FILTER 8
OSCILLATOR
MULTIMODE FILTER/8-PHASE
INTRODUCTION
Representing the cutting edge in modular analogue voltage controlled filter (VCF) design, Filter 8 offers more possibilities and higher fidelity in 12 HP than ever before.
Starting from the classic OTA-style 4-pole
cascaded lowpass (LP) topology, Filter 8 features
separate LP outputs, each with their own
character. Additional filter responses are
achieved by pole mixing: 1-pole highpass, a
special band boost and notch response, phase
shifter and 4-pole bandpass. All outputs are
available simultaneously.
Key to Filter 8's performance is the innovative
resonant feedback circuit. By increasing the
resonance, all outputs will resonate at the filter
frequency, without any low frequency response
loss. At higher resonance levels, self-oscillation is
achieved, turning the module into an excellent
8-phase sine wave voltage controlled oscillator
(VCO) with temperature and switchable gain
compensation, low distortion, constant
amplitude and accurate frequency tracking over
at least 5 octaves. At sub-audio frequencies,
Filter 8 can function as an 8-output voltage
controlled slew modifier or 8-phase low
frequency oscillator (LFO).
CONTENTS
In the Filter 8 box, you’ll find:
Product card, stating serial number and
production batch.
16-to-16-pin Eurorack power cable.
Mounting hardware: two black M3 x 6 mm
hex screws, two black nylon washers and a
hex key.
The Filter 8 module itself, in a protective
cotton bag.
If any of these items are missing, please contact
your dealer or support@joranalogue.com.
Simultaneous exponential and linear frequency
modulation is possible, for classic FM tones or
chaotic modulation. A hold feature is also
provided, 'freezing' the output voltages manually
or under gate control. This is useful for halting
modulations, or as a sync-like effect at audio
frequencies. The dedicated ‘ping’ input allows
you to easily create crisp percussive sounds with
different timbres.
While rooted in the legacy of classic synthesisers,
Filter 8 provides a new approach to musical
signal generators and modifiers: instead of
simply a VCF, VCO, slew modifier or VCLFO, a
single analogue module can now be any of those,
and anything in between.
1
FILTER 8
OSCILLATOR
CONTROLS & CONNECTIONS
1 FREQUENCY RANGE SWITCH
This switch determines over which frequency
range Filter 8 will operate: low frequency (slew
modifier/VCLFO) or audio frequency (VCF/VCO).
2 COARSE AND FINE FREQUENCY KNOBS
The filter/oscillator frequency is determined by
these knobs. In standard audio mode, with the
fine knob centred, the coarse knob has a range
of 22 Hz to 22 kHz. The fine knob’s range is 5 % of
the coarse knob (6 semitones in audio mode).
In low frequency mode, the total range is
2.8 mHz (a period of 6 minutes) to 180 Hz, with
1 Hz when both knobs are centred.
MULTIMODE FILTER/8-PHASE
3 RESONANCE KNOB
The resonance knob controls a feedback path
from the filter output back to the input, causing
the filter frequency to be emphasised. Filter 8’s
design includes an innovative resonance circuit:
by increasing this parameter, all outputs will start
to resonate at the filter frequency, without any
low frequency response loss.
At higher feedback levels, self-oscillation will be
achieved, turning the module into an excellent
8-phase sine wave VC(LF)O.
4 VOLT PER OCTAVE FM INPUT
This input is used to modulate the frequency in
an exponential fashion, with a standard 1 volt per
octave response. This enables accurate audio
pitch in filter or oscillator mode. In the low range,
the sensitivity is increased to approximately 0.66
volt per octave.
5 EXPONENTIAL FM INPUT AND KNOB
This second exponential FM input includes a
polariser knob to set the modulation depth, with
0 in the centre, +1 volt per octave maximum and
−1 volt per octave minimum in audio mode. In
low mode, the sensitivity is increased to
approximately 0.66 volt per octave.
