Lesson 18

Interference and Hum

The air we breath... and all around us we are surrounded by radio waves of different frequencies. Ok... we don’t really breath them in... they either pass right through us, or go around us. But they are there, and they carry energy. When they travel in the path of a conductor, the conductor can absorb their energy as voltage fluctuations. Remember lesson 3,... about how sound flows electrically as voltage fluctuations... so when you amplify this signal, you will also amplify the interference. That’s actually how a radio works, the antenna is tuned to pick up the waves within a certain frequency range, those waves induce a small voltage into the antenna, and that voltage is amplified so we can hear it. 

Wait a minute... if any conductor can pick up the radio... how come I don’t hear it every time I plug in an guitar or microphone?
Aha... that’s a good question and I’m glad you thought of it. The truth is... they do. To pick up an audible radio station, the cable has to be just the right, perfect blend of materials, length, and matching radio station. High impedance unbalanced cables, such as guitar cables are especially susceptible, but the chances of everything matching up perfect are quite slim... though not impossible. There can be the rare instance where a guitar amp or a keyboard plays a radio station. I’ve seen it happen a few times.

Balanced cables are less prone to this, because they are designed to be resilient against interference. As explained in Lesson 11 on analog cables, that’s the whole purpose of balancing them. 

The other source of interference is from 120 volt electrical lines. The nature of alternating current is that the voltage fluctuates between 120 volts positive current, and then it switches to 120 volts negative current. It does this 60 times per second. Now, what happens is that this rapid swing of energy emits an electrical field, and that electrical field will  be induced into any nearby conductor. That is how transformers work, they are comprised of 2 coils of wire right next to each other, and when current is passed through one coil, it basically jumps into the other coil with different properties of voltage, impedance, and current. That’s the point of a transformer, it provides an isolated copy of the signal, with precise control of the electrical properties, depending on the design of the transformer. Only alternating current will create an electrical field that radiates and can be absorbed into another conductor. DC current does not do this. 

So anytime you have a 120 volt electrical cord running somewhere, be mindful not to run any microphone cables along side it. Always separate them by at least a few inches for xlr cables. Guitar cables, and other unbalanced cables will need more space, at least 6 inches, because they are more prone to picking up interference. So Run all your electrical cords together, and all your audio cables together. If you need to run an electrical cord across an audio cable, do it at 90 degrees to minimize this effect.

If you are using a wall wart for a audio device, avoid running any cables near the box thing.. it houses a transformer and will have a bit of a stronger electrical field. However, the cable coming out of the wall wart will be DC voltage, therefore it will not have an electrical field. and Here’s an example of using a wall wart near audio cables.

Another source of interference is poor grounding. The whole idea of proper grounding, such as a grounding shield on a guitar cable, or pretty much any piece of gear, the outer case is always grounded. This is so that this grounding shield pics up the interference, and routs it to ground, and then it’s gone forever. Sometimes, a signal wire isn’t properly isolated from the ground. Since all the unwanted noise, hum and interference is captured and drained into the ground... the idea is that this noise never makes it to the signal wire. But, if there’s a path for it to travel from the ground to the signal wire, all that garbage will make it into the signal. This is the worst to troubleshoot, because it can be difficult to isolate the specific part where the where the problem is.

Microphones can also be sensitive to interference. Condensers especially, because the capsules convert air pressure fluctuations to electrical voltage fluctuations, and the capsule emits an extremely weak signal. The capsule itself, component or a wire can act as an antenna, pick up the interference, or radio station, and send it along with the signal from the capsule to be amplified. This is why a good microphone, such as any iSK microphone, will have the circuitry completely encased in a vindictive body which acts as a faraday cage and doesn’t let in any interference. 

There’s a lot of sources of interference. Radio stations, 2 way radios, tv, remote controlled devices, garage door open pets, cordless phones, cell phones, microwaves, fluorescent lights, certain light switch dimmers will corrupt the electrical on that breaker. Apartment buildings are often problematic for interference, because there’s just so many of these sources all crammed into a neat little package called an apartment complex. 

Fluorescent lights are the worst for interference. If you have them in your studio... get rid of them, use something else. They emit a strong broadband field of basically white noise in radio waves, and your gear will pick it up in small amounts, whether you notice it or not, it’s there.