APOLLO VIEW RABBIT HOLE User manual

RABBIT HOLE

2
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.

3
TABLE OF CONTENTS
SPECIAL THANKS ............................................................................................................................................ 3
LIMITED WARRANTY ...................................................................................................................................... 4
INTRODUCTION ............................................................................................................................................. 5
ORIGIN ............................................................................................................................................................... 5
WHAT IS IT? ........................................................................................................................................................ 5
BLOCK DIAGRAM ........................................................................................................................................... 6
INSTALLATION RABBIT HOLE .......................................................................................................................... 7
POWER ............................................................................................................................................................... 7
TUBE .................................................................................................................................................................. 7
KNOBS ................................................................................................................................................................ 8
CV CONTROL ...................................................................................................................................................... 9
SWITCHES .......................................................................................................................................................... 9
RABBIT HOLE FUNCTIONALITY ..................................................................................................................... 10
EXPERIMENTATION .......................................................................................................................................... 10
WARMING ....................................................................................................................................................... 10
DISTORTION ..................................................................................................................................................... 10
SUBHARMONICS .............................................................................................................................................. 10
WONKY VCO .................................................................................................................................................... 10
FEEDBACK ........................................................................................................................................................ 10
TRIMMERS ....................................................................................................................................................... 11
CURIOUSER .................................................................................................................................................. 12
DESIGN ............................................................................................................................................................. 12
CURIOUSER SWITCHES ..................................................................................................................................... 12
INSTALLATION CURIOIUSER ......................................................................................................................... 13
POWER ............................................................................................................................................................. 13
TRIMMERS ....................................................................................................................................................... 13
CONNECTING CURIOUSER TO RABBIT HOLE .................................................................................................... 14
PATCH IDEAS ............................................................................................................................................... 15
SPECIAL THANKS
Thomaas Banks Thomass Banks
Ben Wilson DivKid
Sam Battle Look Mum No Computer
Tom Wiltshire Electric Druid

4
Limited Warranty
Apollo View Modular Ltd warrants this product to be free from defects in materials or
construction for a period of one year from the date of purchase (proof of purchase/invoice
required).
This warranty does not cover any damage caused by misuse of the product, or any
unauthorised modification of the product.
Apollo View Modular Ltd reserves the right to determine what qualifies as misuse at their
discretion. Examples of misuse include, but are not limited to:
1. Exposure to extreme heat or moisture
2. Malfunction resulting from wrong power supply voltages, backwards or reversed
eurorack bus board cable
3. Issues related to third party products
4. Any condition resulting from incorrect or inadequate maintenance or care.
5. Damage resulting from misuse, abuse, negligence, accidents or shipping damage.
6. Dissatisfaction due to buyer’s remorse
7. Normal wear and tear
8. Damage to the product caused by excessive physical force or abuse of the product,
removing knobs, changing faceplates
This warranty does not cover any other causes determined by Apollo View Modular Ltd to
be the fault of the user, and standard service rates will apply.
Apollo View Modular Ltd agrees, at its option during the warranty period, to repair any
defect in material or quality or to furnish a repaired or refurbished product of equal value in
exchange without charge (except for a fee for shipping, handling, packing, return postage,
and insurance which will be incurred by the customer). Such repair or replacement is subject
to verification of the defect or malfunction and proof of purchase as confirmed by showing
the model number on the original dated sales receipt.
Apollo View Modular Ltd implies and accepts no responsibility for harm to person or
apparatus caused through the operation of this product.
Please contact jon[email protected] with any questions, requests for a return
to the manufacturer, or any needs & comments.
https://www.apolloviewmodular.com/

5
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

6
Block Diagram
IN A
CV A
IN B
CV B
Vintage
VCA
IN C
CV C
Vintage
VCA
IN D
CV D
Vintage
VCA
OUT
OUT C
OUT D
CV FB
Drive Tube
Vintage
VCA
Cross-
fader
Vintage
VCA
CV Tone
Cross-
fader
CV Mix
Tone
Mix
OWTH
L8
Out
RH/Out
RH/Out
Drive
Tube
Feedback
2 Pole
High Pass
Filter
2 Pole
Low Pass
Filter
LED
Clipping
Curiouser Expander
(GAIN CV)
(GAIN CV)
(GAIN CV)
(GAIN CV)
*
(GAIN CV)
*
(  )
()
FB
FB

