Cavity/pickgard shielding

[Cagey]

pickup as an antenna - sure. That's the problem with single coils and no shielding will get rid of this. But shielding the cavities should get rid of any extra noise that would come in through unshielded wires, right? And at least for hum buckers that should be the major source of noise.

You would think. But, mean ol' Mr. Reality has shown us over and over again that it doesn't work. If you really want a quiet guitar, you need to shield the wires individually, and keep them as short as is practical.

The problem is that amplifiers (and special effects) have very high input impedances because the signals a guitar generates are very small. You need a brick wall for them to run into in order to drop any signal across an input stage. That makes them ultra-sensitive.

You can see it in action if you've ever played with an oscilloscope. They typically have input impedances that are in excess of 10MΩ, so all you have to do is touch the end of the probe with your finger and you can see the large voltages induced in your body from the magnetic fields in the surrounding environment. If you set the trigger right to make the waveform seem to stand still, you can also see all the harmonics riding on that wave, which are of higher frequencies that can sneak into smaller openings in your shielding. It may not sound like a higher frequency when it gets amplified because you're only hearing single cycles riding on a lower frequency, so it comes through sounding like 60hz hum, but that's what it is.

 

[line6man]

You can see it in action if you've ever played with an oscilloscope. They typically have input impedances that are in excess of 10MΩ

Most general purpose oscilloscopes have 1M Ohm input impedances. ~20pF capacitance is also common.

 

[Cagey]

Most of the oscilloscopes I've played with were not general purpose, so maybe I'm a bit off there. But, the observation stands. You'd see the same sort of thing even on an old B&K Precision or EICO 'scope.

 

[Jumble Jumble]

Also there's the fact that the pickups aren't in the faraday cage, theyre popping out of it.

I can deal with the tiny amount of noise induced in hum bucking guitars and I don't do any shielding at all on those.

 

[line6man]

Most of the oscilloscopes I've played with were not general purpose, so maybe I'm a bit off there. But, the observation stands. You'd see the same sort of thing even on an old B&K Precision or EICO 'scope.

I've had various scopes from the '50s, '80s and '90s and I've not seen high impedances like that often. It's absurd for a lot of applications. I'd imagine some of those high end Tektronix scopes probably have a variable input impedance, though. A lot of multimeters these days have low impedance input modes. Fluke makes a number of meters with 50 Ohm voltage measurements.