AJH Synth Gemini 2412 User manual

User Guide
Thank you for purchasing the AJH Synth Gemini 2412 Dual VCF module, which like all AJH Synth
Modules, has been designed and handbuilt in the UK from the very highest quality components.
We hope that it will help and inspire you towards creating some great music and soundscapes!
Comprising two completely independent Vintage SEM style lters, with a central mixing bus for
both audio and frequency control voltages which allow the two VCF's to be combined in both
serial and parallel modes, and VCF1 can be inverted against VCF2. So the two VCF's can be com-
bined to behave as a 24dB SEM lter, vocal formant lter and much more!
Each of the two lters are based on the vintage SEM lter, with FET transistor buers and feature
Low Pass, Band Pass and High Pass outputs and a useful Vary output which can be faded from LP
to Notch to HP. All outputs can be used simultaneously.
The two lters can be used independently, or can combined in the following modes:
Dual (Parallel) Mode - The input to VCF1 is normalled to VCF2, and the two VCF’s are placed in
parallel, the output mix of VCF1 (OUT 1) against VCF2 (OUT 2) is controlled by the setting of the
Audio Mix control, with the output routed to the D-OUT jack.
Dual (Parallel) INVERTED Mode - this is the same as Dual mode, however the output of VCF 2 is
inverted against VCF 1.
Cascade (serial) Mode - The output of VCF 1 is normalled to the IN 2 jack, so that the two lters
are cascaded, signals are rst processed by VCF 1 and then by VCF 2. In this case the Audio Mix
control crossfades between the ouput of VCF 1 (single VCF) and VCF 2 (dual cascaded VCF’s).
Again, the Cascade output is routed to the D-OUT jack.
Module width is 28 HP of Eurorack space and it is compatible with standard Eurorack cases. The
height of the panel is 128.5mm and depth is 24mm . There are four mounting holes at the
corners of the module and we provide 4 of M3 rack xing screws along with a Eurorack compat-
ible power cable. Current consumption is 50mA from the +12V supply rail and 45mA from the
-12V supply rail.
All AJHSynth modules are covered by a two year guarantee against manufacturing defects.
Note:
It is very important that the power supply ribbon cable is connected correctly, see the “adjustment and
calibration” section for an illustration of the correct orientation.
Dual Vintage style State Variable Filters
www.ajhsynth.com
2412

The Gemini 2412 is a very comprehensive and extremely versatile dual lter module. Although it is a
complex design, it becomes very easy to understand when we break it down and examine its compo-
nent parts and the relationship between them. The diagrams and text here will get you up to speed
quickly so that you can explore all of the possibilities and capabilities of your Gemini 2412. It comprises:
1 - Two fully independent Multi Mode VCF’s (VCF 1 and VCF 2)
2 - Audio Cross fader / Panner module (Mix Bus)
3 - Dual Frequency CV source with external input and panning mixer (Mix Bus)
4 - Audio Inverter (Mix Bus)
5 - Parallel / Serial Connection switch (Dual / Cascade mode)
The block diagram below shows how the above component parts are connected together and the signal
ow between them. It can be seen that either Input 1 or the output of VCF 1 can be routed to the input
of VCF 2 via the Dual Cascade switch. This connection is made using the internal switch on the IN 2 jack
socket, and is connected if no jack socket is connected, but if a patch cable is insered into IN 2 then this
connection is broken and the external signal from the patch cable is routed to the input of VCF 2 instead
- the technical term for this is normalising.
Overview
VCF 1
GEMINI 2412 DUAL VCF - Block layout and signal ow
MIX BUS
VCF 2
IN 1 (Audio In)
+5V
Cuto Freq.
FCV IN
INVERT
VCF 1 Mode
Res Pot
1V/Oct
RES CV
+5V
MIX CV
+5V Cross Fader
Audio
Dual Frequency CV Source
FCV Level
Attenuverter
IN 1 Level
IN 2 (Audio In)
IN 2 Level
SIGNALS:
INPUT AUDIO SIGNAL PATH:
DUAL AUDIO SIGNAL PATH:
FREQUENCY CV SIGNAL PATH:
RESONANCE CV SIGNAL PATH:
INTERNAL VOLTAGE CONNECTIONS:
BP Out
D-FCV Level
D-FCV MIX
D-FCV
+10V
Res Pot
RES CV
+5V
HP Out
LP Out
Out 1
Vary Pot
Dual / Cascade
Switch
Self Oscillation
Jumper
Self Oscillation
Jumper
Invert Switch
D-Out
VCF-1
VCF-2
Audio Mix
Cuto Freq.
FCV IN
INVERT
1V/Oct
FCV Level
Attenuverter
INVERT
LP
BP
HP
VR
VCF 1 Mode
BP Out
HP Out
LP Out
Out 1
Vary Pot
LP
BP
HP
VR

