Mosha Eurorack Modules #6: VCF

The VCF is a 6 HP module that has an LM13700 based filter using a mixture of designs by Look Mum No Computer, Réne Schmitz and Moritz Klein. It is nearly 1V/oct, but not quite, but the voltage control circuit is simpler than most. There is a -12V to 12V offset that can be added manually to the CV input, and a separate knob for resonance. There are two inputs. The entire VCF part is using the gold colors, the purple colors are a separate filter that isn't voltage controlled, but uses a two-colored LED for a more gritty response in the feedback loop. It also uses 10nF instead of 1nF capacitors.

A full circuit diagram is included in this blog post, however, this may not exactly match the built version and there may be errors in it. Although multiple circuits suggest to use the buffers internal to the LM13700 there is some trouble making sure that they are offset correctly, and it is therefore easier to use the TL074. This also provides a cleaner sound. 

Mosha Eurorack Modules #5: VCO

 

The VCO is a 6 HP module that has a CEM3340 based design. The first input is the linear FM input. The gold input is the 1V/oct (which can be controlled with the golden knob). The third input is the soft sync. The purple input is the PWM modulation, which can be attenuated with the bottom knob and separately controlled with the middle knob. From top to bottom the outputs are the sawtooth wave, the pulse wave and the triangle wave. The sawtooth wave is at 10Vpp the other waves are louder and softer (they are buffered, but not amplified copies from the original CEM3340 output).

The design of this is a mix. It uses the basic and advanced schematics from Look Mum No Computer, the variety of schematics on the electric druid site, and the schematic from Non-Linear-Circuits.

As you can see, there is only one input (which may have been a mistake) there is no high frequency tracking. When in doubt, the design choses the simplest solution, however, the FM input and sync input were kept.

Mosha Eurorack Modules #4: Double VCA

 

The double VCA is a 6 HP module that has two VCA circuits and a mixing circuit. The two VCA (gold and purple) are identical and organized vertically. The VCA level is only controlled by CV, the knobs are part of the mixer that mixes the VCA with a third input.

The circuit is based on the vintage VCA of electric druid. It uses the "more practical current source" as described there, since no mixing of current sources is needed.

The mixer is a simplified version of the Doepfer A-100 do-it-yourself page. It doesn't have the part that inverts it back to normal and it doesn't have an offset voltage. In fact, it doesn't even have a knob to control the volume of the third input.

Mosha Eurorack Modules #3: Attack/Release and Decay

 The Attack/Release and Decay is a 6 HP module, that, as the name implies, as an Attack/Release section and a Decay section. The output voltage is 8V to 0V. The Decay (in purple) accepts either triggers or gates as inputs, the Attack/Release (in gold) section requires a gate input.

There are are knobs to control the Attack, Release and Decay, and two red LED indicating the current voltage on each. There is also a single CV input for the Attack/Release section. A positive voltage will affect the Attack (the higher the voltage, the shorter the attack) and a negative voltage will affect the Release (the lower the voltage, the shorter the release). At 0V the CV doesn't influence the outcome.

The circuit diagrams shown are mostly used for the simulations, the actual diagrams are slightly more complicated. However, they give an idea on how things work. The Decay takes the gate input which is low-pass filtered by a large capacitor and a potentiometer. A diode prevents it from triggering negative voltages and the output is buffered. There is a second buffer for the LED.

The Attack/Release circuit diagram is even less complete, as it doesn't show how the potentiometers are individually connected to the outputs of the microcontroller in order for them to be controlled separately. 

In order to ensure that there is a clean input for both the Decay and the Attack/Release there is a PIC10F206 microcontroller that accepts the messy input signal and cleans it up. For the Decay this means converting a short trigger into a longer gate output and ensuring that the capacitor is discharged quickly (over a 100 Ohm resistor) when the signal needs to be reset. The source code is here for the Decay and here for the Attack/Release.

The reason for a micro-controller solution was to have more control over the outputs and how they work. However, a solution using an OPAMP in comparator mode would work equally well (or even better, as it would allow higher voltages without amplification, which could be done using the buffer. The CV control was done using a light sensitive resistor and an LED, with the LED being hooked up to the CV. By having one LED forward and one LED backward it was possible to control both with a single jack.

Mosha Eurorack Modules #2: CV Sequencer

 

The CV Sequencer is a 6 HP module that, as the name implies, sequences control voltages in the range of 0V to 10V in either 15 or 16 steps. The switch has three settings: 15 step playback (top), 16 step playback (middle) or record (bottom). Record mode can also be activated using a CV in the top right jack, so it can modify its own content.

