The Coven LFO Eurorack Module is a 4HP, easy to build DIY kit that offers square, saw, triangle and stepped random outputs. It features a single frequency knob, mode button, an FM and a Sync CV input, and four LFO outputs, each with a different output frequency.
Introduction
In this step-by-step guide we will cover how to build the Coven LFO module from start to finish. To complete this build you will require a CCTV Coven LFO diy kit, a few tools and a little bit of soldering experience. You should set aside approximately 2 hours to complete this build. The soldering skill level required is about a 5/10. There is only one surface mount component that needs to be soldered and the rest are through-hole components. The tools required to complete this build are:
+ A Soldering Iron
+ Solder
+ Side cutters
You may also find that a pair of tweezers and a set of pliers come in handy, though they are not essential. We will first start by soldering all of the resistors to the larger PCB and from there we will begin soldering the larger components in the order outlined in this guide. After assembling the larger PCB we will begin assembling the smaller PCB, and finally attach the two boards together using header pins. Keep on reading to get started!
The bill of materials, which outlines all the parts included in your kit, can be seen in the table below.
Beginner Tip: As you finish soldering components, strike the spreadsheet entry with a marker or pen so that you know what part of the build has been completed.
Let begin our build!! Pictured below are the two PCBs that come in your kit:
We will start by populating the larger PCB. First, solder the 330 Ohm resistors R1, R5, R9, R13, R17, R18, R19 and R20 (8 total). The values of the resistors are printed on the PCB and written on the paper packaging so you can easily locate the components and where they need to go. Bend the resistor legs tightly to the body as seen in the picture below, so they fit nicely through the holes in the circuit board.
The resistors are not polarity sensitive, so their orientation is not important, but you will receive synth DIY brownie points if all the stripes point in the same direction. Solder each leg to the bottom side of the board then clip down the leads using side cutters.
Next, we will solder the 10k resistors of which there are six (R3, R7, R11, R15, R25, R26). Follow the same procedure as for the 330 Ohm resistors.
Bend the resistors legs out slightly like below so they stay in place while you are soldering.
Finally, we will solder the 60.4k resistors (R2, R6, R10, R14) and the 75k resistors (R4, R8, R12, R16, R21, R22, R23 and R24) in the same manner. Now all of your resistors should be soldered like in the picture below.
Now that we have soldered all the resistors to the larger PCB, it is time to solder the diodes, D1 and D2 to our PCB. These diodes provide reverse polarity protection. Note, the diodes are polarity sensitive, meaning their orientation is important. The stripe on the diode represents the cathode. Make sure to line up the stripe on the diode with the stripe printed on the PCB as seen below. Solder the leads to the bottom of the PCB then trim the leads down using side cutters like we did for the resistors.
Next, we will solder the XIAO, pictured below, to our PCB. The XIAO is a SAMD21 based, low power microcontroller which is 100% Arduino compatible. It features a USB-C port which can be used to supply power to the board and to download code. The XIAO in your diy kit comes pre-loaded with the code required for this project so you don't need to worry about loading any. If you are interested in modifying the code however, check out the section "How to Hack Your Coven LFO" at the end of this build guide for more information.
The XIAO is a surface mount component and the trickiest component to solder so take your time with it. Care must be taken to make sure the pads of the XIAO are aligned with the pads on the PCB (the footprint for the XIAO is labelled U2). You may find it helpful to solder a corner on each side of the XIAO to make sure it is properly aligned before soldering the rest of the pads. Ensure the USB C port hangs off the edge of the board.
Add a small amount of solder to the top of a pad on the XIAO then heat the corresponding pad on the PCB, making sure that the solder has flowed through the hole on the XIAO and is making a good electrical connection. You should see the solder melt through the pad holes on the XIAO when you heat the pad on the PCB. The XIAO is the heart of our module, so take your time making sure each pad is properly soldered. Once you are finished soldering the XIAO, your PCB should look like this:
Next, we will solder the quad op-amp (part number TL074, the 14 pin IC) and the dual op-amp (part number MCP6002, the 8 pin IC).
Note that the IC’s are polarity sensitive. Make sure the notch on the IC lines up with the notch on the footprint of the PCB like this:
Solder each pin of the ICs to the bottom of the PCB making sure that they are sitting flush with the PCB. Again, solder 2 corners first to make sure you're seated properly before moving on. You do not need to trim the pins down after soldering. Your PCB should now look like this:
Next, we will solder the capacitors. We will start with C1-C3 which are 100nF ceramic capacitors (the small brown capacitors). These capacitors are not polarity sensitive, and the leads should fit nicely through the PCB holes without much fuss. Solder each lead to the bottom of the board in the same way we have with all the through hole components. Clip down the leads of each capacitor once it is soldered into place.