VCF 1 Resonance
2
1
VCF 1 Frequency
Vary 1 Control
VCF 1 Mode
Freq CV 1 Level
INPUT 1 Level
VCF 1 Output
VCF 1 RES CV
1V/Oct Input
VCF 1 FCV IN
VCF 1 Input
VCF 1 LP Out
VCF 1 BP Out
VCF 1 HP Out
VCF 2 Resonance
VCF 2 Frequency
Vary 2 Control
VCF 2 Mode
Freq CV 2 Level
INPUT 2 Level
VCF 2 Output
VCF 2 RES CV
1V/Oct Input
VCF 2 FCV IN
VCF 2 Input
VCF 2 LP Out
VCF 2 BP Out
VCF 2 HP Out
6
7
5
4
3
14
13
12
11
10
9
8
5
4
3
10
2
1
6
11
12
13
14
7
8
9
Audio Mix
16
15
Dual/Cascade
D-FCV Level
D-FCV Mix
Invert Switch
D-FCV Input
Dual Output
MIX CV Input
20
21
19
18
17
22
VCF 1
MIXING BUS
VCF 2
Front panel controls

1 VCF Resonance :
2 Vary Control : This control fades from LOW PASS response (fully counter clockwise) to NOTCH response
(control at 12 o’clock) and through to HIGH PASS response (fully clockwise). The resulting
ltering is routed to the VAR position of the VCF MODE switch (5).
In VARY mode the LP setting is 12dB slope, the NOTCH is 6dB slope and the HP 12dB slope
Manually sets the amount of Resonance (feedback) through the lter core. When a patch
cable is inserted into the RES CV jack (8) then this control acts as an attenuator for the
incoming RES CV level.
3 Freq CV Level : This control is an attenuverter, which determines how much of the regular (or inverted)
Frequency CV signal patched to the FCV jack (10) is passed to the lter core to control the
cuto frequency.
With this control at (approximately) centre position non of the Frequency CV signal will
be passed, however as this control is rotated clockwise then increasing amounts of FCV
signal will be passed to the lter core. Rotating the control anticlockwise from centre has
a similar eect, however the FCV signal is inverted.
For example, lets say that a +5V voltage is patched to the FCV Input jack (10). With this
control central then zero volts will be applied as a cuto voltage, but when rotated fully
clockwise +5V will then be passed to the lter as a cuto voltage, thus “opening” the lter.
If the control is rotated fully anticlockwise then -5V will be applied to the lter as a cuto
voltage, thus“closing” the lter.
4 VCF Frequency: Manually sets the cuto of the lter. Any external control voltages introduced from the
FCV Control, 1V/Oct and D-FCV Mix control are summed with this control setting, so in
this case it acts as a +/- oset control.
5 VCF MODE Switch : This selects the lter type that is routed to the OUT jack (7) and to the central mixing bus.
6 INPUT Level : This is an attenuator that determines how much of the signal from the Input jack (11) is
passed to the lter input.
It also includes an overdrive function; if a regular Eurorack level audio signal (10V p/p) is
fed to the Input (11), then full signal level and unity gain corresponds to a setting of
around 8 on this control, and if it is rotated further clockwise then the output level will
still rise, but the lter core is driven into distortion and an “overdriven”sound is created.
7 OUTPUT :
8 VCF RES CV : Patch a voltage or signal (modulation from an LFO or audio rate) to this input to exter-
nally control the lter Resonance. The amount of this signal passed to the lter core is
determined by the setting of the VCF RESONANCE control (1), which acts an an attenuator
when a patch cable is inserted into this jack socket.
The accepted signal range for this input is 0 to +5V. Negative signals will be ignored and
positive signals higher than +5V will be capped to +5V. Maximum permissible voltage to
this input is +/-12V.
Filter output. This is downstream of the VCF mode switch, so the lter type determined by
the MODE switch (LP, BP, HP or Vary) is routed to this jack. Please note that this output is
available simultaneously with other output jacks (12, 13 & 14). For example, if the MODE
switch is set to LP then this output will be low pass, however a low pass output will still
also be available from the LP OUT jack (12)
Note: This is the only way to get a Vary output from the front panel, as Vary does not have a
dedicated front panel output jack.
Description of individual lter controls
VCF1 and VCF 2 are identical and independent, these labels apply to both.

