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As far as I know, there're two kinds of humbucker:

picture of two humbuckers

The left one (called open coil), and the right one (called covered), are both humbuckers. What makes them different?

6

From Sweetwater:

  • Covers may cause feedback problems with high-powered amps
  • The pickup’s magnetic field may be affected by the cover’s plating materials; brightness may be reduced by nickel, chrome, or gold plating
  • Electrical interference may be picked up by uncovered pickup coils
  • Covers may reduce 60-cycle hum
  • A nickel-silver cover may have less capacitance, which can affect brightness (tone might have more bass/low-frequency)

From Gibson Forum:

Lots of vintage Gibsons have lost their original pickup covers, because back in the days when amp distortion was harder to achieve it was commonly believed that removing the covers increased output. In fact removing the covers decreases capacitance, so it's more likely to create the 'illusion' of loudness by lifting the high frequencies.

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  • This all makes sense to me except for “Covers may reduce 60-cycle hum.” Isn’t the whole point of humbuckers that they eliminate that hum by combining the signals of the two coils? Shouldn’t that apply to covered and open coil humbuckers alike? – Alex Basson May 7 '19 at 9:59
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The main difference is covers increase capacitance to further "buck the hum". As with "humbuckers" vs single coils in general, this has the added effect of taming the highs or muting the pickup clarity. Just like singles are quieter yet clearer, but also noisier than humbuckers, uncovered hums are noisier but clearer than covered hums (though not quieter in output if it's the same pickup, obviously). Humbuckers are when you put two reverse wound single coils together, it has the effect of canceling out the high frequencies, making it more powerful and loud, because it is doubled, yet half as clear. This also cancels out most of the hum in those frequencies. The cover then completes the hum elimination, especially when soldered to the baseplate, creating a complete metal "shield" (faraday cage) around the pickup. 60 cycle hum and rf noise are not the same as microphonic feedback. You will buck the hum and rf noise as good as it gets, but the microphonics will be similar or worse. The best way to kill microphonics in covered hums without extra potting is to make sure everything is really tight, including the humbucker itself in the mounting ring or pickguard. Using surgical tubing instead of pickup height springs can help. Feedback is reduced with pickup closer to strings, increased with pickup further away, and this includes microphonics. I think part of this is because when the pickup is closer it is really tight in the mounting ring. The microphonics are worst when the pickup is sunken into the pickup cavity, so really loose with a hollow area around it to create echo.

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A "humbucker" has two coils, which are wound in opposite directions. Since the ambient noise will be at different polarity in the two coils, summing the signal from them thus subtracts the ambient electrical noise. Whether they have a cover or not is mostly aesthetic.

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