SynthCube was having a sale, so I picked up a DIY kit for Erica Synths DIY Swamp. It’s a version of Grant Richter’s Wogglebug circuit. I can’t believe it’s been twelve years since I designed and built my own Wogglebug circuit from information Grant had posted online. I guess I’m lucky, because Erica Synths is changing their DIY line.
Parts Acquisition
From SynthCube I bought the PC boards, Panel, and special sample and hold IC (ALPHA 1100CK2), which Erica Synths chose instead of the usual LF398. The remaining parts are quite standard and in fact I found all of the 1/4 watt resistors in my stock, plus most of the capacitors and the vactrols. The rest were easily ordered from Mouser.com. The pots, jacks, switches, and knobs came from Thonk. Thonk carried the exact knobs that Erica Synth uses. The only variance from the specified parts is that I used knurled jack nuts instead of hex nuts. I have a special wrench for the knurled nuts and I like the look better.
Block Diagram
I drew my own block diagram for the module for ease of seeing the differences from other Wogglebugs, including my own.
Erica Synths added two features: the tone outputs from the 4046 oscillators can be switched between square and sawtooth waveforms; an external audio input to one side of the ring modulator (digital XOR) that replaces the square tone from the swamp oscillator.
Also of note: the input to the sample and hold is a mix of three sources. The smooth saw tone is cross faded with the S&H output (the cluster function) with a panel control, but the swamp CV output is also mixed in.
The only missing feature is an input jack for the S&H to override the internal sources.
Construction
The module was very easy to build, consisting of all through hole parts. The two circuit boards sandwich together with header pins and sockets on each side. Somewhat unusual is that the parts side faces outwards on each boards. I used organic, washable solder (Kester 331) for all of the work, except for the pots, switches, jacks and LEDs, which went on last.
Demo Recordings
The filter used was the Vult Freak stereo filter in various modes.
Nice post! How much work would it take to add the external input for the S+H circuit? Looks like there’s room on the panel…
Hi Benton! Adding the S&H external input would take a shallow jack, since there are parts on the board behind either end of the top row of jacks. I will look into it. Thanks for the suggestion.
There is not enough clearance behind the panel to add another jack. I checked it with the shallowest jack I have. So, no mod for S&H input.
If there’s room for a switch, perhaps the Audio In jack could also be routed to the Ext S+H input?
A DPDT switch would be needed, because the normal switch on the jack would have to be switched too. A miniature switch might fit. Might require a lot of tearing up of the PC board, since the socket is soldered into the board.
Ah, probably too much work then. For me, I would be happy to have the Audio In jack also feed the external S+H input
One jack can’t feed both, because each has a different normalized input. I thought to replace the audio in with S&H in, but I’m going to leave it as is. After all, in the Eurorack box this is currently in I have access to as many as twenty-two sample & holds already.
I just built a Swamp. No magic smoke. And it is making bizarre waveforms as was expected. The time knob should adjust how much feedback from the phase comparator into one of the VCOs as is depicted by your diagram but I am not sensing any change of the waveforms when I move the time knob. At what settings of the other inputs and knobs is a turn of the time knob more significant? IE how do I check the functioning of the Time knob?
Thank you for your write up of the Swamp.
Jeff, the Swamp Range knob interacts with the Time knob. Try setting them closer together. When slower, the Swamp CV will be more wavery, and when faster more woggly. The Swamp, Ring, and Swamp CV outputs will be affected. Keep Rate in the middle at first while you adjust these two. In general, just play around with different settings of all the knobs.
Thank you for the tip. It is working as expected which is surprising when you look at the quality of my solder joints.
I just built one, it was working (kinda) but after I unplugged to diagnose since a few parameters seemed off then plugged back in to my rack the whole rack wouldn’t power. Luckily it was just the wall adapter died, the power supply was fine. Tried again in another rack that has the uzeus power, and it started blinking blue so I unplugged. I did a continuity test, checked polarized parts, all checked out fine. Seems like a power issue, even reflowed most of the joints.
