Another 4hp digital LFO kit for Eurorack, the CCTV Coven LFO is an addition to my Noise Rack.

Coven LFO is a 4 HP modulation source in an easy-to-build DIY kit. Four bipolar CV outputs provide synced channels ranging from ridiculously slow to audio rate.

Selectable waveshapes include:

Triangle
Saw
Square
Stepped Random Voltages

An FM input (FREQ) allows you to modulate your modulator – you can even self-patch it for more complex waveshapes.

A Sync input allows Coven LFO to become your go-to clock divider and rhythm section.

Divisions between the channels can be selected from the arhythmic default (3,7,11) to more traditional rhythmic divisions (2,4,8, and 4,8,16). In all modes, the top most output is the base frequency from which the others are divided and synced to.

Available as a DIY kit with just 60 components and as a complete module. Firmware is provided in Arduino format for hackers and tinkerers.

Output Frequency
Max = 100 Hz
Min = 69 minutes period

Sync Frequency
Max = 1kHz
Min = 71 minutes period

Output Voltage
Bipolar -10V to +10V

Caveats

The FM input responds only to positive voltages.  This makes for some wacky wave shapes if self-modulating!  Negative voltage at the FM input does not change the frequency.

The output is digitized, resulting in steps that can be heard when modulating an oscillator frequency.

The Kit and Build

The build instructions are linked from the product page. It’s an easy build and the instructions are complete.

As usual, I try to solder as many parts as possible with organic flux solder, departing from the prescribed build order. All parts but the XIAO and the three right angle headers were soldered in the first pass and then the board washed. The panel PCB was soldered with regular no-clean.

Modification

This is the second LFO I’ve built that has a default 20V p-p output. (The first was the Double Dragon.) That seems much too big, and so I changed some resistors to reduce it. The schematic is provided on the build page, so it was easy to discover the gain and offset resistors for the four outputs.

I changed R2, R6, R10, and R14 to 30K and R4, R8, R12, and R16 to 60K. These values result in an output of about 9V p-p, roughly centered on 0V.

The only other change I made was to swap the knob for a black Davies clone knob, which looks better with the other modules in the rack.

Update October 25, 2023

Finally got around to building the second one. This time I used 68K for R4, R8, R12, and R16, which reduced the positive offset. Still not quite centered. I made another change, to capacitors C4-C7. These are supplied as 15nf, are 10nf on the schematic, but I used 4n7, one third of the ones supplied. The purpose of these caps is to smooth out the digital steps in the triangle and saw waves. They also take quite the edge off the square and random stepped waves.

The scope readout shows the rise time reduced from 980us to 300us, just what you’d expect from using caps with 1/3rd the value. You could use 1nf caps to get the rise time down to around 10us. You’d hear the steps more when modulating a VCO, but the square and random step waves would be sharper.

Updating the firmware

I’ve already described the procedure in a past comment, below. The trickiest part is shorting the tiny RST pads, as if doing a double click I made a little connector, soldered to those pads, where I can plug in a button. Made it a lot easier. The only software update available at this time fixes a bug where negative CV on the FREQ input was ignored.

Though it’s hard to read, the Joranalogue TEST3 meter shows a peak +12V current draw to be 38ma.

Patching an external clock to SYNC forces Output 1 to follow the clock. If you set the divisions to 2/4/8, then you’ve got a nifty clock divider instead of an LFO! You could cascade two of these modules by patching the /8 output to the next one’s SYNC and get /16/32/64.