Noise - Shielding - Cables (Again)

[rabidgerry]

Hello, I've a few questions I'd like to hopefully have explained.

I've been reading through some of the troubleshooting guides on Guitarnutz website and came across a few stumbling blocks to do with noise.



Now one of these is to do with "PROPERLY SHIELDED" guitar cords or leads.

Any music cable I own says it is shielded, i.e "noiseless" (usually) somewhere along the side of the lead or else it says "shielded".
However I have discovered through use of the "Troubleshooting Noise" guide on guitarnutz that my cables aren't actually shielded according to this guide and one of the tests to prove if guitar leads are "properly shielded or not". This is the resistor test of placing a 10k resistor across the connections of a guitar cord.

"Unplug the other end of the cord from the amp (or the stomp box it is plugged into).
If the noise level dropped significantly the problem is the cord. Replace the cord. Note, it's not unusual for an unterminated cord to pick up some noise, but if the cord is well shielded it shouldn't pick up much. If you want to be certain whether the noise is coming from a poor cord or just because the cord is unterminated, you can place a 10k resistor across the free end (without holding it in place with your fingers, which will introduce noise). If you do a lot of this sort of thing I recommend a 1/4" inline jack with a 10k resistor across the terminals.
If the noise level didn't drop significantly, the cord is okay, proceed to the next step. "

So I did this resistor thing right and yes the noise got quieter, so if you read the quote above it says
"If you do a lot of this sort of thing I recommend a 1/4" inline jack with a 10k resistor across the terminals."
Do a lot of what sort of thing? huh???

Is this statement trying to tell me that putting a 10k resistor across "poorly shielded cables" is the answer to making them quiet?

Please can someone clear this up for me. If this is what it is implying then rather than buy a load of really expensive "quiet" guitar leads then I', just gonna wire a whole pile of 10k resistors to my leads since it made it quiet whilst the cord was unterminated.

Ok next up, why do all my "probably lesser quality" guitar cables say they're shielded if they are not actually?

and what makes and qualifies for a good quality, quiet and properly shileded guitar cable? Any links to brands of cables please list for me as that will be useful also.

Ok more questions.

I shielded partially the inside of an old zoom multi effects pedal I own just to see if I could get away with reducing the noise reduction setting a little. From my early tests it didn't make any difference. However when I attached a wire to the baseplate of the pedal (which is metal and in connection with the internal shielding) and then attached the other end of the wire to the ground connection of a guitar cord (which was plugged into the input of the pedal) noise dropped a noticeable amount (not tonnes but certainly a useful amount). So would this indicate that I need to attach the shielding to the ground of the input?

I'm also very aware I might blow myself up or die from electric shock so I'm kinda afraid to do much more. Is there a danger of the shielding electrocuting me?


By the way I shielded this pedal as its plastic housing all apart from the metal baseplate.

Thanks for any assistance people I hope you all had a good Christmas and New Year

[JohnH]

That 10k resistor idea is just a test. Hes suggesting if you test alot of cable you might like to wire a resistor into a jack socket so you dont need to keep attaching aligator clips. You dont want any resistors permanently on your cords! I cant confirm how efective that test is.

For your shielding, providing you are just dealing with low voltages, you are safe. No risk in a passive guitar, but in an active unit, you wouldnt want to short the grounded shielding to the power supply, risking damage. Anything that has mains voltages in it however, beware of danger!

I get by fine with standard shielded instrument cable from the electronics store at about $2 per m, and normal metal jack plugs - they work fine and quietly.

John

[sumgai]

Gerry,

First, check your PM, if you haven't already done so.


Somewhat in order......

The 10KΩ resistor imitates your pickups and controls. If your cable is fully connected such that you can play, the approximate amount of impedance across the cable is likely to be 10 - 15KΩ, perhaps a bit more if the pickups are really hot, or if all the coils are in series. A 10KΩ resistor substitutes for this quite well.

