Watts and Ohms

Posted by Ben Edwards on January 18th, 2012

We have had several questions about guitar amps – what “watts” and “ohms” means, and that sort of thing. David Stewart’s Guide To Specs had a really good article that answered this question.

Guitar amp watts and ohms.

Here are three common and related questions. In guitar amps how do the “watts” and “ohms” relate to the sound? Is louder necessarily better? And, do more watts and more ohms equal louder?

Let’s start with the third question. Watts relate to power. How much power or energy an amp is capable of putting into a speaker is rated in watts. How much energy a speaker is capable of handling is its wattage rating. Ohms relate to how much resistance (impedance) there is to the transfer of this energy. Everything has some resistance to the flow of electricity – some materials (insulators) much more than others (conductors). If a speaker has “more ohms” then it actually has a higher resistance to the transfer of energy. This means that (all other things being equal) it will not be as loud as an otherwise identical speaker with a lower impedance rating.

This does not mean that lower ohm rated speakers are “better.” It is simply a functional difference that can be used to one’s advantage when putting together a system. In some situations it is desirable to have multiple speakers, but when multiple speakers are added in parallel the net load on the amplifier can drop to dangerously low levels. (There are a couple of other Tech Tips about this in the archives if you want more background.)

That’s one example of where higher impedance speakers can be a benefit. All other things being equal, the transfer of more energy (more watts) to a speaker will result in louder sound. People generally buy higher wattage amps because they need more volume. There are other factors such as speaker sensitivity, efficiency, etc., but in general more watts means more volume.

But does it mean better – or even different – sound quality? From an engineering perspective ideally the sound shouldn’t change…but it does. Loudspeakers of all makes and models have different operational characteristics at different energy levels. There are many, many variables involved in this: a very complex set of dynamics with regard to electricity in coils, magnets, and the resulting magnetic fields, combined with the complex mechanical dynamics of getting an object to vibrate in a very tightly controlled fashion against the resistance of air inside a cabinet and the space it is placed within, not to mention the additional magnetic energy put back into the coil of wire (and ultimately the amp) as a result of the speaker moving.

This is a science (art?) that is far from perfect. Tremendous improvements have been made in the past couple of decades. It is now possible to buy loudspeakers that are very linear (though still far from perfect) when operated within their optimal range of tolerance. In the old days speakers would take on radically different sonic characteristics when driven near their operational limits (this is still true of modern speakers today, only the area of significant non-linearity is much, much closer to the upper limit of where failure will occur).

For engineers designing and operating systems where accurate sound reproduction was a significant concern, this was an obstacle that could be worked around to some extent – they simply made sure the energy transfer to the speaker kept it in or near its ideal operating range. Well, guitarists have been notoriously not interested in “accurate” sound reproduction. It didn’t take them long to discover the sonic changes that occur when speakers are driven up to the edge. And, as they say, the rest is history.

To finally answer the question, with certain speakers there can be a desirable sonic benefit to driving them with more power (watts). Your results will vary dramatically depending on many factors, including choice of amp, speaker, age (and operating history) of speaker, cabinet, and any other variable that could change the sound of your guitar. Whether these changes in sound result in “better” sound is of course up to the individual.

What about amps? All of these same concepts apply, just for different technical reasons. Much like speakers, every tube and every transistor has more and less linear ranges of operation, not to mention other components in the amp. Even when a transistor or tube is operated entirely within its “linear range” the combination of components that make up an amplifier can still behave differently at different levels. Again, in the most modern and highest quality designs these changes are extremely minute.

In the older days one could hear a significant difference in a tube amp when driven as compared to how it sounded at lower levels. Are these changes always considered improvements? No, usually not…but to guitarists…well, you get the idea. Now, when combining these amps and speakers together one has created a unique and dynamic beast with an incredibly complex set of interactions.

Turn up the volume of an old Marshall stack to significantly high levels and you will most certainly hear dramatic changes in sound, even before what most of us would normally characterize as conventional distortion sets in. Are these changes the result of amplifier non-linearity or speaker non-linearity? Yes. Any rock guitarist who has been around a while will tell you there is nothing like the sound of an old Marshall (or any of a number of other amps of the era) wound up. Even the most current and sophisticated modeling techniques are just beginning to skim the surface of addressing the variation in sounds that occur when a certain pickup on a certain guitar, with certain strings, going into a certain amp at a certain volume (and tone control setting) reacts with a certain set of speakers.

Posted by Ben Edwards.

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