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Class D, power consumption, power output and creative figures

andruca

andruca

Mar 31, 2004
3,422
4,299
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Madrid (Spain)
www.andresdemarco.info
#1
Lately I've been trying several class D amps and combos and this led me to some reflection and, of course, quite a bit of confusion. Things started some 6 months ago at a small club gig with a metalcore band I sub for. Shared backline with 2 other metal bands and one of them shared their bass amp, a TC Electronics BG250 15" combo. The thing could barely keep up with the small venue (in which I had never used my old 150W Peavey TNT past 2 on the master in previous occasions). Now, I know class AB is less efficient so, it's natural that my TNT's power consumption states 500W (for producing 150W output, 30% efficiency), or for the case my Hartke 3500 head draining 740W from the power outlet to produce its 350W rated power output (little under 50% efficiency). In both cases power output is clearly stated RMS.

I don't know in depth the reasons why class D seem to be abandoning the RMS rating standard (other than marketing guys taking over in product specs) and of course would love for somebody to enlighten me in that aspect. So, what I'm recently doing more and more is check out what do class D amps state as power consumption. But here is where things become more and more perverse. The aforementioned TC BG250 combo states a power consumption of "70W@1/8 of rated power". Now c'mon, you don't specify things in such a weird manner if you're not trying to hide something (and as to my experience, those 250W output are plain crap, I can confirm). Another example, the Peavey MiniMega reads "Typical power consumption 200W", with a rated power output of 1000W I don't even know what "typical" means, is it in standby?, is it an average figure? I'm a BIG Peavey fan, yet can't be on their side on this one. In both cases "RMS" is nowhere to be seen either in marketing material, specs at online stores, nor the user manuals themselves (altho' Peavey states "continuous" and "measured up to 1%THD" -in which case it has to be draining WAY more than those "typical" 200W from the outlet-, I certainly would prefer Peavey lying to me about power consumption than output of course).

Certainly other manufacturers publish what look like more realistic figures. Another couple examples of this case:

- The Tech21 VT 500 (rated 500W output) states a 600VA power consumption
- The Ampeg SVT 7 Pro (rated 1000W output) states a 1050W drain from the outlet

Both figures are much more reasonable and logical thinking of their rated output and a 80-95% efficiency figure a class D amp is suposed to have.

"A priori", I know I must double check published power output with power consumption ratings (and even then I still can't asure, as I explained, I also use the fuse value as an additional checking mechanism) and definitely won't trust a manufacturer that publishes specs such as power output or power consumption with dubious "names" but my question is still there and rather simple:

Is RMS rating falling out of fashion for a technical reason (i.e. not fair to apply to class D for some reason I ignore) or just because of a marketing driven power output figure race?

I know RMS rating is obtained using a continuous 1khz sine wave signal, connecting an 8/4/2/whatever ohm load to the output and measuring voltage between cables before a 1% (or was it 0,1%?) THD can be measured. And besides I strongly suspect TC Electronics is over-rating this BG250 thing, I mean, I probed it against a similar 15" class AB combo rated 150W RMS from maybe 1990 and it was put to GREAT shame.

Please be so kind as to try providing actual technical data so we can all know what figures to take into account and what not to or presume suspicious/misleading. Things like "my BG250 is freaking loud" are not helpful really ;-)
 
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#3
andruca said:
...states a power consumption of "70W@1/8 of rated power". Now c'mon, you don't specify things in such a weird manner if you're not trying to hide something...
Click to expand...
Actually, it's common to give power consumption at 1/8 power in the pro SR world, where 1/8 rated power is considered "typical with occasional clipping" and 1/3 rated power is considered "severe with heavy clipping".
 
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#4
wcriley said:
Actually, it's common to give power consumption at 1/8 power in the pro SR world, where 1/8 rated power is considered "typical with occasional clipping" and 1/3 rated power is considered "severe with heavy clipping".
Click to expand...

Ok, didn't know that. Still, and excuse my ignorance, if that was the power consumption with occasional clipping isn't it very unlikely the amp puts out anything remotely close to what traditionally would be considered 250W RMS?
 
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#5
andruca said:
Is RMS rating falling out of fashion for a technical reason (i.e. not fair to apply to class D for some reason I ignore) or just because of a marketing driven power output figure race?
Click to expand...
The term "RMS" is defined by regulation, notably the FTC Amplifier Rule (and possibly similar rules in Europe). As with any rule, there are good and bad reasons for not following it. The bad reason is, of course, to publish inflated ratings, and there have been cases where amp companies have had some 'splaining to do when somebody measured their output power and published the results.

On the other hand, possibly the main legitimate problem with the RMS rating is that it's a continuous rating -- the amp has to sustain its rated output for something like 15 minutes. But music signals are not continuous, and some amps are now rated in excess of the power that can be drawn from a wall outlet. Also, in an idealistic engineering sense, an amp that can work under conditions where it will never be operated, is over-designed, i.e., you're paying for materials and weight that you will never use. So, makers have endeavored to come up with ratings that are meaningful for signals that are not continuous, i.e., typical music signals.

