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Shielding/Star Grounding Troubleshooting Help Needed

#21
I also have a 90's J standard with same sized single coils. Having the volumes at the same level never made any difference. It's always been noisy as heck especially at gigs. I decided to install a push-pull series/parallel switch to increase the tonal options of the bass, but also thinking that it would have a hum-canceling effect. Guess what, it turned out to be even more noisy! It would quiet down considerably if I touched the strings or pickguard which it never did before, so I guess that's a plus.

I was about ready to spend $150 on new noise canceling pickups cause I was pretty much sick of the hum. Instead, I decided that I would try shielding the bass. I got some copper adhesive and shielded the cavities and the bottom of the pickguard and now it is dead quiet! It makes no difference where the volume levels are or if it is in series or parallel mode. It's always dead quiet!

I didn't bother star grounding anything. In fact, it is wired exactly like the directions posted here:
[Invalid or Expired Link Removed]
 
#22
Agree that the hum is in a open lead - there is some wiring error or non-connection.

re: Hum canceling - the magnet polarity is irrelevant - you can change this with the wiring of the Pup.

To test the output polarity of the pickup, connect a sensitive multimeter in DC mode to the output of the pup. Place a steel tool almost touching the pup, and pull it away quickly. This will create a DC pulse going either positive or negative and tell you the phase of the Pup wiring. Note the wire arrangement for this - which wire is going to the Red lead of the meter. Then you can decide how to wire them for in or out of phase.

I have had no problems with star grounding. After all cavities and covers were shielded with Stew-Mac Copper (the adhesive is conductive so you don't have to solder all the pieces together) all grounds were connected to a single screw in one cavity. Pickup grounds, pot grounds, cavity grounds, and output jack grounds all to one point so that there cannot be any ground loops. This P/J bass was so quiet I thought is was not working when I first plugged it in.

For Jazz basses, it can only be hum-bucking when both pickups are set the same. Otherwise the Pups cannot cancel. I got tired of this and just replaced both Pups with Fender SCN hum-canceling types, now all is quiet and they sound great, too!

BTW, I found that my computer was a BIG cause of hum if I was within 3 feet or so. They radiate like crazy.
 

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#23
bongomania said:
Holy freaking Clamato!!!!!!

My ADJV with hum-cancelling SCN pups has been giving me endless noise grief since I bought it, and the (otherwise very good) tech I brought it to was not able to figure out what was wrong. I have tried various high-end preamps in there and they never sounded quite right or worked quite right. I shielded the holy heck out of all cavities and channels per the instructions in this forum, and it made no difference. So in a fit of frustration I recently pulled all the electronics, wired the pickups in series directly to the output jack, and twisted all wire pairs for maximum rejection, and kept the wires as short as possible.

No improvement. How is it possible? I have been pulling my beard out trying wrap my head around how it is even possible for such a heavily shielded, twisted-pair, humbucking pups situation to still have so much hum.

Guess what, they are the same polarity. I had no idea to look for that until seeing that vid, so thanks for posting it. Now I have to figure out whether it makes more sense to buy some magnets and DIY the polarity reversal, or send the pups off to someone else, or say screw it and buy some new pups.
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Absolutely correct. You would think the manufactures would consider this issue when designing an instrument. A simple compass reading at the poles tells the story. If you take the front pup out and flip it upside down you will hear no buzz which tells you the winding is going in the same direction. Im still trying to figure out why they call them noiseless. Maybe to a deaf person???
 
#25
bongomania said:
Granted that star grounding did not solve my hum problems at all, so it is not the relevant solution here- but all audio is AC, the output of pickups is AC.
Click to expand...

I'm referring to ground loops and mains powered gear. You can't have a ground loops with a passive or even battery powered bass.

A ground loop occurs when you have two pieces of AC powered gear with the power derived from two different AC circuits, and one of the grounds has a higher potential. When you plug them together via audio patch cords current flows from the ground with the higher potential to the one with the lower resulting in audible hum. This is why DI's have ground lift switches. You only want one ground connection at a time.

Instruments only have a single connection to ground via the output jack. They can't have a ground loops because all the grounds on the inside are at the same potential and exit to one point. So it makes no difference if you ground the back of every pot, and then wire them together. It's all part of one ground plane.

Star grounding in instruments is supposed to stop ground loops from forming, except you can't have a ground loop in the first place.
 
#26
Actually, you can surely have ground loops in passive gear. It occurs from EMI hum induced in different parts of the circuit, if they have different resistances caused by multiple paths to ground. It is the same concern we have in the design of PCB layouts for audio and RF circuits.

In addition, there are internal currents in the circuit that are created by the pickups - they can also be affected by circuit loops.

Granted, the result is usually much smaller than AC power induced hum, but we are dealing with very high gain situations that can make a small hum audible. If the cavity is shielded this is probably moot, but it does no harm to be careful.
 
#27
markblasco said:
Yes, the pickups should be reverse polarity. If your other stock pickups are wired opposite, why not try swapping one of them from one set to another, and see if that works?
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I thought of this, but unfortunately, the compass reads "south" on all four pickups. Dammit.

I was just reading on another forum that one of the cost cutting measures in the MIM jazz basses (especially the older ones) is to basically mount two neck pickups (no reverse polarity).

And the bummer is that the bridge cavity rout is thus too small for most bridge jazz pickups, which are a little wider than the neck ones. I didn't even know they were different sizes, to be honest.

