Why Your Preamp Hums and How to Fix It

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What Is That Humming Noise?

If you’ve ever experienced a humming noise in your music when playing a guitar or bass through your amp, your instrument is likely suffering from some unwanted vibrations or electric charges that are creating a buzzing sound. These vibrations can pre-exist in the instrument, particularly if it's made out of wood, or they may only appear when the instrument is amplified.

Regardless of what is causing your instrument to hum, a few simple tips can help eliminate guitar humming.

Isolate the Vibrations

The first step to eliminating the noise is to determine the source. Put your ear near the speaker to try and isolate the noise and determine where it’s originating from. To do this, you will need to play the instrument while the amp is on.

Most of the time, the noise will not come from the instrument but from the speaker or the amp itself. If this is the case, the vibration will only be heard when playing the instrument, and not when the amp is unplugged. If the sound is coming from the speaker, the buzzing will be heard whether or not the instrument is plugged in.

What Could It Be?

There are several reasons why a guitar preamp will hum:

If your amp is plugged into the same outlet as a light switch, you are probably experiencing 50/60 Hz AC Hum. This particular frequency hum comes from normal household electricity and is a byproduct of the power grid. It is not the result of harmonics produced by your amplifier.

The AC hum is not audible when your amplifier is plugged into an outlet that isn’t shared with a light switch but is apparent when both are plugged into the same outlet, and the light switch is turned on. If you have an amp that makes a humming sound when you have the switch on (usually the standby switch) then you most likely have an issue at your house. You can try using a line conditioner on your amp. This will not eliminate the hum but might help to reduce it.

So, what is the cause?

When you run your signal through a preamp, you may be making it easier for external noise to be amplified. If you’re a musician, you’ve probably dealt with this issue. It’s common in most instruments, especially when the stage is small. The stage may have a certain amount of acoustic space that causes enough noise to be picked up on the preamp and transmitted through the cable to the amplifier, which in turn modifies the instrument’s sounds, making it difficult to tune it or record it properly.

There can be several causes for some noise problems. If you’re experiencing hum every time you run your signal through your preamp or if your amplifier starts to hum, blare out, or at least get deafening, then the best thing you can do is to have your gear tuned by an expert. In case you can’t justify the cost, you’re going to choose a DIY option. If that’s the case, you will at least be able to figure out the cause and how you can fix your amplifier without breaking the bank.

Ground Loops

Preamp hum in your high-quality, interconnect-powered mixer or soundboard is a result of a ground loop. A ground loop is a current path between a hot (“hot” is the technical term) wire and a ground wire. These wires may be in the same or different devices. The current is called ground loop current. Other names for ground loop are ground loop voltage, ground potential problem, or EMI (electromagnetic interference) caused by a ground loop.

The two basic causes—and solutions—of a ground loop are connected equipment and unbalanced grounds. A ground loop occurs when the same wire is used for both a hot lead in one piece of equipment and a grounding wire in another piece of equipment. Because this wire is also a hot lead into and out of the other piece of equipment, the end of this wire is not “at ground potential.”

The voltage at this end of the wire is between “ground potential” and the voltage level set by the voltages present at the two ends of the wire. “Ground potential” is often the same as zero volts. This “non-zero” voltage is called the ground voltage level.

How Do Ground Loops Occur?

Ground loop hum occurs when two pieces of audio gear are connected together with a single cable and share the same ground.

The audio signals from two different gears are carried over the cable, being amplified in each gear at certain frequencies. Since the grounds are also connected, the two grounds add together, resulting in a "grumbling" or "humming" sound in the background.

This is because the two different frequency signals are being amplified at the same time and are also out of phase with each other. If your gear has only one outlet, another audio device can cause ground loop hums.

When placing your preamp near a TV or video monitor, the TV's internal power supply can create a ground loop. This is because the video equipment uses the house AC voltage as returns, but the house ground is also used to protect the equipment from electrical surge damage.

How Do I Get Rid of It?

A humming preamp can be caused by a variety of problems. The important thing is to figure out what the problem is before you decide how to fix it.

The causes of humming could be anything from a loose connection inside the preamp to a faulty component. Depending on the cause, you should either be chasing that specific component or the entire preamp.

For instance, if the main problem lies with the actual guitar your preamp is hooked up to, you should replace it. However, if the problem is a loose component, such as a jack, you should be troubleshooting it instead of replacing the entire preamp.

Another common problem is that your guitar may feedback. This can be fixed by routing the guitar’s cables through the floorboard, or by placing a foam underneath the cables.

What If I Can’t Accommodate a Single Socket?

Traditionally, if you need to cover a hum – in a grounded preamp or a single point – you’ll likely need to use an isolation transformer. These are sometimes affordable, but they’re much more complicated and bulky than their Ground Loop Isolators. So we recommend using the latter wherever possible.

