Revised potmeter wiring. Trigger when not used shorted to Gate via internal switch in trigger input socket.)

Print only:

This design works very well and does the job it needs to do. It has a few little quircks though. The potmeters for instance. I used normal linear type potmeter and that works but it would be better to have reversed logarithmic or anti-logarithmic potmeters because the difference between 1 second and 10 seconds on the Release for instance is only a few degrees of turning the knob. But once you're used to this it's not really a problem. The Sustain potmeter is at its maximum at about 2/5th of its maximum throw. If you turn it further the Sustain level rises but the attack won't be able to reach it. So if you have Attack set to, for instance, one second, it will rise normally and after one second it will suddenly jump to the Sustain level. I've got some oscilloscope pictures below to illustrate this.
The external trigger input is normally shorted out through a switch in the Trigger input socket. So if there's no trigger cable attached, the trigger for the chip is provided by the Gate signal through the 3nF capacitor. The AS3310 needs a simultaneous gate and trigger signal to function. So if you plug a cable into the trigger input but you don't provide a trigger signal, the Attack parameter of the ADSR will not work! So this is not a malfunction, this is how it's supposed to work.

I added a few extra's to this design. First there is the option to output a signal that is twice the voltage of the normal output (10Vpp instead of 5Vpp). You can use this, together with a passive attenuator in the mixer panel, to drive a filter's resonance or other parameters of the synth. Then there's also an inverted signal output, to add to the options of driving parameters of the synth. This goes from 0V to -10V.
All this takes place at the opamp on the lower left of the circuitboard. You can use the old favourite TL072 for this or the TL082. Pins 1,2 and 3 together with the two 100K resistors form the 2x amplification. You can use other resistor values as long as you use 2 resistors of the same value. Then the input signal is split at the non inverting input and goes, via a 100K resistor to the other side of the chip where the opamp is set up as an inverting buffer. Both opamp outputs have their own output jack socket. The normal 5V ADSR output is a separate socket (of course).

And finally I added a manual trigger option, at first I added it so I could put a gate signal on the gate input for test purposes, but then I thought this would be handy to have anyway so I added the switch to the final panel. I decoupled the manual trigger signal from the gate input socket with a Schottky Diode so no voltage goes into the circuitry that delivers the normal gate signals so as not to damage it (although this is probably not necessary). I used a Schottky diode because their voltage drop is only 0.2 Volts so it doesn't detract too much from the usual 5 Volt gate signal.

Here's the E.G. mounted in my synth. You can see that I doubled the output sockets. There's 2 outputs for normal 5Vpp ADSR and 2 for either 10Vpp or Inverted -10Vpp I also installed a Gate output and a Trigger output. The Trigger output is connected to the Gate output via a 3nF capacitor and the Gate output is simply switched in parallel over the Gate input. I will however install a opamp buffer for the gate output in the near future.:

Here are some oscilloscope screenshots showing the function of each variable:
This is the normal envelope CV at 10Vpp.

Varying the Decay time:

The picture below shows the quirck I mentioned earlier where the Sustain level is set higher than the Attack can reach and so after the Attack cycle has finished the Decay is skipped all together (because there is no Decay if the Sustain level is higher than the Attack level) and the envelope jumps to the Sustain level. You can clearly hear the jump in volume in the audio. You can use this to your benefit though because it sorta has a percussive quality to it. Anyway, if you don't want this, just turn the Sustain down a bit. Problem solved. You can also limit the maximum resistor value of the Sustain potmeter by adding a resistor or trimmer to pin 3, but you'll have to experiment to find which value works best.

Lowering the Sustain level:

Increasing the Release time:

And finally switching between the inverse envelope (which was set to 0 to -5V in my ADSR but to 0 to -10V in the stripboard layout) and the 10Vpp envelope.

Okay, that's it for this one.
I hope you enjoyed this article and leave a comment please if you found this helpful! Much appreciated! Also, any questions? Put them in the comments or contact me on Facebook. I'm a member of the 'Synth DIY' Facebook group and the LMNC Discussions FBgroup and also the "Synth DIY for non engineers" Facebook Group all under my own name Eddy Bergman.
Stay tuned for more build reports and click 'follow' to be notified of new posts to this website.

Synthesizer Build part-3: TRIANGLE TO SINEWAVE CONVERTER

This article has been re-written at 11-11-2020.

After I had finished the VCO I wanted to add a Sinewave option to it. The first design I had posted here was a bit sketchy so I now present a new layout here. This layout has been made using the schematic of the Thomas Henry CEM3340 Deluxe VCO, which has a sinewave output. Btw, you can find that schematic in the 'files' section of the 'Synth DIY for non engineers Facebook Group'.

