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AJH Synth Gemini 2412 User Manual

Dual vintage state variable filters
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2412
User Guide
Dual Vintage style State Variable Filters
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.
www.ajhsynth.com

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Summary of Contents for AJH Synth Gemini 2412

  • Page 1 User Guide Dual Vintage style State Variable Filters 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.
  • Page 2 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)

  • Page 3: Front Panel Controls

    Front panel controls VCF 1 VCF 2 VCF 1 Resonance VCF 2 Frequency Vary 1 Control VCF 2 Mode Freq CV 1 Level INPUT 2 Level VCF 1 Frequency VCF 2 Resonance VCF 1 Mode Vary 2 Control INPUT 1 Level Freq CV 2 Level VCF 1 Output VCF 2 FCV IN...
  • Page 4 Description of individual lter controls VCF1 and VCF 2 are identical and independent, these labels apply to both. Manually sets the amount of Resonance (feedback) through the lter core. When a patch 1 VCF Resonance : cable is inserted into the RES CV jack (8) then this control acts as an attenuator for the incoming RES CV level.
  • Page 5 Description of individual lter controls (cont.) VCF1 and VCF 2 are identical and independent, these labels apply to both. This is a frequency cuto modulation input with a xed control level corresponding to 1V/Oct INPUT : one volt per octave. It can be useful to patch a pitch cv to this input so that the lter automatically “opens”...
  • Page 6 Description of Mixing Bus controls Controls mixing and con guration of the two individual lters The Audio Mix control cross-fades between the outputs of VCF 1 and VCF 2. It’s exact 15 Audio Mix : 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.

  • Page 7: Dual Output

    Description of Mixing Bus controls (cont.) 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.
  • Page 8 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.
  • Page 9 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.
  • Page 10 1 - Con guration as two independent VCF’s 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.
  • Page 11 GEMINI 2412 - PATCHING EXAMPLES 2 - Dual Parallel VCF’s Inverting Mode, for Band Pass Vocal sounds In this patch we are using the Gemini in Dual Inverting Mode, so the Dual and Invert switches are both in the down position.
  • Page 12 GEMINI 2412 - PATCHING EXAMPLES 3 - Cascade Mode - creating a 12dB BP lter by combining LP and HP lters In this patch we are using the Gemini in Cascade Mode, so the Dual/Cascade and Invert switches are both in the up position.
  • Page 13 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.
  • Page 14 GEMINI 2412 - PATCHING EXAMPLES 5 - DUAL Mode - mixing and ltering two audio input sources For this patch we are using the Gemini in DUAL Mode, so the Dual switch is down and the Invert switch is up (o ).
  • Page 15 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.

  • Page 16: Adjustment And Calibration

    Adjustment and Calibration Important note: 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.