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GUITAR
AMPS PAGE
SECTION 5: POWER
(WATTS RMS)
STEP
1: DECIDE HOW MUCH POWER (WATTS RMS) IS NEEDED
The sound we actually here is acoustical
energy in the form of sound waves generated by the loudspeaker, which in turn
is driven by the amplifier.
The loudness of the sound is expressed either
as a Phon or in SPL.
Note:
In the real world SPL is called “volume”. The “volume control” on an amplifier
adjusts the signal voltage supplied to the amplifier input and that controls
the power output - because everything after the volume control has a fixed
amplification ratio (gain) and is non-adjustable. A “gain” control is not a
volume control – its purpose it to fix the gain ratio from signal input to
signal output.
The unit of measure is the dB or decibel.
It follows than that to produce a specified
sound level (SPL) we need to know the efficiency of the loudspeaker in SPL at 1
Watt electrical input, multiplied by the desired SPL loudness.
e.g. if the speaker has an SPL efficiency
rating of 100 dB at 1 Watt rms input, then to produce a 20 dB increase in SPL
we will need 100 x 1 = 100 watts rms input.
As a guide 20 db is the full dynamic range of
a vynil recording – i.e. from softest to loudest.
The average band will typically play at
around 100 db - comfortably loud.
But to increase the SPL by another 20 dB
lifts the power requirement to 10,000 Watts rms. This is realm of the massive
sound arrays we see at outdoor concerts.
Back in the real world the guitarist needs to
make some up-front decisions because the range of practical amplifier power
outputs is limited to a range of about 1 to 100 watts rms.
Note:
Specialty guitar and bass amps in the range to 400 to 500 Watts have been made
and are still made but they are very heavy and not for the weak or feint
hearted. Just the power transformer and output transformers will weigh in at
about 30kg. This power level is also a challenge for the DIYer because
commercial output transformers are not available off the shelf so must be
custom designed and manufactured.
The following graph shows how sensitive the
human ear is to frequency.
A shown in Section 6: The Loudspeaker the full frequency
range of a guitar including harmonics is from about 80 Hz through to 3 kHz
Omit the harmonics and the fundamental
frequency range reduces to 82 to 660 Hz at the 12th fret.
In other words, ALL of the sound produced by
a guitar falls within the frequency range where our hearing is at its best -
i.e. 1000 Hz to 2000 kHz.
This frequency range is comparable to the
human voice so explains why traditional public address amplifiers (PA) were
very successful in the early years of amplified guitar.
The above graph shows that in that frequency
range our hearing sensitivity increases
as the sound level increases – the louder we play the better and more evenly
the audience hears it.
Note: The graph
shows that at all frequencies the bass guitar will appear to be louder than the
guitar – the difference reducing with SPL (i.e. as volume/loudness increases) -
but because bass loudspeakers have an inverse SPL to power input ratio in the
lower range, the power required to equal the guitar SPL will be many times
higher. This is discussed separately.
Estimation of User needs for final amplifier
electric power output is necessary to determine the core design specification
requirements for critical components such as tube type, output transformer,
power transformer, chassis size, ventilation and gross weight.
These in turn, determine needs for B+ voltage
and current, grid bias voltages, rectifier and electrolytic capacitor voltage
ratings etc.
Hence, a very apt starting point is to
determine power needs.
However since there is no reference point to
start from this can be a tricky exercise.
It is assumed you will be using loudspeakers
- which are transducers used to convert alternating current electrical
energy into acoustic, audio or sound pressure energy. Loudspeakers are
notoriously inefficient and fragile devices and great care should be taken in
their selection, mounting, installation and operation.
Generally speaking, listening to recorded
music in an average room at a comfortable level requires only about one watt
RMS - which is surprisingly loud with a typical 85-90 dB SPL efficiency
loudspeaker - even louder with 100 dB SPL efficiency units.
Note:
SPL on its own does not convey the volume of air being pumped. The SPL rating
is a measure of "loudness" determined at 1 watt electrical
input (amplifier output) at 1 metre from the speaker face. Power used to be
measured with a single frequency rms signal, but nowadays all kinds of rating
changing techniques are used by loudspeaker manufacturers to rate electrical
power input - e.g. music power, white noise, pink noise, peak power, programme
power, etc - all expressed in "watts".