9 1V/Oct INPUT : This is a frequency cuto modulation input with a xed control level corresponding to
one volt per octave. It can be useful to patch a pitch cv to this input so that the lter
automatically “opens” at this rate as the pitch gets higher. This input can be used simulta-
neously with the other frequency control inputs FCV and D-FCV.
Also, when the lter is in SELF OSCILLATION mode then applying a pitch control voltage
to this input allows the lter to be used as a Sine wave VCO with reasonably accurate
tracking over a four octave range, however do note that there is no temperature compen-
sation on this, so it is calibrated to only track well at regular room temperature (21
degrees C)
10 VCF FCV Input : This is the main frequency control input for the lter, and voltages or signals patched to
this input control the cuto of the lter at a level (and inversion) set by the Freq CV Level
control.
The FCV input can be a xed voltage or a waveform, either at low frequency from an LFO
or similar for modulation or at audio rates from a VCO.
The accepted signal range is -5V to +5V. Any voltages higher or lower than this will be
capped to +5 or -5V. Maximum permissible voltage to this input is +/-12V.
13 BP Output : This is the Band pass output, the slope is 6db per Octave.
11 VCF Input : This is the audio input to the lter, and it is routed to the Input level control (6).
In no input is patched to VCF 2 Input (IN 2) then it is normalised to Input 1 if the
Dual/Cascade switch is set to DUAL, and it is normalised to the OUPUT of VCF1 Mode
switch if the Dual/Cascade switch is set to CASCADE. Plugging a patch cable into IN 2 will
defeat this normalising and the external signal patched in then takes priority.
Please note: This lter (like many vintage synth lters) is intended for processing audio signals
only as it is AC coupled - so is therefore it is not suitable for processing control voltages
through the audio input. It won’t cause a problem, the input capacitor simply blocks any DC
voltages.
12 LP Output : This is the Low Pass output, the cuto slope is 12db per Octave.
14 HP Output : This is the High Pass output, the cuto slope is 12db per Octave.
Description of individual lter controls (cont.)
VCF1 and VCF 2 are identical and independent, these labels apply to both.

15 Audio Mix :
16 D-FCV Level : The D-FCV control allows a second cuto voltage to be sent to both VCF 1 and VCF 2, and
the D-FCV MIX control determines how this frequency CV is split between VCF 1 and VCF
2. This is a hugely powerful feature, as the cuto frequency of both lters can be altered
using a single control, and varying amounts of CV directed to each lter to create
asymmetric sweeps.
An external control voltage can also be patched to the D-FCV jack, and in this case the
D-FCV control becomes an attenuator for the incoming external D-FCV signal.
The Audio Mix control cross-fades between the outputs of VCF 1 and VCF 2. It’s exact
function depends on the setting od the Dual/Cascade switch (18)
In manual mode, with the control fully anti-clockwise, only the output of VCF 1 will be
heard from the D-OUT output.
With the control (approximately) centred at 12 o’clock, an equal mix of the outputs of VCF
1 and VCF 2 will be heard at the D-OUT output.
With the control fully anti-clockwise, only the output of VCF 1 will be heard from the
D-OUT output.
External control of audio mixing is possible by applying a control voltage to the MIX CV
Input (22) and in this case the audio mix control becomes an attenuator for the incoming
CV voltage.
18 Dual / Cascade: The Dual / Cascade switch determines how VCF 1 and VCF 2 are combined together in the
Mix Bus. This allows them to be can be connected in Dual (Parallel) or Cascade (Series)
DUAL - VCF 1 input is normalised to VCF 2 input, so that the same signal is presented to
both lters, and the AUDIO MIX control (15) cross fades between the outputs of VCF 1 and
VCF 2, and the resulting mix is sent to the D-OUT jack. It is also possible to connect a
dierent signal or waveform to VCF 2 Input (IN 2) and in this case the audio mix will
cross-fade between the individually ltered outputs of VCF 1 and VCF 2.
CASCADE - The output of VCF 1 (OUT-1) is connected to the input (IN 2) of VCF 2, so that
the Signal is rst processed by VC 1, and then passed into VCF 2 for further treatment. The
Audio Mix control still cross-fades between VCF 1 and VCF 2, so in this case it cross fades
between one or two VCF’s.
For example, lets say both VCF’s are set to Low Pass mode, with equal control of the lter
cuto on each. Now each lter has a cuto slope of 12db/Octave, however because we
are treating the sound twice then this cuto slope is doubled by the second lter, so we
can now have a SEM style lter with a four pole 24dB/ Octave slope! And we can freely
crossfade between the 12dB and 24dB variants!
17 Invert Switch: This switch inverts the output of VCF2 with respect to VCF 1, so that when both lters are
combined in DUAL mode one lter output is inverted against the other, which causes a
dierent set of frequency and phase cancellations from non inverted. It is only eective
when the Dual/Cascade switch is set to the Dual position.
Description of Mixing Bus controls
Controls mixing and conguration of the two individual lters

