Rossum EVOLUTION User manual

EVOLUTION
Variable Character
Ladder Filter
Copyright 2016 Rossum Electro-Music LLC
www.rossum-electro.com
Operation Manual

2 |
Contents
1. Introduction 3
2. Module Installation 4
3. Overview 6
4. Input and Output 7
5. Frequency Control 8
6. Q Control 10
7. Genus Control 12
8. Species Control 14
9. Specifications 15
10. From Dave’s Lab: The Evolution of EVOLUTION 16
11. Acknowledgements 19

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1. Introduction
Thanks for purchasing (or otherwise
acquiring) the Rossum Electro-Music
Evolution Variable Character Ladder Filter.
This manual will give you the information
you need to get the most out of Evolution.
However, the manual assumes you already
have a basic understanding of synthesis and
synthesizers. If you’re just starting out, there
are a number of good reference and tutorial
resources available to get you up to speed.
One that we highly recommend is:
Power Tools for Synthesizer Programming
(2nd Edition)
By Jim Aikin
Published by Hal Leonard
HL00131064
Additionally, for the technically and/or
historically minded among you, check out
Section 10, The Evolution of EVOLUTION, for
Dave’s account of how Evolution came to be.
Support
In the unlikely event that you have a problem
with your Evolution, tell us about it here:
http://www.rossum-electro.com/support/
support-request-form/
… and we’ll get you sorted out.
If you have any questions, comments, or just
want to say “Hi!,” you can always get in touch
here:
http://www.rossum-electro.com/contact-us/
… and we’ll get back to you.
Happy music making!

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2. Installation
As you will have no doubt noticed, the rear of
Evolution is a circuit board with exposed parts
and connections. When handling Evolution,
it’s best that you hold it by the edges of the
front panel or circuit board. It’s not particularly
easy to blow up, but why take chances?
More specifically, the biggest risk (to the
extent that there’s a risk), is damage by static
electricity. Particularly on dry, cold days (or if
you’ve just shued across your shag carpet in
fuzzy slippers), make a point of touching the
metal panel first, before touching any other
part of the module.
While all Rossum Electro-Music modules are
protected against reverse polarity damage,
both to your module and your system, care
should still be taken to connect the power
cable correctly. (For more detail on our
unique protection method, check out Dave’s
discussion of Circuit Protection in Section 10.)
Plug the included 16-pin connector into the
header on the rear of the module such that
the red stripe on the cable (the -12V side) is on
the same end of the header as the “Red Stripe
`-12V” text on the PCB.
Evolution requires, at most, 85mA of +12V
and 75mA of -12V.
We have included both M3 and M2.5 (for
vector rails) mounting screws. Use what fits
your system.
If rack rash is of concern to you, use the
included nylon washers when mounting
Evolution in your case.

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6 |
Taken together, Evolution gives you
everything from the platonic ideal of the pure
classic ladder filter to an almost unlimited
selection of alternative filter characteristics.
In the following sections, we’ll look at each of
Evolution’s functions in turn.
3. Overview
Evolution is a pure analog lowpass filter
that oers — in a single module — the
characteristics of a wide variety of synthesizer
filter types.
At the core of Evolution is Dave’s unique
implementation of Bob Moog’s iconic ladder
filter from the original E-mu Systems 2100
LPF module (which, incidentally, Dave counts
as his favorite sounding of all of the filters he
designed). Check Section 10 to learn what
made the 2100 (and, consequently, Evolution)
so special.
For Evolution, Dave has designed new
capabilities that not only allow you to dial in
all of the outstanding qualities of the original
2100, but combine to let you create the sonic
characters of other synth filters that are not
otherwise possible with traditional ladder
filters.
They include:
>A Genus control that allows voltage
control of the number of filter poles,
allowing a range of entirely new and
striking audio eects.
>A Species control that allows voltage
control of the signal level into the ladder,
letting you control the intensity of the
filter’s characteristic distortion.
>Voltage controlled resonance with a
variable Q Level Compensation control
that lets you control the balance of the
resonant signal and the frequencies
below the cuto frequency (which would
otherwise be attenuated as the resonance
is increased).
>An extremely accurate and temperature
stable frequency control exponential
generator, rivaling the specifications of the
best analog VCOs

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4. Input and Output
The Input and Output jacks let you get audio
into and out of Evolution.
Evolution is DC coupled throughout, so you
can use Evolution to process CVs as well as
audio.
The Input expects a signal level up to 20Vp-p
Depending on the Input level and the settings
of the various controls, the Output provides a
signal level of up to 20Vp-p without clipping.

