****UPDATE**** SIMPLIFIED SCHEMATICS FOR MAKING THE STRING SYNTH

Below is a simplified schematic for the string synth...  Sorry it's not the best quality I'll try and take a better picture later, but hopefully this should be of help those of you new to schematics that want to try and build this synth. 

The connections where it says 'goes to keyboard', you can see elsewhere in this blog for another detailed diagram of where and how these can be wired up to the keys.   As for the potentiometers, I *think* I've drawn them the correct way (if you make this and the pots are working the opposite as you'd expect, swap around the ground and 5v connections on each pot).  If the drawing is incorrect, let me know and I'll re-do it. Even if this version is wrong, nothing will break & it'll still work OK (just the pots will work in reverse). 

You can see where the pots 5volt and ground connections need to go in the diagram, I didn't join them in the picture because the wires would start overlapping and become confusing.  It's pretty obvious what to do though.

Also drawn is the audio out headphone socket, with the positive voltage/audio signal coming from the output D11, which then goes through the resister/capacitor (which acts as a filter to reduce some of the noise from the Arduino).  The other side of the audio goes to the ground signal.  If you don't know how to wire up a audio jack, maybe google 'audio jack pin-out' or 'audio socket wiring diagram'.

If you plan on adding the analogue filter, you'd leave off the audio jack and have the previously mention positive wire go into the input of the filter.  You'll have to learn to read the schematic for that I'm afraid, as I couldn't really simplify it without it becoming more complicated than a normal schematic.  Give me a shout if you get stuck though.

Good luck! 

P.S

I'll draw a simplified schematic for the Arduino drum machine soon.

****UPDATE PART 2******

Below is a wiring diagram to add a bypass switch to the filter;

...And if you were struggling to figure out how to wire up the Pots for the filter, I've drawn out a simple picture showing how (I've had to do this whilst away traveling, so it's partly drawn from memory but I'm pretty certain it's correct)...

Obviously the drawing only shows how the pots are wired and not the full schematic.  Go look at the proper schematic for the rest of the plans, but this will give you a good idea of how to wire up the pots.

Powering the filter from a 9v Battery will give it it a bit more 'oomph', so if you'd like to do that, the below illustration will give you an idea.  Further, should you want an LED indicator showing if the filter is switched on or not, see the chequered box;

I only had SPDT switches available when I built the synth, so that's why there's separate switches for both the on/off & bypass, but you could easily just use one 2PDT to save space.  You could even add an audio in jack at the start of the filter to process other audio.

Good luck!