Audiobus: Use your music apps together.
What is Audiobus? — Audiobus is an award-winning music app for iPhone and iPad which lets you use your other music apps together. Chain effects on your favourite synth, run the output of apps or Audio Units into an app like GarageBand or Loopy, or select a different audio interface output for each app. Route MIDI between apps — drive a synth from a MIDI sequencer, or add an arpeggiator to your MIDI keyboard — or sync with your external MIDI gear. And control your entire setup from a MIDI controller.Download on the App Store
Audiobus is the app that makes the rest of your setup better.
Cheap Synth as a Midi Controller?
My goal was to buy this cheap synth to use it as a midi controller, but (why, I don't know), the knobs are not sending Midi Out to control VST or iOS app.
The synth cost less than a similar midi controller.
My question: Is there a cheap synth that you can use as a midi controller?
Not really, synths are not designed to be midi controllers. Every synth uses it own midi cc's, if they support sending out midi cc at all. Most don't and the ccs are pretty much never editable (neither for input or output).
If you want a midi controller, just buy one in approximate range.
There are lots of options like midi twistor if you want knobs or 16n faders if you like faders.
I was considering exactly the same to control a REPRO-5!!!
Then I've set up the bill of components and work required to do that insane soldering and wire cutting (or switching) work and I decided to not give it a go.
Too much work, not to speak of additional tweaks like linearizing pots etc.
A better option might be to use a camera and image processing to detect knob and switch positions but that won't work live as long as the fingers cover the control.
My workaround so far has been a Windows tablet with REPRO running on it. Limited polyphony but it works.
The Arturia MicroFreak can be used as a Midi controller. Problem is that its controls do not really match perfectly to a virtual analog synth but it works. Much better is the DIY Model D controller.
I still have 4 sets of the PCBs that I could sell because 5 sets was the minimum order quantity at the PCB service in China. I would sell a set for $10 plus shipping from Germany. Assembling it is not very difficult if you have some soldering experience. The most difficult is to obtain all the parts which cost less than $100.
Artura microfreak, novation bass station 2 (has some NRPN options) but both 300/400 euro range new.