Hexaphonic Pickup Project

[Veronica Merryfield]

A coil does not act like an antenna but it will pickup some EM field radiation, but in an entirely different manor to an antenna. However, the signal that is picked up is not common mode. As I said, if the amplifier is close to the coil (you mentioned in the pickup with the coil), then the common mode pickup is close to zero, for most cases. If you are trying to use the guitar in a high EM field environment, then common mode pick might be an issue, but you'll have other issues that will out weigh the common mode.

Differential pickups are designed such that the coil without a magnet in the core detects the signals that the other coil with the magnet picks up in addition to the variable reluctance derived signal from the string movement. This is not common mode with regard a differential op-amp.

Cancelling in this way is hard because no matter where the cancelling coil is located it will be in a different place from the desired pickup, will be wound different and will have different coil characteristics because the core material is different. You best bet might be to have a method to adjust the cancelling signal level on an individual basis and allow for null cancelling.

A low impedance coil will give better results since it will be intrinsically less susceptible to picking up the background signals. Since you are worried about the characteristics of pickups with large turn count (L, C and R), this would be a good route for you to take. Whist some designs use a current transformer, you could use a single turn and an current detector - either a sense resistor, or an active device such as a hall sensor for example.

If you are really brave with the DSP, you could use a steered correlation algorithm to remove the back ground noise from the signal of interest using the null coil signal for steering.

[Joel de Guzman]

A coil does not act like an antenna but it will pickup some EM field radiation, but in an entirely different manor to an antenna. However, the signal that is picked up is not common mode. As I said, if the amplifier is close to the coil (you mentioned in the pickup with the coil), then the common mode pickup is close to zero, for most cases. If you are trying to use the guitar in a high EM field environment, then common mode pick might be an issue, but you'll have other issues that will out weigh the common mode.

Differential pickups are designed such that the coil without a magnet in the core detects the signals that the other coil with the magnet picks up in addition to the variable reluctance derived signal from the string movement. This is not common mode with regard a differential op-amp.

Cancelling in this way is hard because no matter where the cancelling coil is located it will be in a different place from the desired pickup, will be wound different and will have different coil characteristics because the core material is different. You best bet might be to have a method to adjust the cancelling signal level on an individual basis and allow for null cancelling.

A low impedance coil will give better results since it will be intrinsically less susceptible to picking up the background signals. Since you are worried about the characteristics of pickups with large turn count (L, C and R), this would be a good route for you to take. Whist some designs use a current transformer, you could use a single turn and an current detector - either a sense resistor, or an active device such as a hall sensor for example.

If you are really brave with the DSP, you could use a steered correlation algorithm to remove the back ground noise from the signal of interest using the null coil signal for steering.

Again, EMG active pickups also have two coils connected to an Op Amp in a differential manner, also very close to the coils. Did EMG got it wrong too? I don't think so. Any kind of wire or coil or loop, will act like an antenna. It is true though that low impedance coils are less susceptible to EM.

Also, don't forget another benefit of a balanced design: the fact that the inputs are floating isolates them from any noisy ground reference. That is a very important factor. Ground lifting and isolation is inherent in a balanced design. People go to great extents lifting and isolating the noisy ground.

But you are right that it will still pickup EM albeit to a lesser degree. For that matter, it might interest you to know that the design (damn, can't link here!) "take(s) advantage of the multiple coils and have the coils alternate from north-south pole orientation, clockwise winding for the 1st, 3rd and 5th coils and south-north pole orientation, counterclockwise winding for the 2nd, 4th and 6th coils. That way, the sum of the pickup outputs will cancel some more of the noise the same way humbuckers do."