Oscillators. More is More! TH-VCO1

The Very Beginning of the Signal Chain

Voltage controller oscillators, or VCO's, are the root of everything in subtractive synthesis. That's the place where the sound is born and everything after that is just modifying it. VCO really creates the sound. It is also the component that defines pitch and thus makes melody and harmony possible. Rest of the signal path is mostly about timbre. (And rhythm of course.)

I started my modular synth by building a Doeper DIY Synth so I only had one VCO. It really limited things so I had to get more. Things got a bit better when I added another Doepfer DIY Synth to the rack but 2 oscillators is still not that much.

There's two reasons to add more oscillators. First one is to be able to play multiple notes at the same time to generate harmony. The second one is to configure multiple oscillators to play the same note to generate more complex sounds. Analog VCO's are always more or less out of tune so the sound will get thicker and thicker the more oscillators you add to the mix.

As a trance music lover I have been fascinated with the "super-saw" sound of Roland JP-8000. It's a digital synth but the idea of the sound is to emulate 7 sawtooth oscillators slightly out of tune against each other. I want to do the same with real analog oscillators! Or why not more than 7 oscillators! I want my modular synth to be huge so I need lots of VCOs!

DIY VCO Alternatives

VCO's are quite difficult DIY modules. At least to me they are way too complicated to be built on veroboard and because VCO is responsible for the pitch it's also defines how well the synth stays in tune so temperature compensation needs to be taken into account and some components need to be matched precisely.

There are lots of nice DIY kits available. Unfortunately most of them are quite expensive so I went with my "quantity over quality" philosophy again and started looking for cheaper alternatives. I probably ended up spending more money with my DIY hacks than those readymade kits would have cost but at least it felt cheaper when buying the first components...

As with the previous diy modules I started with the ASM-1 synth. It has a relatively simple VCO with saw and square outputs. This time I wanted to have a proper PCB so I ended up ordering 2 ASM "Ultra" VCO pcb's from Marc Bareille (Papareil Synth Labs). It's a slightly improved version of the ASM-1 VCO. Unfortunately I haven't had time to build these yet so these will be discussed in another blog post some day.

While searching for the PCB's I also found Thomas Henrys nice VCO-1 DIY module and especially Tom Whitwell's Eurorack version of it. He has done a 2 pcb + panel design and published all design files in github.

How to get the PCB's?

I had never used any PCB manufacturing services before. After some googling I found http://dirtypcbs.com/. It seems to be the cheapest alternative to get custom PCBs done. The idea is interesting. They use some unspecified cheap manufacturers in China and give no guarantee about the quality. There's a risk that you might get faulty pcb's but on the other hand if everything goes well it's really cheap. They don't even guarantee the amount of boards you will get. It's "about 10". May be more or may be less.

As Tom Whitwell had shared his design files I decided to just give it a shot. I first ordered the component board only to minimize the risk. After about 3 weeks I found 11 nice looking PCBs from my mail box. Awesome! So in this case "about 10" meant 11. One extra for free!

Component boards arrived! Dirtypcbs.com works!

I first planned to use only the component board and do a custom panel but after thinking about all the pain and mess of drilling several aluminium panels I decided to order also the rest of the PCB's designed by Tom. A potentiometer board for pots and jacks and a dummy-pcb as the front panel. For the panel I did some modifications myself to add labels for pots and jacks. Luckily Eagle design software is free for this kind of hobby use.

All boards ready for soldering!

Again about 3 weeks later I got mail. 11 potentiometer boards and 12 panels! Great! I'll use the extra panel for some other DIY project later. Unfortunately the panel was not perfect. Some of the labels I added are too close to the potentiometer and will be partly hidden if I use knobs. But that's just a small cosmetic issue.

Soldering Time!

Usually I'm able to build most of the modules by just using the components I happen to have around. With this one I realized pretty quickly that I need to order the exact BOM. Most of the resistor values are rare values outside the regular E12 series. So I made an order to Reichelt and waited yet another week. After that it was pretty quick build.

I still had to put two resistors in series in some places because the resistor values are so rare.

Potentiometer board ready. I used cheap ALPS mini potentiometers. They have position marking in the shaft so knobs are not mandatory. I might try different pots in other builds as these do not feel very good.

All boards Ready! I used IC socket only for the expensive THAT340 chips.

Notice the tempco resistor on top of the THAT340. I didn't have any chemicals to properly attach them but it seems to be quite stable also by just pressing it against the chip.

Mechanics!

I really like the fact that there's zero jumper wires in the module. Panel is held in place by the jacks and PCBs are attached with a connector and screws. Really solid at least in theory. Unfortunately I didn't pay attention to the height limitations and ended up with a really crowded build. The power supply capacitors should be mounted horizontally and IC-socket+THAT340+tempco combination is too tall. I'll probably leave the socket out next time. On the other hand the potentiometer PCB now firmly presses the tempco against the chip so the thermal connection is better.

Not much headroom downstairs!

Tempco pressed against the THAT340 chip. PCB screws not yet in place.

Ready!

All done!

Unfortunately I ordered slightly wrong type of trimmers. They need to be tuned from the sides. This might be good for some eurorack cases but in my case it's really difficult. It would be much easier to turn the trimmers directly from the backside. I had to temporarily install the module with some extra space on the side to be able to tune it. I'll reorganize my rack after it's properly tuned...

Installed. Had to leave some room to access tuning trimmers.

Initial tuning was easy and unit seems to be quite stable. Unfortunately the ALPS pots feel quite wobbly and make tuning it more difficult. I should try to find better 9mm mini pots for the other units I'm planning to build. I also need to trim the sine wave output to produce better sine.

There seems to be some issues with the linear FM input. If I feed triangle wave into it it behaves like square wave and frequency just makes huge jumps instead of smoothly going up and down. Have to debug what's going on there.

I also miss the sawtooth wave already! to me saw is the most fundamental waveform in analog synthesis and feels weird not to have that. But maybe I'll get used to that. I anyway have two oscillators in my Doepfer DIY synths producing sawtooth waves if needed.

I will definitely build more of these. I still have 10 sets of pcbs and parts ordered for 4 oscillators so expecting to have more of these in my rack very soon. But probably need to build some mixer modules first to be able to utilize all of these sound sources.

Here's an oscilloscope demo of the sine output: https://www.instagram.com/p/BVNbO1nlnAI/

And here's three oscillators playing a simple arpeggiator pattern: https://www.instagram.com/p/BVQAfzrFU2s/. TH-VCO1 produces triangle wave, one doepfer plays sawtooth wave and another produces square wave.

Edit: My version of the panel design is now available in github: https://github.com/tvainio/TH-VCO1