Say you have two pieces of audio gear in your rack: a preamp that’s grounded, and an outboard piece that’s not. If you’re experiencing a hum when you connect these two pieces of gear, you’re going to need an isolated output transformer for that single point of contact.

What If the Humming Persists?

If the humming is part of the preamp itself, then it could be related to a grounding issue – either internal or external. Check the subwoofer’s ground to make sure it is connected to the right location in your system (i.e., to the ground screw on the back of the receiver). If you’re sure the subwoofer is connected correctly and the humming continues, then continue to step two.

If you’re still having problems, the humming could also be due to a ground loop. If you have a lot of surge protectors connected to your system, chances are you’re probably in the presence of a ground loop. This loop can be caused by two or more pieces of equipment doing the same thing. For example, there can be two pieces of equipment plugged into the same outlet but connected to a surge protector. With a ground loop, you’ll find that the humming and buzzing start in one location and then gradually shifts to the other location (this movement is your ground loop). To break the ground loop connection, check all of your equipment. Start with the computer that you’re using and work your way up to the preamp.

How to Troubleshoot Ground Loops

When you use multiple pieces of equipment together in your studio, it will send an electrical current from one piece, through the next, back to the first, and so on. This loop will be completed through the ground in your wiring, either in the cables themselves or in the walls. This loop will contain a guitar signal, bleed from one track and other noise, which you won’t want to be recorded. When you record, you want the signal from your instruments, not noise and bleed from other tracks or your surroundings.

Loud ground loop hums can be caused by several different kinds of grounding problems. These include improperly grounded power supplies, improperly grounded audio equipment, a loose connection in the ground wire, a short in the ground wire, or a problem with the electrical wiring in your studio.

To fix a ground loop hum problem, you need to find its source. If your home is properly wired, there shouldn’t be a ground loop hum where you can’t find its source.

The first step in fixing the ground loop is to find the offending piece of equipment that is causing the interference. This may be the equipment that is wired in a different, isolated electrical circuit.

If you don’t have a separate circuit just for your audio equipment, then it may be that piece of equipment that is connected to the ground wire in the electrical wiring.

Ground Loops in Audio

"Humming" in a guitar or amplifier can come from many different causes. An instrument plugged into an amplifier is a circuit, meaning that it can build up electrons as a result of the electric current moving through the wires. If there's enough current in the wires harnessing a guitar or amplifier, the electrons can flow freely.

However, if the guitar or amplifier isn't grounded (meaning that the amplifier shouldn’t sound like it has electrical "hissing"), it can produce a buzzing sound. It will continue to produce this sound until all of the electrons are grounded – fixed.

Ground Loops in Video

Ground loops are pretty common, and most electronic equipment generates its own ground current. Ground loop hums can infect all of your gear and mics. The effect is usually more noticeable if your mixer it's connected to a computer or a house’s AC outlet.

To solve this issue, first, make sure that each piece of equipment is plugged into its outlet, and that there is only one cord coming from that outlet. This helps minimize ground loops.

If you still hear a constant, low buzz, you can ground the equipment with a simple plug adapter, and in some cases, it’s as easy as moving your gear a few feet apart.

AC Line Noise

Before getting into hums caused by the pickups or instrument cable, there are a few other points of potential noise to consider. The most common culprit is AC power line noise, which is usually heard in one specific channel.

To eliminate the hum caused by the power, you’ll need to begin by locating the point where the electrical ground wires from both the amplifier and the instrument meet. Strip off the covering from the wires and connect them together. This eliminates AC loop noises.

Ideally, your instrument’s ground wire should have a soldered connection. If it doesn’t, try plugging the instrument directly into the input jack of the amp or preamp.

Also, try and avoid using extension cords. If you must use one, try switching to one made from a higher gauge wire and make sure the plugs are firmly connected to each end.

What Should I Do About AC Line Noise Interference?

You may think that you're overloading your preamp channel if you connect your guitar to the Input jack on the front of the preamp and get a noisy, scratchy sound through your amp. It may even sound like the noise is coming from the preamp itself and not your guitar, but it isn't. If you have a noisy AC power outlet and the problem is getting worse, your preamp is picking up the noise, and you do have a preamp overload.

The cause of the 60 Hz AC noise problem, in most cases, is a faulty power transformer in the amplifier. The transformer could be shorted, or it could be mounting improperly to a metal surface. The best course of action is to have a qualified technician look over your amplifier and likely change out the power transformer.

A different cause of 60 Hz AC interference is a poorly designed preamp. The preamp could be receiving this interference directly, or it could be passing it on to other equipment in your chain, such as a guitar amp. Many preamps are designed to process certain kinds of noise like buzz or hum. Ideally, passing the noise through the preamp should invert it, which isn’t that easy to do. If the preamp is set up to playback noise in a way your amp doesn’t like, the solution is to power everything down, unplug everything, and power everything back up in a different order.