7
Installation Rabbit Hole
POWER
Rabbit Hole is supplied with a 10-pin power cable already placed in the keyed 10-pin
shrouded socket. Should the power cable become disconnected, simply reconnect into the
shrouded socket using gentle pressure, ensuring correct alignment of the ‘key and lock’ the
red stripe should be to the left when looking at the back of the module.
The module has reverse protection diodes, which will simply divert the reverse current to
ground in case of incorrect installation.
TUBE
Rabbit Hole comes with a 12AU7 tube already installed, which might require re-centring
after shipping. The gain factor of the 12AU7 is 20. This has been selected for its specific
headroom and characteristics to produce the widest range of tones from saturation through
to total distortion. If you fancy participating in some tube sniffing or need to replace the
tube due to wear, it might help to unscrew and separate the PCBs. Then gently lift off the
old tube holding near the base, then replace it with a new tube using gentle downward
force into the tube socket with a slight circular rocking motion, ensuring the pins are aligned
correctly with the socket. The Tube typically reaches a maximum operating temperature of
approximately 45oC (113oF).

8
KNOBS
Gain A - Volume control for channel A vintage VCA. Gain A also affects the
saturation/distortion of the signal through the Tube circuit.
Gain B - Volume control for channel B vintage VCA. Gain B also affects the
saturation/distortion of the signal through the Tube circuit.
Tube - Tube bias voltage control. When L8 is switched Off, it is the bias for just PIN 3; when
L8 is switched On, it is the bias for PIN 3 & PIN 8
Drive - Adds more gain to the mixed signal A+B (+C+D with Curiouser) by an operational
amplifier (op-amp) before the tube stage. It has a maximum gain of x 32. Medium settings
can cause the op-amp to clip, and higher settings cause asymmetrical wave folding. The
hard-clipping of the op-amp is smoothed out by the Tube stage, which follows. Drive should
be used in tandem with the Gain A & Gain B to achieve the required amount of
saturation/distortion. In practice, Drive sets the upper limit of saturation/distortion of the
Tube circuit, and Gain A & Gain B are the finer CV-able control of the
Drive/saturation/distortion.
FB - This is a VCA placed in the feedback path. This control works in tandem with FB switch,
which needs to be switched on (down). It has two functions, feedback duties and tuning
when the module is used as a VCO.
• FB knob has an interesting range of effects over the timbre and is highly interactive
with other settings.
• Inverting the Output attenuverter’s polarity (ACW from the mid-point) has a
significant effect on the Feedback timbre.
Tone – This is an overall tone shaping circuit for the ‘wet’ Tube signal. After the Tube circuit,
the signal is split and passes in parallel through an active 2-pole low pass and high pass
filters. These are then recombined through a crossfader. Tone is the crossfader control for
blending between the high passed and low passed signals. (For further info, see TRIMMERS)
Mix – After the two vintage VCA channels are mixed and before the Tube stage, the signal is
split into Dry and Wet. Wet then goes through the Tube and Tone stages; Dry goes direct to
Mix. At this point, the signals are recombined through the Mix crossfader, blending
between Dry and Wet signals. It should be noted that Dry is exactly the signal that arrived at
the inputs. The vintage VCAs impart some colour on the sound, and the Gain of the VCAs
will obviously affect the amplitude of the Dry signal at Mix. (For further info, see
TRIMMERS)
Out – Attenuverter for more flexibility. This is perfect when using Rabbit Hole as a drum
voice mixer and plan to mix with a bassline. The phase of Rabbit Hole’s output can be
flipped to fix kick/bass phase issues. Additionally, this has an effect on the timbre when
using the FB feedback circuit. Clockwise from 12 o’clock, the output signal is in the same
phase as the input signal. Anticlockwise from 12 o’clock, the output signal phase is rotated
180o with respect to the input signal.

9
CV CONTROL
Rabbit Hole has CV control for:
• Gain A
• Gain B
• FB
• Mix
• Tone
CV control has been optimised for a 0-5V control voltage. Each CV input has an attenuator
to scale the response to the incoming control voltage. 0-5V was selected as this is the lowest
possible CV signal in Eurorack; if using 0-8V or 0-10V simply use the built-in attenuverters to
achieve the optimal range. Attenuverters have been used for total flexibility. This is
especially useful for creating ducking effects, e.g., drums against a bassline. The Gain CV has
been normalled. A CV signal present at CV A is passed through a switching jack to CV B and
then on through any connected Curiouser Modules. Therefore, using one CV signal, it is
possible to control the gain of all channels at once, and each channel's gain will be
dependent on its attenuverter setting. Inserting a patch cable into a channel will break the
normalled CV connection.
SWITCHES
Switches are Off when up, On when down.
L8 (Pronounced Late) – This affects the point of connection for Pin 8 of the vacuum tube
(cathode of triode 2). In the Off position (up), Pin 8 is in its default connection direct to
ground. When On (down), it is connected to the same node as Pin 3 through the Tube Knob
circuit. This has the effect of changing the character of the valve and, at lower gains, can
produce subharmonics.
NOTE: At some settings (mainly when OWTH is On), switching between L8 On/Off
can cause momentary audio dropouts. This is normal behaviour due to the change in
voltage at Pin 8.
FB – Feedback Switch. Changes the timbre/character of the signal and should be used in
tandem with the FB knob when engaged can cause sub-harmonics. FB must be switched on
when Rabbit Hole is used as a VCO.
OWTH – Off With Their Heads. This engages the LED Diode Clipping Circuit and hard-clips
the output signal.

