Qual é a potência apropriada para um amplificador?

Appropriate Wattage

Hi folks. Welcome back to The Corner. This week we’re going to have a discussion about amp wattage. What’s appropriate and what’s too much? How do you know if your amp will keep up with a drummer? We’ll try to answer these questions and more as well as provide some insight as to power (wattage) vs. volume (db). It may surprise you to read about the difference in volume between a 100 watt head and a 30 watt head. So let’s have a look at the guitar amp and see if we can find out what the appropriate wattage is for you.

The first thing we need to do is to have some basic technical knowledge drummed into our heads. We are going to use two different units in this discussion. Wattage (power output) and db (decibels). Wattage is the measurement used to indicate power. It’s derived from multiplying the voltage and the current. That’s it (in a simplified manner). All guitar amplifiers will give you their output power in terms of wattage so we will use the relationship between wattage and db for this article to keep things as simple as possible.

Decibels (db) are a ratio between two power levels. It is a very common measurement used to describe loudness since a very large amount of ratios can be described using the decibel. It’s also a good thing to keep in mind that a decibel is one-tenth of a Bel. A full Bel is perceived as twice as loud to most people. So, in order to compare two different output powers, we must first make sure both powers are expressed in wattage (or the same unit if comparing voltage or SPL). The basic equation for this is:

Código:

          P1
10 x log  ———
          P2

Where P1 and P2 express the output power of the two different sources. It’s also very important that both output measurements are taken with the same load on both amplifiers. Taking 30 watts at 8 ohms is different than 30 watts at 16 ohms, so in order for us to get a proper figure out, both powers need to be measure with the same load on the output. Let’s take a look at a 50 watt head vs. a 30 watt head. First we take 30 divided by 50 which gives us .6. Then we take the log of .6 which is .222 and multiply it by 10 which gives us 2.22 db. So a 50 watt amplifier is only 2.22 db louder than a 30 watt amplifier. For reference, twice the output power is equal to a 3 db change, and 3db is barely enough for the human ear to discern the change. So one more for fun, we’re going to compare a Fender Deluxe Reverb that has a nominal output of 22 watts to a Marshall 100 watt Super Lead. Now we all know the Marshall is louder but after running these through the equation we find that the Marshall is only 6.576 db. Considering that 10db is a nominal figure for what most people hear as twice as loud, then the Marshall is not even twice as loud as the Deluxe. Surprised?

Here’s a handy little chart I got from http://www.analogrules.com/dbwatts.html:

0 db increase = same power

3 db increase = 2 x power

10 db increase = 10 x power

20 db increase = 100 x power

30 db increase = 1000 x power

40 db increase = 10,000 x power

So the difference in the Marshall and the Deluxe is over twice the power (as stated by their outputs in watts) but not twice as loud. Also, in order to get twice the perceived volume of the Marshall, we would need an amplifier that had 1000 watts of output. Interesting stuff isn’t it?

Now let’s have a look at decibels and put them into a relation so you have a reference point for all this db nonsense. I got this chart from http://www.gcaudio.com/resources/howtos/loudness.html and find it very handy referencing db to sounds I am familiar with.

Weakest sound heard	0dB
Whisper Quiet Library	30dB
Normal conversation (3-5')	60-70dB
Telephone dial tone	80dB
City Traffic (inside car)	85dB
Train whistle at 500', Truck Traffic	90dB
Subway train at 200'	95dB
Level at which sustained exposure may result in hearing loss	90 – 95dB
Power mower at 3'	107dB
Snowmobile, Motorcycle	100dB
Power saw at 3'	110dB
Sandblasting, Loud Rock Concert	115dB
Pain begins	125dB
Pneumatic riveter at 4'	125dB
Even short term exposure can cause permanent damage – Loudest recommended exposure WITH hearing protection	140dB
Jet engine at 100', Gun Blast	140dB
Death of hearing tissue	180dB
Loudest sound possible	194dB

A live rock concert comes in somewhere between 110 and 140 db. So the difference of 6.576 db in relation to the chart is quite nominal. In fact to the human ear it will be a pretty slight difference in volume.

Choosing an Amplifier Based on Wattage

Now that some of the technical jargon is explained, let’s talk amps. There has been a trend toward lower wattage tube amplifiers in the past few years and for good reason. Let’s face it; most of us are not playing in front of 5000+ people on a regular basis, if at all. That coupled with the trend of a lot of modern music to move away from the arenas and into smaller, more intimate venues has changed the needs of a lot of professional guitarists. If you’re rolling into the club that seats 100 people with a 100 watt Mesa stack, you’re probably going to get some strange looks. The fact of the matter is, most 15 watt amplifiers are going to produce enough sound to be considered LOUD.