8 |
5. Frequency
Evolution’s Frequency
section provides
manual and CV control
of Evolution’s cuto
frequency (i.e., the
frequency above which
the amplitude of the
signal is attenuated).
FREQUENCY Knob
The FREQUENCY
knob allows you to set
Evolution’s initial cuto
frequency (I.e., the cuto
frequency when no
CVs are present). The
actual cuto frequency is
controlled by the sum of
this knob and all present
control voltages.
Without CVs present, the
range of the control is
from approximately 22Hz
to 22kHz. With CVs, the
available range is from
.02Hz to 40kHz.
When Evolution is in
self-oscillation mode (see
the Q section below),
this control sets the initial
frequency of oscillation.
NOTE: Refer to the
Genus section below
to understand how the
Poles setting aects the
oscillation and resonant
frequencies.
1V/OCT CV Input
The 1V/OCT CV Input is a calibrated full
level control voltage input that is summed
with the value of the FREQUENCY knob
and the FREQ CV2 and FREQ CV3 inputs.
When properly calibrated (as it will be when
delivered), Evolution’s 1V/Octave tracking
is accurate over a 10 octave range (better
than many Eurorack VCO modules). When in
self-oscillation mode, Evolution serves as an
extremely high quality sine wave oscillator.
FREQ CV2 Input
The FREQ CV2 Input is a control voltage
input that is modified by its associated
attenuverter and then summed with the value
of the FREQUENCY knob and the FREQ CV3
and 1V/OCT inputs.
When the attenuverter knob is set to its “0”
position, no control voltage is passed to
the filter. As the knob is turned clockwise
from 0, the amplitude of the control voltage
increases until, at maximum clockwise
rotation, the full amplitude of the signal at the
FREQ CV2 Input is passed through and results
in a nominal 1V/Oct response.
As the knob is turned counter-clockwise from
0, the signal at the FREQ CV2 Input is inverted
(e.g., a CV of +2.5V becomes -2.5V). The
farther counterclockwise the knob is turned,
the less the attenuation of the inverted
signal, until, at maximum counter-clockwise
rotation, the full amplitude of the inverse of
the signal at the FREQ CV2 Input is passed
through, also at a nominal 1V/Oct response.

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FREQ CV3 Input
The FREQ CV3 Input is a control voltage
input that is modified by its associated
conventional attenuator and then summed
with the value of the FREQUENCY knob and
the FREQ CV2 and 1V/OCT inputs.
When the attenuator knob is set to its
maximum counter-clockwise position, no
control voltage is passed to the filter. As the
knob is turned clockwise, the amplitude
of the control voltage increases until, at
maximum clockwise rotation, the full
amplitude of the signal at the FREQ CV3
Input is passed through at a nominal 1V/Oct
response.

