A good number of my parts came in on Friday, and I spent yesterday putting together a prototype of my circuit. I'm missing the new blend pot, but I did get the AD706's, the 10K 1% tolerance resistors, and some other things. I chose 1% tolerance to be precise with the relative gains between neck and bridge.
Armed with these new parts, a 12V wall wart, a breadboard, and some solid core hook up wire, I decided to implement part of the circuit, specifically the summing amplifier. The neck/blend/bridge switching will be done manually on the breadboard, and the series/parallel/coil cut will be implemented later when I'm ready to dismantle the electronics all together.
For a power supply, I snipped the barrel off of a 12V wall wart. This will give me +-6V, which is well within the +-18V max that the chip will take. I hooked up the power supply with 10K resistors (I will use 1M at least later to cut down on current draw). 9V will be a little less of course.
To hook up to the guitar, I had to isolate the feeds from the neck and bridge pickups. I did this by disconnecting the 3-way switch from the volume pots. Since each pickup has its own volume pot, I traced the wires back to the pickups and identified which was which. I marked these with a sharpie.
I used alligator clips to tap into the guitar's circuit at the appropriate places. For the pickup signals, I clipped onto the (newly disconnected) wipers from the pickup volume pots. This allowed me to still use the pots to adjust blend and volume. For the ground, I just picked a place. For the output from the circuit (through the tone pot to the amp) I clipped into the wiper on the tone pot, which has previously been connected to the output of the 3-way switch.
Modified Guitar Wiring
With alligator clips. Green=Gnd, Red = Neck, Black = Bridge, White = Output
It was very easy to connect to the solder lugs. Alligator clips are a must for doing all prototyping.
Guitar hooked up to circuit. Yes I sat and played it like that and had the courage to hook it up to my amp (with very low volume that is).
This is the circuit. Very simple. Up on the top left of the board, I hooked up the 12V supply. It's hooked up through an ammeter with the yellow and red alligator clips. The two capacitors are the power supply. You can't see the resistors underneath. The small vertical jumper is the ground connector. (actually that is a virtual ground too). The chip is of course the 2 AD706's. Far left blue rail is -4.5V. Left red rail is 4.5V. Right blue rail is ground.
From a different angle, you can see the two adjacent resistors with the neck and bridge pickups connected (red and black). Take a moment to admire the exactness of the wiring lengths. You can also see the feedback resistor next to the yellow alligator clip.
The circuit pulls 1.397 milliamps no matter what I'm playing, even with no playing. This will be reduced because I will use higher resistors for the power supply and a lower supply voltage. Probably 750uA or so of that current is consumed by the opamp. 600uA is from the power supply. That will drop to 4.5uA when I use 9V and 1M resistors. So hopefully, it will consume less than 1 mA. A quick look at an Energizer 9V datasheet says that the capcity is 600mAh at constant 10mA current draw. So we should get at least 600 hours of operation per battery (probably a lot more).
Here's a picture of my whole setup. I'm working on the kitchen bar. Soon I'll be married and she's promised me a whole room for my stuff. :) This is the first time I've gotten to use my new soldering iron (though most of this was on a breadboard). I love it. It has digital temperature control. Nice.
Conclusion and Performance
I am thrilled with the performance. The virtual ground works as expected. I can turn one pickup all the way down without affecting the other pickup at all. Right now the volume varies quite a bit because I can adjust each volume separately, but the blend pot should counteract that volume variance, because both will be adjusted simultaneously.
I connected the neck output directly to the output of the guitar as a test. The sound was a lot muddier and I lost a lot of treble. This is solely because of the additional load on the pickup. I had the tone pot connected to the output of the opamp, and the sound was crisper and brighter. So providing a constant 500K to the pickups definitely helps the sound, and it's easy to hear the difference by simply connecting the tone pot in parallel. I expect great performance from the full circuit. To clarify, connecting the tone pot and capacitor adds a parallel path for the current to flow. Like an inductor, a capacitor's impedance varies with frequency. So for high frequencies, it's like having 250K instead of 500K. For lower frequencies, it's not as bad, but it drops the load impedance to the pickup. This is what the original setup was like.
The only drawback was that the opamp's output did have a little more hum. However, nothing was shielded, and the hum got louder based on the position of the bridge pot. So I expect that proper shielding and the elimination of the long wire runs strung out of the guitar will reduce the additional hum.
So I declare victory. I am eagerly awaiting the arrival of the rest of the parts. Although, it's looking like I will have to wait a longer time to get some new wire from Digikey. The stuff is $50 bucks for 2 pair 4mm wire 100', and I was hoping I wouldn't have to spend that much when I need so little. Oh well. I can keep the rest.
Thanks for reading,