19 D-FCV MIX: This control pans the D-FCV signal - to VCF 1 only (Control fully anti-clockwise) or VCF 2
only (Control fully clockwise) or it can be panned and a mix of the signal is sent to both
lters, with the signals being split equally between VCF 1 and VCF 2 when it is in the 12
o’clock, mid way position.
20 D-FCV Input: The D-FCV voltage can be either an internal or external control voltage, for external
control a CV should be patched to the D-FCV jack (20), and when the patch cable is
inserted the D-FCV Level control becomes an attenuator for the incoming CV signal.
With the D-FCV control fully clockwise the eective control range for the D-FCV control
voltage is -10V to +10v, and the maximum permissable voltage to this input is -12V to
+12V.
21 Dual Output: The D-OUT is the output from the MIX BUS, which is where the two VCF’s are mixed
together. This is the only output that combines the mix of VCF 1 and VCF 2, so to use the
DUAL, CASCADED and DUAL INVERTED modes this jack must be used as the output.
All of the other outputs on VCF 1 and VCF 2 are still active and available, however they are
obviously individual to VCF 1 and VCF 2 and only a single lter will be heard when any of
these are used.
22 MIX CV Input: The cross-fading between VCF 1 and VCF 2 can be controlled with an external CV by
patching a control voltage to this MIX CV jack. When the patch cable is inserted the
AUDIO MIX control (15) then becomes an attenuator for the incoming CV signal.
With the AUDIO MIX control fully clockwise the eective control range for the MIX CV
control voltage is 0 to +5V. Any negative voltages are seen as 0V and voltages higher than
+5V are capped at +5V. The maximum permissable voltage to this input is -12V to +12V.
Description of Mixing Bus controls (cont.)
Controls mixing and conguration of the two individual lters
(Cont.)

SELF OSCILLATION MODE
Using the GEMINI 2412 as a Sine Wave Oscillator
Usually 12dB state variable lters such as those in the Gemini 2412, Oberheim SEM and other synthesisers do not go into
self -scillation at high resonance settings, this is because of damping in the lter core itself which prevents this from
happening. However, we have included some additional circuitry that can be connected into the lter using a PCB
mounted jumper on the back of themodule. The eect of this circuitry is to overcome the internal damping and force the
lter core into self oscillation, and it then behaves as a Sine Wave Voltage Controlled Oscillator (VCO) that tracks
1V/Octave over (approximately) a 4 octave range.
See the “Adjustment and calibration” page for information on the location of the self oscillation jumpers.
There are a few points to note about self oscillation mode:
1) Because we have to force the circuitry into self oscillation the resulting signal level is quite high - around 19V p/p
rather than the usual 10V p/p that most Eurorack modules produce, this is not a problem as long as it is feed into
another module that has a level control - for example the AJHSynth MiniMod VCA - this has input level controls and
simply turn these down to the point where the are of similar volume to other VCO’s etc and there will be no problem. If
you feed it at full output into other Eurorack modules without attenuating the level then you may get some ugly clipping
distortion. With this in mind, if using the GEMINI 2412 in CASCADE Mode with VCF 1 as a Sine wave oscillator and then
feeding the output into VCF 2 then do keep the IN 2 LEVEL control turned down to 5 or lower to prevent overloading the
input of VCF 2.
(Cont.)
1 V/Oct CV SINE OUT
0
0 0
5
00
10
5
05
00 LPLP
DUAL VCF 2
0