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6. Q (Resonance)
Evolution’s Q section
provides manual and CV
control of the height of
the resonant peak at (or
near) Evolution’s cuto
frequency, as well as the
level of the frequencies
below the resonant peak.
(See the discussion of
the eect of the Genus
setting on the frequency
of the resonant peak
below.)
Q Knob
The Q knob allows you
to set the initial height
of Evolution’s resonant
peak (I.e. the height of
the peak when no CVs
are present). The actual
height of the resonant
peak is controlled by the
sum of this knob and all
present control voltages.
This control produces a
voltage range of between
0V and 7V.
When the Q knob is
turned clockwise to
about 4V, the filter will
enter self-oscillation
mode, producing a pure
sine wave. In this mode,
Evolution acts as a very
high-quality sine wave
oscillator.
NOTE: The purest sine
wave and the most
accurate tracking occur
when the filter is just
barely into oscillation and
the Genus control is set to
4 poles.
ANOTHER NOTE: Refer to the Genus
Section to understand how the Poles
setting aects the oscillation and resonant
frequencies.
YET ANOTHER NOTE: Also refer to the
Genus Section to understand how the
Q setting aects oscillation during transitions
from one pole setting to another.
Q LEVEL COMPENSATION
In a traditional resonant ladder filter, as the
height of the resonant peak is increased, the
levels of the frequencies below the resonant
peak are attenuated. When the cuto
frequency is swept manually or by a control
voltage, the result is the familiar classic “wah”
we’re all familiar with.
In many cases, that’s exactly what you want,
as at high Q, you can hear the filter pick out
each overtone (or “frequency” of noise) as
you sweep through the audio spectrum.
However, if what you want is to add some
resonant spice to a pad or fat bass sound, the
lower frequency attenuation that results from
turning up the Q results in sucking some (or
most) of the guts out of the sound, leaving it
sounding thin.
Luckily, Evolution gives you a choice with the
Q LEVEL COMPENSATION control.
With the control turned fully counter-
clockwise, there is no compensation and the
filter acts exactly like a traditional ladder filter.
As you turn the control clockwise, the
amplitudes of the frequencies below the
cuto frequency are progressively boosted
until, at full clockwise rotation, they are at
their full (pre-attenuation) level.
Simply dial in the exact level of compensation
you want for each particular patch and you’re
good.

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Q CV2 Input
The Q CV2 Input is a control voltage input
that is modified by its associated attenuverter
and then summed with the value of the Q
knob and the Q CV1 Input. (If you haven’t
already read the explanation of how the
attenuverters work back up in the FREQ CV2
Input section, you can check that out now.)
Q CV1 Input
The Q CV1 Input is a full level control voltage
input that is summed with the value of the Q
knob and the Q CV2 Input.

12 |
7. Genus
Evolution’s Genus
control gives you real-
time manual and CV
control of the number
of poles in the ladder.
It is is, as far as we’re
aware, unique among all
implementations of the
classic ladder filter.
The number of poles
defines the slope at which
Evolution attenuates
frequencies at increasing
distance from the cuto
frequency. This parameter
is expressed in dBs of
attenuation per octave.
For example, a lowpass
filter with a slope of
24dB per octave (the
most common slope of
classic Moog-style ladder
filters) means that a signal
one octave above the
cuto frequency will be
attenuated by 24dBs, a
signal two octaves above
the cuto frequency will
be attenuated by 48dBs,
at 3 octaves 72dBs, etc.
Expressed as filter poles,
each pole provides 6bB of
attenuation.
The slopes available in
Evolution are:
3 poles = 18dB/oct
4 poles = 24dB/oct
5 poles = 30dB/oct
6 poles = 36dB/oct
Not only does being able to select the
number of poles give Evolution a variety of
filter characteristics, the ability to modulate
the number of poles in real time (including
at audio rates) opens up entirely new sonic
vistas.
Pole Selection and Resonant and
Self-Oscillation Frequencies
For technical reasons that you’ll find
explained in Section 10 (if you’re so inclined),
dierent pole selections result in dierent
resonant frequencies relative to the cuto
frequency.
The resonant and oscillation frequencies at
the various pole settings are:
3 poles: resonant frequency is 10
semitones above the cuto frequency
4 poles: resonant frequency is equal to
the cuto frequency
5 poles: resonant frequency is 6
semitones below the cuto frequency
6 poles: resonant frequency is 10.5
semitones below the cuto frequency
Consequently, with Q turned up, modulating
the number of poles results in a dynamic
shifting of the resonant frequency. This
provides unique and striking eects, both
at moderate LFO-like rates and at audio
frequencies.
The same principle governs Evolution’s
oscillation frequency when in self-oscillation
mode. Modulating the Genus parameter
while in self-oscillation mode will result in
various arpeggiator-like patterns (defined by
the amplitude and speed of the modulation).