Online UPS

Your utility power may be perfectly clean and stable, but you may still have issues with hum and noise if you're not playing through a UPS (Uninterruptible Power Supply). All electronics produce ripple and noise–that's just a simple fact all of us who have dragged home consoles, tube amps, etc. have learned to live with. If you're getting interference over a small area (say, from neighboring tube amps) a heavy-duty UPS can solve the problem. Those beefy units are still not enough to cover most whole-house situations, however. Using multiple cheap UPSs from internet mail-order houses is not a smart move to do. Plus, online UPS rental sites don't always offer the same level of protection that a gear-specific UPS will. To do it right, you need to buy a whole-house UPS. This one's a champ, for starters.

Isolation Transformer

Whether you’re an experienced musician or a hobbyist, you must know how important isolation transformers are.

Transformer Types

You recognize a transformer by the black metal bands wrapped around its body. The box-like device has two inlets and two outlets. The primary inlet is connected to the 120-Volt (VAC) outlet, which feeds the secondary inlet. This inlet is connected to the instrument or device you want power for.

Isolation transformers isolate you from the dangerous voltage that’s present at the outlet and give a clean output to the device. They provide essential power for electrical and electronic devices that are sensitive to it, such as your guitar preamp.

How do isolation transformers work?

It's the same principle as all other transformers. An alternating current flows through a coil and the voltage in the second coil which will have a current induced in it, will be proportional to the ratio of windings between the two coils.

What Are the Uses of the Isolation Transformer?

An isolation transformer is a basic piece of equipment that can be used across a variety of musical applications. Whether you are messing around with different guitar effects pedals or are experimenting with other electronic DIY inventions, an isolation transformer is something that can be used to create a safety barrier between the power source and the circuit you are building. Thus, it holds great significance in helping beginners take the first few steps in building their own setups.

An isolation transformer is a device that contains two coils of wire (primary and secondary) surrounded by a fiberglass casing. The coils of wire are attached end to end, and the device uses an isolation transformer to provide a path to the ground while preventing feedback and static bursts.

It is very different from a standard audio transformer, as it is used for signal and not for electrical isolation.

The primary winding of the isolation transformer is connected to the circuit you want to be energized. In the case of guitar effects pedals, the primary winding is the board that powers the pedal, and the secondary winding is the parts of the pedal that you want to be electrified, such as the switch and the components. The secondary winding is connected only to the power source, while the primary winding has a path to ground through the casing of the transformer. The casing has a direct path to the ground, and this path removes the energy from the circuit that can cause problems with feedback and static electricity sparks.

Multimedia Setup Wiring

When installing a new multimedia speaker system that includes speakers or speakers/amplifier combination unit in a home theater or sound system, don’t rush the wiring process. To get the most out of the system you are installing, take the time to plan out the necessary wiring needed to get the sound to your ears. For the most part, if wired properly, you won’t notice a big difference in the audio output. However, it could be the difference between sounding crystal clear and woefully out of tune.

Here are a few tips to help create a dependable, efficient, and top-notch sound system. As you’re planning out your wiring, keep the following in mind:

  • Use a high-quality wire. While it does cost more, the increased sound quality and reliability are worth it. By spending a little extra, you won’t have to worry about blown speakers or rattling sound.
  • Wire your speakers and amplifier for the highest wattage it can handle. It will prevent distortion and blown speakers.
  • To help minimize electrical interference from surrounding appliances, turn off all components until the power on your system is complete. This will ensure your system wiring has the cleanest, highest output possible.

Rule Number One

Preamps Have DC Power Supplies, which generate noise.

Like record players, preamps were once built with AC power supplies if people wanted to connect their turntable to another amplifier (although this was only ever used for commercial applications).

I say were, because no turntable manufacturer still offers this feature on their decks. So why does your preamp still have an AC power supply, and does it still generate noise?

Because no one was going to buy a premium turntable that was hardwired to only work with its internal amplifier, let alone one that wouldn't work with any other amplifier. And this is why you will find AC power supplies in so many high-end turntables.

As for the humming, the DC power supply in your preamp not only generates noise, but the noise can also get into your music, and cause added hiss in your LP’s upper-mids and mids. If you notice an excessive mid/upper-mid hiss emanating from your preamp, one of the solutions is to disconnect the preamp’s AC power cable, which will eliminate the additional noise.

Rule Number Two

The second reason a preamp hums is interference.

Some people believe that disconnecting your speaker will eliminate the hum, but this is not the case. If an AM/FM radio station is playing too loud, you will hear the sound over your stereo. But if the music isn't loud enough, the hum will bleed over. Disconnecting your speakers won't do anything.