The bit about "do this often" is referring to how often you test your cables. Even those you bought some time ago, and use daily, they should be tested once in awhile, say every month or two. That's where the inline jack comes in handy. Always test your new cables right away, and return them (if you can) if they don't live up to the test results you expect.

No, you don't want to add 10KΩ resistors to each cable, whether it be inside your guitar (one resistor for all cables) or inside the plug of each cable. That not only defeats the design and reason for matching the guitar to the amp/effects pedals impedance-wise, it'll also kill a bit of tone and volume together. Bad ju-ju, don't do that.


Cable shielding quality.....
That's a tough one. For my money, the only answer is: "it depends". It depends first and foremost on what you accept as "good enough" in terms of hum reduction. If you were to hang a simple piece of wire out of your amp's input jack, you'd get a whale of a lot of noise, eh? Now do that with your shielded cable - how much noise reduction was there? Most importantly - was it enough reduction to make you happy, or at least accepting of the cable's performance?

In many cases, if not the vast majority, guitar cables are constrained by a trade-off of impedance versus shielding capability. IOW, the more shielding effectiveness, then the greater the chance of tone-robbing capacitance. (Errr, that is where the capacitance is coming from, the interaction between the cable's two conductors (usually arranged co-axially). And at AC of any frequency, capacitive reactance is another word for impedance.) Hence, the trade-off values usually favor a noise reduction value somewhere in the neighborhood of 80 - 90% of the theoretical maximum. This also allows, not coincidentally, the player to be able to carry the cable (lighter weight, much lighter), bend the cable (too much shielding can be very stiff), and in general not worry about what'll happen if someone steps on it during a particularly active moment of joy. (Again, the trade-off is that if you go for the maximum shielding material, it becomes more fragile, comparatively speaking.)

Don't mistake this for an endorsement of the expensive cables that make all sorts of unprovable claims. Many threads/topics can be found on this forum, and others, denouncing in scientific thoroughness the invalidity of those claims, and that's all I'll say on the matter here and now. I'll add only that I am happy with my 'middle of the road, quality-wise' cables, but then again, I always travel with at least two spares.


As to the shielding of a plastic-boxed effect pedal, that's always a good idea, as you've found out for yourself. Running an extra connection between any metal parts, such as the baseplate and the jack's ground, is also a good idea, it certainly can't hurt. In fact, you don't need to do both jacks, one will be sufficient - they're already connected together via the circuitry.


And finally, what harm can you suffer from shielding? Well, once again, it depends. For the most part, shielding is benign, but it is electrically connected to something somewhere that might cause problems for the user. Best to treat it like you would a hot soldering iron.... with respect!

I guess the best answer is, if you're in doubt, ask! Each case has it's pro's and con's (the ol' trade-off trick), so we might give one answer in Case A, and a different answer in Case B. Best to leave it at that, and rely on you to ask ahead of time, all right? (Although there's no shame in trying something on your own, and asking afterwards why it didn't work! Experimentation is how we all learn. )

HTH



sumgai

[rabidgerry]

Ok, then I reckon my cables are probably poor - middle of the road variety.

And thanks for clearing up the 10k resistor query I now know what is meant by it.

I;m not sure the shielding of the pedal really worked but its not fully electrically connected so this may be why. I haven't had time to solder connections to each bit of copper tape that require a join. I will do though and see what happens.

[newey]

Rabidgerry-

There's a lot of variation in cable quality, and it's not necessarily related to the price.

I bought a 20' cable at Radio Shack for very little coin. It's a sort-of pseudo-vintage type, with a faux cloth covering which is in turn covered in a tough clear plastic coating. It's the quietest cable I own, less noisy than more expensive 10' ones I have from Whirlwind.

And, in addition to the quality of the shielding, don't buy them longer than you need.

But, more importantly, to heck with the noise, crank the amp up to 11 and blast out! It's your birthday!

Afterward, you've earned that pint of Guinness. Happy Birthday!