This is an ongoing post while I develop the “No Knobs” Eurorack module, one of the modules included in New DIY Eurorack modules in design phase.
The objective
The main objective of this design is to create a new Eurorack module, which is a compact, stable VCO with enough functionality to make it worthwhile. The original aim was to have it work in a 2hp Eurorack format, but that’s been ditched for 3hp.
Inputs: I wanted a minimum of 1V/Oct, linear FM, sync, and PWM inputs.
Outputs: As a minimum, I wanted triangle, saw, square, and pulse wave outputs.
I’d considered a sine output too, but as one of the aims of this module is to keep it compact, I thought I’d give it a miss this time.
All of the above can be achieved using the Alfa AS3340 VCO chip, which is a modern remake of the widely used CEM3340 IC. Using this IC also gives scope for having extremely good temperature compensation, 1V/Oct tracking, and simple tuning.
Why “No Knobs” are used in the design?
I’m pretty new to Eurorack, synths, and music production, but one thing that has caught my eye has been generative patching. Generative patching, being the method of altering sounds through varying control voltages, with little manual input.
A compact VCO with plenty of CV inputs, and no way of manually altering the pitch/PWM levls simply seemed like a logical idea. So due to the lack of knobs to wiggle, it was christened “No Knobs”.
Why 3hp, and not 2hp?
I had initially aimed for 2hp, but after a bit of research, decided against it.
The design of a 2hp or 3hp module with so many feaures requires quite a lot of PCB space, which means that the PCB has to be mounted perpendicular to the front panel, and not parallel like you see in larger modules. By having a perpendicular board, the height of the board and components combined becomes a bottleneck.
Most components when mounted on the PCB would fit within the 2hp’s 10mm width, but some of them are really pushing the limit.
ICs mounted in sockets would be grazing the neighbouring Eurorack module to start with. This wouldn’t be much of an issue if the IC was to be mounted directly onto the PCB, but the idea of an IC socket is to allow for ICs to be easily swapped out. I’ve recently had a bad batch of LM13700N chips, so I’ve seen how frequently ICs can be swapped out, and I really don’t like the idea of desoldering 8, 14, or 18 pins.
The other main component which would cause bother at 2hp is the power header. I know the company 2hp utilise a single row of 5 pins, rather than 2 rows, mounted at a right angle, to achieve fitting a power cable to the board, but this just felt really flimsy. I’ve instead opted for a right angle, shrouded 10 pin socket. By using a shrouded socket, it should mean that I don’t need protective diodes on the power rails, as it should be impossible to plug in the power ribbon the wrong way round.
There’s also the ergonomic factor. 3hp isn’t much wider than 2hp, but the possibility to get your fingers in to patch the module is pretty important. Eurorack in general is pretty small, and 3hp is still pretty tight, but with any luck a lot more manageable.
For the above reasons, I’ve gone for 3hp, and not 2hp!
Front panel and component layout
I’ve spread the inputs out while giving a slither of extra room at the top and bottom of the panel, to allow anything like logo and module name to go. I’ve used the planned jack locations as the starting point for the PCB design.
Electrical design
My design is based on the AS3340 IC, the circuitry provided in the datasheet, circuitry specified by Rob Hordijk related to easy VCO tuning (https://cabintechglobal.com/tune3340), and some other general circuitry.
Normalising wave amplitudes
I’ve been aiming to normalise the output amplitudes from this design. The saw output has an amplitude of 2/3rd the input voltage (+12V), whereas the triangle has an amplitude of 1/3rd, and the pulse has an amplitude of 12V.
Bu using a TL074 opamp, I’ve increased the amplitude of the triangle output by a gain of 2, making it match the saw. That’s used 1/4 opamps on the IC.
To achieve a square wave, I’ve used a 1/3rd voltage reference and the saw wave form with a comparator, to get a 50% duty cycle. I’ve then used a second opamp to reduce the 12V square to match the saw and triangle waveforms. That’s opamps 2/4 and 3/4 now used on the IC.
And as I’m using the saw output as a reference for the comparator, I wanted to buffer the saw output to avoid any issues, meaning that’s 4/4 opamps used.
As I’ve not got any opamps left on the TL074, and very little space left, the pulse output has been left at 12V.
If the opamp to buffer the saw wave isn’t needed, I could reuse it to bring down the saw wave to match the rest of the outputs. That’ll be a future release though.
1/3rd voltage reference
I’ve been pretty undecided here. One option I seriously considered was to use the voltage reference from pin 6 of the AS3340 IC, and just buffer it, so it could be used throughout the circuit.
Another option, which I think I’ll end up settling with, is to simply use a voltage divider off of the +12V power rail to ground. I’m going to use 3x 100k resistors, with a reference taken between the middle resistor and that closest to ground.
Tuning
The addition of 4 trimmers, a couple of test points, and a shortable pin header has made life a bit more straightforward when it comes to tuning the VCO. This method is thanks to Rob Hordijk and the tuning method he published.
The extra components needed does indeed make tuning easier, but makes the PCB layout a little trickier.
Some of the values specified in the circuit don’t seem quite right to me. The 1M8 resistor in the reference pitch circuit, for example, is quite a bit bigger than the 1M5 resistor in the same location found in other classic synths, according to an Electric Druid analysis, although the Sequential Profit synths used a 2M2 resistor instead.
This is a point in need to exploration when my next batch of prototype PCBs arrive. I have already considered that the resistor value won’t be an on hand value, so I’ve built in space for 3 series resistors, with the view of using a 1M, 470k, and then whichever other smaller resistor needed to get the reference pitch near to where it needs to be with the adjacent trimmer.
An update will follow when I’ve decided on arrive resistor values…