# Humbucker-style microphone interference cancelling

For a recent experiment, I decided that it would be a good idea to try to add some electric guitar distortion effects to my otherwise Classical flute playing.
In order to do this, I attached a small electret microphone to my flute, which is powered by a phantom power unit and provides an audio signal to a guitar amplifier. A rough idea of my setup is shown in the following circuit diagram:

simulate this circuit – Schematic created using CircuitLab

(I did not include component values as I don't think they are that important in this problem, but I can add them if required.)

This setup seems to work quite well, I am able to add as much distortion as I want using the amp. However, there seems to be a bit of interference (mains hum, etc.) present, as there probably would be in proper electric guitar setup. My friend, who plays electric guitar, immediately noted that I should try to implement something similar to a Humbucker pickup, which is a type of guitar pickup that cancels out most interference, but with microphones instead of pickups.

My understanding of how a Humbucker pickup works is that instead of having just a single pickup, there are two, wired in series or parallel. The pickups have opposite magnet configurations (ie. one has all magnets with the north poles pointing upwards, and the other has magnets with the south poles pointing upwards) and wire coils wound in opposite directions. Any interference induced in the coils is then cancelled out because the interference from each coil is equal and antiphase in relation to one another. The signal from the guitar string remains unaffected because the although the coils are wound in opposing directions, the magnets are also in opposite polarity.

Based on that understanding, I came to the conclusion that it isn't possible to replicate the Humbucker setup with my microphone, because there is no equivalent to "winding the coils in opposite directions" or "reversing the polarity of the magnets" in an electret microphone. Simply connecting two of my microphones in series or parallel (which I believe is what my friend was thinking) would increase the audio signal but also the amount of interference, due to the microphones (and therefore interference) being in phase with each other.

So my question is, is it possible to replicate the concept of a Humbucker pickup but with microphones instead of guitar pickups?

(And if so, will it actually make a difference? Is the perceived interference I'm getting actually from the microphones themselves, or another source such as wiring?)

• I think most people use balanced lines to counter that kind of interference Feb 2, 2016 at 13:23
• @PlasmaHH I did some very brief research on balanced lines, but I still don't understand how they actually help reduce interference. "Ok, so we have two antiphase lines that have relation to ground. But doesn't that just mean interference is now affecting 1 extra line?" In any case, I'd prefer to keep with using my current electret microphones (I'm assuming balanced lines require a different type of microphone?). Feb 2, 2016 at 13:29
• yes, but in the end, you subtract one line from the other, thus cancelling out that interference taht influenced both Feb 2, 2016 at 13:32
• @PlasmaHH Oooohhh right, now I understand, thank you. So it would end up sort of being compared on the inputs of an Op-amp or some similar device, in which case like you say the interference would be subtracted from itself Feb 2, 2016 at 13:38
• Assuming you want to continue with the electric guitar amp, the simple starter is to make sure that your shielding is good everywhere; the complex (and \$) method would be a balanced microphone to a preamp with balanced input and an unbalanced output to the guitar amp. Feb 2, 2016 at 14:40

is it possible to replicate the concept of a Humbucker pickup but with microphones instead of guitar pickups?

A humbucker guitar pickup is intended to directly cancel the induced voltage caused by magnetic fields. The magnetic fields that a guitar is susceptible to will be barely a problem for a microphone because the coil is normally much smaller and shrouded by ferromagnetic material that would cause external alternating magnetic fields to bypass the coil.

A guitar pup on the other hand, has an "open" sensitive area in order to project a static magnetic field that can be "modulated" by the movement of the "iron rich" strings. It is much more susceptible to hum because of this.

If a microphone is picking up a lot of hum then, the first question to ask is if the cable feeding the microphone is balanced and is the microphone driving a balanced signal. I suspect that the answer here is no because clearly, an electret is a polarized microphone and one end needs to connect to ground whilst the other end feeds the signal to the amplifier.

I'd try using a conventional moving coil microphone and a balanced cable and balanced input module. A guitar produces an unbalanced signal and the amp is also unbalanced (because it doesn't need to be balanced).

If the magnetic field hum were picked up by the electret - you'd have to use another electret and invert its signal in order to cancel the main microphone's hum but, the problem you have here is that both microphones need to be in close vicinity to "receive" the same level of hum and this means acoustic cancellation and therefore poor levels for the signal you want to keep.

Balanced line with phantom power

simulate this circuit – Schematic created using CircuitLab

Figure 1. Balanced line with phantom power.

If you are happy with the sound with the mic you've got you could use a balanced phantom power arrangement with a pair of microphone transformers, 10 kΩ to 600 Ω, centre-tapped on the lo-Z (600 Ω side) Wire up as shown in Figure 1.

How it works

• The supply for the FET is fed in at the centre-tap at the amplifier end, splits both ways and travels up on both signal wires. The centre-tap arrangement means that the opposing DC currents cancel out and don't affect the core.
• At the mic end the procedure is reversed.
• Any induced hum should now be common to both signal wires and cancel out at XFMR2.

The biggest problem with this might be the additional bulk of the transformer at the flute end.

Edit: @supercat suggests R2, C2 to improve voltage stability. See comments.

• Could the circuit be improved further by adding an RC filter in series with the supply feeding R1 [instead of having R1 connect to the center tap directly, connect to a node with one side of a cap to ground and a resistor to the center tap]. Feb 2, 2016 at 22:15
• It wouldn't do any harm. It might help if there was high common-mode noise. I guess many 48 V (standard AFAIK) phantom-powered microphones would have had a dropping resistor anyway so they would have decoupled. Let's make that a yes. I'll update. Feb 2, 2016 at 22:20

It's probably a grounding problem. For example, if you are actually plugged into a pedal or something powered from a wall-wart, the ground is going to be polluted with noise. Try plugging directly into the guitar amp so that your ground is not connected to anything else.

Also, try disconnecting the battery and ground the very end of the signal cable. Meaning short out the electret leads. Do you still get noise? If yes, it's not the circuit, it's just the guitar amp or whatever else you're connected to.

If you ground the input and the noise goes away or even get's noticeably better, then it's outside the amp. In this case you need to use a shielded cable. The balanced circuit proposed previously would probably have absolutely zero effect (not unless the cable is really long and runs by noisy stuff). But if you just use a shielded cable, that might make a difference. And the shield should extend as close to the electret as possible.

So the circuit might look something like the following:

The circles represent the ends of the shield. So from the right to the left you have the guitar amp plug, then a piece of mono shielded cable like a regular unbalanced guitar cable and then a battery and then a piece of two conductor shielded cable to the electret mic circuit.

You might even use some foil tape to extend the shield to completely encapsulate the capacitor and resistor (using also some shrink tube to prevent the foil from shorting things out).

But my best guess at this point would be that, if your gain is high enough to create fuzzy distortion, then it's just the guitar amp or something connected to it (if this is the case, the noise will not be significantly reduced by shorting the electret leads).

A humbucker connection cancels the pickup of magnetic fields, which is inevitable with an open faced guitar pickup.

An Electret Mic however is entirely electrostatic (ei there should be no magnetic pickup), so a humbucker cannot do anything.

Judging by your circuit, you do not have a continuous metallic cable shield running from the plug to the body of the mic.