Now we will solder the 15nF film capacitors (the small square capacitors). These capacitors are not polarity sensitive and should fit nicely through the holes in the PCB without bending their leads. Solder the leads to the bottom of the PCB then trim them down using side cutters. Your PCB should now look like this:
Next, we will solder the three 22uF 35V electrolytic capacitors to our PCB (C8, C9 and C10). These capacitors are polarity sensitive, so we need to be careful that we get the orientation correct. Make sure that the negative lead (which is marked with a white stripe on the capacitor) DOES NOT go through the hole labelled positive with a (+) on your PCB.
Up next is the positive voltage regulator (part number 78L05) that is used to power our XIAO with 5V. The footprint is labelled as IC2 on your PCB. This component is polarity sensitive so we must make sure that the flat edge of the voltage regulator is lined up with the flat edge of the footprint printed on the PCB. You will need to bend the center leg of the regulator back so that it fits through the center hole. Solder the leads to the back of the board and trim them down using side cutters once soldered in place.
The last component that we will solder on this board before moving on to the smaller PCB is the eurorack power header which is the 5x2 pin header.
You can locate where the header goes by looking for the “RED STRIPE” text printed on the PCB. The short legs of the header pins go through the holes on the PCB and solder to the bottom of the board. You do not need to trim these leads down. Once your 5x2 header pins are soldered to the board, your PCB should look like this:
Now we will move on to populating the smaller PCB. We will first start by soldering the six 1/8” jacks to our PCB (labelled as CV1, CV2, OUT1, OUT2, OUT3 and OUT4). You will see that there is only one way for the jacks to fit through the holes in the PCB. You may need to squeeze the legs of the jacks a little bit, so the legs fit through the holes. Make sure that the jacks are sitting flush with the PCB before soldering the leads to the bottom of the board.
Once you have the six 1/8” jacks soldered to your PCB, it is time to solder the momentary switch to the footprint labelled S1. It looks like this:
You will notice that the spacing of the leads on the bottom of the switch are such that the switch will only fit onto the PCB in one orientation. You may need to bend the leads of the switch a little bit so that they fit through the holes in the PCB. Make sure the base of the switch is sitting flush to the board before you solder it into place. Your smaller PCB should now look like this:
Next, we will solder the 9mm potentiometer to the top of our smaller PCB. You can locate where this component goes by looking for “VR1” labelled on the circuit board. Again, make sure that the potentiometer is sitting flush to the board before soldering into place.
Now we will solder the four LEDs onto the PCB. The LEDs look like this:
Note that the LED’s are polarity sensitive so we must also be careful that we insert them into the PCB with the correct orientation. The shorter lead of the LED should go through the hole with the flat line marked on the PCB. Place all four LEDs onto the PCB but do not yet solder them. The LED’s will be soldered with the panel cover in place so that they line up with the holes on the panel cover nicely. Place the panel cover onto the PCB and secure it using the included nuts for the six 1/8” jacks and the 9mm potentiometer. Once the panel cover is in place, we will begin soldering the LED’s. Push the LED’s up as far as they will go so that they are sitting proud through the holes of the panel cover. Once you have soldered the LEDs to the bottom of the PCB, trim the leads down using side cutters. You can now put the knob on to the 9mm pot.
The last step is to attach the larger PCB to the smaller PCB using the 90-degree header pins included in your kit. Start by soldering the short end of the header pins to the larger PCB like this:
Then solder the smaller PCB to the protruding pins coming off the larger PCB that you just soldered into place. The pins from the larger PCB will only fit with the smaller PCB in one orientation like this:
If you find it tricky to get your soldering iron to these pins, you can always take off the panel cover which makes accessing them a little easier.
Congratulations, you have now completed your module! Turn off your modular case, plug it in (where the red stripe of the ribbon cable lines up with the stripe on the power header footprint), and modulate the day away with your new CCTV Coven LFO module!
Troubleshooting – If your build is not working correctly, try these troubleshooting steps:
1.) Ensure that your XIAO’s pads are properly aligned with the pads on your PCB and each pad is soldered and making a good electrical connection. You can try reflowing the solder on each pad and add a bit more solder if you are unsure or you are having an issue such as an output or your mode select button not working.
2.) Do a visual inspection of each of your solder joints. Make sure to keep an eye out for any solder bridges between pads or components. If you find a solder bridge, use your soldering iron, a piece of copper braid or a solder sucker to remove it. If you see a component that does not have enough solder, add a little more and reflow the joint.
3.) Ensure you have the correct polarity for your diodes (D1 and D2), your LED’s (LED1-LED4) and your electrolytic capacitors (C8, C9 and C10).
User Guide
LFO Outputs
The LFO outputs are designed so that the first output (the output closest to the potentiometer knob) is the fastest and each output below is slower based on a division of the first output. The default divide down divisions are as follows:
Set 1
Output 1: Base frequency
Output 2: #1 frequency divided by 3
Output 3: #1 frequency divided by 7
Output 4: #1 frequency divided by 11
You can change these divisions by pressing and holding the waveshape select button for longer than 3 seconds. Note, if you hold down the button for longer than 3 seconds, the division sets will cycle through. There are three division sets in total that can be accessed in this manner.
