• The magnet height. How close the individual magnets are to the strings will determine how loud that string is. On pickups that have adjustable pole pieces that’s not so much of a problem, but on pickups with fixed pole pieces (like a Fender Strat or Tele) that could cause a slight imbalance in the string output.
As an example, prior to the late 1960s, most guitarists used a wound G string, so the fixed height of the magnets on a Strat were different to compensate.
• Pickup cover. Metal covers on humbuckers can cause a resonance that results in feedback problems at high volumes. That’s why many of the early rockers removed their pickup covers, and why many guitars and pickups are sold that way today.
• Pickup potting. Many pickups are sealed in wax to eliminate vibration induced signals that make a pickup microphonic. The heat from the hot wax can weaken the magnet though, thereby changing the pickup’s sound.
• Potentiometers. Although not exactly a part of the pickup itself, the volume and tone pots are part of the electronic circuit along with the pickup and can affect the sound. The higher the resistance of the pot, the more high end will pass.
Fenders use 250k ohm pots, Gibson uses 500k, and many other manufacturers use 1 Meg pots.
There are other factors such as winding direction, magnetic polarity, and the type of bobbins used, but their contribution to the final sound is subtle at best.
As if the known factors in building a pickup weren’t enough, consider the many intangible factors as well. For instance, most pickups loose their magnetic strength over time because of environment and electrical interference.
Pickups can become weakened or demagnetized completely by leaning your guitar against an amplifier with large transformers, or even from taking your guitar too close to the train motor of a subway (as happened with Andy Summers of the Police).
Another intangible is the fact that tolerances of just about every component were much looser until the 1990s. While the difference was indeed subtle, add enough components at the edge of their tolerances together and you suddenly get a pickup that sounds different even though it’s made the same.
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Bobby Owsinski is an author, producer, music industry veteran and technical consultant who has written numerous books covering all aspects of audio recording. For more information be sure to check out his website and blog.