The 3 LFOs submodules use the exact same circuit. Due to my insistance in not employing any specialty chips, I had no hope I could ever fit all the 3 circuits on the main PCB, since all the jacks ocupy so much space already. The solution: 3x identical submodule PCBs!
They make the module 10cm deep in total, which maybe is a lot… I’d be curious to hear if someone finds this a dealbreaker. My rack is the classic one from LMNC’s YT tutorial, so it has more than enough depth to accomodate this.
The circuitry is derived from the Befaco “Slew” (!) module. I wasn’t very happy with the performance of some VCLFO examples I tried on the breadboard. Then I noticed this in Befaco’s schematic when I was researching ideas for my VCSL module:
It turns out this 4 transistor core thingy (exponential integrator) can be useful when making a lot of synth circuits. Too bad I didn’t run into this concept while designing my VC ADSR!
I adapted Befaco’s circuit heavily and stripped all parts that make it a slew limiter. The result is a VCLFO can also do VCO duties very well, with good V/Oct and temperature compensation.
The CV Amount knobs even serve as “Fine tune” when nothing is connected to the CV inputs. There’s also a hard sync input and switches to select between 3 frequency ranges: fast, super fast (audio rate), and very slow. It can also listen to pitch CV coming from the power distribution bus on the back, which saves on a lot of cables when using it as a tripple VCO.
Hmm, I wonder why they drew the transistors QQ2 and QQ1 that way. It seems to me that these are 2 pairs of transistors (one NPN and one PNP) in parallel. When drawing the left bottom transistor on the RHS and the right bottom transistor on the LHS this becomes clear. Is this some sort of controlable current source (the schematic is not complete, so I’m not sure)?
Can you provide a reference to the full schematic?
When I have uninterrupted time, I do tests and breadboarding… when I’m physically tired, I design the PCBs on the sofa… When I’m mentally tired I just design front panels for future projects
It seems they also had an older Slew module that’s not being made anymore, but the schematics are still there:
In this older version you see they positioned the transistors in a different way, but it’s still the same principle.
This looks to me like a linear-to-exponential converter that can both source AND sink current, hence you can control the angle of both the rising and falling slopes of the integrator with independent CV for each. The +10V reference and the 100k to 1k8 voltage divider (resulting in 18mV at the base) hints at making this a 1V/Oct compatible linear-to-exponential converter, although you would need a trimpot and thermal compensation to make it track well. That’s what I did to adapt this into a VC(LF)O, among other things.
I work in Eurorack dimensions but can have 12cm deep modules I think the deepest module I’ve made is about 10cm because I just slap things together without having to worry about dainty skiffs!
