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Dimarzio Area J - pot selection

zenrad

zenrad

Supporting Member
Feb 4, 2009
1,075
1,001
Bergen County, NJ
#1
Just installed a set of Dimarzio Area J's in a MIM Deluxe J bass with the intentions of removing the Fender preamp making it into a passive bass. I bought 3 CTS EP886 500K pots (audio taper from what I've recently learned) and like some here have mentioned the volume knobs do very little until the last 1/4 turn of their rotation. So I'm in search of 500k linear pots with a solid shaft, but I can't find a CTS linear 500k with a solid shaft anywhere.

What are users of this pup using for pots? The Dimarzio tech actually wasn't much help - he told me to swap the output and pickup wire on the volume knobs (output on the middle lugs, pickups leads on the outside) and while it would kill both pickups if one was turned all the way off it would "fix" the taper :hmm: :spit: He actually seemed puzzled by the stock Jazz bass wiring.

Right now I'm back to using the original Fender preamp and controls, but I really liked the sound of the passive pups and would love to get it wired up that way if I can find the right parts. The only 500k linear pots with round shafts at Mouser are Alpha and Bourns pots. Never used a Bourns pot, but I've had acceptable results with Alpha pots in the past in effects boxes and amps but I would think that CTS would make their same 500k pot with a linear taper.
 
#2
zenrad said:
So I'm in search of 500k linear pots with a solid shaft, but I can't find a CTS linear 500k with a solid shaft anywhere.
Click to expand...

EP-0220-008.jpg

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#4
yep, the brass sleeves work like a champ (i get them from allparts), except that you'll want to always put the set screw against the gap in the pot shaft, rather than against the splines, or you'll just pinch the split shaft together when you tighten the screw.

you'll need to orient the pots so that the knobs point the way you want, which can be a headache if there's not enough room in the control cavity.

if not, your choices are having the knobs not face where you want them to, routing a little wood so the pots are clear to be turned, or jamming a spacer in the gap of the pot shaft so you can tighten the knob's set screw from whatever direction you need to without closing up the split.
 
#5
Before ordering the sleeves and 500K linear pots I was thinking - these pups were designed to be direct replacements for stock pickups, so I wanted to see what they would sound like with 250K pots. I went through my spare parts bins and found a couple of 250K audio taper pots, wired them up along with the 500K CTS tone pot and as I expected based on other posts here the taper issue wasn't as noticeable. I also discovered I liked the sound much better than the 500K pots.

I went to a nearby guitar repair shop at lunchtime and paid a bit too much for three 250K CTS pots :) , grabbed a 30-year-old Sprague orange drop cap from my parts bin (scavenged in the early 90's from an old amp...a 600v but it fits) wired it as a Vol-Vol-Tone and it worked out tone-wise, but I found that the taper problem, while better, was still a bit of a nuisance especially when blending the two pickups.

Since I really liked having a blend knob rather than two volumes, I took the 250K blend pot from the original active electronics and wired the bass as Vol-Blend-Tone. This configuration has the output connected to the center lug of the volume pot (rather than the outer leg) which makes it sound like an audio taper should, smooth and even increase in volume as it's rolled open. I'm curious to hear how a linear taper would behave as a tone pot with this configuration, but I don't feel like pulling it out again because it's working fine as it is and I want to play it for a while.

So I've settled on

Vol - 250K Audio taper
Blend - 250K MN taper
Tone - 250K Audio taper
.047uf cap

Now I have an unused jack on the side of the body and battery compartment, but I may put a preamp back in one day if it can be switched off to allow me to use the passive sound of the bass, which now is just great!

:bassist:
 
#6
nice.

a linear volume will be smoother still, especially wired as you have it, with the pickups going to the far lug, and the center lug as the output.

a linear tone will do nothing until it's almost all the way off, so don't bother with that.

a trick i've been doing for a little while now is to run an "unloaded" blend pot. as it stands, you have 3 250k loads on your pickups (1 from the vol, 2 from the blend pot), so you're running a bit darker than stock.

the trick is, you just disconnect all the ground connections of the blend pot!

it will no longer bleed off any signal to ground, so will cause no loss at all. it will still blend from one pickup to the other, by virtue of the series resistance it places in front of one pickup or the other, but it no longer "loads" the signal with parallel resistance. i think of it as "dimming" one pickup or the other when turned.

as a bonus side effect, it seems to pan more slowly when turned, making the blending more controllable.
 
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#7
Walterw - thanks for that tip. I tried it and it does seem like the bass has a bit more bite to it. I really like it. :bassist:

I'm trying to understand the theory behind this, not sure this is correct, but I'll guess that three 250K parallel resistive loads would equal a total resistance of a little over 80K if my math is correct (but it's probably not) - and that would cause me to lose some highs.

With only one resistive load from the volume pot that would bring it back to a 250K load.

If I understand this correctly, with normal blend pot wiring you're shorting the input to ground when you turn it in one direction, until you get no volume at all. When you remove the ground wires from the pot you're gradually lifting the ground as you turn the pot until you get no sound.

What I don't understand is what is exactly happening when you remove the ground wire from the blend pot. I drew up a schematic but I just don't get it. How does removing the ground connection get rid of the parallel resistance, and what does lifting the ground do differently than shorting to ground?

I know a little about electronics but there are some basic things that still escape me. :(
 
#9
line6man said:
Not true. When the ground is removed from the blend pot, you are simply varying the resistance from the input to the output. The signal is no longer diverted to ground at all.
Click to expand...

Thanks for the explanation - what I still don't understand is how removing the ground wires creates a series resistance rather than a parallel resistence. I'm just trying to learn about the theory behind this as I'm basically a solder jockey and not an engineer.

Meanwhile my bass sounds great and the help I've received is much appreciated, so thanks again walterw and line6man for the pointers!
 
#10
zenrad said:
Thanks for the explanation - what I still don't understand is how removing the ground wires creates a series resistance rather than a parallel resistence.
Click to expand...
when a volume pot is wired normally, it's creating both series resistance and parallel "bleed-off" as it's turned down.

disconnecting the ground wires just leaves the series resistance that's already there when turned down. the series resistance doesn't "lift the ground", it just puts more and more resistance between the pickup's hot and the rest of the circuit, gradually blocking it off.

the trade-off is that you won't turn a pickup all the way off at one extreme or the other. (if you tap the "off" pickup with a screwdriver, you'll hear a little noise.)

playing-wise, though, i only hear the "on" pickup when the pan is thrown all the way over, so i don't think it's an issue.

i'm glad you tried this trick; it struck me as an obvious way to solve the issue of blend-pot loading, but i've never seen it done, so i'm waiting for the feedback that it doesn't work for whatever obscure reason i overlooked.
 

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