DIY: Modifying the Doepfer Dark Energy: LFO2 Out

So you just got yourself a Doepfer Dark Energy semi-modular analog synthesizer and have wondered why in the world they didn’t bother patching out more elements.  They went about 85%, then ran out of room on the front panel, I guess.  Or maybe they’re planning an expander or something.  Who knows?  All I know is that there are a few things I wouldn’t mind having access to:  LFO2, OSC out (pre-filter)…uh, well that’s it I guess.  (The device is already very full-featured and I love it.)

UPDATE 05242013:

Soon after I published this guide, I sent an email to Dieter Doepfer, the man himself, asking about why getting the raw waveforms was not possible from the DE.  (Of course, this makes perfect sense to me now that I understand how to read schematics!)

Hello -d,
to carry out modifications you should have some electronic experience. The Dark Energy uses a CEM3394 for sound generation. You find the data sheet in the internet. You will see that the VCO waveforms are not accessible because they are internally processed. Only the triangle is available at the VCO capacitor.  But you have to add a buffer. Otherwise the VCO will become unstable. If you own a newer Dark Energy the buffer is already on board (if JP13 is available you own a new version with buffer for VCO triangle out and VCF out).

Best regards,
Dieter Doepfer

The CEM3394 is a “synth-on-a-chip”, so the raw oscillator goes through the VCF and VCA, to the output; there’s no way to bypass or “patch into” this path.  Doepfer does note that accessing the triangle wave (the oscillator used for linear FM) is possible, but you’d have to buffer it (send it through an op-amp).  Unfortunately, I have a version 1 DE and I don’t feel like soldering on my own op-amp.

For the technically inclined, read all the details of the CEM3394 chip, the heart of the Dark Energy (and other legendary synths, like the MC-202, SH-101, etc.), here:

Now, back to the guide on how to get LFO2 out of the box:

With a little guidance from the Dark Energy technical manual (page 3), I found that JP10 is “LFO2 out,” but was slightly confused by the paragraph above, for JP8:

Connects the output of the internal inverter to the socket /LFO1 “/” means inverted, i.e. the factory default is inverted LFO1, can be used for other applications (e.g. inverted ADSR or LFO2 output in combination with JP9 or JP10, or direct output of LFO1 or LFO2 without inverting)

Do I have to use JP10 in conjunction with JP8?  Do I have to connect JP10 to JP8?  Will it affect LFO1?  I said, forget it, I’ll just try my luck with connecting JP10 to a jack and see how it goes.

Before doing that however, I got out my trusty multi-meter and tested JP10 to see if there was anything happening there.  Not having the full schematics, I wasn’t sure exactly what ground to use, but a little probing helped me to decide on the GND pin of the pot controlling LFO2 speed (the top pin, pictured below).

inside: tip goes to gnd pin of LFO2 speed pot

Inside: the tip of the jack goes to gnd pin of the LFO2 speed pot.

The multi-meter indeed confirmed that I was getting alternating voltages of -3.3 and +3.3 volts at JP10 (pulse waveform).  Great!  The voltage is a little low, but it’s something.

Doepfer was nice enough to include a 2-pin wire for the glide “upgrade” (which is nothing but a potentiometer and a knob for 10 EUR), so I used that for the connection.  One wire (tip) was soldered to the GND pin of the LFO2 speed pot; the other wire (ring) was simply placed over JP10.

Another quick test with the multimeter at the jack and we’re done!  Almost!  Where do we put the jack?

dark energy front lfo2 jack

Using the hole meant for the "glide" pot.

The obvious answer is the hole meant for the glide potentiometer…but will the 3.5mm jacks (274-0251 @ radioshack) be too small?  (Side note: people like to make fun of RS, but they’re pretty good, in a pinch.)  The jack just barely fits and swims around a little.  A wider washer would help with this.

inside the dark energy, pins soldered

Inside close-up.

After screwing the nut in tightly and closing up the bottom lid, I powered the machine up and performed my first test: patching LFO2 to VCO FM.  Perfect!  Mission accomplished.  You just added about $100 of value to your DE.

This was an easy modification that I totally recommend if you’d like to have an additional modulation source for your DE or other external modular gear.