10
Rabbit Hole Functionality
EXPERIMENTATION
The best way to achieve results with Rabbit Hole is through experimentation. That said, for
specific results, the following approaches can be taken.
WARMING
To warm a signal, Drive should be at a minimum. Gain, Tube & Mix knobs to taste. Using
this technique can bring warmth and phatness to any sound but is particularly useful for
drums.
DISTORTION
Drive should be set anywhere from 12 o’clock to fully CW. The Drive will hard clip the signal
but the subsequent Tube circuit will round this off. If hard clipped tones are required, the
OWTH switch should be On
SUBHARMONICS
To achieve sub-harmonics with Rabbit Hole, the L8 switch must be engaged, and various
knobs tweaked to find the sweet spot.
Switch L8 On, OWTH On, FB Off. Patch an Oscillator to IN A, and set Gain A to its midpoint
(12 o’clock). Turn Drive fully anticlockwise; Tube should be set to the 3 o’clock position. You
can set Tone to taste, Mix fully clockwise for just sub-harmonics, or you can choose to mix
the root note back in. Altering Gain A then affects the interval of the sub-harmonics
generated. Rabbit Hole will step down octaves, and related intervals with some fine-tuning
around the above settings.
TIP: Sending CV modulation to the Gain A allows you to ‘play’ these sub-harmonics. Mix &
Tone can be CV modulated too. It is glitchy, weird and wonderful. Try adding a second
oscillator to In B for even more complex and glitch tones.
NOTE: If FB feedback is switched On, you can still achieve subharmonics. However,
the resulting subharmonics are not as stable/reliable as the previous method, but it
can be fun nonetheless.
WONKY VCO
1V/OCT input to CV TONE (scaled using the TONE CV Attenuator), FB switch in ON (down
position), Tune pitch with TONE Knob. 1V/Oct tracking over a few octaves is achievable with
some fine-tuning. Play around with all the knobs, at this point, they all affect the pitch. The
frequency of this can be slowed right down to LFO territory, and the OWTH can be used to
create square waves.
FEEDBACK
FB will add additional timbres to colour the sound further. It is interdependent on the
polarity of the Output attenuverter.

11
TRIMMERS
The two trimmers on the back PCB control the centre point for the crossfade for both Mix &
Tone; these have been ‘Factory Set’ but can be tweaked to adjust the crossfade point if
desired. These are 25 turn Bourns trimmers; making adjustments will change the centre
point for the crossfade away from the knob’s initial 12 o’clock position. If you experiment
with this setting and find you wish to reset to a factory setting, it will require a voltmeter.
The Test Points near Mix and Tone should be used for their respective setting. They need to
be referenced to ground. The connection pins to Curiouser can be used to access a ground
pin. The middle two pins of the Curiouser connection pins are both ground. The ‘Factory
Setting’ voltage for each Test Point is -4.00V with respect to ground.
On the side of the module are an additional two trimmers for ensuring the vintage VCAs
don’t leak/bleed signal through. These should not require any adjusting. Each vintage VCA
has its own requirement due to slight differences in the chips arising from manufacturing
tolerances.

12
Curiouser
DESIGN
Curiouser is essentially a duplicate of the two input channels of Tom Wiltshire’s vintage VCA
from Rabbit Hole. When connected to Rabbit Hole, these extra inputs are fed to the mixer
before the Tube stage. Curiouser’s VCAs can also be used Standalone. This enables
Curiouser to be used as the final VCA for a synth voice which has been patched through
Rabbit Hole; see ‘3 Oscillator Synth Voice With Tone Animation’ in Patch Ideas.
CURIOUSER SWITCHES
When the switch is in the Up position, the signal is fed to the mixer on Rabbit Hole. When
the switch is Down, the signal is fed to the direct Out on Curiouser.

