EMG Pickup Installation in a Gibson Explorer (part 3)

Posted by Andy on November 19, 2007
Guitar

EMG pickups require you to replace most of the electronics in the guitar, including most of the wires, the volume and tone potentiometers (“pots”), and the output jack. At this stage in the process, you basically rip out every part of the guitar, and replace it all with the shiny, brand-new, American-made EMG parts! America rules!

A potentiometer is basically a three-terminal variable resistor. Because there are three terminals, there are several different ways to wire it up. I must admit, in my original posting of this blog, I described how I thought it worked in a guitar, but I was dead wrong, so I have edited this post… Of course, I could still be wrong, but I just don’t have a huge desire to draw out the circuit diagrams and try to figure it all out completely, so please correct me if I’m wrong.

In a three-terminal pot, the two outside terminals are connected to each end of the internal resistor, and the middle terminal is tied to a sweeper that can be rotated to contact any point along the internal resistor. If you were so inclined to connect a signal path from one of the outer terminals to the center terminal (and leave the other terminal floating), you’ll basically just have a two-terminal variable resistor.

In a guitar, the volume pots work as a parallel “bleeding” resistor on the signal path. The signal current enters one of the outer terminals and exits the middle terminal, but the other outer terminal is actually tied to ground, which gives you a parallel, variable resistance path to ground. This parallel path acts to bleed current from the signal path, varying on how far you’ve turned the knob. In a guitar, to maximize the output volume, you turn the pot so that the signal passes from the outer terminal to the middle terminal with “no” resistance, and very little of the signal is bled to ground through the maximum-resistance parallel path. To lower the volume of the signal, you turn the pot so that some of the signal will pass through some resistance, and more and more of the signal bleeds to ground as the parallel path resistance is lowered. For zero volume, you turn the pot so that the signal path to the output jack faces a huge series resistance, and the parallel path to ground has no resistance, which causes all of the signal to bleed to ground.

With passive pickups, the potentiometer values for the controls are usually pretty high 250kOhm -> 500kOhm. Passive pickups have lower current outputs, so you have to be extra careful with power bleed as the signal travels from the pickup to the amp. As you bleed current, the signal power gets lower and lower, and more and more noise can begin to play a role in the signal. Active pickups, such as EMGs, normally work with with much lower-valued 25kOhm pots. There are probably many reasons for this, which I will attempt to describe… An EMG pickup is internally powered and the pickup output is boosted, so it can afford to work with a lower-valued bleed path because you’re not as concerned with losing precious signal current. With a passive pickup, you want a very large resistance in the bleed path, so that you bleed as little current as possible when you’re set to maximum volume. In an ideal world, using a huge resistor is great, but with real potentiometers, value tolerances and part “performance” can degrade as you work with parts of higher and higher resistance values. Most electronic components are manufactured with some sort of value tolerance, maybe 5% or 10%. With a resistor, this means that if the part is listed as 500kOhm, the resistance is actually some random value between 475kOhm and 525kOhm (500k plus/minus 5%). With an even larger value, like 1MegaOhm, the actual value will be somewhere between 950k and 1.5M. You can see that as the resistance value gets larger, the range of actual values gets larger as well. This might not seem like a problem, since we’re working with a variable resistor anyway, but the part “performance” can also degrade with larger resistance values. With a larger pot, the resistance sweep has a much wider range to cover, so there is a greater chance that the pot will will have undesired spikes, hot-spots, or undesigned variances in the value sweep. I also read somewhere that high-valued pots tend to have a spiky response to high frequencies. I’m not sure if that’s a specific characteristic of the pot hardware, or just a result of the inherent capacitance of any electronic component paired with a very large resistor.

The circuitry in the EMGs is probably tuned very carefully to work with a specific output “load,” so they probably try to use the lower pot values to get the better value tolerances, and maybe a “smoother” sweep from zero to maximum resistance. Another thing I learned is that pots can have different “sweep” (or “taper”) characteristics. You can get a pot with a linear sweep, where the resistance value increases linearly as you turn the knob, or you can also get an “audio potentiometer,” which has a non-linearĀ  (logarithmic) sweep, which gives you more fine-grained control as you get to higher volumes. For example, with a linear pot, when you turn the knob to 50%, you’ll get 50% of the volume. With an audio pot, there is a logarithmic sweep, which means if you turn the knob to 20%, you might actually get 50% of the volume, but the remaining 80% of the pot turn is available to finely adjust the upper 50% of volume.

Anyway, I hope that wasn’t too boring, but I thought it was interesting. Here are a few more pictures. This one shows the control cavity. You can see the wires coming in from the pickup cavity – the cable from the toggle switch, and the two braided cables from the pickups.

Old controls closeup

The Explorer has 2 volume knobs (one for each pickup) and a “master” tone knob, which affects both pickups. The pot on the left is the neck volume, the middle one is the bridge volume, and the right one is the tone control. The tone control is another pot, but instead of a bleed resistor, it acts as a simple low-pass filter with a variable resistor and a capacitor. The capacitor sits across one of the resistor terminals and ground – it’s that round, orange thing soldered on the right pot.

Here’s a closeup of the left side:

Left control (neck volume)

The Explorer control placement is a little confusing in terms of the signal path. The pickup signal comes down through the braided wire through the volume pot, then gets routed back up to the toggle switch, and if the switch is in the right position for that pickup, the signal comes back down from the toggle switch, through the tone filter and finally out the output jack. One other interesting thing in this picture is the “string ground” wire. Near the top-right of the picture, you can see a silver wire that goes up into a little hole. This wire is tied to the ground of the controls (which are ultimately grounded by the amp, via the guitar cable), and apparently is connected somehow to the screws that hold the bridge piece. The bridge piece connects to the strings, which effectively grounds the entire guitar, strings and all. I read that if the strings are not grounded like this, there is a potential electric shock hazard. EMGs are internally grounded, so the EMG instructions say that you shouldn’t connect this ground wire when you replace the controls. To be honest, I don’t completely understand this, but oh well… I ended up just taping the wire up to the inside of the cavity with electrical tape when I took out the controls.

Here are some blurry closeups of the middle and right controls:

Middle control (bridge volume)

Here’s the tone control:

Right control (tone)

The black cable near the bottom connects down to the output jack.

That’s definitely enough for one post, thanks for reading!

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