World's Most Obsessive PBass Shielding/Rewiring Job?

[rjb]

Ancient History:

Many moons ago, I picked up a '73 PBass at a swap meet. It had been hot-rodded in typical '70s fashion: brass nut, Badass II bridge, cream DiMarzio pups, and an aluminum pickguard with two mini-toggles added for serial/parallel wiring and .1uF/.02uF (or was it .047uF/.01uF?) tone cap select. It was always kinda noisy, and the smaller cap was burned open- but I liked the "no load" tone switch, and played the bass "as is" for quite a while. Then, one day, I plugged it in and no sound came out.

So I opened it up and rewired it and- since it already had an aluminum pickguard on top- shielded the pickup and control cavities and the tunnel between them. Then I plugged it in, and it was *really* noisy. I concluded I didn't know what the hell I was doing, and the bass languished for years.

Recent History:
Recently, I decided to take another shot at it. In the mean time, I've learned a bit about electronics and grounding and shielding, and have read some textbooks and too many websites (some of which are listed at the end of this post) that proffered interesting but sometimes confusing, confused, and/or contradictory facts and/or opinions.

The Request:
I've let the words stew and fester awhile, and now have a plan and a passel of parts. The schematic below and its notes essentially outline the plan. The goal is to use wiring and shielding techniques to make the PBass "as noise-free as feasible" without "spending a fortune", resorting to “extreme measures", or "killing the tone". I realize some of the particulars of my proposed project are non-standard practice and, for once in my life, thought it might be interesting to ask for help sorting the reasonable ideas from the bone-headed ideas *before* I finish the job. So, let the design review begin!



Your comments solicited. More to come.
Thanks,
-rb
-------------

References:

RaneNotes
Grounding and Shielding Audio Devices: Grounding and Shielding Audio Devices
Pin 1 Revisited: Pin 1 Revisited
SCIN: Shield Current Induced Noise: SCIN: Shield Current Induced Noise
Sound System Interconnection: Sound System Interconnection

Bill Lawrence Website
External Interference (Hum, buzzes, static; You have a friend in Downey):
Bill Lawrence Website
For Tele Lovers (Cu plus Al foil shielding): Bill Lawrence Website

Music Electronics Forum Threads
Anyone know about a tonally transparent cover: http://music-electronics-forum.com/t...orum.com/t2614
Anyone tried shielding their Strat?: http://music-electronics-forum.com/t6132/
Basic Question: http://music-electronics-forum.com/t17066/
Grounded polepieces vs. closed covers: http://music-electronics-forum.com/t16248/
How to shield a single coil pickup: http://music-electronics-forum.com/t4490/
Reverse wind-reverse polarity question...: http://music-electronics-forum.com/t17063/
Shielding a single coil: http://music-electronics-forum.com/t13532/
Shielding pickups: Shielding pickups /
Solder all grounds to bridge, optional: http://music-electronics-forum.com/t19673/
Why do you need a string ground: http://music-electronics-forum.com/t4887/

[rjb]

OOPS, TOO LATE Department

Installment 1: Pickup Modifications

I actually started working on the project before getting around to uploading the original post.

Here are some pics of the PICKUP MODS.



Objectives:
> To shield two loop antennae (the pickup coils) from RFI, while avoiding the creation of tone-killing eddy currents in a closed loop around the coils, using a marginally novel configuration of copper foil liners inside the pickup covers.
> To avoid pops and annoying static cling by providing a discharge path from the pickups' pole pieces to ground.

Notes:
> I used TinyCAD's symbol editor (free from SourceForge) to draw the cover liner templates.
SourceForge.net: TinyCAD - tinycad
> The overlapped areas of the “half- liners” are separated by insulating tape, similar to the way you'd wrap a pickup coil to prevent eddy currents.
> Wax-potting plastic bobbins (with magnets secured by hot glue) in a DIY double boiler was NOT the smartest thing I've done in my life. But everything cleaned up OK. And the slight warping is hidden by the covers.
> While the covers were off, I figured I might as well treat them to a Scratch & Swirl exfoliation and Turtle Waxing. These indulgences made no appreciable difference, but the wax may have helped keep new hot glue from adhering to the fronts of the covers.
> Using the braided shield from a piece of faux-vintage coax, I bussed the pole pieces to the upper pickup mounting hole, which will be tied to "chassis". I re-encased the whole shebang in new hot glue, and verified continuity between each pole piece and the mounting hole. The scheme worked great. The second time. The cure was forming the braid so it makes contact with the threads as well as the back of each screw.