13
Installation Curioiuser
POWER
Curiouser is supplied with a 10-pin power cable already positioned in the keyed 10-pin
shrouded socket. Should the power cable become disconnected, simply reconnect into the
shrouded socket using gentle pressure, ensuring correct alignment of the ‘key and lock’. The
Shrouded pin header is on the inside of the rear PCB panel. When correctly orientated, the
red stripe should be to the bottom when looking at the back of the module.
TRIMMERS
The two trimmers on the side of the module are for ensuring the vintage VCAs don’t
leak/bleed signal through. These should not require any adjusting. Each vintage VCA has its
own requirement due to slight differences in the chips arising from manufacturing
tolerances.

14
CONNECTING CURIOUSER TO RABBIT HOLE
Curiouser and Rabbit Hole both have two sets of 2 x 3 connecting pins. This is so multiple
Curiouser modules can be daisy-chained into one Rabbit Hole, and the pin’s positions
provide flexibility to choose which side of Rabbit Hole to place Curiouser module(s). The
only important thing is the orientation of the cable. Please use the 6-core cable provided,
with the red stripe at the top edge of the modules.
Don’t get too lost down the Rabbit Hole…

15
PATCH IDEAS
following are a few ideas to inspire
Experimentation with the additional parameters not explicitly mentioned in the patches is
encouraged
Warm Drum Mix
Route drum mix to inputs of Rabbit Hole and Curiouser. Ensure RH/Out switches are UP to
send the audio to Rabbit Hole. Adjust the Gain controls to get the mix you desire. Set the
Tune bias high and a moderate amount of Drive. Adjust the Tone and Mix controls to suit.
This will give a warm, phat drum mix full of character.

16
Aggressive Drum Mix
Drum mix routed the same as the previous example. This example has increased solid state
Drive, and the OWTH hard-clipping circuit is engaged. This will deliver a much more
aggressive sound.
Exploring further, try experimenting with the L8 and FB.
Note: Hi-hats and cymbals don’t sound great with heavy distortion. It is best to mix hi-
hats/cymbals to the drum mix after Rabbit Hole has processed the other voices.

17
Warm Drum Mix with Accents
This patch extends on the Warm Drum Mix Patch to include accents. The example here uses
the snare, but you could send accents to any or all of the inputs as each channel has its own
VCA. The Gains have been reduced, and the Drive knob turned up to compensate. A periodic
accent (attack decay AD envelope) is occasionally being sent to accent the snare. Increasing
the Gain of the snare drives it into the Tube harder and gives a more characterful tone when
the accent is present.

18
Sidechained Bassline
In this patch, we take advantage of the CV attenuverters. A warm drum mix has been
achieved. A bassline is patched into an input on Curiouser. The trigger signal from the kick
drum is used to generate an AD envelope using other modules. This AD envelope is then
sent to the CV input for the basslines Gain. The bassline's CV attenuverter is in the
anticlockwise position, which means it is inverting the incoming AD envelope CV signal.
Therefore, when the Kick drum hits, the Gain of the bassline is reduced, creating a sidechain
ducking effect.
Note: The CV signal from Curiouser’s top channel is normalled to the bottom channel (check
the block diagram for a visual guide). The bottom channel’s CV attenuverter is set to zero
(12 o’clock position), so this normalled CV signal does not affect the Gain of the Toms.

19
3 Oscillator Synth Voice With Grouped VCA Accents
This patch again takes advantage of the CV normalling through Rabbit Hole and Curiouser
(check the block diagram for a visual guide). 3 oscillators have been mixed to make a voice,
e.g. Osc 1 and Osc 2 triangle waves tuned to the same pitch but detuned from each other by
a few cents. Osc 3 a sine, tuned one octave down.
The Gains are all set to their midpoint at 12 o’clock, and each channel's CV attenuverters are
clockwise. We have patched an accent AD envelope to CV A; due to the CV normalling, this
CV signal is sent to the CV for each channel (a patch cable at a CV jack would break the
normalling). When the AD accent is present, each channel's Gain will increase, adding more
character to the mix.
Note: this normalling could be utilised with all the Gains turned completely down and ADSR
envelope patched to CV A to control the whole Rabbit Hole & Curiouser system as an end of
voice VCA.

20
3 Oscillator Synth Voice With Tone Animation
This patch has the same oscillators patched to the inputs as the previous patch. The
difference is that we are using the bottom Curiouser channel as the end of voice VCA.
Therefore, we patch the Out of Rabbit Hole to the In of the bottom channel of Curiouser,
which is being used independently of Rabbit Hole; notice the RH/Out switch is in the DOWN
position. The Gain of the bottom Curiouser channel is fully down. The VCA is being opened
by the ADSR signal present at the CV input; the CV attenuverter is in the clockwise (positive)
position.
For fun, a bipolar LFO is being used to modulate the Tone control.
Exploring further, we could modulate the FB & Mix with additional LFOs or envelopes.