SELF OSCILLATION MODE (cont.)
AUDIO RATE MODULATION
2) The frequency control circuitry is NOT temperature corrected, so it will only track 1V / Octave at a given temperature (it is
factory calibrated to 21 degrees celcius ambient). If you use it in much colder, or much hotter conditions then the SCALE
trimmer may need to be adjusted to correct the tracking to the ambient temperature level.
3) The sine wave is not a perfectly pure waveform, it has some slight clipping to the top and the bottom of the waveform -
this gives it a very slightly brighter sound than a pure sine wave.
4) No input is needed for self oscillation, simply connect the pcb jumper, turn the Resonace control around to 10 and set
the Frequency control so that the oscillation can be heard at the desired frequency. Use the LP output
With the above points in mind it is a very useful addition, and each VCF can be set to Sine Wave VCO mode individually, so
VCF 1 could be used as an oscillator and the output passed into a wavefolder or similar circuit to add harmonics, and then
this signal can be passed through VCF 2 and ltered.
The V-Scale buer can be a useful addition if the self oscillation mode is used often and accurate 1V/Octave scaling is
required - it allows scaling to be adjusted from the front panel without having to remove the GEMINI 2412 from the
Eurorack case and adjust the small trimmers on the back of the module.
The addition of a Gain Switch Multi (a 2HP wide module) allows the self oscillation jumpers to be switched from the front
panel. The two switches on Gain Switch Multi are connected to the jumpers on the rear of the GEMINI 2412, so that self
oscillation mode can be selected from the front panel without having to remove the Gemini from the Eurorack Case.
Switch UP activates self oscillation mode, switch DOWN is regular SEM style lter. Self oscillation can be individually
selected for either or both VCF’s.
The GEMINI 2412 has a wealth of modulation inputs, including FCV 1, FCV 2, Resonance CV, D-FCV, Audio Mix, 1V/Oct etc,
and all of these handle xed control volages and modulation waveforms from LFO’s for low frequency modulation,
however all of these inputs can also be used with audio rate modulation too, which can sometimes give very interesting
and unexpected sounds, so just because it’s a modulation input don’t be afraid of patching in a VCO or other audio rate
signal for some extra sounds!
(Cont.)

Here the Gemini 2412 is congured as two completely independent VCF’s in a two voice setup. VCF 1 of the Gemini is used as
part of a Lead Voice and VCF 2 is used in a Bass Voice. We do not need to use the centre Mix bus here, however the D-FCV and
D-FCV MIX controls are still active, so can be used for additional frequency control if desired - if not required then keep the
D-FCV control set to zero.
VCF 1 is the Lead voice, and here we are using the LP output which is patched to a VCA. One envelope controls Gemini VCF 1,
the second envelope controls the VCA. Here we are splitting the Lead synth Pitch CV with a passive Multi and feeding 1V/Oct
to the lter cuto too, so that as the pitch gets higher the lter opens more. Frequency CV control is also added from the
Envelope Generator which is patched into the FCV 1 Input - turning the FCV 1 control clockwise from centre position gradu-
ally adds more of the Envelope CV voltage to the lter, turning it anti-clockwise from centre adds the inverse of the Envelope
CV, so it will close down rather than open up the lter cuto point.
The bass voice is fed to Gemini VCF 2 from the lower VCO. This time on the Gemini we are using the OUT 2 jack, and the lter
type used by OUT 2 is set by the four way rotary switch. We have selected VAR (Variable) as the Mode, so in this case the VAR 2
control also comes into play - we have selected 0 (Fully anti-clockwise), which gives us a Low Pass conguration, but turning
the VARY control clockwise will take us to a NOTCH lter at the 12 o’clock position and on to a High Pass lter with the VARY
control fully clockwise.
Note: The numbers used to illustrate control knob positions do not relate to the markings on the module itself, but are simply a scale from zero to 10 with 5
being the control centre position. For attenuverters the control positions are -10 to +10, with zero being centre position for the control knob.
10
10
1V/Oct IN 1
Gate 1
Output 1
Output 2
1V/Oct IN 2
10
0
0
0
0
0
4
5
8 8
2
4
5
10
10
10
0 0
0
0
4
+4 +6
5
0
2
2
2
3 3
8’
00 VAR
0
0
00
0 0 0
Gate 2
GEMINI 2412 - PATCHING EXAMPLES
1 - Conguration as two independent VCF’s
Using the Gemini 2412 as two independent VCF’s
32’
0
00
0 0 0
8’
0
00
0 0