Still more to come.
-rb

[rjb]

Oops, Too Late Department, Installment 2

Installment 2: Pickguard

Here’s a detail of the PICKGUARD SLOT



Objective:
> To minimize unintended tone changes caused by eddy currents in the 1/16” thick aluminum pickguard, by cutting a slot from the pickup cutout to the pickguard’s edge.

Notes:
One project begets another: When the world gives you alligators, try making alligator soup; then change burning bridges in midstream. (Hunh?)
> I cut the slot with an electric sabre saw. Took about 5 seconds.
> I then filled the slot with a spacer made of some polymer clay I had on hand. Bad move.
I baked the clay and pickguard together, not realizing the guard’s paint would bubble in a few random areas at 275°F. Not wanting to repaint, I baked the pickguard at various higher temperatures, in an attempt to achieve a “uniformly ugly” look. Not liking the results, and resigned to stripping and repainting “someday”, I replaced the original spacer with a stronger one of epoxy putty.
> As for “someday”, I’m thinking “hammered brass” spray paint would go well with the brass nut and natural finish body.


Here are some pics of the PICKGUARD SHIELDING


Objective:
> To blindly follow Bill Lawrence’s assertion, by double-shielding the bass with both aluminum and copper tape.
“Aluminum has some strange properties, and it’s the only commercially available metal I know of that can eliminate the buzz caused by light dimmers. An inch thick copper or brass shield cannot reduce the buzz caused by light dimmers but .003 thick aluminum foil can! This is known some thirty years and the reason why Belden introduced double shielded cable ( Copper braid plus aluminum foil).... It helps quite a bit when you shield your guitar with copper and aluminum foil.”
Bill Lawrence Website

Notes:
> None of the aluminum or copper tape I have on hand has conductive adhesive.
> I originally wasn’t going to apply aluminum tape to the anodized aluminum pickguard, but changed plans when I found no conductivity between the pickguard’s mounting holes.
> Mating surfaces of the folded-under aluminum tabs are coated with Ox-Gard conductive grease.
> A layer of clear packing tape between the aluminum and copper layers serves three functions: holding down the tabs; containing the grease; and preventing galvanic action between the dissimilar metals.


To-Do List

• Body Cavity Shielding (Aluminum & Copper)
• Wiring Harness (with "controversial" grounding system)

Later,
-rb

[Steve Conner]

Great! That looks like it's going to be a real engineer's bass.

I'd question the need for a plastic TRS jack isolated from the instrument, though. I prefer the build quality of the metal Switchcraft jacks, so I'd use one, and have the connection between "audio ground bus" and "chassis" ( ) be there.

I can't see why it shouldn't be a direct connection, all of the RCs seem pointless, unless you're doing it to protect yourself from shocks on stage or something.

I also don't believe the Bill Lawrence thing about double shielding.

Also, you can not shield the pickup coils from magnetic interference, only electrostatic pickup, in which respect they are more like capacitor plates than loop antennae. Shielding them magnetically would get rid of the sound, too. The only thing that gets rid of the magnetically induced noise is a humbucking configuration.

[FunkyKikuchiyo]

That looks like a lot of fun!

I question the effectiveness of reducing the static pop from rubbing the instrument. I've seen some very, very elaborate shielding/grounding things in place to try to eliminate it, but none of them have been effective. Dryer sheets or a sloppy guitar polish to coat the offending finish/plastic to prevent the popping to begin with is the only thing that has ever worked for me. Think about if you're out in dry weather and your slacks are sticking to your legs (happens to me all the time, I have hairy legs), does touching a large conductor help dissipate the charge and make your slacks stop sticking to your legs? Same idea.

In support of Steve's comment on the jack, P-basses are as prone to jack breakage/issues as any other instrument I've seen because of how the cable end likes to stick out - so I think it would pay to go rugged. I'd normally suggest reinforcing the underside of the guard, but since you have such a rugged guard I wouldn't bother.

Also bear in mind that the aluminum guard will have effects beyond the electronics. It is a large reflective surface under the strings, and will change the acoustic tone of the instrument. If you're liking it then great... I'm not saying it is a bad thing... just a thing. I'd never put a metal guard on a guitar (tried it twice, sounded horrible both times) but on a bass it can be kinda nice.

[rjb]

Where's my magnifying glass?

I've noticed that the schematic in my original post isn't very easy to read with the online viewer. One work-around is to right-click to save the file on your computer, then use Windows Picture & Fax Viewer, Microsoft Office Picture Manager, or such, to view and enlarge the image to a more legible size.

