APOLLO VIEW RABBIT HOLE User manual

RABBIT HOLE
DIY BUILD DOCUMENT V1.1

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Warning!
This module gets
loud
RABBIT HOLE is capable of producing 24Vpp
Total Maximum gain over 1000
Ensure input and output gains are turned down before use
(Note: Output Gain is an attenuverter, the minimum Output
signal is achieved by setting the knob to the 12 o’clock position)
Nominal Eurorack audio levels are 10Vpp. Rabbit Hole can produce peak-to-peak voltage 2.4
times greater than standard Eurorack audio levels. This could potentially damage your
speakers and ears. Please exercise caution, and use the Output Gain attenuverter. Even
when the Tube stage is not used, the Dry signal can reach an Output voltage of 15Vpp.
Specifically, an input audio signal of 10Vpp with maximum Input Gain on the Vintage VCA
and maximum gain on the Output attenuverter will produce an output signal of 15Vpp. This
additional gain is available to balance the output signal for lower Tube and Drive settings.

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TABLE OF CONTENTS
SPECIAL THANKS ............................................................................................................................................ 3
INTRODUCTION ............................................................................................................................................. 4
ORIGIN ............................................................................................................................................................... 4
WHAT IS IT? ........................................................................................................................................................ 4
TOOLS ............................................................................................................................................................ 5
ESSENTIAL .......................................................................................................................................................... 5
OPTIONAL .......................................................................................................................................................... 5
CONSTRUCTION GUIDE .................................................................................................................................. 6
PARTS LIST ..................................................................................................................................................... 7
ASSEMBLY STEPS ........................................................................................................................................... 8
CALIBRATION ............................................................................................................................................... 23
MIX & TONE CROSSOVER CALIBRATION ........................................................................................................... 23
VCA BLEED CALIBRATION ................................................................................................................................. 25
SERIAL NUMBER .......................................................................................................................................... 26
SPECIAL THANKS
Thomaas Banks Thomass Banks
Ben Wilson DivKid
Sam Battle Look Mum No Computer
Tom Wiltshire Electric Druid

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Introduction
ORIGIN
I would like to shout out to Look Mum No Computer who first drew my attention to the
Matsumin Valvecaster distortion pedal, through his Safety Valve module.
It is from experimenting with LMNC Safety Valve 2.0 design that Rabbit Hole developed. The
aim was two-fold. Firstly, to have more accurate CV control over the gain/distortion of the
system and secondly, to create a broader range of tube effects, from subtle saturation
through to total audio destruction. On this journey of experimentation and discovery of
tube circuitry and different tube characters, I truly fell down a Rabbit Hole. The module took
on a life of its own, revealing more inherent features, namely sub-harmonic generation and
self-oscillation.
Special mention also needs to go to Tom Wiltshire (Electric Druid). While designing a
suitable VCA circuit to complement the characteristics of the vacuum tube circuit, I
discovered Tom’s design-a-eurorack-vintage-vca-with-the-lm13700 design. I didn’t want any
old clean sounding VCA. I had already made the LM13700 datasheet VCA and found it was
indeed too clean for my purposes. With permission from Tom, his design has been
implemented for all the VCAs in Rabbit Hole and Curiouser, and they sound fantastic. To
quote Tom, "Since I was designing something that was specifically intended as a “vintage
VCA” and because I like the soft differential-pair distortion of the chip, I deliberately
tolerated several percent of distortion in the circuit. It adds character! Anyway, if you want
a really clean VCA there are lots of other better options than the LM13700. It should be
used for its strengths, I reckon."
WHAT IS IT?
Rabbit Hole is a 16HP summing mixer with characterful valve saturation & distortion. It is a
hybrid solid-state and vacuum tube design. It has two input channels with independent
vintage VCAs. The outputs of the VCAs are summed and passed to a solid-state Drive circuit
which at higher gain settings hard clips the signal before entering the starved cathode dual-
triode vacuum tube. The OWTH - Off With Their Heads diode LED clipping circuit can be
engaged for further distortion.
Rabbit Hole is capable of adding subtle warmth or total audio destruction. The Curiouser
expander module adds an additional 2 inputs. Multiple Curiouser modules can be daisy-
chained to suit your mixing requirements.
• Expandable audio mixer
• Glue for your drum mix
• VCA to a synth voice
• Add subtle warmth and character or total audio destruction
• Capable of generating Subharmonics
• Self oscillates with extreme settings