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GENUS Knob
The GENUS knob allows you to set the initial
number of poles in the ladder. The actual
number of poles is controlled by the sum of
this knob and all present control voltages.
A swing of 5 volts (either from the Genus
knob or via the CV inputs) will cause
Evolution to cover the entire range of poles.
NOTE: The selection of poles is not a
discrete four-step process, but oers a
continuous transition between settings (as
indicated by the LEDs described next).
ANOTHER NOTE: With Q set just at the
point of self-oscillation, slowly moving
from one pole setting to another will result
in Evolution ceasing to oscillate when the
setting is between poles (i.e., when two
adjacent LEDs are lit).
However, with Q set to maximum, oscillation
is continuous during transitions between pole
settings.
If you’re modulating Genus during self-
oscillation, choose the Q setting that results
in whichever of these options you want for
your patch.
Or modulate Q as well for even more
complex eects.
GENUS LEDs
The four LEDs indicate in real time the
current number of eective poles as defined
by the sum of the GENUS knob and the
two CVs. Any time two adjacent LEDs are
lit simultaneously, it is an indication that the
Genus parameter is between those two pole
settings.
GENUS CV2 Input
The GENUS CV2 Input is a control voltage
input that is modified by its associated
attenuverter and then summed with the value
of the GENUS knob and the GENUS CV1
Input. (We’re pretty sure you know how the
attenuverters work by now.)
GENUS CV1 Input
The GENUS CV1 Input is a full level control
voltage input that is summed with the value
of the GENUS knob and the GENUS CV2
Input.

14 |
8. Species
Evolution’s Species section
provides manual and CV
control of the level of
the signal into Evolution’s
distortion circuitry.
As Dave explains it:
“The ladder design, as
I implemented it, has
no inherent distortion
for signals far below
the cuto frequency.
The characteristic
timbre of the ladder
filter comes primarily
from distortions of
frequencies near
and above the cuto
frequency. The
degree of distortion
depends on the
signal amplitude.
Consequently, it is
sonically interesting
to modulate the signal
amplitude going
into the ladder, and
modulate the output
signal with the precise
inverse gain. This is the
function of the Species
control. A high voltage
into the Species input
will cause the filter to
distort more audibly.”
So, briefly, the Species section gives you
control over the ladder’s “good” distortion.
NOTE: Because of the way Species is
implemented, with Q feedback within
the VCAs that control the Species function,
Evolution’s resonance is unaected by the
Species setting (I.e., the higher the Q setting,
the less obvious the Species setting). However,
the amplitude of any self-oscillation will be
inversely proportional to the Species setting.
Consequently, when using the Evolution as an
oscillator, the Species control can be used to
amplitude modulate the output. If oscillation
is combined with an input signal, the results
become even more interesting.
SPECIES Knob
The SPECIES knob allows you to set the initial
level of the signal going into the ladder and
provides a range of 10 volts. The actual signal
level is controlled by the sum of this knob and
all present control voltages.
With no CVs present, the range of the knob
from full counterclockwise up until the 12:00
o’clock position results in subtle distortion.
The range past the 12:00 o’clock position
results in more pronounced distortion.
SPECIES CV2 Input
The SPECIES CV2 Input is a control voltage
input that is modified by its associated
attenuverter and then summed with the value
of the SPECIES knob and the SPECIES CV1
Input. (Attenuverters, blah, blah, blah.)
SPECIES CV1 Input
The SPECIES CV1 Input is a full level control
voltage input that is summed with the value of
the SPECIES knob and the SPECIES CV2 Input.

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Controls
Initial Frequency
Initial Q
Q Level Compensation
Initial Genus (# of Poles)
Initial Species (Distortion Input Level)
Signal Input
1x 3.5mm mono socket - 100K Impedance
Signal Output
1x 3.5mm mono socket - 1K Impedance
Frequency CV Inputs
1x Attenuated - min 50k Impedance
1x w/Attenuverter - 100K Impedance
1x 1V/OCT Full Level - 100K Impedance
Q CV Inputs
1x Full Level - 100k Impedance
1x w/Attenuverter - 100k Impedance
Genus CV Inputs
1x Full Level - 100k Impedance
1x w/Attenuverter - 100k Impedance
Species CV Inputs
1x Full Level - 100k Impedance
1x w/Attenuverter - 100k Impedance
Power Requirements
+/-12V via 16-pin, Doepfer-style connector
Current Draw
85mA +12V, 75mA -12V (maximum)
Dimensions
16HP (W); Rear of panel to the top of the
power connector: 25 mm (D)
9. Specifications
Supplied Accessories
1x 16-pin, Doepfer-style cable
4x M3 screws
4x M2.5 screws
4x Nylon washers
1x Quickstart Guide

