AJH Synth Gemini 2412 Manuel utilisateur

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

VCF 1 Mode

BP Out

HP Out

LP Out

Out 1

Vary Pot

VCF 1 Resonance

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

Audio Mix

Dual/Cascade

D-FCV Level

D-FCV Mix

Invert Switch

D-FCV Input

Dual Output

MIX CV Input

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

00 LPLP

DUAL VCF 2

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.

1V/Oct IN 1

Gate 1

Output 1

Output 2

1V/Oct IN 2

8 8

0 0

+4 +6

3 3

8’

00 VAR

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 0 0

8’

0 0

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