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Internal (electronic) Noise
All electronic amplification adds noise to the signal being amplified. This noise would be an absolute minimum at "absolute zero" temperature (-273 degrees Centigrade) but that temperature is a little difficult to reach... At room temperature there is noise arising from electron motion within resistors, transistors, vacuum tubes/valves and other circuit elements even when they are carefully selected for lowest noise performance.
The noise can be minimized by design and component selection but it cannot be completely removed. We build our electronics with the older components, like 1/8 watt resistors, 8-pin DIP op-amps, etc. They are bulkier than Surface Mount Components. But we can test them for noise. We take the time and energy to test for noise many, in some cases most, of the components of a module. It is much cheaper than selecting for low noise after they are put together.
The internal (electronic) noise level increases with gain and it is most easily heard in the treble frequencies. All treble boost will add noise to the signal. Well designed electronics built with well chosen and noise tested components minimizes this noise contribution.
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. To reduce hum and noise from external sources to the lowest level use a cavity fully shielded in metal foil which is properly grounded to the output jack.
shielding with metal foil
Solderable 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.
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:
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- -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.
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- - 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.
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- -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.
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- 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.
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- -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 also.
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