External Hum and Noise
All of our active electronics modules are shielded with metal foil. If necessary they can be used in an unshielded instrument cavity, but a shielded cavity is always preferable. To reduce hum and noise from external sources to the lowest level use a cavity shielded in metal foil which is properly grounded to the output jack and bridge. Poor ground connections are the top reason for electronic noise.
High frequency noise that goes away when the treble is turned down is usually due to a ground problem between the preamp and the bridge. Please check that the bridge is well grounded. Make sure that the preamp has a solid connection to ground. A potential source of high frequency noise is your cell phone. The cell phone in the pocket will get me every time.
Note that Clyde is quite a geek about noise immunity, having spent many years as the responsible regulatory compliance and electromagnetic compatibility engineer and manager for several high-tech companies, so this gets kind of deep:
Instrument cavity shielding is often misunderstood. It is easy to conclude that a cavity should be completely covered with grounded copper foil, so that is a reasonable approach when professing to novice technicians. But technically, only the top and bottom of the cavity require shielding. It is important to understand that a cavity shield serves two purposes.
First, it is a signal reference for all of the electronics inside the cavity. Every conductor in the cavity carries an electromagnetic signal that has a corresponding return path. A proper shield will act as that return path, while an improper shield will require that the return path divert through other conductors. While this is not a problem for proper circuit operation, it does allow each conductor to act as an antenna. For a cavity shield to act as a signal reference, it need only be a single grounded plane of a good conductor, like copper.
Second, the shield acts as an isolator for electromagnetic interference and radiation. Generally, the technician is not concerned about electromagnetic radiation because the internal electronics are both relatively low power and low frequency. Electromagnetic interference, which is radiation from other electronics such as lighting, equipment, and radio transmission, on the other hand is often a problem for instruments. The instrument cavity needs to be properly shielded to block this interference. A proper shield is a conductive cage that will block interference that is in the frequency range that will produce audio frequency signals in the instrument electronics. This shield can be as simple as 2 planes of copper on the bottom and top of the cavity tied together by a conductor at each end of the cavity. This creates a cage with an aperture of about 150mm which will block interference signals that have quarter wavelengths longer than the aperture. A quarter wavelength of 150mm equates to a frequency of 500MHz, which is significantly beyond the audio spectrum.
Shielding with metal foil:
Solder-able metal foil (copper or brass) should be used. All parts of the metal foil shielding should connect to ground by firm metal to metal contact (cover plate screwed down to metal "lip") or by soldered connection. Understand that any foil that is not connected to ground will act as antenna - making noise problems worse. Again, poor ground connections are the primary source of noise.
Shielding with conductive paint:
Good shielding paint properly applied is reasonably good though not as good as metal foil. Extra care must be taken in grounding it. To make a good connection from the paint coat to the ground of the output jack, try the following:
- Strip the outside insulation and the inside guts from a 2" or 3" length of our 1 conductor shielded cable (if you have grounding braid available you don't have to do this, of course). Flatten it and scrape it on both sides for about 1/2" with a sharp knife blade so that you see a lot of "bright" metal.
- Staple it or screw it with a very small wood screw to one of the painted sides of the cavity. Give it 2 or 3 coats of the shielding paint over the scraped section, the paint will contact a lot more of the metallic conductor than if it was a single wire.
- Solder the other end to the ground lug of the output jack. While you solder, grip the braid near the jack with long nose pliers or an alligator clip so that the heat does not travel down the braid and bother the "join" to the side wall.
The quality of paint shielding can be checked with an ohmeter ( a multimeter or volt/ohm meter set to Resistance and to the X1 scale). The meter probes should be freshly sanded (lightly with 320 or 400 grit) to remove oxidation from the metal. Adjust the zero setting with the probes touching each other.
Touch the probes to the painted surface at points 1" apart. The reading from the meter should be in the region of 10 to 20 ohms, the lower the better. The best paints and silver inks will measure 1 to 2 ohms. Metal foil will measure less than 0.1 ohm. Make sure that you get a low reading to the output jack ground and bridge also.