So It sounds to me like I'm going to need a pickup upgrade and do some minor routing to get the true J bass sound and quieter operation I'm after...
 
#28
You just reverse the leads on the one that you want to be out of phase. Simple as that.

Just be aware that this also changes the sound, as some audio is always canceled as well as the hum. You might want to put in a switch to select in or out of phase.
 
#29
Two pickup out-of-phase will sound very thin, with virtually no low end. Not very useful for bass.

You can have the magnets recharged in the reverse polarity. Some pickup makers can do this for you if they have a magnet charger. Then all you would need is to reverse the polarity of the pickup by swapping the two leads.
 
#30
samuelzero said:
Hello,
*Replaced mini pots with CTS 250K audio taper pots. Replaced cheap ceramic cap with Orange Drop. Replaced output jack with switchcraft.
Click to expand...

Can I ask why you decided to use an orange drop over the other cap?

Just so we are clear on why I'm asking, I think it's the biggest voodoo waste of money in the guitar electronics world, but it doesn't happen as often with bassists. Just curious why you decided to do it.
 
#31
Angus said:
Can I ask why you decided to use an orange drop over the other cap?

Just so we are clear on why I'm asking, I think it's the biggest voodoo waste of money in the guitar electronics world, but it doesn't happen as often with bassists. Just curious why you decided to do it.
Click to expand...

I think any film capacitor is better than a ceramic, but even then you would be hard pressed to hear a difference when it comes to a passive tone control. Even the little green film caps in Radio Shack are on par with more expensive caps, and good quality caps are not expensive (including orange drops). These expensive "music caps" are a big waste of money. You can go to mouser.com and get the EXACT same caps for a fraction of the cost (read pennies).

Also the small pots are not worse than the big ones just based on size. You can get cheap large pots that are worst than the small ones. Many of the small ones are very good, such as the ones that come with EMG pickups. It's not the form factor that contributes to quality.

having said all that, most of the electronics on cheaper instruments are fairly poor quality.
 
#33
Angus said:
Can I ask why you decided to use an orange drop over the other cap?

Just so we are clear on why I'm asking, I think it's the biggest voodoo waste of money in the guitar electronics world, but it doesn't happen as often with bassists. Just curious why you decided to do it.
Click to expand...

First off, I'm also a guitarist...

I went with the Orange Drop because I wanted to be able to take a side in the "voodoo" debate based on real experience.

I swear my tone control does sound different now, though it could be "gear placebo" or other factors, such as the nicer pots.

Either way, I spent 50 cents on it, so I hardly consider it a "big waste of money."

And further, there's just something philosophically satisfying about using high quality components, even if they don't make that big a difference.
 
#34
DaveLS said:
Save yourself some time and money and read my above post:

http://www.talkbass.com/forum/showpost.php?p=6673126&postcount=21
Click to expand...

Hi Dave,

Thanks for your post. Though I never responded as such, I am going to do just what you did.

I think if there is an issue with my wiring, it lies in the ring terminal I used for the star ground. So I'm getting rid of it and going back to soldering to the pot casings.

Even if I did a stellar job with the ring terminal thing, it's not a very convenient setup... To swap pickups, the pots have to be removed from the plate to get the ring terminal off, then I'd have to fiddle around with trying to solder a ground wire to it while it's still attached to the bass. Doesn't make sense.

So yeah, I'm on board. I really don't wish to drop $160 on pickups for a bass I spent $100 on, and I've already shielded the shite out of the pickup and control cavities. Also, I usually don't play with both pickups full up, so the reverse polarity/reverse phase thing may not be such a big deal anyway.

Once my wiring is tight, hopefully my bass will be as quiet as yours...
 
#36
Nino Valenti said:
Did you shield the pick up cavities? If so, did you connect the pickup cavities to the control cavity? If no, do so. I had this same problem and it drove me nuts. :)
Click to expand...

Yes, and Yes. All cavities are grounded and connected together, and the copper foil comes out over the edge to make contact with the pickguard, which is also covered in copper foil (the kind with conductive adhesive).

I've checked with my multimeter to make sure the cavities are common.

I must have a loose ground somewhere...
 
#38
Gio S said:
You just reverse the leads on the one that you want to be out of phase. Simple as that.
Click to expand...

I'd like to get some more commentary on this subject before I go buying more pickups or sending mine off for re-polarizing, etc. It seems as though "polarity" and "phase" are being used interchangeably in this thread, at least enough to where I'm confused. Of course they have different meanings, but the mechanical function of pickups has me in doubt.

On a technical level, what is the exact relationship between the magnetic polarity of the pickup poles and the directionality of the two wires from the pickup?
 
#39
bongomania said:
I'd like to get some more commentary on this subject before I go buying more pickups or sending mine off for re-polarizing, etc. It seems as though "polarity" and "phase" are being used interchangeably in this thread, at least enough to where I'm confused. Of course they have different meanings, but the mechanical function of pickups has me in doubt.

On a technical level, what is the exact relationship between the magnetic polarity of the pickup poles and the directionality of the two wires from the pickup?
Click to expand...

I don't understand it enough to explain it to you (or to myself), though I believe that you have to have both of them reversed for it to work in a humbucking context.

Just swapping the wires when the pickups have the same magnetic polarity results in the thin, phase cancelled sound referenced above.

But also having the magnetic polarity of one of the coils reversed cancels the hum without the out-of-phase tone.

Any of the experts want to weigh in on this?
 

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