Post by JohnH on Aug 1, 2019 21:10:08 GMT -5
Cancel that Hum
How to find pickup combinations with low noise
These notes describe the issues relating to hum and noise in guitar pickup combinations, along with some circuit ideas to illustrate them. I have presented the reasons why some combinations are better than others, and how to set up combinations with two, three or four pickup coils, in-phase or out-of-phase, with low noise.
Reducing Noise
Most of these notes will be about pickup combinations that cancel hum, but the complete package for a low noise guitar should also include:
- Correct grounding to a single point
- Appropriate shielding
- Circuitry that does not leave partially disconnected or bypassed coils, with one or both ends connected to the output even when the coil is not active
That last point I have not seen written, but I believe it is important. Try attaching a loose piece of wire to the tip of a guitar lead and plug it in to an amp at high volume, to see what I mean.
Hum and Humbucking
Hum is induced in a magnetic pickup from the electromagnetic waves produced by the electrical equipment which surrounds us. It is at mains frequency, (50 or 60 Hz) and is induced in all conventional single coil pickups, and in each coil of a two-coil humbucking pickup. The trick with humbucking, is to arrange things so that the phases of the sound signals from each coil reinforce each other, while the hum signals cancel out.
There are two characteristics that affect the phases of the signal and of the hum. These are the polarity of the magnet and the direction of the winding. The phase direction of the signal is affected by both of these, so reversing one will reverse the signal phase, but reversing both will leave the signal as it was. The hum however is only affected by the winding direction, not the magnetic polarity.
Hence in a humbucking pickup, the second coil is reverse wound and reverse magnetic polarity, which is still in-phase with the sound signal from the first pickup, but is out-of-phase with respect to the hum, which therefore cancels out. I will show this on diagrams like this:
The ‘+’ indicates the phase direction of the sound, so these two are in-phase for sound. Red indicates reverse wound and reverse polarity, so these two coils cancel out hum, to form a humbucking pickup, with the coils connected in series.
The hum-cancelling also works in a parallel arrangement like this:
Out of Phase Sounds
If however, we use switches to reverse the connections to one of these coils to get an out-of-phase sound, we loose the hum-cancelling and result in a hum-adding arrangement (doing this to a humbucker is actually quite a funky sound, but you have to live with the hum!).
A similar hum-adding result is obtained in a parallel arrangement:
To obtain an out-of phase sound with low hum, it is necessary to find two coils with the same winding direction, and connect them in reverse:
parallelseriesThree single-coil pickups
Here is basic wiring for a three-pickup Strat style guitar, with a 5-position switch (ignoring the volume and tone pots):
The middle pickup is reverse wound and reverse polarity with respect to the other two, so that the intermediate switching positions neck/middle and middle/bridge are in-phase and hum cancelling.
Based on the points above, an obvious missing combo, to provide with extra switching, is neck/bridge. If this is done out-of-phase, it will also be hum-cancelling, and provide a very different sound to the others. This could be done in various ways with a DPDT toggle switch, or maybe as part of a push-pull pot:
Hum cancelling with three active pickups
How can this be possible, surely only two of the three pickups can cancel out hum, leaving one un-cancelled? It is actually possible however, in a series/parallel arrangement like this:
Above is a version with all pickups in-phase with respect to sound. In this diagram two ‘normal’ (blue) pickups are in series (hum positive). These are in parallel with a reverse wound pickup (hum negative). The single reversed pickup cancels the hum from the two series connected ones!
How does this happen?
Imagine each pickup as a small power source (eg a battery), in series with a resistance. Just think of the hum signal, so the blue pickups are out-of- phase (for hum) with the red one. Now think of each power source separately, and how much of the voltage it produces appears between the ground and output.In the red pickup, 1/3 of its hum signal is lost across its own resistance, and 2/3 appears across the output:
In each blue pickup, only 1/3 of its hum signal appears at the output:
The red pickup provides hum x 2/3, each blue pickup provides hum x -1/3, so all added together, no hum!
For a hum-cancelling combination with one pickup out of phase, move the reverse wound (red) pickup into the series group. Note the phase of each pickup, so that the series pair make opposite hum to the parallel coil:
In conclusion, three-pickup hum cancelling series/parallel combinations are possible in three versions:
- Middle pickup in parallel, neck and bridge in series (all sound in-phase)
- Middle pickup in series with neck out of phase, with bridge in parallel
- Middle pickup in series with bridge out of phase, with neck in parallel
Twin-coil Pickups
Here is the basic dual humbucker switching. Everything is in-phase and hum cancelling. Note I’ve shown the reversed coil in a different position on each pickup. This doesn’t affect the basic switching but is important when considering switching for other options like single-coil.
One of the most popular switching options with dual humbuckers is coil-cut to give single-coil sounds. To let the resultant combination of single coils be hum-cancelling when used together, make sure one of the coils which is active after coil-cutting is reverse wound with respect to the other.
Out-of-Phase with coil cut
To get an additional out-of phase sound with two single coils, it is best to arrange for both active coils to be wound the same way ie, on one of the pickups, the ‘other’ coil is cut (different to the in-phase option above), so that it makes a hum-cancelling out-of-phase option instead of a hum-adding one. Making this work, in addition to an in-phase option is a puzzle, but here is one version. This circuit reverses the phase of the whole bridge pickup, as a way of changing the coil which is the one to be cut.
This circuit provides series-connected neck or bridge pickups, separately or together, in-phase or out-of-phase, or single coils separately or together, in or out-of-phase. Also, you can select either of the bridge coils separately (they sound slightly different). Apart from the single coil only options, all the combinations are hum cancelling.
Four coils active
Obviously, with two humbuckers both in-phase, the combination is hum cancelling.
This is also true in an out-of-phase arrangement where both coils of one pickup are reversed, because each cancels its own hum:
Now going back to that funky sounding series out-of-phase wiring of a two-coil pickup, where each pickup is out-of-phase with itself. On their own, each pickup wired this way has high hum. But if you combine both pickups such that there is balance overall — the hum cancels.
Three coils active
Finally, on the same basis that applies for three single coils, you can coil-cut one pickup and put the other out-of-phase with itself, to obtain another hum-cancelling combination:
This is quite useful, because it sounds a lot like the single coil only, with plenty of high harmonics, but with no hum. Most other ways of taming single-coil hum tend to lose high harmonics.