The second set of divisions are as follows:
Set 2
Output 1: Base frequency
Output 2: #1 frequency divided by 2
Output 3: #1 frequency divided by 4
Output 4: #1 frequency divided by 8
The third set of divisions are as follows:
Set 3
Output 1: Base frequency
Output 2: #1 frequency divided by 4
Output 3: #1 frequency divided by 8
Output 4: #1 frequency divided by 16
Waveshape Selection
You can change the waveshape of the LFO by pressing the white button that is just under the frequency control potentiometer for 1 second or less. The waveshapes in order are triangle, saw, square, and stepped random. Note that pressing this button for longer than 3 seconds changes the divisions as outlined above.
Sync Mode
Sync-mode allows a user to send in a trigger or gate pulse train and the output frequencies will snap to the input frequency based on the division set selected. For example, using division set 1, if you send in a clock with a frequency of 10 Hz, Output 1 would have a frequency of 10 Hz, Output 2 would have a frequency of 3.33 Hz (10 divided by 3), Output 3 would have a frequency of 1.43 Hz (10 Hz divided by 7) and Output 4 would have a frequency of 0.90 Hz (10Hz divided by 11). These outputs will be synced in time to the input clock. The module will automatically detect if you send in a pulse and snap into Sync-mode. To get out of the sync mode, you must first unplug the sync input and then move the frequency potentiometer. You will know that you are successfully out of Sync-mode when you see the output LED’s changing in frequency when you move the potentiometer.
The sync input has a threshold voltage of 1V, so bipolar and unipolar clocks will trigger just fine.
Frequency Modulation (FM)Input
The frequency modulation input allows a user to input a control voltage that is additive with the potentiometer frequency setting. Note that if you are in Sync-mode, the FM input will be ignored, as Sync mode takes precedence over the frequency knob and frequency modulation. To get out of Sync-mode, just remove the CV input to “SYNC” and twiddle the frequency control potentiometer.
Technical Details
Current:
35 mA @ +12V
10 mA @ -12V
5V Required? No
Dimensions:
Width = 4HP (20mm)
Depth = 42mm
Output Frequency
Max = 100 Hz
Min = 69 minute period
Sync Frequency
Max = 1kHz
Min = 71 minute period
Schematic
How to Hack Your Coven LFO
The Arduino project file for the Coven LFO can be found here:
If you are interested in modifying the code, you will first need to download the latest version of Arduino IDE which can be found here:
Now, follow these instructions so that Arduino can recognize our board, the Seeeduino XIAO.
Next, we need to install an additional library so that your code will compile. This library can be downloaded directly from inside the Arduino IDE by going to Tools> Manage Libraries...
Once the Library Manager opens you can use the search bar located at the top to find the library we will need to install. Using the search bar, type in “FlashStorage_SAMD” and install the latest version. Note that this code was written with version 1.3.2 of the FlashStorage_SAMD library.
Select the Xiao Board: Tools -> Board -> Seeed SAMD -> Seeeduino Xiao
Once you have the Arduino IDE installed and recognizing your XIAO and have installed the “FlashStorage_SAMD” library, you are now ready to compile the code and modify it until your heart and ears are content.
Read the comments in the code for more info on what to HACK
UPDATE DECEMBER 12 2023
A few builders have brought up the "stair stepping" at lower frequencies, so I sat down to try to find the cause of this.
The output is 8 bit, so only 255 steps define the entire waveshape. Because the triangle wave needs to go up and down in one cycle, it actually loses a bit of resolution, which explains why the effect is more prominent in this mode. At slow speeds each step hangs around for long enough for us to hear the separate steps, resulting in a stair stepping effect.
I was able to increase the resolution from 8 bits to 9 bits, which reduces this effect, but it does not eliminate it. I've uploaded this new file to github: https://github.com/cctvfm/covenlfo
I cannot increase the resolution any more without a hit to the sample rate. The resolution of the output is tied to the sample rate with the equation Sample Rate = 48Mhz / (2* (2^n)) where n is the number of resolution bits. I was actually running the generator at half speed, so increasing the resolution to 9 bit and doubling the generator speed is an improvement with no side effects, however any further increases will affect the sample rate. For example, 10 bit output would reduce sample rate to 23 Ksps, while currently 9 bits is at 46 ksps.
This is all tied in with the output filter, the 15nF and 60K4 resistors create a 1st order low pass filter to smooth out the steps, tuned to 180Hz. This works well at most frequencies, but isn't enough for the lower ones. Changing the 15nF capacitors to a larger value would reduce the stair stepping even further, however it would reduce the amplitude at higher frequencies. If you only wish to use the LFO at slower speeds (or the amplitude hit doesn't bother you) 47nF or 100nF box film capacitors. The other downside to increasing this capacitor will be a rounding of the square and random outputs.