So why are there 100-200 watt guitar amps? First of all, back in the heyday of rock music, the sound systems that were used were not as high quality as what we have now and did not reproduce sound quite as well. A 100 watt amplifier was necessary in a lot of cases to carry the guitar sound in larger venues. Second of all is clean headroom. A 15 watt amplifier and a 30 watt amplifier are going to be really close in volume, they have approximately a 3 db difference in level which is just at the level that the human ear can hear the difference but the 15 watt amplifier is going to start saturating and distorting earlier. For someone who uses the amplifier for overdrive and the volume of the guitar for attenuating between clean and dirty, a 15 watt amplifier is going to be loud enough for most venues, perhaps even too loud for some. An 8 watt combo is an ideal amount of power for the volume knob player. It’s loud enough to get by the drummer and if the venue is big enough, they’ll most likely have it mic’d anyway so there’s no need to drag out the half-stack. For the country player or someone who uses pedals for their overdrive tones, a 30 watt amp might be more suitable. Even a 50 watt amplifier is going to be a good bet but getting past 30 watts is not always necessary. Take an AC30 and crank it to the point of saturation. It sounds great but man is it LOUD!

Venues are also a consideration. Keep in mind that if a venue is well organized, they are aware of the size and the amount of volume needed. If it’s small enough that the PA system only has vocal mics, you definitely can use any amplifier that is loud enough to practice with. If they have a full PA system with mics for the amps and drums, then more than likely the sound guy will still want your amp fairly low so the front of house has a balanced sound. Ever been to the show where the amps were mic’d and the guitarist still had his/her Marshall stack cranked? Not only is this uncomfortable for the 40 people directly in front of that amp, but the overall front of house mix is thrown off balance and if you move about the room you’ll notice that the guitar is overpowering in some areas and non-existent in others. A good rule of thumb that I’ve found from both running sound and playing live is that you want to find the right tone at a volume that you are comfortable standing in front of and stick with it. If the venue is bigger than usual, let the mics do the work. Any solid soundman will have some sort of monitor mix for you so there’s no reason to make ears bleed.

The trend toward lower wattage amplifiers has another upside. The iconic guitar sounds we all want to achieve in a large part come from the 60’s and 70’s. Due to their lack of high powered public address systems, these guys needed to crank their 100 watt amps up to be heard and pushing them to their limits is what generated that sweet tube saturation and tone that we all lust after. This day and age, cranking up a Marshall stack is not only going to garner dirty looks and hands over ears, but it may result in not being invited back to that ultra-hip club it took two years to get a gig at. The smaller wattage amplifiers allow us to push our amps to their limits at a volume level that is comfortable for both you and the band as well as the audience and the soundman. If it’s not loud enough, the soundman will make it louder. You don’t have to be the loudest instrument on the stage (drummers have that luxury, beating on things and all) to be heard in modern venues.

One last note before I wrap up this long-winded article. A lot of low powered combo amplifiers come in a 1×12 configuration and even though it might be loud, it just doesn’t have the push of a large 4×12 cabinet. If you want to make your low watt amp sound bigger and be perceived as louder, grab an extension cabinet. A nicely made 2×12 extension cabinet can make all the difference in to that thin sounding Deluxe Reverb. A larger enclosure and the extra driver allow more air to be moved and will give more of that satisfactory punch and “big amp” feel to the little guy.

If you’re still unsure about this, then by all means keep that 100 watt Plexi, but try this. Go to your local music store and find the lowest wattage tube amp they have and hook it up to a Marshall 4×12. A Vox AC4TVH or Z.Vex Nano Head would be ideal. Set the AC4 on 1 watt mode and crank it up. You’ll be surprised by how quickly the sales representative comes in and asks you to turn it down. You might also be surprised at how good it sounds.

Hopefully this article has helped spread some uncommon knowledge about the wattage rating on amplifiers. Next time you look at that boutique combo that’s rated at 8 watts don’t be so quick to dismiss it as “too small for live use” and plug it in. It might just surprise you at how well it keeps up and the fact that you can crank it and make the tubes work for their money results in sweet, singing tone that tube amplifiers are known for. Thanks for reading folks. Until next time, in The Corner.

Guest Contributor: Rusty Wiseman