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Articles: Guitar amplifier power & volume


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Which will suit you?

What wattage will work?
Why does your 10W amp seem so unbelievably loud in your bedroom, but gets drowned out when used in a bigger room? How come your friend's 25W amp drowns out your 50W amp? Why can't you get your 50W Marshall which sounds so good on stage to sound good in the studio? What size amp should you be using anyway?
To answer these and other questions, you need to know more about guitar amplifier power.

Generally speaking
A guitar amplifier's power rating, measured in Watts (W), has a direct bearing on how loud it is - and how loud it is determines how well it will work in a given situation. It is easy to generalise and say that a valve amp of 5W or less works for a practice or studio amp, 15W - 30W for playing with a band (depending largely on the drummer) and 50W and up for large stage applications. However, there are a few things which can change these requirements and a few concepts you have to be able to understand to make the right choice.

The factors affecting volume

How changes in volume are heard
The most important thing to know is that the difference in volume level between two different power ratings is logarithmic. The human ear doesn't hear changes in volume evenly, and as a rule of thumb, (all else being equal) it takes ten times the power to double the perceived volume. What this means is that a 100W amplifier is only twice as loud as a 10W amp, and the 10W amp is only twice as loud as a 1W amplifier! This also means that the 1W amplifier is a quarter as loud as the 100W amplifier and that two 10W amplifiers will be as loud as a 100W model.

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A valve. A.K.A. tube

Valve vs. solid-state
The second important thing to know is the difference between valve (tube) and solid-state amplifiers. Ever notice how a valve amp will overpower a solid-state amp rated at the same power level? That's because (thanks to the origin of amps being hi-fi) an amp's power is measured at the "onset of clipping" (when it starts distorting). A valve amp starts clipping fairly early in it's power range and a SS amp very late. So, quite simply, a valve amp has a lot more power after it starts clipping and will often put out more than double it's rated power by the time it is cranked. The valve amp also compresses the guitar sound quite a bit when it's driven, so the average power output is much higher - and our ears interpret higher average power as louder. So if you are looking at solid-state amps, you will need at least twice the power rating as the equivalent valve amp to do the same job.

Speaker sensitivity
Speaker sensitivity makes a big difference to the volume level of an amplifier, with every 3dB SPL (decibels of Sound Pressure Level - the true indicator of volume) difference doubling the apparent volume of a sound. This means that a speaker rated at 97dB SPL will be twice as loud as a speaker rated at 94dB SPL and half as loud as one rated at 100dB SPL (all common ratings for guitar speakers). This means that you can that you can make an amp louder or quieter just by changing the speaker used to one with a greater or lesser sensitivity.

Number of speakers
Doubling the number of speakers used on an amplifier also adds 3dB SPL to the volume, making the amp twice as loud. So changing from a 1X12" configuration to a 2X12" will double the volume, as will moving from a 2X12" cab to a 4X12". Although if you plan on adding more speakers to your amp, make sure that the impedance of the amp is set correctly, as an impedance mismatch can damage the amplifier.

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"No, you can't have one for your bedroom!"

Type of music
Some styles of music require more amplifier power than others - and this isn't just down to a simple matter of volume.

Clean volume - Most guitar amps distort when turned up and guitarists using exclusively clean sounds will often need to use higher powered amplifiers. This is so that they can keep the amp turned down fairly low to keep the sound clean and distortion free, while still getting the volume they need.

Downtuning - Many players are also now tuning to lower pitches, often using baritone guitars, seven-string guitars or simply downtuning standard guitars from the traditional E tuning. Because lower pitched notes need more power to be perceived as same volume as higher pitched notes (which is why your bassist gets a bigger amp than you do), amplifiers used for downtuned guitars need to be more powerful - often double the usual power requirements.

Size of room/stage
The size of the rooms you are playing in is also an issue - simply put: larger rooms need more amplification. The large amps we still find these days were mostly developed when PA technology was still in its infancy, and guitars were not mic'ed up to go through the PA. As the concerts grew larger, the instrument amplifiers had to grow in size and players started using multiple amps to cope with the added requirements of huge venues. The downside to this was that the stage volume became ridiculous and a whole generation of rock guitarists developed serious hearing problems.

With modern PA systems being what they are though, it is possible to mic up a smaller guitar amp to augment the volume going out to the audience while keeping the onstage levels manageable. The PA helps not only to make the guitar sound louder, but also to spread the sound more evenly over a room, so that instead of having all the instrument sound coming from behind the musicians loud enough to push all the way to the back of the room, the majority of the sound is coming from multiple speakers in front of the stage. So when reinforced by the PA, the primary factor when choosing an amp is the size of the stage, rather than the size of the room.

Even with a smaller amp on a large stage it is possible to add the guitar sound into the monitor mix to make sure that the other musicians can hear it clearly. In fact this approach is preferable, so that your amp is your personal guitar monitor, rather than pushing large amounts of sound across the stage (so that the bassist on the other end of the stage can hear it clearly over his bass amp, which is pushing out large amounts of volume, etc., etc., etc.) - all with the vocal and drum mics picking up the time delayed leakage in the middle, affecting the quality of the FOH (Front Of House) PA sound immensely. For the same reasons, the development of In-Ear-Monitors (IEMs) has also improved stage sound immensely.

Taming the volume
As musicians, we end up playing in different situations and different size venues which require different size amps. The problem is that for most of us, it isn't practical to have multiple amps for multiple situations such as personal practice, band practice and all the different size venues. Small practice amps are relatively inexpensive, so getting a second amp for personal practice is rarely a problem, but it's rarely possible to have a variety of different sizes of quality valve amplifiers on hand to cover all situations. Also, many players have a signature "sound" which may rely heavily on a specific amplifier to create. So there are a few ways you can make a given amp a bit more flexible.

Head and different cabs
If you have a guitar amplifier in a "head" format, where the amplifier is in it's own cabinet, separate from the speakers, it is possible to have a few speaker cabinets of different speaker configurations and to "mix 'n match" as the gig requires. For instance, you could use a 1X12" speaker cabinet for band practice, a 2X12" for smaller club gigs and small stages and a 4X12" (or even the 2X12" and the 4X12") for larger venues.

Master volume
Many modern amplifiers have master volume controls, which let you turn up the preamp (so it distorts) and then use the master volume to turn down the level reaching the power amp. This works particularly well with solid-state, modelling amplifiers and modern high gain valve amplifiers, but many valve amplifiers, particularly the classic models rely on power valve distortion as much as preamp distortion to shape the tone, so fitting them with a master volume usually affects the tone adversely.

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A THD Hotplate attenuator

Power attenuators
Power attenuators are devices that can be fitted between the output of a valve amp's power amp and a speaker to absorb some of the amplifier's power and effectively turn the volume down after the power amp. This allows you to bring the volume of the amp down while still getting power amp distortion. Some valve amplifiers are now being made with built-in attenuators (Fender Princeton Studio, Peavey Windsor Studio,

While they are a great tool to tame an amp that's a bit too loud for a specific application, attenuators are unfortunately not a cure-all, as they work best to bring the volume down by relatively small amounts - so trying to get a Marshall 100W down to bedroom volume (while possible) may affect the tone badly.

Some popular attenuators are the THD Hotplate, Weber Mass and the Marshall Powersoak.

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