In this patch we are using the Gemini in Dual Inverting Mode, so the Dual and Invert switches are both in the down position.
We are feeding the audio into IN 1, however do note that there is no patch cable in the IN 2 input - it is not needed as in DUAL
mode IN 1 is normalised (connected) to IN 2 internally, so the single input is automatically patched to both VCF 1 and VCF 2.
Note that the Envelope OUT 1 is connected to a Multi, so that we can patch it to both FCV 1 and D-FCV inputs simultaneously
- also, with the FCV control we have it set to -9, so it is applying a negative control voltage to the lter cuto (because the FCV
control is an attenuverter). But we are also routing this same envelope out CV to the D-FCV Input, and with the D-FCV Control
set to 8 most of this voltage is passing through, because the D-FCV Control acts as an attenuator on external CV voltages and
it is almost fully open.
We are taking the output from the D-OUT jack, so that we can cross fade between VCF 1 and VCF 2 using the Audio Mix
control. As both VCF 1 and VCF 2 are set to Band Pass and have quite a large amount of resonance added we get some classic
vowel sounds, and sweeping between the lters gives interesting tonal variation. Try dierent settings with the Invert Switch,
D-FCV Mix and Freq 1, Freq 2 and FCV 1 controls to vary the sounds, and also try HP and Notch settings too for further
variations.
Also, try controlling the Audio Mix by patching a second Envelope (or other control voltage source) to the MIX CV jack, and set
the AUDIO MIX control fully clockwise to get the full range of control, just back it o a little if the CV voltage is peaking too
high so that you get a full sweep from VCF 1 to VCF 2
Note: The numbers used to illustrate control knob positions do not relate to the markings on the module itself, but are simply a scale from zero to 10 with 5
being the control centre position. For attenuverters the control positions are -10 to +10, with zero being centre position for the control knob.
GEMINI 2412 - PATCHING EXAMPLES
10
10
Gate 1
Output 1
10
0 0
0
8
4
2
8 8
1 0
8
2
-9 0
7
50
55
16’
00 BPBP
DUAL INV
7
0
00
0 0 0
2 - Dual Parallel VCF’s Inverting Mode, for Band Pass Vocal sounds
Dual Parallel VCF’s Inverting Mode, for Band Pass Vocal sounds
1V/Oct IN

In this patch we are using the Gemini in Cascade Mode, so the Dual/Cascade and Invert switches are both in the up position.
By feeding a signal rst through a LP (low pass) lter, and then feeding it through a second lter set to HP (High pass) we
create a BP (band pass) lter. The rst question would be.... why bother? We already have a BP lter on each of the individual
lters........BUT the individual ters are each 6dB slope when set to BP lter, wheras if we combine the two lters together we
can get a BP lter with a 12dB slope, which has a higher Q and covers a narrower band. And of course we also have the option
of fading from LP response (VCF 1 only) and BP responce (VCF1 and VCF 2 in series) by using the AUDIO MIX control, and we
can automate this cross fading by patching an external CV to the MIX CV jack.
We are feeding the audio into IN 1, however do note that there is no patch cable in the IN 2 input - it is not needed as in
CASCADE mode OUT 1 is normalised (connected) to IN 2 internally, so the single input is automatically patched to both VCF 1
and VCF 2.
The D-FCV IN input is fed from the contour generator, and here we have the D-FCV Mix control at mid point, so that the D-FVC
control signal is being sent equally to VCF 1 and VCF 2.
We have also included a variant on this patch (lower picture) which shows the inverse output from an envelope generator
being used as the control voltage to automate the control of the Audio mix from Low Pass to Band pass response, obviously
other modulation sources can also be used for this, and the eective range of the control is 0V to +5V to cross fade fully when
the Audio Mix control is set to fully clockwise. .
Note: The numbers used to illustrate control knob positions do not relate to the markings on the module itself, but are simply a scale from zero to 10 with 5
being the control centre position. For attenuverters the control positions are -10 to +10, with zero being centre position for the control knob.
GEMINI 2412 - PATCHING EXAMPLES
3 - Cascade Mode - creating a 12dB BP lter by combining LP and HP lters
0
1
3
3
0
0
10
Gate 1
Output 1
10 10
0 0
0
3
5
3
8 8
1 0
5
2
00
4
55
510
16’
00 HPLP
CASCADE VCF 2
5
0
00
0 0 0
Cascade Mode - creating a 12dB BP lter by combining LP and HP lters
With added CV control of AUDIO MIX
Cascade Mode - creating a 12dB BP lter by combining LP and HP lters
10
10
Gate 1
Output 1
10
0 0
0
3
5
3
8 8
1 0
5
2
00
4
55
510
16’
00 HPLP
CASCADE VCF 2
5
0
0
0
0 0 0
1V/Oct IN
1V/Oct IN

