[diy] LIL' MONO SYNTH - A proper introduction

video upload by planeteatrr

"You can find more information on this little oddball over here: https://modwiggler.com/forum/viewtopi...

All the files for having the circuitboard and the frontpanel produced will be made available soon. If you can solder you can easily build this on one sunday. Complete production costs for one unit are around 70€/$ or less. I will add a link to the production files in some weeks.

Cheers
Sebastian"

Additional details:

"Since a year or so I fantasize about developing a series of small analog synthesizers, that will fit in guitar pedal or other types of enclosures, with frontpanels made of FR-4 pcb material. The synthesizer I am about to describe is basically the archetype and prototype for the more advanced synths. The next one will be based around Electric Druids VCDCO and TAPLFO chips and another one will include the famous CEM3340 vco ic paired up with a vcf that is a weird mixture between a Wasp vcf and an Oberheim Sem vcf.

But let's have a look on the first one of these synths. The main goal for me was that it can fit into a small aluminium die cast enclosure, without the need to use smt componentes or small 1/8 watt resistors. This meant I needed to have a low parts count. The result is that I used the most basic circuits for each individual function and merge them together into one circuit. I really did not expect much from it, but I am actually very suprised that the output of this progress is much more versatile and interesting (but with a nice and big "oddness-factor") than I thought.

Though I took my HEF4046 (The IC needs to be a Philips/NXP or Motorola, TI ICs will track very poorly, those HEF/MC ICs are obsolete as through hole parts but they are available in hugh quantities at e.g. ebay) vco design and built all the other stuff around it. I decided to take the most simple vca design I know which is the infamous single diode "vca", that is often used in lunetta-type synths. I was suprised that it is actually working with sawtooth waves aswell. It gets a bit weird and distorted when the envelopes fade in or fade out, but I really like the sound of it. Since my vco design has a sawtooth output that is one octave higher than the squarewave output, the squarewave output can be used as suboscillator. Both waveforms are mixed together and send in to the vca, which comes before the vcf. The vcf should actually have been a twin-t notch filter, but I left out the highpass part and it became a (sort of bridged T) low pass filter, which has quite some similarity to tim escobedos q&d vcf. This thing sounds dirty but when you use the right values for the components it can be tamed and can sound quite creamy at high resonant settings. For the envelope generator I actually planned to use a 555 timer ic, but that would have had to many components. Though I decided to build the most simple envelope generator that just contains a comparator at the input and a buffer at the output. It's not perfect, but it works just fine. There is no modulation for the vcf beside the envelope modulation, but this alone sounds quite nice (there will be a jack for cv).

There are some downsides to this simple circuit:
* The "vca" is not a real vca and distorts the signal (which I actually like)
* The "vca" has a little bit of signal bleedthrough, which is completly gone when using the env mod
* The vco is not very precise, but works solid over 2 to 3 octaves or even 4 octaves (you need to live with some tuning error, somehow it depends alot on the used cv-interface, actually the cv inputs needs to be buffered :roll:)
* The envelope generator is not that good but if you are aware of its sweetspots it can do a good enough job
* There is an overall oddness to some controls, but it works much better than I expected
* It has no midi connection (only cv inputs)

The pros are:
* The gerber files will be available for free
* It will be very small for a fully functioning, east coast style analog synth. It is literally half the size of a korg volca device
* It has a suboscillator
* The output is actually quite powerful and it "sounds big"
* It sounds unique and quite versatile
* It can be hooked to other (semi)-modular devices.
* You can play it polyphonic when using several of those synths and a multichannel midi2cv interface
* Built cost will be under 50€($)
* I developed the synth in 5 evening sessions or so ;)

There are some issues in the initial schematics, but I will update them soon...

Gerber files will be uploaded in spring/summer and I will most likely sell some pcbs and frontpanels."