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Why Passive?
My new Schecter Tempest guitar came with EMG 85 and 60 pickups installed. The guitar was manufactured in 2007 as a “ Hellraiser” model. Put EMGs in a guitar and give it an evil name and it becomes a metal guitar. I gave the EMG’s a couple of weeks playing and practicing and while they sound very good there was something missing. The EMG tone is very consistent and even throughout the range of the volume control and the tone controls but lacks a certain “feel” compared to a passive pickup. Some of the differences have to do with the way a passive pu interacts with the volume controls, tone controls and when both pickups are selected, the way you can blend the two PU for different sounds. This is due to the fact that passive PU are high impedance devices and form a LCR tuned circuit that creates tone changes as the controls are manipulated. I like the tones from passive circuits and I don’t hear the same tones when using the EMGs…so they had to go.
Mechanical changes
In order to convert the guitar to passive pickups all the internal controls must be changed to the higher impedance values required. The only thing I did not change was the selector toggle and the output jack. The output jack has a ring connection used to switch the internal battery when a plug is inserted into the jack (just like an effects pedal). I want to use the internal battery later.
I removed the EMGs and the internal controls. DSCN0207[1]One thing I noticed about the stock wiring is that the tone controls were wired different for each pickup. The bridge used the standard “modern” wiring while the neck used “50’s style” with the tone control connected to the volume pot wiper. DSCN0211[1]
Here is the empty control cavity.
DSCN0213[1] There is a very thin layer of conductive paint applied that scratches very easily. I don’t know how effective the paint is but two coats would provide better protection against scratches.
After removing the pickups controls and wiring the holes in the body had to be enlarged to accommodate the 3/8 bushing of the new CTS pots. The stock Asian pots have a smaller bushing as you can see. DSCN0210[1] I used a step drill bit to enlarge the holes after taping the top to prevent chipping. In this case I used a hand drill to slowly enlarge the holes with good results.
DSCN0215[1]
I knew I was rewiring the entire control cavity so I decided early on that all wiring would be shielded with a single point (star) ground and no huge solder lumps on the back of the pots. I used a small piece of vector board to terminate all the connections and provide a place to install a buffer circuit later. Here are the pots all wired up ready to go.
DSCN0224[1]
The tone pots have only two connections required but the board end of the connection has a grounded shield. Here is the finished wiring.
DSCN0228[1]
Circuit design
I have never been a fan of typical tone control circuits and sometimes I disconnect them completely. I wanted to have something a bit more useful in this guitar. I had been reading about the Bass contour control in Reverend guitars and the PTB controls on G&L guitars. The bass contour sounded like a good option to coil splitting to get a leaner tone from a humbucker. After some experimenting I decided on the following circuit:
• Volume control (500k) for each pickup
• A treble roll off control for the bridge pickup with a 500k no load pot
• A bass contour control for the neck pickup (500k reverse audio)

The bridge tone control uses a .01ufd cap and with the no load pot I can completely take it out of the circuit. There is a slight but noticeable difference with the tone pot on 10. The neck pickup bass contour control uses a .002ufd capacitor and a reverse taper audio pot. The reverse taper is required to get the control to work properly.
I did have a fortuitous mistake when wiring the circuit by connecting the bridge pickup out of phase with the neck. Nothing new there as I get the standard out of phase tone in the selector middle position. However when I used the bass contour control in the selector middle position with the pickups out of phase I found a whole new range of tones. Effectively this gives a half-out of phase tone with lower end added to the thin out of phase tone. This not a new idea having been used by Bill Lawrence in the Gibson L6-S and more recently in the Peavey Omniac, but both of those are static settings where as the bass contour control is variable from full out of phase to something in between.
I haven’t decided to keep the out of phase tone but the bass contour is very useful in cleaning up the neck pickup low end while retaining the noise cancellation of the humbucker.

More to do

I will post more as this project progresses. Two things that are still outstanding are the string grounding and the buffer circuit. Most EMG equipped guitars do not have string grounding as the pickups are very quiet without it. The passive high impedance Duncan’s require a string ground to control noise especially with high gain or overdrive pedals. The buffer will be added to keep the passive circuitry from interacting with various cables and pedal inputs and maintain consistent tone and control response no matter what it’s plugged into.

Parts list
Seymour Duncan SH-15 Alternative 8
Seymour Duncan SH-6n Seymourizer
• 2 ea. CTS 500k volume pot
• 1 ea. CTS 500k no load tone pot
• 1 ea. CTS 500k RA bass contour pot
• .01 ufd tone capacitor
• .002 ufd bass capacitor
• Two conductor shielded cable
• Small Vectorboard or perf board material

Reference

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