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Tools
ESSENTIAL
• Soldering Iron - a solder station with temperature control is best. Some components
on this build are connected to the ground plane and require a large amount of heat.
A low Wattage soldering iron that plugs directly into the power supply will not be
good enough to achieve an effective solder joint. When soldering pots and jacks,
370oC is best; for everything else, 340oC (this is dependent on the solder you use, so
check out your solder’s data sheet).
• Solder - We find thin is best, around 0.6mm. We use a lead-free rosin core solder.
Lead is easier for beginners, but be sure to wash your hands.
• Voltmeter (multimeter) – To calibrate the Mix and Tone controls. Jumper wires
(male to female 2.54mm) and crocodile clips can assist with this as you need three
hands.
• Solder Mat
• 7mm & 10mm Nut drivers
• Bananut driver
• Flux Pen
• Cleaning Brush (an old toothbrush will do)
• Side Cutters
• Pliers
• Solder Sucker
• Watchmaker screwdrivers, Phillips ≅2.3mm and flathead ≅2.3mm
OPTIONAL
• Masking Tape (It can help hold components in place when flipping the board over to
solder)
• Male-to-female jumper wire (for the calibration steps)
• Crocodile clip (for the calibration steps)

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Construction Guide
There are already excellent soldering guides in existence, so we will refrain from reinventing
the wheel here.
If you need some guidance, please check out the Moritz Klein x Erica Synth Build
Documents. The soldering appendix is an excellent resource.
Or, if you prefer videos, this is a pretty good guide.
In most cases, components can be placed onto the PCBs, and then the board can be flipped
upside down and laid facedown on the soldering mat to solder the legs to the back of the
PCB.

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Parts list
Component
Qty
Faceplate
1
Faceplate Screws
4
Front PCB
1
Back PCB
1
Tube
1
Tube Socket
1
Trimmer 100k Side Slot Adjustment
2
Trimmer 100k Top Slot Adjustment
2
8.2nF Poly Capacitor
2
68nF Poly Capacitor
2
Standoff
2
Standoff Screws
4
01 x 17 Pin Header
1
01 x 17 Pin Socket
1
01 x 11 Pin Header
1
01 x 11 Pin Socket
1
Straight Shrouded Power Header
1
02 x 03 Pin Header - Expander Connection pins
2
Thonkiconn Jack
8
Bananuts
8
Knobs
7
A100k Alpha D Shaft
3
B100k Alpha D Shaft
2
A500k Alpha D Shaft
1
B500k Alpha D Shaft
1
Black Hex Nut
7
B100k Song Huei Tall Trimmer
6
SPDT Switch
2
DPDT Switch
1
Power Cable
1

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Assembly steps
Note: when instructed to solder things in place, it is also implied to cut the legs off the
components with legs once soldering is complete. This is only capacitors and trimmers.
Once the legs are cut, you can apply heat again (and solder if required) from the soldering
iron to make an aesthetically pleasing solder joint.
Front PCB Face Front PCB Rear
Back PCB Face Back PCB Rear

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1. Find the Front PCB. Place the 2x5 pin shrouded power header onto the rear of the
Front PCB. Ensure the key slot is towards the bottom of the PCB. Flip the board over
and solder in place.
2. Attach the standoffs to the rear of the Front PCB with two M2 screws. Put the other
M2 screws to the side for now.

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3. Take the Back PCB. Place capacitors on the face of the Back PCB. The two 68nF in the
top two footprints and the two 8.2nF in the bottom top footprints.
4. Place the two 2x3 pin headers in the footprints on the rear of the Back PCB. Flip the
board holding them in place (masking tape is your friend here) and solder them in
place. A flux pen can help get a good solder joint here; a light wipe over the pads is
all that’s required.

