The circuit was originally published on electricdruid.net. People were struggling with VCAs not closing correctly.
In fact the original circuit can´t deliver dead silence so i´ve been modifying it to get proper results.
At first i´m advicing to start assembling when you got all the parts and tools you need and of course i´m assuming you got all the skillz you need for soldering and desoldering! Following the documentation you will see which parts i´ve been using. In almost every case i´m using common parts you can easily find out there.
Step 1 – Resistors
Follow the BOM and designators “R” on the PCB and put the correct R to its footprint. You see, the resistors are vertical alligned to safe space. The orientation doesn´t matter. Take care for R10, R16, R18 & R19! They´re in ohm.
Step 2 – Capacitors
You got 2 Values of those yellow ceramic caps (1nF and 100nF), the designator on the PCB is “C”. At first choose the 1N caps, the rest is 100N. Orientation doesn´t matter.
Step 3 – Transistors
In best case these 2N3906 PNP transistors should be matched. I´ve been widening the pad distance a bit, but take care! The pads are very close to each other. Take a deep breath, respect the orientation and make them sit tight.
Step 4 – IC Sockets
Sure, you can solder the ICs directly onto the PCB, but i´m not advicing to!
Little trick, attach the sockets with some sticky tape, at first just solder one pin. Make sure it sits tight and plane on the PCB. In case you disrespected the orientation, don´t worry!
That´s why sockets safe your life 😉
Step 5 – Diodes
You got only two diodes to mount. RESPECT THE ORIENTATION!!!!
Step 6 – Electrolytic Capacitors
Two 47uf caps. You can choose 10uf aswell or something in between. Make sure they´re capaple of at least 25V. The rule says twice the voltage you feed. In case you do something wrong here one of the caps will blow up or explode.
RESPECT THE ORIENTATION!!! Round hole minus leg.
Step 7 – Eurorack Power Pin Header
That guy is attached on the other side of the PCB!!!! The 3 inner pins are ground and connected to the ground plane which is all over the PCB on top and bottom. You may will experience ground related pins to be a bit harder to solder. That´s because these pads cool down very quickly due to its thermal connections. Especially for the pin header i´m advicing to make the iron hotter. You see on my “beautiful” solder spots the iron was already a bit too hot.
Step 8 – Thonkiconns
Yes, it´s time! Now we come over to the funny parts. The thonkies have 3 pins. You can´t place them wrong but make sure they sit tight and plane on the PCB, otherwise you will have a hard time attaching the panel. First solder the middle pin, check if it´s placed well, solder the other pins.
Step 9 – Potentiometers
The pots should snap in and sit tight and plane on the PCB! Again, solder one pin, check its placement and solder the other legs.
The two Alpha pots must be 100k, but can be linear or logarithmic. A question of taste … The Song Huei pots can be something between 10k-100k. They simply attenuate the incoming CV signal, while the level pots give a specific current and can´t be changed! I´m advicing to choose the values i´ve been using or you dive into the simulator to calculate the right balance (again).
Step 10 – ICs & LEDs
Place the ICs to their sockets and make sure you respect the orientation. You see, i´ve already placed the TL072 wrong verse 😀 The mark on the PCB corresponds to the mark on the IC.
In case you want to know what happened to the TL072 after powering 😀 :
Place the LEDs to its place but don´t solder them yet! They need to be adjusted with the panel. The orientation matters! Long leg round hole. You can choose your desired color but choose bright ones. More mcd means more brightness.
Step 11 – Panel Mounting & LED soldering
So, here we are. Placing the panel should be easy in case all the parts sit where they should!
In case you don´t have the right tools to mount the nuts use some pliers BUT be careful not to scratch the panel! At first just mount two nuts to make sure the panel is placed well, than solder the LEDs. Make sure they correspond to the indicator graphics ….
I´m using an old tooth brush to clean the PCB at least.
Step 12 – Quick Check
Check all your solder spots, reheat the critical ones. Check the power pins of the pin header with a multimeter to make sure there is no short!
VCA not closing correctly.
This happens when the chosen parts fall out of the tolerance range too far. The calculations i´ve done are very theoretical and based on mathematical optimums. The real outta world isn´t that precise. In most cases this is caused by the transistors or the resistors R31 & R32. Swap the resistors to 1M and you should be safe 😉