We may reasonably then ask the question
"what is a watt"?
Or, "when is a watt not a watt"?
To provide for the minimum full dynamic range
of recorded music - i.e. 20 dB – over a starting point of say 0.5 Watts average
a total power rating of 50 Watts RMS will be a reasonable target to enable
transients to be reproduced without too much audible distortion above normal
listening requirements.
For
every 3 dB increase in SPL the electrical power must double.
For
every 3 dB decrease in SPL the electrical power is halved.
In this age of digital recording, musicians
have found it necessary to resort to effects to produce saleable product, hence
it is common nowadays for CD's to incorporate extreme levels of low frequencies
of all kinds of waveforms, to produce that "thumping" brain-deadening
sound so beloved of today's teenie-boppers.
Unfortunately this means that to attain realistic
sound reproduction free from audible distortion, a 50 W RMS amplifier and a 50
W RMS loudspeaker system (25 W RMS per channel - stereo) is mandatory. This
equates to 75 watts EIA, 100 watts peak or up to 800 watts PMPO.
Of course a lesser level of power may be
usefully utilised but transient peaks may be cut-off or truncated, with a
corresponding loss of realism.
It all depends upon how loud the amplifier is
played. Obviously is the amplifier is capable of 100 Watts it is not necessary
to drive it to that level. Experienced guitarists know what power their amp
will need and usually use the lightest and most compact amp in their stable –
make life easier.
A more detailed explanation of this
requirement is provided by Dave O'Brien, formerly from McIntosh Laboratories,
in his description of his "Spectral
Fidelity Test for Intermodulation Distortion" (IMD). IMD is what causes the sound to become
"fuzzy" at high volumes.
See also an interesting overview at http://www.axiomaudio.com/archives/power.html
Note:
Recording technologies have always been far ahead of playback technologies.
Examples of just how good recording technologies and standards have been in the
past are easily heard by listening to recordings made way back in the 1940's
and 1950's, or even earlier in some cases. (Some of my CD's include original
recordings that go back to 1910). When transcribed to CD format and cleaned up,
these recordings contain signal information not previously heard with
conventional playback equipment. The information was always there on the original
tape, but just not accessible. However thanks to advances in audio recording
and transcription technologies, now it is.
Except as otherwise determined from the
following information about loudspeaker performance, when used in a typical
domestic home situation, an amplifier/loudspeaker combination of less than 100
W RMS will simply produce overload distortion in all of its forms on transients
and/or intermodulation distortion on steady state heavy programme material such
as loud rock music.
The electric guitar can produce very high half-wave
amplitude transient (very short time) signals because the plectrum (pick) is
pushed away from the player when normally playing a note hard. The amplitude of
the peak signal can be as high as the string is deflected over the pickup’s
sensitive area – beyond which distortion will be evident.
However this
distortion may not be heard in practice because the amplifier may actually
simply chop the top off the transient peaks, in which case everything else will
sound just fine up to that power output level where the whole system collapses
through severe overload. What will be heard is a modified sound, but the
modification may not be discernible to all but the most critical listener.
However, a rational approach to Guitar
Amplifiers is that if you cannot hear the distortion then don't worry about
it!!
If you can hear unintended distortion then
turn the volume control down a few notches – or use a more powerful amplifier
that will tolerate your playing excesses.
LIVE STAGE PERFORMANCE
Commercial guitar amp systems for live stage
work come in wide range of options.
From the humble 5 watt practice amp to the
gargantuan systems used for large venues, the musician has a myriad of choices
available.
Large PA/sound reinforcement systems are beyond
the scope of DIY so are not described here.
Of course the size of the venue, floor
coverings, crowd numbers (people soak up power) and indoor v outdoor conditions
(outdoor offers no reflected sound) affect the minimum power needs.
IMPORTANT: It is of no use
connecting a powerful amplifier to small loudspeakers or inefficient
loudspeakers or connecting with undersized cables where losses will be high
(potentially up to 50% or more)
Generally speaking, for indoor venues:
·
more speakers deliver
better sound
·
more speakers
deliver better dispertion into the room.
