Hi I've owned mother 32(sold it now) and had some experience with 'traditional' pianos
I've been trying to get to know more about modular synth and found out about niftykeyz
Since I have some experience with keyboard styled instruments, I thought it will be a good idea to start with that
My objective for this build is to make a 4 voice synth (think of moog's grandmother with 4 voice)
I own a beatstep pro, so I don't think I'll need a different clock source or sequencer
Honestly, I don't think this system would be all that much fun to play with - you barely have enough modulation sources to control your STOs' volume, leaving you with one mod source (either the A-140 or one of maths' function generators) and whatever the NK provides for actual modulation of all the possible places you could modulate - the STO inputs, the BIA's and AI's inputs, the filters' frequency and the squawk dirty's other inputs. At this point, once you've gotten a fun patch going, you have zero modulation sources left to actually do anything with that morphagene.
Personally, I'd probably scratch the idea of going for four voices entirely and build for one or two voices and a lot more modulation (and attenuators, VCAs and all the other utilities that come with that :P). If you do want multiple voices, it's probably better to get a standalone synth (such as the dreadbox nymphes) and, if the synth doesn't have one, a MIDI keyboard. You'll have a much more balanced and affordable system.
If you insist on making a polyphonic eurorack, I'd go for voices with a lot smaller foot print - BoBeats' test setup (seen at about two-three minutes in and onwards - the embed doesn't care for the timestamp apparently), for instance, used two Winterbloom Castor&Polluxes and a tiny VCA. I'll try and reconstruct his setup as a modulargrid link later, if I remember to.
Edit: I've reconstructed BoBeats' setup now:
Note that this is still just a very simple four-voice synth without lots of options in terms of modulation or patching, which at least in my book wouldn't justify the (roughly) 2.5k€ price tag when something like the aforementioned dreadbox nymphes does basically the same stuff for less than a third of that.
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For modulation, I like Batumi quad LFO, Quadrax, and Maestro. Those all have lots of modulation features to spice up patches and for me, I burn through modulation like crazy. Take Noise Engineering modules for instance. Noise Engineering modules beg to be modulated with many points for modulation sources. If you have just one, that easily takes up four modulation patch points.
I got my NiftyKeyz recently and I love it. I'm just starting on this journey too, having bought a Minibrute 2S a few months earlier. Here's my rig:
I'm in the process of ordering he five modules to the right of the Dreadbox Nostalgia - hopefully it'll get here by next week.
Having the delay and phaser really help add depth to the sounds. I haven't figured everything out yet, but using the arp on the NiftyKeyz with the delay, I've gotten some pretty cool sounds out of it. I'm looking forward to having three voices when the Castor & Pollux gets here! I haven't even tried linking it up with the Minibrute yet. It's a good thing the Steppy has four separate outputs. Don't forget that you have an additional modulation source from the Niftykeyz itself - it's built-in LFO, controlled from the modwheel.