I was wondering about expression pedals as well, here is some info that Arturia support provided:
The less the resistor value will be the less the efficient range will be (which may let a deadzone at the end of the pedal) but will react in a more linear way.
- Pedals under 10K would be better to avoid to prevent any potential difficulties to reach the maximum 127 value.
- 10K pedals will be working fine
- 25 K should be also another good compromise between linearity and efficient range as it only let around 7% of deadzone at the end with a correct linearity.
We would though also recommend avoiding pedals over 25K.
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So I found a pedal maker called mission engineering who build high quality and versatile expression pedals, some with multiple outs, and the "aero" shape is a really cool design.
https://missionengineering.com/product-category/products/expression/The Arturia support said that they often use the M-Audio EX-P themselves in house.
https://www.sweetwater.com/store/detail/PedalExp--m-audio-ex-p-expression-pedal