·
larger diameter
speakers deliver better sound
·
the more speakers
there are in a cabinet the better the sound and acoustic efficiency
Think of it like mono v stereo.
In my experience 12 inch drivers sound better
than 10 inch, 15 inch drivers sound better than 12 inch, 18 inch drivers sound
better than 15 inch. The more cabinets the higher the acoustic efficiency.
However for general venues such as weddings,
parties, dances, hotels and clubs a single 12 inch speaker with a 40 W tube amp
is usually adequate for a guitar amp.
Guitar rigs fitted with two or four 10 or 12
inch speakers and a 100 Watt amplifier are wasted unless the venue is large or
the sound is very loud. Heavy metal players sometimes like to push the system
into overdrive to obtain a certain level of distortion – but this means an SPL
of at least 100 dB.
See SPL Warning Notice
re hearing damage.
One of the curious aspects of modern band
sound systems is that the typical band will have two fifteen inch PA speakers
set high on pedestals either side of the stage for voice, one to eight 12 inch
loudspeakers for guitar, one or two fifteen inch loudspeakers for keyboards,
and sometimes two eighteen inch subs for drums - but just ONE fifteen inch
loudspeaker for bass guitar !!
The resultant tonal and volume balance
between instruments is obvious.
Food for thought !!
Consider this testimonial from John. W Ashton
at Reverb.com
7 years ago edited
I am 62; made my living playing from 1974 - 2006. My experience has been that "tone is in the ear of the beholder."
We start with an inspiration; mine was Jimi. So I got a Stratocaster, but couldn't afford the Marshall. First pro amp was a 65 Deluxe Reverb. Through the years I owned and played lots of different Fender combos, mainly below 40 watts. I had a brief affair with a 50 watt Marshall half -stack and another stint with a Road 100 watt solid state 4-12 (anyone remember those?). By the time I was in my 30's I was playing in medium sized clubs and getting the evil eye from the sound tech at load-in. "F#%k off! This is my tone! Deal with it!" But I had an epiphany at Herman's Hideaway in Denver when I set up a Super Reverb with a Vibrolux Reverb on top, and the sound guy said "too much! You want to be in the mix or not?" I realized that if my stage volume was too loud, he was gonna pull my level down or even out altogether, and I wasn't going to be in the mix out front. So back to the Deluxe Reverb, then a Rivera Super Champ. With a mic and a monitor mix, it was as loud as I needed, and totally in the sound tech's control. My advice is if you are playing live, keep your stage volume reasonable so you are going to be heard. Go to some shows where there's a sound tech running the system and listen to the guitarist's sound in the room. Is it in the mix, or just in the room? If it's just in the room, that guitarist isn't doing themselves any favors up there. Check out how his/her amp projects into the room like a flashlight, sounding thin and harsh in the beam and flat and dull outside that beam. "If the sound man ain't happy, ain't nobody happy". Unless thrash and crash is your sound, you need a mix! Write lyrics? Want people to hear them?? Then control your stage volume. The way to do this is with an amp that's 40 watts or less, and tilt it so it's facing your head, where your ears are, not the back of your knees. Use a god mic (I bring my own) and think about a small baffle placed a foot or so in front of the speaker(s) to stop your amp projecting into the room, where your sound is dissipated and all that's left in your tone is harsh treble. Even if you are in an instrumental trio, your audience should be able to hear the tone you have invested so much of yourself to achieve.” (end quote) Source: https://reverb.com/au/news/the-evolution-of-fender-amps
40 Watts is a realistic power output for an
amplifier using a pair of 6L6G or EL34 tubes and seems to be to most commonly
used rated power level for commercial tube guitar amplifiers over time for
general purpose usage in live venues without sound reinforcement.
If you like distortion from overdriving the
power stage then it is easier and less expensive in replacement tubes to achieve
with a smaller rather than larger power amp.
REMEMBER - ALWAYS TAKE CARE WHEN WORKING WITH
HIGH-VOLTAGE - DEATH IS PERMANENT!!
Contact:
"electron"
Email: contact
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This page last modified 12 July 2023