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Dirty-South Blues Harp forum: wail on! >
Watts
Watts
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Rick Davis
1451 posts
Mar 12, 2013
6:28 PM
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Somebody asked me in another thread why I use the term “watts at clipping” to describe the power an amp makes. I thought I’d devote a new thread to this topic. It might get long.
It may surprise you to know there is no hard and fast formula that amp makes use to calculate exactly how many watts their harp amps make. In 2009 I researched an article I was writing on this topic by contacting all the big harp amp makers at the time to find out how they arrived at their wattage claims. Some of these amp makers were quite annoyed that I even asked the question.
I believe we need a standardized method for calculating amp power so the numbers are meaningful and consumers can make rational, informed decisions. Wattage numbers in amps are like horsepower figures in cars. There is a powerful incentive to fudge the numbers upwards – higher than your competitors – because many buyers are greatly influenced by it.
You've probably seen wattage claims for boom boxes or car audio or computer speakers that seem, um, unlikely. They sometimes claim hundreds of watts when the truth is a small fraction of that. Harp amp makers have been more restrained in their numbers, but still there is no real standardization.
Those of you who like vintage hifi gear will recognize this phrase: 110 watts RMS per channel @ 8 ohms from 20 to 20K Hz with .1% THD. I found that harp amp makers borrow parts of that formula to arrive at their wattage spec.
This is what they do: They drive the amp with a test tone – a sine wave – and measure the AC output at the speaker tap by watching the waveform on a scope. You may have opinions about what the proper method should be, but that is what they actually do.
One problem is that the frequency and amplitude of the test tone is not uniform (and certainly not announced by the amp maker) and it can make a big difference. Another problem is the amount of deformation they tolerate in the wave form at the point they claim as their wattage number. In other words (as in the hifi formula), how much distortion is included in the number?
This is where the question arose in the other thread: What the heck is wrong with distortion? We love distortion in our tone, right?
Indeed we do, but for the sake of arriving at a meaningful wattage number that allows us to make real comparisons we have to stipulate the level of distortion in the test. The easiest and best way to do that is to measure the amp’s power at the point that the sine wave begins to clip. That is a very good indication of the amp’s true strength.
Can the amp make more power beyond the point of clipping? Sure. But depending on the design of the amp the distortion (the deformation of the sine wave) can start to sound unappealing pretty quickly. Amp makers could just crank everything to the max and report that number but it would be unrealistic and meaningless for musicians, and there are lots of ways to juice the max number. We want to know: How much clean power can the amp make? How does it compare with other amps?
Here is what I am suggesting as a standard for harp amp makers: Drive the amp with 150mvac@130Hz and measure power as peak clean voltage into the appropriate true non-reactive load.
Is it a perfect formula? Probably not, but it is not meant to be. We need to insist that amp makers use a standard, uniform and verifiable method of calculating their wattage claims, and this method is a good place to start that conversation.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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PeterG
12 posts
Mar 12, 2013
6:59 PM
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Correct me if I'm wrong, but isn't the main reason folks are concerned with wattage because they want to be assured of sufficient volume capability with a desired tone and they assume more watts will equal greater volume? In that case, wouldn't it be better to measure an amp's db at maximal or what one might consider optimal volume? Just a thought.
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Rick Davis
1452 posts
Mar 12, 2013
7:08 PM
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Sure, and that is exactly the reason we need a standard and verifiable measurement.
Measuring SPL would be good, at the rated power.
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-Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
Tip Jar
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CarlA
298 posts
Mar 12, 2013
7:34 PM
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Are we talking about the neighborhood in Los Angeles???
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5F6H
1579 posts
Mar 13, 2013
12:04 PM
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I did type a long response, all 3000 odd characters, but it got wiped by capture ...again :-(.
Just to say W ratings are diagnostic states for techs to pinpoint issues, most amps, whatever the instrument, are rated by "rule of thumb". Relatively few schems back up quoted W and, for those that do, test procedures change from amp to amp, even within amps from the same manufacturer. Manufacturers might not have any control over wall AC where the amp is being used, which will affect actual performance (wall AC is nominaly rated, we're supposed to be 230VAC here but are often way over 240VAC...other parts of Europe may see 5W less from a "45W" amp? But then, on that stage, they are all getting the same VAC and the field is levelled).
Loud enough is simply loud enough. W tests, as far as musicians on stage are concerned, are pretty moot. SPL (standardised test & acceptable tone) trumps Watts.
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Last Edited by 5F6H on Mar 13, 2013 12:12 PM
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tmf714
1555 posts
Mar 13, 2013
12:43 PM
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"We need to insist that amp makers use a standard, uniform and verifiable method of calculating their wattage claims, and this method is a good place to start that conversation. "
Never happen-why you may ask?
Too many variables,especially with harp amps.
Any change in the power stage-i.e. tubes,bias etc. that cause a change in plate voltage directly correlates to wattage.
With all the tube swaps available,tube types and bias set hot or cold,you would be putting the manufactuer under the gun-having to state claimed watts with each tube,each ma of bias-it could be a quite lengthy list-and for what?
I personally am fine with John Kinder stating my 6-10 HarpKing is putting out 100 watts-and my 4-10 is putting out 70 watts-whos to dispute it? And like I said -why? If John states that,and the amp sounds good,I'm happy,as I think most would be.
Last Edited by tmf714 on Mar 13, 2013 12:44 PM
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