But since it isn't polite to ask someone to jump through hoops to do you a favor, here are the notes from the schematic:

Notes:

1. Pickup Mod

a) Copper foil shield liners are installed in the DiMarzio pickup cases in a configuration intended to prevent eddy currents from flowing in a closed loop around the coils:

Two "half-liners" are overlapped across each cover's center, separated by insulating tape; each half-liner is individually bonded to "chassis" via mounting screws.

b) Each pickup's pole pieces are bussed with copper braid and bonded to the top half-liner.

c) The pickup coils are potted in an 80:20 mixture of paraffin wax and bees wax.

2. Body Shielding

a) The aluminum pickguard is slotted to block eddy currents from looping around the pickups.

b) The pickguard's underside areas over the control cavity and around the perimeter of the pickup cavity are faced with one layer each of insulating tape and copper foil.

c) The pickup and control cavities are double-lined with aluminum and copper foil shielding, separated by a layer of insulating tape.

d) An assembly of braided copper cable shield, wrapped with Teflon plumbers tape, and surrounded by a "lasagna pan aluminum" tube, connects the pickup and control cavities.

e) Pickguard mounting screws are coated with Ox-Gard conducting grease to prevent galvanic action and corrosion between the copper and aluminum layers.

f) All pickup, switch, and pot cases are bonded to the shielded enclosure ("chassis"); 18AWG bus wire also daisy-chains switch & pot cases to ring terminal TP1 near ouput jack J1.

3. Wiring

a) Signal & Return (AKA Pickup Common) wires are treated as SIG+/SIG- pairs, twisted to minimize circuit loop area and reduce susceptibility to EMI pickup.
It is noted that guitars produce single-ended, unbalanced signals- not differential, balanced signals; therefore, use of SIG+/SIG- nomenclature in this schematic is not strictly kosher. So sue me.

b) Nylon TRS Phone Jack J1 isolates phone plug's shell from aluminum pickguard.
A jack with PCB mounting tabs was used only because no Panel-Mount Jack was available from Mouser.
(I've since found that Antique Electronic Supply carries insulated PMJs).

c) R3, R4 (470 Kohm 1/4 W carbon) and C3, C4 (10nF/1KV ceramic disc) are for RFI supression and personal safety.
Component leads are cut as short as possible to minimize inductive reactance.
Arguments could be made for eliminating R3 & R4, or for replacing R3/C3 & R4/C4 with a direct connection from J1-Shield to TP1.

4. Operation

a) When a standard mono instrument cable is plugged into J1, J1-Ring is shorted to J1-Shield, tying SIG- to Cable Shield at a single point; TP1 (chassis ground) is coupled to that single point via R3/C3 & R4/C4.

b) When a custom stereo-to-mono cable connects J1 to an amplifier, SIG- ties to Cable Shield at the cable's far end (the amp's input).
This configuration may (or may not) yield better RFI protection by separating cable shield currents from the signal path.

c) If a custom stereo-to-mono cable connects J1 to a stomp box, all bets are off.

Updated Schematic
I found some spelling errors in the original; here's the latest REV:

[rjb]

Great! That looks like it's going to be a real engineer's bass.

Um, thank you, I think.

I'd question the need for a plastic TRS jack isolated from the instrument...
I can't see why it shouldn't be a direct connection, all of the RCs seem pointless, unless you're doing it to protect yourself from shocks on stage or something.

Please see Note, uh, 3c. The plastic jack and RCs are supposed to be for shock protection; I thought it would be *really* pointless to isolate the bridge wire, while tying the pots and switches (with metal shafts and knobs) to the coax shield (which I have seen done). The pickguard is missing chunks of enamel- so could also be a "potential shock hazard". But, on the other hand, most guitars have no shock protection at all. I'll log your vote for "direct connection".

I also don't believe the Bill Lawrence thing about double shielding.

I'm not sure I do, either. And I guess I'll never really find out- I don't have the means or inclination to do an A/B test with two identical '73 PBasses.

Also, you can not shield the pickup coils from magnetic interference, only electrostatic pickup, in which respect they are more like capacitor plates than loop antennae. Shielding them magnetically would get rid of the sound, too.

I guess I'm confused and/or misinformed; I thought the coax shield, body shielding, pot and switch shells, and pickup shield all protect mainly from radio frequency EMI.

The only thing that gets rid of the magnetically induced noise is a humbucking configuration.

These pickups (and, I think, standard PBass pickups) are in a humbucking configuration; the two units are RWRP from each other.

[rjb]

I question the effectiveness of reducing the static pop...

I can't say from experience, but read somewhere on this forum that a) grounding magnets and polepieces reduces static pop, and b) polepieces are potential antennae that should be grounded. Your mileage may vary (as may mine).

...slacks are sticking to your legs (happens to me all the time, I have hairy legs)

Um, that's more information than I really need....

does touching a large conductor help dissipate the charge and make your slacks stop sticking to your legs? Same idea.

Two points:
1) I can't say, as I don't experience the "sticky slacks" problem.
2) Wouldn't it be more like dragging a tow chain, or cabling your leg to the plumbing?

...prone to jack breakage/issues... because of how the cable end likes to stick out - so I think it would pay to go rugged.

I do prefer rugged to flimsy, but was thinking plastic specifically for electrical isolation from the metal pickguard. (I did try wrapping the threads of a metal jack with Teflon tape, but that didn't work).

Regarding jack issues, how about using a sturdy right-angle plug to reduce rotational torque?

...aluminum guard... is a large reflective surface under the strings, and will change the acoustic tone of the instrument.

Thanks for that point. I should have thought of it, but didn't. I bet that has a lot more effect on the sound than the eddy currents I was focused on!

I'm not saying it is a bad thing... just a thing.

Yup, just a thing that came with the package. Maybe someday I'll replace it with a plastic guard.

[FunkyKikuchiyo]

I played a P-bass with a gold anodized guard a while ago (Fender did a run of them on the Mexican instruments) and I really liked the sound. It basically made the strings sound newer than they were. It kept a bit more growl because of that. On a strat is ends up giving you all sorts of highs that you'd rather not deal with, but on a bass it was nice... unless that goes against what you're seeking.

If Steve wanders back to this thread he'll explain the interference issue WAY better than I will, but I'll have a go anyway. In a guitar you're going to hear hum that is 60 Hz and 120 Hz. 60 Hz is the hum that is cancelled out through the humbucking effect and is heard as a low rumbly hum. 120 Hz is the RF stuff and is heard as more of a sizzly hum, and that is the stuff that tames down when you touch the strings, thus improving the ground. Shielding in my experience does absolutely nothing for 60 Hz hum, but 120 Hz hum can be almost completely eliminated. Sometimes I wonder if this is because of actual shielding or if it is just because you're increasing the overall mass of material wired to ground, but that's another matter.

On the static, I'm starting to think we're talking about two different things. Are you talking about the sound when you touch the pole pieces? If so, then you're right and ignore what I said! That comes because the pole pieces aren't grounded and you're introducing all kinds of noise using your body as an antenna every time you touch them. Guitar pickups do this too, but bass players are so much more tactile with their playing that it'll drive you nuts.

What I thought you were talking about initially is the effect you sometimes get wherein if you rub the pickguard (typically plastic) or the lacquer you get a series of very small pops coming out of the amp. You can even rub the back of the instrument and get that effect. I've only ever had the problem in dry weather.

[rjb]

...P-bass with a gold anodized guard... really liked the sound... unless that goes against what you're seeking.

Confession: I'm not much of a bass player. What I should do is: get the thing assembled, practice a whole lot, THEN start thinking about fine-tuning tone with pickguard swaps and such.

On the static, I'm starting to think we're talking about two different things.

Or maybe I was mistakenly combining two different things into one. Thanks for the clarification.

[big_teee]

I'm way late on the thread, but would like to add a couple of things.
You may have already fixed your P-Bass Noise problem.
I had a thread a while back where I rebuilt a pair of 71 P Bass pickups.
I made them like original. P Bass Pickups if stock are already humbuckers.
If Polarity is correct and they are properly wired together with the stock grounding plate underneath, They should be relatively quiet.
Here's the link to the thread with pictures of the pickups I re-built.
These went back in a vintage bass and shop owner wanted no cosmetic restoration.
These were real quiet after installing, and there was no additional grounding or shielding added.
Pictures are at end of thread.
http://music-electronics-forum.com/t21850/
Good Luck,
Terry

[rjb]

I'm way late on the thread... with the stock grounding plate underneath...

STOCK GROUNDING PLATE?!!!! Man, I wish you had replied earlier!

This bass came with no grounding plate, and I had no idea such a thing existed. From a shielding perspective, I'm puzzled why someone would remove a grounding plate and add an after-market (paper label "Music Barn $10") aluminum pickguard.

Checking online, I see I could have a new Fender plate delivered to my door for $7.50. Would've been a lot simpler than my aluminum/packing tape/copper sandwich! But since I have no requirement to "keep it stock", I don't think I'll turn back now.

Oh well. Ya live, ya learn.
-rb