2
FILTER 8
OSCILLATOR
6 LINEAR FM INPUT, KNOB AND AC SWITCH
In addition to standard exponential FM,
simultaneous linear frequency modulation is
possible as well. This form of FM is well-known for
being more ‘tonal’ in VCO mode, limiting the
perceived shift in fundamental pitch as the FM
depth is increased.
This FM input also features a polarised depth
knob. The sensitivity ranges to approximately
±7 % frequency deviation per volt.
Enable the AC switch to AC-couple the LFM
input. This rejects any DC offset or very low
frequency content that may be present in the
modulation signal, preventing a fundamental
pitch shift from appearing during audio rate
modulation.
7 RESONANCE MODULATION INPUT AND
KNOB
The resonance amount can be modulated
through this input, with +5 V corresponding to
maximum resonance. The modulation knob
range is bipolar, just like the FM inputs.
MULTIMODE FILTER/8-PHASE
9 SIGNAL INPUTS
Connect the signal(s) to be filtered here. These
identical inputs are mixed together, which can
simplify certain patches as it often removes the
need for a separate mixer module.
Filter 8 can clip and distort with certain input
signals, especially if they are high-level, rich in
harmonics and resonance is added. This is
because all outputs are unity gain (when
compensation is disabled), and removing
harmonics can actually result in a signal’s
amplitude rising. This distortion can be used as a
deliberate sonic effect. If you prefer to avoid it
however, simply lower the amplitude(s) of the
input signal(s).
To use Filter 8 purely as an oscillator, simply set
the resonance to maximum and leave the input
sockets unpatched.
8 GAIN COMPENSATION SWITCH
By default, each output is at unity gain with
respect to the signal inputs. This is very useful
when processing audio or CV. However, when
Filter 8 is used in oscillator mode, all outputs
resonate at different amplitudes. For this reason,
the outputs are equipped with gain
compensation stages, enabled using this toggle
switch. With the compensation turned on, all
output signals will have the same amplitude.
3
FILTER 8
OSCILLATOR
10 LOWPASS OUTPUTS
These are the outputs from the different stages
inside the 4-pole lowpass filter core. Frequencies
above the filter frequency (also known as ‘cutoff’,
‘corner’ or ‘centre’) will be attenuated. More
stages means more attenuation, so each output
has its own character: LP1 has the softest filter
response (−6 dB per octave), while LP4 has the
steepest (−24 dB per octave).
Electronic filters do not just affect amplitude, but
phase as well. The phase shift at the filter
frequency, relative to the input signal, is printed
above each socket. Every additional filter stage
adds 45° of phase shift. As a result, the sine waves
generated at the outputs in self-oscillation will
be separated by 45° between adjacent stages.
The LEDs at the output sockets show the realtime output voltages, lighting up red for positive
and blue for negative. Note that DC offset
voltages may be visible at the outputs, especially
at extremely low frequencies. This is a result of
the circuit topology and is considered normal
behaviour.
MULTIMODE FILTER/8-PHASE
11 POLE-MIXING OUTPUTS
Using pole-mixing, additional filter responses are
derived from the standard lowpass stages. The
first is a 1-pole highpass output, offering relatively
gentle highpass filtering. It will resonate at 225°.
The second pole-mixing output is a special band
boost and notch response. This boosts
frequencies just below the filter frequency, and
cuts those just above it. At high resonance
settings, wavefolder-type sounds can be
generated. It will resonate at 270°.
The image below illustrates the amplitude
responses of the four lowpass outputs,
superimposed. The vertical dotted line denotes
the filter frequency; the horizontal unity gain.
Both axis are logarithmic.
Next is the phase shifter output. It consists of a 3pole allpass response, mixed together with the
input signal. This creates a combined highpass
and notch response, essentially a one-stage
phaser effect. It will resonate at 315°.
4
FILTER 8
OSCILLATOR
The final pole-mixing output is a 4-pole
bandpass response, selecting a band of
frequencies around the filter frequency. It will
resonate at 360° (equal to 0°, so no phase shift).
