Discrete OTA VCF User manual. January 5, 2016 by Rutger Vlek

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1 Discrete OTA VCF User manual January 5, 2016 by Rutger Vlek Disclaimer When you buy a Eurorack module, you buy a part of a bigger and highly flexible system. River Creative Technology modules have been designed with the utmost care for your safety, as well as the safety of the module itself and the other parts of your Eurorack system. However, it is important to realize that I cannot foresee all potential risks and wrongdoings related to operation in your specific system. In the case of DIY products, I attempt to provide you with the information to safely and successfully construct your own modules, but still assumes you have sufficient skills in handling potentially lethal voltages and the required construction tools safely. Therefore, River Creative Technology cannot be held responsible for any harm to you/others, or to your/their equipment (including electric shocks, fire, damage, malfunctions and accidents involving soldering or mechanical assembly). I hope you understand. By buying a River product, I assume you are aware of this. 1 Features The Nucleus is a voltage controlled low-pass filter in a compact but highly versatile 8HP Eurorack package. The filter core is built around discrete transistor OTAs, as is the resonance VCA (allowing for CV controlled resonance via RES input; with CVs ranging from 0 to +5V). It provides switchable 12 and 24 db/oct filter slopes, and three distinct resonance character modes. The threeway resonance character switch allows you to select either: K-style, Neutral and Q-comp. To be able to hear the difference between these modes, the filter needs to resonate a bit (so turn up the resonance knob). 1

2 Neutral Classic negative-feedback resonance, reminiscent of many vintage 4- pole designs. Capable of stable self-oscillation. Inherently with a loss of low-end at high resonance. Q-comp Similar to the neutral mode, but compensated for loss of low-end by injecting more input signal at higher resonance levels. This provides a firm and stable resonant sound, with slightly different saturation character. K-style The beefiest of all modes, with a thick mid range and fat low-end, and a more aggressive resonant sound going way beyond self-oscillation. I leave it up to you to guess what K stands for... In each of these modes, the low-frequency resonance behaviour can be further tailored with the Solid bass feature: Solid bass When enabled, the solid bass feature gently rolls off the intensity of the resonance peak towards the lower frequencies (independent of the setting of the resonance control). This keeps the bass cleaner and tighter, while still allowing the mid and upper frequencies to boldly resonate (very good for punchy percussive sounds and basses). It combines well with all three resonance character modes, but with different results for each mode. The Nucleus is capable of self-oscillation (and beyond) and features a gently saturating input stage when higher signal levels are used. Envelope (unipolar: for signals between 0 to +5V) and FM (bipolar: for signals between -5V to +5V) modulation options are provided, each with their own attenuvertor, and with a switchable linear or exponential FM response. A 1V/Oct CV input allows for keyboard tracking duties (with CVs ranging from 0 to +10V, and normalized to +5V when unused), as well as serving as a generic extra CV input. 2

3 Nucleus FREQUENCY Cutoff frequency control K-STYLE Resonance character mode Toggle solid bass resonance behaviour NEUTRAL RESONANCE Q-COMP SOLID BASS Resonance control 0 EXP Attenuvertor for envelope modulation - LIN db/oct 24 Envelope modulation CV input (unipolar) Toggle VCF slope between 24 (4-pole) and 12 (2-pole) db/oct Toggle between exponential or linear cutoff frequency modulation 0 12 IN V/OCT Attenuvertor for frequency modulation depth Frequency modulation CV input (bipolar) RES VCF audio input VCF audio output Tracking 1V/Oct CV input River Resonance CV input Creative technology Figure 1: Nucleus front panel 2 Technical specifications Dimensions width: 8HP (40.34mm) height: 3HE height(128.5mm) depth: 25 mm (skiff friendly) weight: 72 g Power requirements +12V -12V +5V not used Built-in protection against reverse voltage on -12V and +12V lines. Power connector: 5x2 standard Doepfer-style. Pin 1 is -12V (red stripe, 3