The best way to fix this is to try to block the signal. If there is a specific station that is problematic, try to pull in the station on your stereo using a tuner. If you can tune in to the signal, you can try to re-tune the antenna so it is playing the same station. If this doesn’t work, try tying a piece of metal wire across the back of the tuner.

RF Interference

A build-up of static electricity in the room may cause an audible hum. Any material which holds a static charge can cause RF interference. So, in addition to humans, the hum may also be caused by carpeting, clothing, plastic plants, etc.

Resistive Coupling

Resistive coupling is another possibility. It occurs when you touch the preamp and other electrically conductive objects. Resistive coupling can also occur by placing an RF absorber pad between the preamp and the offending object, like a plastic plant. You can purchase RF absorbers from an electronics supply store. Use a non-conductive item, like a sponge, to prevent the pad from touching the preamp.

USB/HDMI Cable Noise

If you are using a long cable such as the ones found on many Android phones, the cable can cause a lot of noise on your rack-mounted amplifiers and preamp. Here are some tips to reduce the noise

  • Try to use the shortest cable that works.
  • Make sure to use a high-quality USB cable. Many cables are cheaply made and not shielded.
  • If you have a lot of audio gear, try not to run the cables parallel to power cables as the magnetic interference can make the problem worse.

Mechanical Hum Vs. Electromagnetic Hum

Non-technical people often incorrectly refer to any type of electrical or electronic noise due to the common perception that wires generate noise. In reality, this is is not the case. For example,–with solid-state, magnetic technology, it’s the electrons moving in the wire and the mini-magnets in the process of moving electron charge that create the sounds. On the other hand, the noise created from moving electrons in windings is a source of electromagnetic hum. Although this is different than a mechanical hum, both types of electrical noises are usually referred to as hums.

Speaking of magnetic noises, hums are caused by magnetic interference created by appliances that run on alternating current. Most of these appliances are electric. For example, the magnetic field created by a running electric motor can affect the flow of electrons inside a wire, thus creating a humming sound.

The reason why these sources of electric power hum are created is because of the electromagnetic field that’s generated whenever electricity flows through a wire. The frequency of the hum depends on the frequency of the power source.

Why Do Transformers Hum?

Transformer hum occurs because of the magnetic field generated by the core of the transformer. You may find the noise more noticeable when using a magnetic pickup such as a piezo pickup.

A common trick known to most guitarists is to turn the amp up louder and keep the preamp volume low. This will only partially fix the problem since it still increases the magnetic field intensity.

Here are some solutions for the common causes of transformer hum:

  • Use a shielded cable to connect your pickups to the guitar amplifier. It’s best to use a shielded cable with professional-grade XLR connections or a shielded guitar cord that closes the magnetic loop, and goes from the input jack to the volume pot, to the tube amp, and back to the output jack.
  • A noise gate can be used to eliminate the hum.
  • If you have a tube amp, you can connect two grounded power supplies instead of one single power supply. This will help block the hum caused by the ground current.
  • Narrow pickups like single-coil pickups will have a lower noise level than wide pickups like humbuckers.

Troubleshooting

Audio equipment doesn’t come cheap. You bought your condenser mics, your pre/amp, your monitors, and your acoustic guitar. You busted your ass saving up for each one of these products and finally bought them.

But do you know that all this might go down the drain because of a single hum?

You should be very careful. Sometimes, the hum is the first site of trouble when there’s a bigger problem with your setup.

Overheating devices can cause a short circuit and give off a humming sound. Or you could have a very loose ground wire in the line or a bad connection. The first thing you should do is always check that the AC cord is secure and connected to the outlet. Since the outlet itself tripping could always cause this to happen, it’s always better to make sure that you’re on a secure outlet.

If there’s still nothing wrong, you should check each cable and make sure that they’re tightly connected so that they won’t come loose each time when the unit is moved.

Also, the AC filter in your home may be faulty, and this could cause electrical hums. When this happens, the device should be plugged directly into a power source, without using an AC filter.

I Can’t Hear It Anymore!

When you start to work on the cables and the electronic equipment, you will find that it is really hard to hear anything! They are connected with semi-conductors, and the voltage flows through them to ”hear” the sound. The problem is that there is a chance that the hum will interfere with the normal operation of the cables.

If you are on stage and the cables create too much noise, it is often impossible to hear your instruments. You cannot hear the metal guitar strings or the soft drums. Even the drummer can’t hear what she is playing because of the loud hum.

How do you repair the hum? Find the external cables or the wiring that generates the hum. The hum is created by electromagnetic waves. There is a coil that creates the magnetic field and the hum. You can find out the exact wires by the process of elimination. You cut off the power and then test the hum. You turn of every cable and test until you find the one that is causing the interference. Unwrap the connectors and cut off the insulation. Problem solved, enjoy your equipment!