By plotting the voltages over time of all outputs
in self-oscillation, the 8-phase relationship
between them becomes visible. With gain
compensation enabled, all generated sine waves
will have the same amplitude. While the outputs
will sound identical on their own, having access
to phase-separated signals opens up many
advanced patching possibilities.
MULTIMODE FILTER/8-PHASE
13 PING INPUT
Filter 8 includes a built-in transient generator to
create ‘filter ping’ sounds. To use this feature, set
the resonance on the verge of self-oscillation.
This point is typically found just past 3 o’clock, as
indicated on the front panel.
A rising edge at the ping input, reaching above
+3 V, will ‘strike’ the filter core, creating a crisp
percussive sound. The amplitude and decay time
are set by the frequency and resonance
parameters. Each output will generate a different
ping timbre. For example, the lowpass outputs
will have a pronounced transient, useful for
synthesising kick drums. The bandpass output
will be the purest, more suitable for bongo-type
sounds.
Gain compensation should be disabled, to avoid
the outputs distorting at the transient (except for
output BB+N). The ping generator is designed to
reliably create ping tones, no matter which kind
of signal is used to drive it.
12 HOLD GATE INPUT AND SWITCH
The hold feature ‘freezes’ the filter core, slowing
it down to near-standstill until the hold is
released. This is useful for halting modulations, or
as a sync-like effect at audio frequencies.
By default, the feature can be manually toggled
using the toggle switch. It can also be controlled
from an external gate signal on the ‘hold’ input.
When the socket is used, the switch enables or
disables the input.
Filter 8’s hold inputs is uniquely designed to be
driven reliably even from weak, slow, bipolar
signals. It features Schmitt action, with a +2 V low
and +3 V high logic threshold.
Additionally, the ping feature can be used in LF
mode to trigger multi-phase decaying sine wave
modulations.
5
FILTER 8
OSCILLATOR
14 VOLT PER OCTAVE TRIMMER
This trim potentiometer is used to calibrate the
module’s pitch tracking. Since it is accessible
from the front panel, calibration can be easily
performed without removing the module from
the system. Each module is individually
calibrated during production; do not adjust this
trimmer if not needed.
Should you find your Filter 8 to be out of tune, set
it to oscillator mode (no input signal, maximum
resonance, range switch to audio, compensation
enabled). Set the coarse frequency knob to
about 20 % of its range (9 o’clock), and the fine
knob in the centre position.
Make sure Filter 8 has been powered for at least
20 minutes at a stable ambient temperature.
Now connect any output to a calibrated digital
tuner. The volt per octave input signal should
switch between 0 V and a precision +3 V source,
toggled automatically or by hand. Using a
dedicated trimming tool or standard 2.5 mm flat
screwdriver, adjust the trimmer until the interval
between both states is exactly 3 octaves.
MULTIMODE FILTER/8-PHASE
15 RESONANCE TRIMMER
The second trim potentiometer sets the selfoscillation amplitude at maximum resonance.
Use the same settings as for the V/oct. calibration
procedure and display the BP4 output signal on
an oscilloscope. Adjust the trimmer until the sine
wave’s amplitude is exactly 10 V
pp
.
Although the volt per octave tracking is
temperature-compensated, the resonance
amplitude is not. It will change slightly with
ambient temperature, as in any other analogue
VCF. If an accurate 10 V
amplitude is required,
pp
recalibrate this trimmer whenever the
temperature changes.
At low settings of this trimmer, Filter 8 will not be
able to achieve self-oscillation. If your module
can’t self-oscillate, an improperly adjusted
resonance trimmer is probably the reason why.
6
FILTER 8
OSCILLATOR
PATCH IDEAS
RESONATING ENVELOPES
Add interest to your envelopes by processing
them through Filter 8. If the envelope has an
amplitude of above +5 V, attenuate it first and
then patch it to a Filter 8 signal input. The
processed envelope is available at the LP1 output.
