This post will cover the DIY build from a factory kit of the Beast-Tek Double Dragon Dual LFO. I bought the full kit from synthCube. Double Dragon is a digital LFO with precision behavior. It uses a PIC microprocessor chip. LFO1 is the main oscillator and LFO2 can have one of eight different modulations from LFO 1 applied to it. LFO2 can also sync to LFO1. Three different ranges are available, shared by both oscillators. They’ll go up to 1.28 KHz in fast mode. Each has eight waveforms, selected by a knob plus CV. The waveforms are:

• Sine
• Triangle
• Ramp Up
• Ramp Down
• Inverted Exponential
• Staircase
• Square
• Noise

The output voltage swings +/-8V. My scope read minimum as -10V and maximum as +10V, due to tiny spikes on some of the waveforms. Each LFO output is indicated by a red-green LED. The oscillators can be used independently, apart from the shared range. But the fun starts when LFO1 modulates LFO2.

Modulation Algorithms

The seven algorithms are:

• LFO1 mixes with LFO2
• LFO1 amplitude modulates LFO2
• LFO1 frequency modulates LFO2
• LFO1 mixes with LFO2 and then wave folding
• LFO1 phase distorts LFO2
• LFO1 is XORed with LFO2
• LFO1 is sampled and held and mixed with LFO2

The algorithm is selected manually and indicated by a seven-color LED. Each algorithm has modulation amount control (and CV), plus a parameter control (also with CV). The combination of waveforms and algorithms, all under control voltages make for a wide variety of shapes and sounds.

Reset / Hold

Each LFO has a reset input that can be changed to a hold function by jumpers for each LFO on the back of the module. There are other jumper options, too.

The Build

I’m unimpressed by the kit package and the build guide. The PC boards seem good quality, but the panel is one of those made from PC board material and it doesn’t make a very good impression. The resistors are a tight fit to the hole spacing. The one standoff listed in the BOM was missing, so I used my own. There were two extra LEDs. Oddly, instead of IC sockets SIP sockets are supplied, to be cut to size. I used my own machined-pin sockets, except for the PIC. The 3.5mm panel jacks are the PJ-301 type and the biggest omission in the build guide was the need to cut off the four little legs. Because if the legs are left on, then the panel cannot sit parallel with the IO board. The build guide has a photo of this, but doesn’t think it’s a problem.

Another slight annoyance was having to cut the 11-pin header sockets that join the board from one long piece. These aren’t that simple to cut (the male counterpart is easy). But I’ve done it before.

This is the first kit I’ve built that has you assembling the IO board and panel as the first steps. I put the panel on temporarily while soldering the panel parts and then removed it for this photo and left it off until final assembly.

The multi-color LED at the top of this board has four leads. It has to be seated as close to the board as possible, in order to stay below panel level. It shines through a window in the panel. I suggest carefully bending the leads close to the body of the LED and then downward, so as not to force them by too much pressure. Maybe that’s why a second of this LED was included.

I soldered the Brain board with washable flux. The 3.3V regulator didn’t line up with its mounting hole, but it really doesn’t need to be screwed down.

I made two cosmetic changes. The shape pots have knurled shafts for accepting those tiny plastic knobs. I quite dislike those little knobs on large pots, especially when there is room. I chose black Davies clone knobs.

I used Bananuts with the color coding I’ve chosen: blue for CV inputs, silver for pulse inputs, and black for outputs.

A Modification to Lower Output Amplitude

Double Dragon has an unusually high output amplitude, around +/-10V (20V p-p). It needs attenuation to be useful. I decided to modify the circuit to lower the output level and it turns out to be simple.

When building the main board, substitute 100K resistors for the 200K called out for R5 and R6. This halves the output level, resulting in the square wave being +/-5V and the sine and triangle a little lower. You can also try any value between 100K and 200K, for levels higher than +/-5V.

If you’ve already built the board, you can put 200K resistors in parallel with R5 and R6. These resistors go between pins 1&2 and 6&7 of U1, a TL072 op amp. I soldered the parallel resistors directly to these pins on the IC socket for U1.

If you want to reduce spikes on the sharp wave forms, you can add 100pf capacitors also across pins 1&2 and pins 6&7 of U1.