4 marked on PCB). +12V GND GND GND -12V x2 male header (top view) Figure 2: Power connector with ribbon 3 Connecting the module Eurorack is a great format, but has one flaw. The power connection standards are prone to confusion. When you connect the module, make sure you line up the power cable on your power supply or bus board with the red stripe pointing to the -12V line, and the same way on the module. This River module is protected against damage from reversed power connections, but I cannot take responsibility for how the rest of your system might handle such an incident. 4 Calibration The Nucleus requires calibration for optimal operation. This needs to be done once for each new Nucleus, and occasionally as time passes (every year or so). Incidentally, you may need to adjust the Nucleus calibration to other changes in your system, such as a new power supply or a different MIDI to CV convertor, or when you put it in a very different environment (e.g. higher room temperature). If you have ordered a fully assembled unit, this has been done for you in the testing and calibration process. However, the calibration may still be improved as the circumstances in the test-environment may have been slightly different from your Nucleus own habitat. If you have assembled a Nucleus yourself, congratulations! And don t worry, calibration is really not difficult to do! First, you need to make sure the Nucleus is set up to be calibrated. Set the switches and knobs to the following settings: FREQUENCY : half way RESONANCE : fully open (clock-wise) SOLID BASS : off (button out) Resonance character : Neutral 4

5 db/oct : 24dB (button out) FM EXP/LIN : EXP ENV attenuvertor : 0 FM attenuvertor : 0 Please disconnect all cables from the sockets, except for two. You ll need to be able to hear the output of the Nucleus, so patch a cable from the Nucleus OUT into whatever module feeds to your speakers or headphones. You ll also need a connection from your keyboard CV output or MIDI-to-CV convertor output (the same line that drives your VCO s pitch) to the V/OCT input on the Nucleus. Since you have (hopefully) set the resonance fully open, the filter is now self-oscillating and producing a sine wave (more or less) by itself. You should now be able to play this tone from the keyboard as if the Nucleus were a VCO. The goal is to calibrate the Nucleus such that this tone tracks the notes you play on the keyboard as well as possible. To judge this, you need a reference. You can use a properly tuned VCO or another instrument for this, or leave it up to an automatic tuner (hardware or software) to judge. You need to unmount the module from your rack, to be able to access the trimmers on the back of the circuit board. These trimmers are labeled V/OCT and TUNE. They are of the multi-turn type (approx. 25 rotations from minimum to maximum), and can be adjusted with a small screw driver. As you ll do this procedure while the module is powered, you ll have to be very careful not to touch other parts of the circuit with the conductive tip of your screw driver. The V/OCT trimmer determines the scaling of the notes over the keyboard, meaning that by turning it you can stretch and shrink the interval between two notes. The TUNE trimmer changes the global tuning of the filter up or down. While it seems tempting to start turning the TUNE and get a the A on a keyboard sound like an A, this is actually the last step of the process. The first goal is to get the scaling right (independent of being globally in tune, with respect to A ). I usually do this by playing alternating octaves in the mid range of the keyboard and turning the V/OCT until the interval sounds like an octave. Then I start turning the TUNE trimmer until an A on the keyboard sounds like the pitch of an A from my reference oscillator. Getting the scaling right with a digital tuner is also nice, when playing the alternating octaves, because you can simply aim for double the frequency values you read out between the two notes of the octave. As most experienced analog synth users can tell you, calibration takes a bit of time, and going back and forth between adjusting the V/OCT and TUNE trimmers. Also remember that you will never be able to achieve perfect tracking. The Nucleus was not designed for super-accurate tracking, and not intended to be used as a precision oscillator either (there are special modules for those tasks). It s normal that scaling becomes worse towards the highest octaves, no matter how well you ve calibrated. For most filter applications, getting the tuning in the right ball park is all that s needed. 5

6 5 Warranty Warranty is governed by Dutch law for all River Creative Technology products. This warranty covers any defect in the manufacturing of this product for one year following the date of purchase. This warranty does not cover any damage or malfunction caused by incorrect use - such as, but not limited to, power cables connected backwards, excessive voltage levels, or exposure to extreme temperature, moisture levels or chemical substances. The warranty covers replacement or repair, as decided by River Creative Technology. Please contact our customer service (info@riversynths.com) for a return authorization before sending the module. The cost of sending a module back for servicing is paid for by the customer, unless Dutch warranty law prescribes otherwise. Modifications to any product, or repair attempts to a pre-assembled product will void warranty. 6