With the low frequency range active, you can
slew the envelope and introduce some
resonance to add ‘bounce’ to transients. Switch
to the audio range to superimpose an audiofrequency component to the low frequency
envelope for a unique combined modulation
signal. Don’t forget to try out the other filter
outputs as well!
HOLD MODULATION
Using the hold input for audio-frequency
modulation creates unique tones, somewhere
between linear FM and standard oscillator sync.
With Filter 8 in oscillator mode (no input, audio
range, maximum resonance), apply an audio
signal to the hold input and engage the switch.
MULTIMODE FILTER/8-PHASE
FEEDBACK DRIVE
Many synthesiser filters include an input
overdrive circuit. While this is not present on
Filter 8, the sound of an overdriven filter can still
be easily created, and with much expanded
sonic possibilities, using feedback patching.
With Filter 8 filtering an external audio signal,
send the LP4 output to an attenuator, and then
back into the spare input socket. The attenuator
controls the amount of ‘drive’.
Experiment with using different responses for
audio output and feedback. Replace the
attenuator by a VCA for voltage control over the
tone.
SAW WAVE OSCILLATOR
Another application for feedback is changing the
waveshape in oscillator mode. Patch the BP4
output to the exponential FM input. The EFM
depth now determines the waveshape. At
maximum modulation depth, the LP2 and BB+N
output signals will be near-perfect saw/ramp
waves.
Tune Filter 8 and the modulation source to a just
interval, and apply volt per octave FM to both to
create ‘bell’ or ‘synthetic bass’ tones reminiscent
of vintage digital FM synthesisers.
If the hold signal source is a pulse wave, you can
additionally modulate the pulsewidth to control
the ‘depth’ of the hold effect.
RESPONSE BLENDING
Filter 8’s multimode outputs are created using
pole mixing, combining the input and lowpass
outputs signals in precise ratios and polarities.
This technique can be recreated externally: using
a mixer module, add together various outputs to
experiment with creating your own responses.
Use a voltage controlled mixer and apply
different modulations to the channels for
dramatic effect.
The other outputs provide additional interesting
waveshapes, ranging from rectified sine waves to
curved saws. Note that the feedback depth also
affects the volt per octave pitch tracking.
7
FILTER 8
OSCILLATOR
MULTIMODE FILTER/8-PHASE
SPECIFICATIONS
MODULE FORMAT
Doepfer A-100 ‘Eurorack’ compatible module
3 U, 12 HP, 30 mm deep (inc. power cable)
Milled 2 mm aluminium front panel with nonerasable graphics
MAXIMUM CURRENT DRAW
+12 V: 75 mA
−12 V: 75 mA
POWER PROTECTION
Reverse polarity (MOSFET)
I/O IMPEDANCE
All inputs: 100 kΩ
All outputs: 0 Ω (compensated)
OUTER DIMENSIONS (H X W X D)
128.5 x 60.6 x 43 mm
SUPPORT
As all Joranalogue Audio Design products,
Filter 8 is designed, manufactured and tested
with the highest standards, to provide the
performance and reliability music professionals
expect.
In case your module isn’t functioning as it should,
make sure to check your Eurorack power supply
and all connections first.
If the problem persists, contact your dealer or
send an email to support@joranalogue.com.
Please mention your serial number, which can be
found on the product card or on the module’s
rear side.
REVISION HISTORY
Revision C: initial release.
MASS
Module: 175 g
Including packaging and accessories: 250 g
8
With compliments to the following fine people,
who helped to make Filter 8 a reality!
Ben ‘DivKid’ Wilson
Boris Uytterhaegen
Jan D’Hooghe
Lieven Stockx
Everyone at Wired Electronics
Sebastiaan Tulkens
Björn Jauss
Gregory Delabelle
Jens Van Daele
Filter 8 User Manual
version 2018-07-05
21st Century Analogue Synthesis—Made in Belgium
© 2018
info@joranalogue.com
https://joranalogue.com/
Loading...