This circuit needs the Triangle input wave to be +/-5 Volt peak-to-peak. You can input a Trianglewave of 0V to +10Vpp but then the input must first go through the 1µF electrolytic capacitor to take away the offset voltage. The Triangle to Sinewave converter will not work properly if you input a 0/+10Vpp Trianglewave without first filtering out the DC offset voltage.

I altered the feedback resistor (Rf on layout) from 10K to 15K to get the amplitude correct with the waveform standard of my synthesizer project which is 0 to +10Vpp. This had the effect that the +/-5V output got a negative offset voltage. What I should have done is change the other 10K that goes from the + input to ground into a 15K also, so everything is in balance again but I put a 1µF capacitor in series with the output of the +/-5Vpp sinewave. The negative pole of the electrolytic capacitor is facing the direction the signal is coming from because I had a negative offset voltage to deal with. Make sure you match the 10K resistors so they all have the same value and if you change the feedback resistor to a 15K make sure you change the other 10K to ground also. Match the transistors too. (Matching them on hfe is good enough). If after all that you still have an offset voltage on the output (unlikely) then you can put a 1µF cap in series with the +/-5V output.

The output amplitude on a dual 12V powersupply is +/-4.2Vpp or 0 to 9.4Vpp. For a dual 15V power supply it is +/-5Vpp or 0 to 10Vpp.

Here is the new stripboard layout. This converter offers a +/-5Vpp output and a 0/+10Vpp output. 

Here is the schematic drawing. I did not include any de-coupling capacitors but if you want to include them then just add two 100nF ceramic capacitors to the voltage rails as close to the chip as possible. One going from +15V to ground and the other from ground to -15V.

Here are two pictures from the oscilloscope. One without offset from the +/-5Vpp output and one with offset from the 0/+10Vpp output. If you look closely at the pictures you see that the scope is set to 2V per division and therefore that the amplitude of the sinewave is 8V. But now that I changed the feedback resistor Rf, that has changed to 10V (even a tiny bit over):

As you can see they are beautiful sinewaves and you can set the symmetry and distortion very accurately with the trimpots on the stripboard. 

It will be easy enough to mount this little stripboard on one of the M3 bolts used to mount the print of the 'Really Good VCO' and thus add a Sinewave output to that VCO. You can tap the Trianglewave straight from pin 10 of the AS3340 (or CEM3340) chip or from pin 12 of the TL074 quad opamp chip. I think that will be even easier. On those pins the Trianglewave is not yet given a +5V offset voltage so it is still +/-5Vpp and therefore doesn't need to go through the 1µF electrolytic capacitor on the layout of the Triangle- to Sinewave converter. 

Okay that's the new version of this article done. If you have any questions please put them in the comments below or on the EddyBergman Facebook Group page.

If you would like to support my projects and the upkeep of this website you can 'Buy me a Coffee' if you like. There's a button underneath the Menu if you're on a PC or Mac. Or you can donate a few bob with Paypal by clicking here.  Thank you!!

Btw, all the comments below upto August 2020 refer to the original Tri- to Sinewave converter article and not to this one. So please disregard those comments.

Synthesizer Build part-2: THE VCO

A word in advance: this article deals with the first VCO I've ever built and is based on the datasheet schematic combined with the LookMumNoComputer lay-out for the CEM or AS3340 chip. I personally had great trouble getting this VCO tuned over a wide range of octaves.  I could also never get really deep notes from this design. I have since found a much better VCO design so if you want to build a simple but excellent working and tunable VCO on stripboard I refer you to Synthesizer Build part-18: A Really Good VCO design.

I'm keeping this article up for my own archive and as a warning for those just starting out not to build this VCO!!!

Here's the original text for the first VCO build:
After having constructed the power supply and the power bus system it is time to move on to the next step. The Voltage Controlled Oscillator. I'm not going to go into details as to how it works etc. There's plenty info online about that.  In order to make this a complete build, not just the circuit board I needed something to mount the knobs and in- and outputs on. So I ordered a sheet of Aluminium, 200 x 1000 X 1.5 mm and powder coated gray/black on one side. That is fantastic stuff to make panels out off and I highly recommend it. You can saw off panels of the right width using an electric jigsaw with a fine toothed metal saw. Make sure you guide the saw with a straight piece of wood or metal to get nice straight panels.

For my VCO I chose the AS3340 chip which is a complete 1 volt per Octave VCO in a chip. It's a clone of the CEM3340 which were used in the 80's in synths like the Prophet 5 the Roland SH101 and many others.
The VCO we're building here will have almost all the options that the AS3340 chip has to offer and those we didn't include are not worth having anyway ;)
The schematic I used is pretty much just the schematic that comes on the datasheet.

This is the one I used:

I used the layout made by Sam Battle, from LookMumNoComputer and did a few enhancements on it. (Look to the one on the right).