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5. Place the tube socket on the footprint on the face of the Back PCB. Flip the board
and solder, and try to ensure the socket is as level (flat against the PCB) as possible,
as this affects the final position of the tube. Note: in the picture below, you can see
the residue from the flux pen from soldering the 2x3 pin headers; we will clean this
off later with the brush.
6. Select the two side adjustment 100k trimmers (not the top adjustment 100k
trimmers, we will solder them shortly). Place them on the face of the Back PCB so
the adjustment screws are outermost. Flip the board and solder; use masking tape
here if you need to.

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7. Lay the Front PCB face down and lay the pin headers and pin sockets onto the rear of
the PCB. Don’t solder yet.
Note: we kindly put the pins into the sockets already. We have a convention to place
the sockets against the rear of the Front PCB; this will help compatibility if there are
any future issues swapping any broken PCB parts around.
8. Locate the two top adjustment 100k trimmers. Place them into the rear of the Front
PCB with the trimmer adjustment screw at the bottom. Don’t solder yet.

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9. Lay the Back PCB face down on the pin headers. Ensure that the trimmer adjustment
screws are poking through the holes on the Back PCB (give everything a wiggle until
everything is correctly aligned, this can be a little fiddly, but we have confidence in
your skills!). Attach Back PCB to the standoffs with the remaining two M2 screws.
10. Flip the assembly over so the Front PCB is face up. Use the pliers to pull the middle
leg of the trimmer up against the PCB and fold the middle leg of the trimmers to hold
them in place. The purpose of this is so that when you solder the trimmers' legs in
place, the trimmers do not drop down toward the rear of the module. If this
happens, the trimmer adjustment screws will be flush with the rear surface of the
Back PCB, but the base of the trimmer will stand a few millimetres away from the
rear of the Front PCB, which will make it prone to misalignment later in the build
process.
11. Solder pin headers and solder the trimmers in place.

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12. Unscrew the Back PCB separate the boards, and set the two M2 screws to one side.
13. We will now populate the face side of the Front PCB, starting with the
potentiometers for In A, In B and FB, which are all A100k (the potentiometers are
clearly labelled with their value). Using pliers to straighten the kinked legs can help
with positioning the potentiometers. The next four steps, 13-16, are all using the
9mm Alpha D shaft pots, which can be identified from the green body.
Don’t solder anything until the Faceplate is on.

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14. Next, place B100k in for the Mix and Tone controls.
15. Next place the B500k for the Tube control

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16. Then place A500k for the Drive control
17. Now place all the B100k tall pots. These can be identified from their blue body.

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18. Populate PCB with Thonkiconn jacks
The orientation of adjacent jacks should alternate by 180o.
When placing jacks, groups of two jacks share the same ground pad.

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19. Screw a nut onto the two SPDT, and the DPDT switches.
20. Place switches with the flat key (on the thread/barrel of the switch) at the bottom.
When this step is done, the Front PCB should look as follows (the red line indicates
the flat thread/barrel of the switch).

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21. Place on the Faceplate. Take care not to dislodge any components. You will have to
begin feeding the faceplate over the Tall B100k Pots, then take care with the
switches. The switches should all be in the Up position.
22. Holding the faceplate on firmly, check the rear of the Front PCB and ensure that all
the legs are through the holes. Take care that all of the jacks’ ground pins are
through, as these spring little legs, especially the bottom row where two jacks share
one pad, are susceptible to popping out. If any components need realigning and the
legs placed through holes, do this before moving on to the next step.
23. Put black hex nuts on first. Place them with the smooth/rounded side down. If you
position them with the flatter side down, they scratch the Faceplate while
tightening.
Note: The washers aren’t strictly necessary but can be used if desired. We chose to
leave them off as we found it more aesthetically pleasing.

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24. Finger Tighten the black hex nuts first before tightening with 10mm hex driver.
25. Then position and finger tighten the Bananuts onto the jacks. Do the final tightening
with the Bananut driver.
26. Then position and finger tighten the switch nuts. Do the final tightening with the
7mm hex driver.
27. Boom! You are ready to solder everything in place, except leave the Cheshire Cat’s
eyes unsoldered. It looks good and doesn’t affect the electrical or mechanical
properties of the module.