Hiss, Hum, Crackle, Buzz - What is a Noise Floor?

Written by Kenny Prochazka

Have you ever bought or built a tube amp, hoping to hear your guitar as gloriously as possible, only to hear an unexpected hiss, buzz or crackle from the amp before ever turning your instrument up or even plugging it in? This is what we call the "noise floor" of an amplifier: the amount of sound the amp circuit puts out without any signal going to it. Some amps are whisper-quiet at idle, while others seem overbearingly noisy. This can result as much from the circuit design as from damaged or improperly installed components.

In most cases, the noise floor of a particular amplifier will have much to do with the amount of gain that is squeezed into the signal. More gain generally equals more noise, although this is a bit of a simplification. Amplifier noise floor is caused by relationships between components as they lead up to, and through, the power section of the amp. Between tubes, resistors, capacitors, potentiometers and wiring, there are a myriad of ways to induce or reduce noise, and we will talk about a few of the major ones here. If everything appears to be connected correctly, but there is more noise than you think is normal, this is a great place to start your troubleshooting process.

Because boosting a guitar's passive signal often requires a few different series' of components, the preamp section of any amplifier is most often the cause of any unwanted noise, which is then amplified even further by the power section. However, a properly matched power section can also minimize extra noise in ways that may be unexpected. For instance, most Fender Tweed-style circuits have very similar preamp sections, but the Tweed Deluxe 5E3 has a much louder perceived noise floor than the Tweed Bassman 5F6-A. This is because the beefier power section of the 5F6-A does not compress the waveform from the preamp as much as in the 5E3, and in turn allows more of the true signal to come through.

What is causing the noise?

A good way to start solving any noise floor issues that you might encounter is by determining what kind of noise you are measuring, and whether it is abnormal:

Hiss: If you hear a hissing sound, the most likely culprits are preamp tubes that could be faulty or old, or simply designed to induce more gain, as well as leaking or old preamp components. (Vintage-correct carbon comp resistors have long been known for producing a noise like frying bacon.) If the preamp tube(s) turns out to be the source of the issue, changing from a 12AX7 to a lower-gain 12AT7 in the first preamp position can make a world of difference. NOTE: Some amp designs have an inherent hiss that should seem to go away when you start playing. As long as it does not interfere with your actual tone, this is usually regarded as normal.

Hum: If you hear a hum measuring around 50-60Hz, it can result from failing electrolytic capacitors or failing power filter caps. However, many of our personal devices (cell phones, lighting, computers, etc.) in the modern era can also cause electromagnetic interference that will come through the power section as a noticeable hum.

Crackle: If you hear a crackling sound, the likely culprits are older tubes, loose sockets, oxidized solder joints or even dirty potentiometers. Any inconsistent contact in the signal can come across as a crackle, so you will want to check everything.

Buzz: If you hear a buzzing that is not a 60Hz hum, it could be an issue with your power supply or with interference from other electrical devices. Alternatively, it could simply be improper lead dress with wires too long or too untidy running conflicting signals next to each other.

How do I lower the noise floor without a soldering iron?

Many of us are not amp technicians, and are understandably a bit uneasy about taking a soldering iron to our beloved amplifiers. However, there are a few things that you can do, without taking your amp apart, to reduce any extra noise:

Try a better instrument cable:  A cheap or poorly shielded instrument cable can induce noise right at the beginning of the amp circuit. As simple as it may seem, this small change can make a huge difference in how much interference gets through to your signal.

Turn down your gain: If you turn down the gain control and increase the master volume, you will end up with a cleaner sound and less background noise. Remember that more gain usually leads to more noise, therefore every setup will require a different balance for the most tone and the least interference.

Try different pickups: Single-coil pickups create more extraneous noise than humbuckers (which buck the hum), purely due to their design. Switching to a humbucking pickup or a different single coil may make all the difference in the world.

Replace your old preamp tubes: Since these are among the first steps in amplifying your sound, and because subsequent sections of the amp make everything they do bigger and louder, they can enormously impact the final sound.

Move away from any sources of electrical interference: Consider moving your amplifier to a different room or a different part of your home or building's electrical circuit. Smartphones, WiFi routers, air conditioners, fluorescent lights and other electronics can create electrical interference for an amplifier to pass along as a hum or a buzz, so more distance between them and your amp will give you the best chance of a cleaner sound. Aftermarket power conditioners can also clean up the electricity from your wall before it causes an issue.

If none of these solve the problem to your satisfaction, it is usually best to take it to a qualified technician for inspection. The job can range from cleaning and/or resoldering contacts to tightening tube sockets and replacing components, but it will be tough to tell without inspecting the circuit. For some amp designs like a high-gain Marshall Style 18W amp, even the best technicians can't do much without changing the character of the amp, and they will usually be very honest about that.

Hopefully you understand a bit more about your amplifier(s) noise floor: what it is, what causes it, and what you may be able to do about it. Some noise is expected with any tube amp, but now you know several minor steps that you can take to minimize and control it. So, what do you do with that knowledge? Get out there, find your sound, and love your tone!