In this patch we are using the Gemini in Cascade Mode, so the Dual and Invert switches are both in the up position.
By connecting two LP (Low pass) lters in series we can double the cuto slope of the lter - many famous vintage synths
such as Moog and Arp used 24dB VCF’s, so we can use the Gemini 2412 to mimic this response too.
We are feeding the audio into IN 1, however do note that there is no patch cable in the IN 2 input - it is not needed as in
CASCADE mode OUT 1 is normalised (connected) to IN 2 internally, so the single input is automatically patched to both VCF
1 and VCF 2.
The D-FCV IN input is fed from the contour generator, this saves having to patch individually to VCF 1 and VCF 2 FCV Inputs,
and here we have the D-FCV MIX control at mid point, so that the D-FVC control signal is being sent equally to VCF 1 and
VCF 2, thus it behaves as a single lter.
Rotating the AUDIO MIX control clockwise from zero to 10 fades between single 12dB LP conguration (VCF 1 only) and
24dB conguration (VCF 1 and VCF 2 cascaded in series). You will not that the 24dB setting sounds “duller” and less bright
than the 12dB LP lter - this is exactly as expected, because we are removing higher frequencies at a rate of 24dB per octave
instead of 12dB per octave. The actual lter LP slopes (with resonance set to zero) for the Gemini 2412 are shown in the
graph below.
Note: The numbers used to illustrate control knob positions do not relate to the markings on the module itself, but are simply a scale from zero to 10 with 5
being the control centre position. For attenuverters the control positions are -10 to +10, with zero being centre position for the control knob.
GEMINI 2412 - PATCHING EXAMPLES
10
10
Gate 1
Output 1
10
00
0
3
5
3
8 8
1 0
5
2
00
4
55
510
16’
00 HPLP
CASCADE VCF 2
5
0
00
0 0 0
4 - Cascade Mode - creating a 24dB LP lter by combining 2 x LP lters in series
Creating a 24dB LP lter by combining 2 x LP lters in series
A comparison of GEMINI 2412 12dB and 24dB lter slopes
1V/Oct IN

For this patch we are using the Gemini in DUAL Mode, so the Dual switch is down and the Invert switch is up (o).
In this example we are looking at mixing and ltering two dierent audio sources to a single output. The audio inputs can be
anything - VCO’s, full synth voices, noise or external audio - however they both need to be at Eurorack Modular levels - i.e. 10V
p/p.
The two audio inputs are patched to inputs IN 1 and In 2 respectively, and here we are using the central Mixing bus to cross
fade and combine the two signals into a single audio output, which is fed to the D-OUT jack.
We can cross fade from VCF 1 to VCF 2 either manually with the AUDIO MIX control (when no cable is patched to the MIX CV
input), or we can use a CV voltage of 0 to +5V, patched to the MIX CV jack, to pan between the two lters. The AUDIO MIX
control acts as an attenuator for this control voltage, so simply set it to a position that gives the desired panning range.
We have also shown a dual envelope generator to control the cuto of each lter individually, obviously this is optional and
and is shown as one of many modulation possibilities.
Please note that even with the lter cuto’s set full open and the resonance set to zero there is still some colouration and
distortion to the resulting sound due to the lter circuitry, and in many cases this is desirable, don’t expect this to be a super
low distortion Pro-Audio mixing solution... It is shown as a combined mixing and ltering modular patch which will add some
character too!
Note: The numbers used to illustrate control knob positions do not relate to the markings on the module itself, but are simply a scale from zero to 10 with 5
being the control centre position. For attenuverters the control positions are -10 to +10, with zero being centre position for the control knob.
GEMINI 2412 - PATCHING EXAMPLES
Gate 1AUDIO 1 AUDIO 2
EXTERNAL CV
OUTPUT
0
5
3
8 8
1 0
5
2
55
0
55
510
00 LPLP
DUAL VCF 2
0
5 - DUAL Mode - mixing and ltering two audio input sources
Mixing and ltering two audio input sources