Kelly, did you measure the resistance to ground from the +12V and -12V pins, when disconnected from power?
Hi Richard,
I am somewhat new to modular synthesis. I just finished building the Swamp module and am still playing around trying to figure out exactly what it can do (it seems the possibilities are almost endless). Your demo recordings have been really helpful! I have a concern though and am asking for your help. I’m not sure if the Rate control is acting as it should. At it’s lowest setting (fully CCW) it is clocking (if that’s the correct term) at around 2hz, then only gradually increases to what I guess is full speed when the knob is at 10 o’clock- after which there is no change at all- it is just super fast. This regardless of where any of the other controls is set. Is this normal? I would think that the lowest setting should be a much lower rate and have a gradual increase in speed throughout the entire range of the control. If it sounds like something is wrong could you give me an idea where I might look to track down the problem (if there is one). I checked and double checked all my solder joints and component orientation and took precautions again static when handling the IC’s. Thank you in advance for any guidance you might provide. -Art
Hi Art,
There are a few things you can check. First, try patching an external clock and see that it does replace the internal clock. The clock LED will still follow the internal clock and CLK OUT will also be the internal clock. But the sample & hold will be externally clocked. Be aware that the rate of the internal clock and the response of the smooth output are correlated, but the correlation is broken when you patch an external clock. With an external clock the rate control and CV will affect only the smooth out response. That’s just something to know.
Now, to fix the too fast internal clock I suggest some possibilities. Double check all the parts around the 555 clock circuit. R69 (10 Meg), R70 (4.7K) and C27 (2.2uf). Make sure they’re the right values and are properly soldered. If you have another 555 chip, try replacing the original one. If you can’t do that, or if it doesn’t make a difference, then the suspect becomes the Vactrol in the clock circuit, VC3. We’ve found that the brand of Vactrol makes a difference in this circuit. You might need to replace it with another make.
Richard,
Thanks for the reply. I did try patching in an external clock earlier. I assumed it would affect the speed of the LED and the clock output, but now I understand it won’t. I guess that’s what threw me. I’m not sure though how to check that the sample and hold is being externally clocked- how would I patch that?Tomorrow I’ll try a different 555 in the circuit and also check those things you mentioned. I also have a 7555 chip- would that be any better/worse as a substitute? The Vactrols I got with the kit (from Synthrotek) are stamped SR VTL and are designated VTL5C3 on the BOM. Don’t know if that tells you anything.
Art, you can check that the external clock is working by looking at the Stepped Out. It should change at the rate of the external clock.
I think the 7555 is a CMOS chip that would probably work. If you got the Vactrols in a kit, they should be the right kind.
Good morning Richard. I checked each of the components (R69, R70, C27) associated with the 555 clock circuit for correct values and polarity, and also reflowed their solder joints as well as the 555 socket joints. I swapped in a different 555 chip, and the CMOS 7555. No change-the internal clock is still pulsing as it was. I don’t know enough about how the 555 works but might a different value for the resistors and/or capacitors safely slow down the rate, without goofing up something else?
I confirmed that the external clock source (a LFO) is indeed being followed properly at the Stepped Out jack.
Art, check for +12V on pins 4 and 8 of the 555. If that’s OK, then you can try a larger capacitor for C27. Try a 10uf. That should definitely lower the rate.
That did the trick!! Now it is much more manageable and I think will lend itself to more creative usage. Thank you so very much for the guidance (and patience!). I can add a couple of more notches to my soldering iron (figuratively of course).
Hey ho
What jacks have a one to one correspondence between Swamp and Wogglebug and what are the outliers? I want to use the Swamp to follow pitch and want to try other patch ideas offered for the Wogglebug.
Hi Jeffrey,
I assume you’re referring to the Make Noise Wogglebug. Since MN doesn’t even give a block diagram, let alone a schematic, I am unable to answer your question.
But as for pitch following, Swamp can’t do it. MN Wogglebug has an extra input that others don’t have.
Right on, thank you.