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