There are presently three dierent types of Eurorack power supplies available - Linear, Switched Mode and Hybrid (Switched Mode
with Linear post regulation).
Linear power supplies, which were used exclusively on vintage analogue synthesisers, are very low noise and very suitable for sensitive
analogue modules such as the Gemini 2412 lter. Likewise, hybrid power supplies such as the Doepfer PSU3 are also very low noise
and equally suitable. Switched Mode supplies are generally usable; however, under certain conditions some switched mode power
supplies can be problematic - this is because of the way in which they work - they switch the power rails at a very high frequency
(typically between 100kHz and 1MHz) and the resultant power line noise noise can be up to 150mV per rail, however it is argued that
this is well above the audio spectrum and is therefore inaudible - but we have found that in some cases (due to load level, asymmetric
loading etc) that heterodyning can bring this down into the audio range, so we may experience up to 150mV of digital hash on the
power rails at audio frequency. The powerline noise rejection of the module itself cannot remove such high levels of noise and it can
therefore nd its way to the outputs of the module itself where it can be heard as low level noise on the output. Thankfully this is a
fairly rare occurance, but if excessive noise is noticed on the GEMINI 2412 outputs then the rst thing to do is try it with a dierent
power supply - ideally a Linear or Hybrid suppy.
Also, there are a few Eurorack modules which are badly behaved and can introduce noise back onto onto the Eurorack power rails too
- the way to nd these is to disconnect all modules from the case, then re-connect modules one at a time while monitoring the output
of the GEMINI 2412 until the culprit is found.
This information relates to analogue Eurorack modules in general - most modern Eurorack modules are digital in nature and much
more tolerant towards power line noise than their sensitive analogue counterparts!
GEMINI 2412 - Specication
Information on suitable Eurorack power supplies:
Dual 12dB / Octave state variable lters, with integrated panning
mixer, Dual Frequency CV source with external input and CV panner,
Audio Inverter and switchable combination modes.
28HP x 3U Eurorack, 141mm x 128.5mm
+12V, -12V and a 10 pin to 16 pin Eurorack power cable is supplied.
Reverse polarity protection is built in.
Power consumption: Positive rail: 50 mA, Negative rail: 45mA
IN 1 & IN 2 - Expected signal level 10V p/p, centred around 0V. Audio
signals only, DC signals are ignored.
All VCF outputs are nominally 10V p/p, noise <35mV
FCV 1 & FCV 2: - The accepted signal range is -5V to +5V. Any voltages
higher or lower than this will be capped to +5 or -5V. Maximum
permissible voltage to this input is +/-12V.
D-FCV: The accepted signal range is -10V to +10v, and the maximum
permissable voltage to this input is -12V to +12V.
RES CV & MIX CV: The accepted signal range is 0V to +5V. Any voltages
higher or lower than this will be capped to 0V or +5V, so negative
voltages are ignored. Maximum permissible v,oltage to this input is
+/-12V.
Description:
Dimensions:
Power usage:
Inputs:
Outputs:
CV Inputs:

Copyright © AJHSynth 2018
A Self Osc 2: Shorting this jumper across both exposed pins connects extra circuitry that forces the
VCF 2 lter core to be self - oscillating at high resonance settings. When the jumper is
only connected to one pin, or removed completely, the lter is in standard vintage
SEM mode and will not go into self oscillation. It is acceptable to have one VCF self
oscillating and the other in vintage SEM mode, or both self oscillating too.
B Self Osc 1: Shorting this jumper across both exposed pins connects extra circuitry that forces the
VCF 1 lter core to be self - oscillating at high resonance settings. When the jumper is only
connected to one pin, or removed completely, the lter is in standard vintage SEM mode
and will not go into self oscillation.
C Freq 2: This trimmer adjusts the freqency centre point for the front panel VCF 2 Frequency
control.
Note: For use by experienced technicians only - Only try small adjustments then test tracking again!
D Freq 1: This trimmer adjusts the freqency centre point for the front panel VCF 1 Frequency
control.
Note: For use by experienced technicians only - Only try small adjustments then test tracking again!
E Scale 2: This trimmer adjusts the 1V/Octave tracking of VCF 2. Turning it anti-clockwise reduces
(shortens) the scaling and turning clockwise increases scaling.
Note: For use by experienced technicians only - Only try small adjustments then test tracking again!
F Scale 1: This trimmer adjusts the 1V/Octave tracking of VCF 1. Turning it anti-clockwise reduces
(shortens) the scaling and turning clockwise increases scaling.
Note: For use by experienced technicians only - Only try small adjustments then test tracking again!
D Freq 1
F Scale 1
The Gemini 2412 module is calibrated after manufacture and under normal circumstances should not require any
user adjustment. Trimmers not documented are for manufacturer’s use only and have tamper paint over the adjuster.
We will not accept any modules returned for repair under warranty because the module has been incorrectly adjusted
by users, to correct this and bring the unit back to full working order full calibration will be required, this is a fairly
lengthy process and in this case will be a chargeable service.
Adjustment and Calibration
C Freq 2
E Scale 2
A Self Osc 2 B Self Osc 1
If you need any help using this module or have any technical questions please feel free to contact us at
[email protected]
Red Stripe aligns
with -12V as shown
Power Cable
Important note: