Question about shielding

[Geoff St. Germaine]

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Ok, so here's what the internals of a potentiometer are going to look like roughly. For explaining, A is the hot output to the output jack, or possibly some other part of the circuit in the bass, W is the wiper connection and the hot input of the signal (either a pickup or a blend knob or something like this) and B is connected to ground. We'll start with the wiper very close to position A. This means that there is very little resistance between the input and output of the pot and most of the signal is sent to the output, there is very little attenuation and the output is "loud". As the wiper is moved towards B the resistance between A and W increases while te resistance between W and B decreases. More signal is now being sent to ground and the signal is more attenuated. Once W is as close to B as possible almost all of the signal is sent to B (ground) and there is basically no output volume.

 

[Worshiper]

ohhhh. Ok I see. I totally understand about the pots now. I'm assuming each pot has to be sent to the ground then?

I also just spend the day looking at the control cavities of my own basses and I learned a lot. The only thing I'm still confused on is to where the ground signal goes. I opened up my bongo and found two ground signals: to the bridge, the other to the back of the control cavity cover, which by the way, was not touching any other surface so the signal would "die"there. My ESP and my p bass only had the ground to the bridge. The p is passive and the bongo and ESP are both active.

And contrary to what was said, only the p bass has a sheilded surface touching the pots.

Can anyone clarify what this mess is?

 

[Suburban]

Dear electrophysisists, please explain this to me:

If I have properly shielded the cavity, then the shaft of the pot will be tightly connected to the shield via the nuts. To me, it seems that the pots then are grounded - no?

If that is so, would it be any improvment to solder a ground wire to the back? Or would it be entering a redundance=safety route? Or would it even be a bad thing?

I've read notes saying each of these, but what is scientifically correct?

 

[nateo]

Dear electrophysisists, please explain this to me:

If I have properly shielded the cavity, then the shaft of the pot will be tightly connected to the shield via the nuts. To me, it seems that the pots then are grounded - no?

If that is so, would it be any improvment to solder a ground wire to the back? Or would it be entering a redundance=safety route? Or would it even be a bad thing?

I've read notes saying each of these, but what is scientifically correct?

I ain't no electrophysicist, but I can speak from a marginally scientific standpoint. In order to minimize interference you need to connect your pots to ground. This we know. Grounding by attaching wires to the pots basically makes the body of the pot act as a tiny Faraday cage, preventing electromagnetic interference from being picked up inside the pot. I doesn't, however, prevent the wires you connected to the pot from picking up that same interference. Some people use "star" grounding to minimize the number of "ground loops" to try and get rid of interference from the wires. If you go ahead and shield your entire cavity and then connect that shielding to ground, the whole thing will act as a Faraday cage. In theory, with a perfect shielding job, there is no longer a chance of electromagnetic interference inside the cavity, thus making the whole "ground loop" thing moot. The real world, though, doesn't do perfect.

That being said, I'd say that a good shielding job is the first priority. Plenty of instruments have been made successfully without it, but it seems to me that the copper shield doesn't require nearly as many solder joints, and those tend to be the weak link in the wiring chain. If you're still concerned about the wiring, go ahead and do what you need to do. Hopefully by this point the interference will have been taken to a suitably low level, and anything else you do will be for that extra little bit of perfection (which the real world doesn't much appreciate).

-Nate

And now for the angry rebuttal...

 

[Dugz Ink]

We seem to have two somewhat related threads going at once; this one, and one on connection ground wires to the chassis of the pot. It wouldn't hurt to read both.

If you build a perfect shield around your electronics, you don't need to ground the pots or have shielding around the wiring. An example of perfect shielding would be the cases that we used for low-input satelite receivers; they were boxes that were made out of non-magnetic metal, all of the joints were a perfect fit, and the pieces were held together with non-magnetic machine screws.

However, your guitar's electronics cannot be built into a "perfect" box, simply because of the layout of a guitar. So there's always an opportunity for RFI/EMI to get around the shielding... through any opening that exists.

Now for the unanswered question: Are the openings in you shielding going to allow enough RFI/EMI to pass to your electronics to be a problem? There's no way to answer that until you play the guitar in numerous environments.

So, nut cases like me will go to extremes to ground and shield everything, in hopes of reducing the possibilities of interference in ANY environment. Is it overkill? Maybe. Am I compulsive? Probably. Do you have to do what I do? No.

There are some guitars out there that have such poor shielding that they might as well not have any, but the owner(s) never play in an environment that has a lot of RFI/EMI... or they simply don't hear the noise over the noise that's coming from their amp.

Of course, if you set up your shielding and/or grounding improperly, then it can work as an antenna and/or conductors that direct the RFI/EMI towards your electronics. So you have to use some logic before you start soldering.

 

[Dugz Ink]

There's been some discussion (in both threads) about different types of shielding, and how to insulate the shielding so nothing touches it.

Here's what I use: copper "roof flashing" from Home Depot. A 50 foot roll (8 inches wide) costs about $15. One side is bare copper, and the other side is coated with tar and paper. (If it conducts electricity, then it's either too low or requires too much voltage for my meters to read it.) I just position it so the paper side is towards my electronics, and set it in place.

The copper is a couple mils thick, and accepts solder VERY well. However, you should scrape back some of the tar/paper where you're going to apply the heat.

I've found that good old Mineral Spirits do a great job of cleaning off the tar, so you can even strip it down to bare surfaces on both sides, if you need.