16 |
10. From Dave’s Lab:
The Evolution of EVOLUTION
The fundamental core of the Rossum
Electro-Music EVOLUTION Variable
Character Filter is Bob Moog’s famous
“ladder” filter, which was described in US
Patent 3,475,623. This circuit uses the
variation of the Bipolar Junction Transistor’s
emitter resistance with current as the voltage
variable element in an RC filter. In the
Moog implementation, four identical stages
each implemented a single real lowpass
pole. The Moog 904A module included
a “Regeneration” control that created a
negative feedback path around the four
poles. Since each pole provided 45 degrees
of phase shift at its -3dB point, increasing this
feedback produced a resonant peak at cuto.
The musical utility of the Moog filter is, of
course, famous.
In 1973, E-mu Systems introduced their
1100 submodule, which was the heart of
their 2100 lowpass filter module. The 1100
used a Moog ladder as its core element,
but I wanted to isolate the innate audio
characteristics of the filter ladder from those
colorations resulting from the input level-
shifters and output amplifier used in the
Moog 904A. I also envisioned a DC-coupled
design with a cuto frequency range well
beyond 10 octaves, as well as eliminating
variations of the height of the resonant peak
or oscillation amplitude with frequency
control voltage.
I level-shifted the exponential generator
to allow the filter signal input to be directly
applied to the ladder base. I then designed a
completely new output stage for the ladder;
this circuit has never (to my knowledge) been
used outside my designs. The entire signal
path was DC coupled and the resonant
feedback path phase compensated.
The 1100 was my favorite sounding filter
(I liked it more than the SSM2040 I later
invented, and kept the 1100 as E-mu’s
modular lowpass in preference to a cheaper
2040 design). The operational range of the
cuto frequency was from about 0.1Hz to
25kHz, with stable Q’s throughout.
In launching Rossum
Electro-Music, I chose a new
implementation of the 1100
filter as the first all-analog
module for our Eurorack
oering, based on its unique
and outstanding audio
characteristics.
I re-engineered the basic 1100 core
using modern available surface-mount
components and then added a number of
features to the original 1100 design:
Ladder filters self-oscillate, and can be used
as VCOs. I was able to design the Rossum
Electro EVOLUTION’s frequency control
exponential generator to be extremely
accurate and temperature stable, rivaling the
specifications of the best analog VCOs. I also
added a novel temperature compensation
circuit for the ladder emitter resistance.
The actual measured specs surprised and
delighted me.
The resonance (“Q”) of the original 1100
was not voltage controlled. I implemented
voltage controlled Q using one cell of an

