Console Project, Part Two: Fader Card-1

I got some faders! More specifically, I got the Bourns PTF01-152A-103B2 model. Basically it's 10kOhm, 100mm travel, linear, AND, most importantly, DUAL GANG. I only got 8 because yikes these things are expensive but also, I'd like to multiplex these in banks of 8 anyway so this will work fine as a starting point.

But Matt!, I hear you say, you SPECIFICALLY said you wanted motorized faders in Part One. Those are NOT motorized!!!

Fair point, imaginary reader, they are not. But, borrowing heavily from the Behringer X32 motor fader design, (and having a boatload of brackets cut on a waterjet...more on that in the future) this SHOULDn't be that hard of a conversion. These were ~$8 a piece and I expect them to cost less than $20 to fully transform.

Why on earth then, would I not buy the pack of 5 pre-made X32 motor faders available on Amazon PRIME of all places? It's $100! That's $20/piece and it's pre-made!

Simple, actually. In the first paragraph, I stated that these were, most importantly, dual gang. Meaning there are two discreet potentiometers in each unit controlled by one fader head. The X32, as best as I can tell, has single gang. If someone else knows better, though, please share. I just realized the X32 fader has 8 pins so I've now become fairly un-confident about that conclusion.

Anyway, next will come designing the fader card. I'll have to make a custom footprint for the faders but everything else is pretty much stock standard so that's cool. This card is completely passive. No onboard intelligence, no power...nothing but interactive components and headers. A proper intelligence board will be designed later.

Here's a pretty general schematic:

And while reviewing this schematic, I actually came up with a pretty glaring problem with my concept as a whole. Pan control. For now, controlling it with this knob in logic is totally fine but, once I add analog channel strips, this becomes problematic. Ideally, each channel strip is mono so if I wanted to pan in logic, I'd have to place each track on separate console channels just to maintain the stereo image. So, what I'll probably do is add a pan disable switch on the intelligence board so that, when analog strips are added, I can turn off the ability for the Fader Deck pan knobs to do anything and make sure every Logic channel is panned center. Then, I can add an analog pan to each channel and THAT stereo image can be sent to the bus section. 

And just as a fun note: I chose a linear fader because that will make the programmers happy when reading its position. BUT, I want to pipe audio through it too and you need a logarithmic or 'audio' taper potentiometer for that. So, to convert linear pots to log pots, place a resistor between the wiper and ground that is between 10% and 20% the value of the potentiometer. Super cool process. Here's an article over it: Secret Life of Pots 

Part One: Console Project, Part One