Amplexus

if you use seperate piezo film elements under the bridge hook each one to an op amp voltage follower. you can use a second op amp to add some gain if needed. pay attention to the impedences. there are cheap pic microcontrollers with built in a to d and usb support and a huge developer community on the web. As far as board layout try protel isis /aries. single sided boards are easy to etch at home, double sided are easy as well, as long as you avoid plated through holes, you have to solder both the top and bottom of the board. Don't cheap out on the op amps and use something like a 741, go with burr brown opa627 if you can afford them. the opa 134 series is not bad as well and a lot cheaper Amplexus

so...it will accept any input and give out an exceptionally low output... Now mixing a passive pickup with an active pickup is notoriously troublesome and the control pots are likely to need scaling back to work properly for the active, but not for the passive...if you buffer after the controls...you are likely to induce noise and scratching from the high pots. A common strategy with opamp designs is to bridge the input to ground with a 1M or so resistor, this gives the input a 1M input resistance similar to the impedance of a guitar amp. This is just one of the many little design subtlties with working with circuits and guitar impedances. Again...not sure I am understanding the application... SO...opamps are often regarded as cold...at the least "overdone". If you find a diagram from a data sheet say of the inside of an opamp...TL071 and family being very common...you will see that it is chock full of diodes, caps and resistors and quite a few transistors. Now...a transistor will give you the same effect, high input, low output...do you really want the guitar signal going through all these extra components? Well...maybe you do...it will give a very cold solid state kind of sound perhaps (different opamps have different characteristics) however, a design like the tillman or fetzer uses a single transistor and a few components to keep it happy, to provide the same kind of effect but a warmer "tube like" distortion characteristic. This may be even more desirable. Really horses for courses I guess...and again depends on the application and what kind of effect you are trying to achieve. Personally, I don't mind opamps for some things and once you start wanting to add variable gain (by altering the feedback ratio) or tone controls or adding numerous stages (a TL072 is a dual opamp in the same size pack for instance and is often easier to work with than the single 071 for the same price even if you only use half of it!)...they also have their own bias circuitry inside that gets rid of pesky biasing trimpots and stuff...they do have a tendency to draw a bit more power, but then it is very little anyway so it hardly matters that much...with more complex circuitry that might have active tone controls (cut and boost) or overdrive (added diodes to produce clipping as in the tube screamer and such) or other processing...there is a lot to be said for opamp circuits. I am not following however the application that your question is asking...generally, you would buffer all of the pickups or signals A & B I suppose, either collectively or individually, or both (as in a mixer circuit) due to potential issues with mismatched impedances, control problems and loading issues. There can also be issues with "popping" when active circuits are switched in and out of a signal...so say you have two pickups and one is active, selecting the non active pickup may well generate a mini power surge that will be loud through a wall of marshalls!!! pete Check out the Burr Brown OPA627, it is fantastic but not cheap, I think you would have a hard time beating it with discrete transistors. if you can afford it is should be your first choice, Analog devices make some good ones as well with a bit hotter sound, Amplexus