For one, his layout is meant for the CEM3340 which uses a 10K pulldown resistor on pin 4, the squarewave output. For the AS chip, that's supposed to be a 51K resistor although I'm reliably informed it doesn't make a blind bit of difference what you use here. There was also a mistake in his design, namely the 10K resistor in the bottom left near the TL072. It is switched in parallel with the 10K on pin 4 making the overall resistance 5K. Just leave the bottom 10 K resistor out.
The 10K trimmer potmeter at the top left of the 3340 needs to be a multiple turn potmeter so you can set it very accurately.

Sam's layout doesn't include the High Frequency Tracking but you really need to include it in your VCO. I first build it without and at first it seemed to work fine but after having completed the whole synthesizer I couldn't get really deep bass tones out of it. That is until I included the High Frequency Tracking. Seems a paradox that something meant for High Frequency adjustments can have so much influence on the bass notes but if you look at the schematics you can see that it pulls the CV voltage on pin 15 down to ground a bit through the 20K potmeter. I kept out the 360K resistor between +15V and CV input because that kicks the VCO into really high notes. I don't know why that resistor is there but it really screws up the frequencies. I left it out but maybe I should have experimented further with that resistor in place. Anyway...
The HF adjustment pot only adjusts about half a note over its full throw so when you first test it it might look as though it doesn't work but it does when you start tuning the higher octaves of the VCO.

Furthermore I gave the buffer for the triangle wave a gain of 2 by adding two 100K resistors to the TL072. That gets the level of the triangle output up to 10V peak-to-peak, in line with the output voltages of the other two waveforms. Btw, you can use any resistor value between 50K and 1M for this purpose as long as both resistors have the same value.

I also added the Positive and Negative Hard Sync options from the Digisound 80 Modular design so that's also available on this VCO.

Here is the layout that I drew and used:

So there we have it. It's become quite a comprehensive VCO with lots of options.
I added a 100K resistor to the +15V input of the Pulse Width Modulation potmeter to get it to work over the complete throw of the potmeter and I added a switch to have the ability to decouple it from the PW Control Voltage if you have PWM controlled by an LFO for instance. You don't have to decouple it but the option is there.

I tested the finished print and everything worked as expected but there was a funny quirck in the squarewave output. Below 1.35kHz there was a strange triangular wave ringing on the downward slope of the square-wave. Here's some pictures of that from my scope:

I opened a discussion about this in the Synth DIY Facebook Group and there were many suggestions but I still haven't figured out the cause. It's not a de-coupling issue anyway.
I suspect that leaving out the High Frequency Tracking I mentioned earlier may be the cause. (Note: I did some more tests and it turns out that it does have a big influence on this issue. Including HF Track with the 360K resistor to +15V almost gets rid of the problem but on low frequencies there still is a bit of ringing on the downward slope but not nearly as much as now.
But as I mentioned before, the 360K resistor really screws up the frequency response so it can not be included. I have heared that there might be batches of chips that have this fault, so it might be the chip. I don't know and don't really care because you don't hear it and everything works fine.

[Edit: In the second VCO I built and now use (see article 18) this ringing is still there but it is much less then in this design. The new VCO has at most 3 spikes in the downward slope of the squarewave. Anyway it has proven to be not a problem what so ever. You can't hear it and it doesn't affect the working of either VCO in any way.]

Although the connection is there in the layout, I did not use the Soft Sync input on my final build. I don't think I'll need it. I did use the FM input. You can connect a second VCO to that for instance.
Here's a look at the finished product, panel and all. The powder coated Aluminium was a great choise and looks so cool. It doesn't scratch easy at all, it's perfect for this project.

I'll explain what's on the panel.
We have the FM input at the top left. The big knob at the top is the Coarse Tune potmeter, below that on the left are the CV1 and CV2 inputs and on the right are the Triangle-, Ramp- or Sawtoothwave and below that the Squarewave outputs. ( I always put inputs on the left and outputs on the right.)
Then there are three inputs to the left of the blue knobs. Those are the Pulse Width Control Voltage input, the blue knob next to it controls its level. Underneath that are the Positive Hard Sync and Negative Hard Sync inputs. The bottom blue knob is the Manual Pulse Width control if you don't use a control voltage. The switch with the diode symbol let's you choose to put a diode in the external Pulse Width input line which de-couples it from the internal PWM control or to bypass that diode and get more range on the PWM control knob. [edit] I have since scrapped this idea and I took out the diode. The switch is now used to turn off or on the manual Pulse Width Modulation potmeter as described above. (It's not necessary but the switch was there so might aswel use it for something). I added one more output which isn't in this picture and that is a "CV out" function to connect the second VCO to the first one. It's simply switched in parallel with the CV-1 input.


Okay, that's it for this one. If you have questions or suggestions please write them in a comment. Next part will be about a filter, probably the Prophet One Low Pass filter.
Stay tuned!