| 17
SSM2164 VCA. Since the SSM2164 is based
on my 1979 design of the SSM2010, this is
an apt choice. (Sadly, the original 2164 is no
longer produced, so a replica source was
needed.) The phase compensation has been
maintained, but I added a “Q Compensation”
control. The negative feedback resonance
path in the 1100, like the Moog 904A, caused
The ladder design, as I implemented it, has
no inherent distortion for signals far below
the cuto frequency. The characteristic
timbre of the filter comes primarily from
distortions of frequencies near and above
the cuto frequency. The degree of
distortion depends on the signal amplitude.
Consequently, it is sonically interesting to
modulate the signal amplitude going into the
ladder, and modulate the output signal with
the precise inverse gain. This is the function
of the Species control. A high voltage into
the Species input will go well beyond the
linear region of the ladder and cause the filter
to distort much more audibly.
Because ladder filters produce their
resonance by feedback, the relationship of
that feedback to the drive VCAs is critical.
The Rossum Electro EVOLUTION places
the Q feedback within the drive VCAs. This
means that the filter’s resonance is unaected
by the Species setting, but that the amplitude
of any self-oscillation will be inversely
proportional to the Species level. When
using the filter as an oscillator, the Species
control can be used to amplitude modulate
the output. If oscillation is combined with an
input signal, the results become even more
interesting. Like the Q circuit, SSM 2164 cells
are used for the drive VCAs.
With these additions, the preliminary design
of EVOLUTION looked pretty complete.
Then Marco asked if it would be possible
to add voltage controlled slope (those
marketing guys are never satisfied). My first
take was that this would not be practical,
because varying the slope usually involves
controlling complex pole pairs, and the
ladder comprises only real poles.
the amplitude of signals in the passband to
be attenuated as the Q increased, which
some users found undesirable. If instead
the signal is inserted into the Q VCA, this
eect is eliminated. In EVOLUTION, the Q
Compensation control allows insertion of
the signal with an arbitrary mix into either of
these inputs, allowing the ratio of direct to
resonant amplitude to be arbitrarily selected.
There is no inherently desirable taper for Q
control. In highly resonant, but oscillation-
proof filters such as state variable designs, it
makes sense to exponentially control Q. But
in ladder filters, oscillation is expected, and
high Q’s without oscillation are not practically
achievable. The Rossum Electro EVOLUTION
implements approximately linear control of
the Q VCA.

18 |
Then I realized that I could
steer the current around
individual ladder stages in an
analog manner, controlling the
number of poles rather than
the slope. A prototype proved
this was both practical and
audibly pleasing.
Since the resonant frequency of a ladder filter
is determined by the 180 degree phase shift
point, it changes with the number of poles:
60 degrees for three poles, 45 for four, 36 for
five, and 30 degrees for six poles. Modulating
the number of poles produces a unique
“bubbly” sound.
Two more tweaks were needed to complete
the circuit. Because the number of poles not
only aects the phase shift for resonance,
it also changes the amount of feedback
required for oscillation, the pole control
circuit needs to control the Q VCA in a
manner such that the same Q control
voltage produces the onset of oscillation for
each pole setting. And since it’s useful but
dicult to tune the initial pole setting to be
in the center of the range (exactly steering
the current to the desired ladder poles), I
added analog controlled LEDs to indicate the
activation of the poles.
We then sent development versions of
Evolution to a number of collaborating
musicians who responded with some
excellent suggestions. These resulted in a few
more circuit tweaks to finalize the product.
Circuit Protection
Eurorack suers from the problem of power
connector reversal. When 10 pin connectors
are used, mis-insertion results in a swap
of +12V and -12 V, and protection is easily
accomplished using various techniques such
as series diodes.
But more systems are providing the +5V
supply and thus use the full 16 pin connector.
When this is reversed, a diode-protected
module is still safe, but the six connected
ground pins in the module will short
together the system’s +5V and +12V supplies,
potentially damaging the power supply and
any modules that use +5V.
To prevent this, Rossum Electro-Music
modules deviate from the standard Eurorack
power connector by leaving power
connector pins 9 and 10 open, rather than
connecting them to ground. When plugged
in backwards, this leaves the system +12V
supply disconnected. Since ground is still
supplied by four pins as well the chassis and
any patch cords connected to the module,
the dropping of these two pins has no
measurable eect on circuit performance,
but it means that if a Rossum Electro module
is accidentally plugged in backwards,
no stress is placed on the +5V supply or
modules that use it.

| 19
11. Acknowledgements
A number of wonderful people generously
provided help, advice, encouragement,
and inspiration during the development of
Evolution.
Many thanks from the Rossum Electro-Music
team to:
Bob Moog, for his inspiring filter design
Jim Aikin
Patrick Brede
Nancy Enge
Josh Holley
Mihai Ionescu
Kurt Kurasaki
Michael Logue
William Mathewson
Gur Milstein
Kevin and Denise Monahan
Trish Neilsen
David Phipps
Bill Putnum
Allan Shaer
Dan Snazelle
Kirk Southwell
Tomio Ueda
Andy Zenczak (and the crew at Gadgetbox
Studios)
And, it goes without saying, but we’ll say it
anyway, our families for understanding all the
late nights and weekends spent not having
fun (or doing chores) with them.