The yellow button can be used to step through, and the jack next to it allows an external clock signal as well. The purple jack is the actual CV that is recorded, but a manual voltage can be supplied with the dial.

Outputs are hexagonal, and the purple output is the actual CV output. The jack next to it is the gate output, which normally should follow the clock. Below are the two indicator LED. The right indicator cycles through 8 different colors for 8 of the 16 steps, the magenta LED on the left has two purposes: it both tracks the other 8 steps, and the fact that the module is in record mode.

The sequencer is built around the PIC16F684 microcontroller with the following pin configuration:

Analog input: voltage to be sampled (RA0, pin 13)
Digital input: clock (RA2, pin 11)
Digital input: write (RA3, pin 4)
PWM output: voltage result (RC5, pin 5)
Gate out (RA1, pin 12)
LED out (RC0, RC1, RC2, RC3, pin 10, 9, 8, 7)
Rhythm select in (RC4, pin 6)
20 MHz crystal: RA5 (pin 2) RA4 (pin 3)

The inputs are merely protected by resistors, there are no protection diodes. The ones inside the PIC16F684 work well enough. The analog input has a jumper that allows halving the voltage using a 1% resistor ladder and buffered to ensure that the impedance is below 10k. 

The PWM output goes through a two-pole RC (100k, 4.7nF) filter network with buffer where the second stage of the buffer is shown in the circuit below. It has a trim pot at the bottom of the module that allows the output to be amplified by 1x to 2.1x. As usual, all OPAMP are the TL074, not the TL081.

The jumper and the trimpot together allow the sequencer to work with a highly accurate 0-5V or a slightly less accurate 0-10V.

The code for the sequencer is available here. The analog imput is sampled multiple times and averaged, to ensure that the lower bits of the value are accurate as well.

Mosha Eurorack modules #1: Triple LFO

The triple LFO is a 6 HP module that, as the name implies, as three LFO. 

• The silver LFO has a -5V to 5V triangle output, a -5V to 5V square wave output and a 0V to 5V square wave output and has a medium speed (~2s to 0.2s).
• The purple LFO only has a -5V to 5V triangle output and a slow speed (~20s to 2s).
• The golden LFO has a -5V to 5V triangle output that can be attenuated with the second knob, and a 0V to 5V triangle output. Each of the LFO has an LED indicator that switches between red and green.

The design is based on the Simple LFO design from David Haillant. Modifications include having three different capacitor values, the replacement of the lower bound resistor with a 1.5k instead of a 470 Ohm resistor and having a fixed output path. What wasn't fixed is the missing 1k Ohm resistor at the output, so shorting the LFO can cause it to reset. For the 0V to 5V square wave a diode was added and a pulldown resistor.

For the 0V to 5V triangle wave the circuit below was added, which basically is an inverting amplifier that adds approximately -5V to the output of the LFO. This causes the LFO to fluctuate between -10V and 0V. The amplifier divides this by 2 and inverts it, resulting in 0V to 5V. Although the circuit mentions the TL081 the entire LFO was built using TL074.

Building my own Eurorack module

 For my birthday I got a NiftyCase with various modules, and I also bought a Mr Phil Ter and a Monsoon for it. However, it lacked envelopes. I started looking for possible solutions, On my youtube channel you can see a bit more about the synthesizer, here I will describe the design and making of the module. The basic circuit I came up with is as follows:

There's two inputs and an output, one of the inputs is the gate signal, which starts the decay, and the other is the control voltage, which uses and LED to control a photoresistor which is in parallel with a potentiometer, both of which control the length of he decay. The output is multiplied by a certain amount (currently 1.55x, but I may go back to the idea of multiplying by 2x as in the circuit above.

The 85k resistor was changed to 1M, and I added a LED and 470 resistor to show the decay working after the OPAMP (an TL072). This circuit worked, but unfortunately most gate signals are terrible, so I had to add a special PIC10F206 circuit to clean that up. This circuit also discharges the capacitor quickly between uses, avoiding a wait time that was necessary otherwise.

The only improvement still needed is handling of triggers: if the gate signal is too short I want to change the behavior into a release instead of a decay, possibly with a check that the output signal has grown small enough.

Here is what it looks like on the outside, which required some drilling, which was definitely a new experience, and quite scary.

By now I learned that the CEO of the company that makes these specific panels isn't a very nice person, so I may have to reconsider my purchasing strategy here.