Sunday, May 16, 2010

Active Guitar Circuit

This guitar circuit is the final result of my guitar mod as described a few days ago.  To reiterate, the goals were:
  •  1 knob that always controls volume.
  • Preserve the neck, blend, bridge option with a single knob to control blending between pickups (preferably with a center detent!)
  • 1 switch for switching between series, parallel, and coil cut for both pickups. 
  • 1 switch for switching between neck, blend, and bridge for output.  
  • Provide a more stable impedance to the pickups. Or at least don't let the impedance fall below 500K.  
  • No new holes/ routing in the guitar. Minimal deepening of original routed area, and only if absolutely necessary.  
The Circuit

The two opamps are AD706's (2 per package).  It is available in an 8-pin dip from digikey.  It pulls about 750uA regardless of supply voltage, and since we're dealing with low drive currents, it doesn't ever pull much more than that from the opamp.

I'll explain the circuit from left to right.  The four voltage sources are the 4 coils in the two humbucker pickups.  N= Neck; B=Bridge.  The first set of switches (labeled series, parallel, coil cut) control how the internal coils of the pickups are wired together.  All four switches reside in a single rotary switch.  The switch is a 4 pole 3 position switch, and the connection scheme is shown below.
 The second set of switches (labeled neck, blend, bridge) controls how the two pickups are wired together.  This is a vast improvement over the original wiring.  Originally, each pickup had its own volume control.  In the middle position on the 3 position switch, the two taps were shorted together.  Here, the two inputs are fed into a summing amplifier (the first opamp).  That deserves its own section...

The Summing Amplifier
The first opamp is connected in a summing (and inverting) amplifier setup.  The two signals from the pickups are summed and amplified (with unity gain) so that we get exactly the same thing we had before, but 180 degrees out of phase.

Maybe you're saying, "But that's the same thing we were doing before."  But you would be wrong.  The only reason to use a complicated opamp in this circuit is because it gains us the all-important "virtual ground."

Because of the ultra high gain of the opamp and the negative feedback connection, the opamp will attempt to hold its negative terminal equal to its positive terminal.  In this case, since the positive terminal is grounded, the negative terminal will be driven to 0V by the opamp (making it a virtual ground).  One very important effect of this is that it eliminates crosstalk between the pickups.  No current will flow from the negative terminal back out to the pickups.  So in theory the two pickups should have absolutely no effect on each other.

Also, because of the ultra high input impedance of the opamp, no current will flow into the negative terminal either.  This is also very important.  This means that the opamp introduces no additional load to the pickup.  So both pickups will see 500K of load no matter what kind of blending or switching we do!

After the Summing Amplifier
Opamps are amazing things!  After the summing amplifier, we can use smaller pot values, because the opamp has negligable output impedance (unlike the pickups).  So the opamp acts as a buffer as well as a summing amplifier.  We hook up the tone and volume pots as we would any other pots, except that in my case they are concentric pots so that I don't have to drill any holes in my guitar.  Tone and volume on the same axis of rotation, but separately controlable.

The second opamp in the AD706 package is mostly there to drive the cable instead of the volume pot.  This will enable us to limit the effects of a high capacitance (long) cable.  If you want to undo the 180 phase shift, hook it up as shown.  Otherwise, you can save 2 resistors and make a simple voltage follower.  I'm still considering doing this instead.

Also, I may decide in the end to up the 50K concentric pots.  I will have to do some math to discover what kind of capacitor I will need to make a decent tone circuit.  That is still open to debate.

Power Supply
The only drawback to this circuit is that it needs to be powered. Here's a good way to get a +- power supply out of a battery.  I'll use a 9V battery and get +-4.5V.  I have played around with the idea of adding a diode in series with the battery to make sure that I don't ever accidentally connect it backwards, even for a second.  I will probably do that.  The current draw by the power supply will be very small with these circuit values.  (4.5uA).  So all together we're looking at less than 1mA to run the whole circuit.

This circuit accomplishes all of the goals set out in the beginning of the project.  Only 2 holes are required for pots as well as 2 holes for switches.  There are 4 original holes in the guitar.  I may have to drill down a little bit to get the switches to mount easily in the current holes, but I'm not too concerned about that.  I've ordered all the parts, and I've done a trial run with some that have come in, but that's for the next post.

Thanks for reading


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