Right now the main rating agency is the risk that somebody will measure the output of your amp and show that it meets no conceivable standard.

Perceived loudness is another matter, since it's affected by the signal content, and by how the amp behaves when it's pushed to clipping. There's also speaker efficiency. If one amp sounds louder than another of comparable rating, I'd first look at the preamp voicing and the speakers.

Power consumption ratings are provided so you can estimate how much gear can be plugged into a circuit.
 
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#7
fdeck said:
Right now the main rating agency is the risk that somebody will measure the output of your amp and show that it meets no conceivable standard.
Click to expand...
So true! There are so many outright bogus wattage claims by more than just a few major manufacturers. All amps should be tested & rated by an independent lab. Same thing with speaker cabinet specifications. I remember BFM pointing out some blatantly false claims by a major manufacturer. If a company will lie about one specification they will lie about them all. Buyer Beware.
 
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#8
fdeck said:
The term "RMS" is defined by regulation, notably the FTC Amplifier Rule (and possibly similar rules in Europe). As with any rule, there are good and bad reasons for not following it. The bad reason is, of course, to publish inflated ratings, and there have been cases where amp companies have had some 'splaining to do when somebody measured their output power and published the results.

On the other hand, possibly the main legitimate problem with the RMS rating is that it's a continuous rating -- the amp has to sustain its rated output for something like 15 minutes. But music signals are not continuous, and some amps are now rated in excess of the power that can be drawn from a wall outlet. Also, in an idealistic engineering sense, an amp that can work under conditions where it will never be operated, is over-designed, i.e., you're paying for materials and weight that you will never use. So, makers have endeavored to come up with ratings that are meaningful for signals that are not continuous, i.e., typical music signals.

Right now the main rating agency is the risk that somebody will measure the output of your amp and show that it meets no conceivable standard.

Perceived loudness is another matter, since it's affected by the signal content, and by how the amp behaves when it's pushed to clipping. There's also speaker efficiency. If one amp sounds louder than another of comparable rating, I'd first look at the preamp voicing and the speakers.

Power consumption ratings are provided so you can estimate how much gear can be plugged into a circuit.
Click to expand...

Thank you so much for your detailed explanation and great points made. This is pretty deceiving from a pure consumer standpoint as there's really no reference point for comparing the different manufacturers' claimed power ratings. Guess I will stick to my "over designed" RMS rated AB amps at least while my back doesn't complain :-( I was considering the power consumption thing or even the fuse value only as a rough way to infer how obscenely a given manufacturer might be inflating their published power output figures. Still don't understand what kind of measurement do the TC Electronics guys do to arrive at the 250W output number. Do they measure transients in whatever music they choose for testing? OTOH I've designed and built some bass reflex cabs and perfectly understand sensitivity and response curves and how box tunning affects them, but still can't see how a (pretty small BTW) cab with a Scorpion speaker such as that of my TNT might be in any way more sensitive than an (also not big) cab with an OEM Eminence driver designed and built maybe 30 years later. I guess I'll have to really try a lot of amps once I decide to "upgrade" to class D :-( Thank god for thomann's 30-day no questions asked money back guarantee as there's little or no stores in Madrid that would let me try an amp at the volumes I usually need to figure out how they'd deliver in real life. Sad.
 
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#9
Linnin said:
So true! There are so many outright bogus wattage claims by more than just a few major manufacturers. All amps should be tested & rated by an independent lab. Same thing with speaker cabinet specifications. I remember BFM pointing out some blatantly false claims by a major manufacturer. If a company will lie about one specification they will lie about them all. Buyer Beware.
Click to expand...

Miss BFM. I was mostly out of TB for 3 or 4 years and when I came back he was sadly gone.
 
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#10
I have discussed this many times in excruciating detail, but it's obvious that it's still not sinking in (or not being read and understood)

The power consumption rating on the back of an amp is required by the safety rating agencies and is specified (if you really follow the rules) as the RMS power consumption at the higher of 1/8 rated audio power will always be based on RMS measurements in the test lab) or the duty cycle that the device was intended to operate at and is marketed to operate at. If the amp is intended to operate under overdriven conditions, then it should be measured under those conditions and duty cycle, so 1/8 power may not be appropriate and a more demanding (by 50%) 1/4 power factor may be used.

This power consumption figure is calculated this way so that typical branch circuit loads can be defined, and so heating/cooling loads can be defined for building industry purposes (like in California, for title 24 energy efficiency calculations in commercial buildings). Each county uses this data as it sees fit, but Europe's CB scheme is much the same.

Many manufacturers of class D amps do indeed use RMS units in their audio power ratings. We do on the Subway amps and the metrics for this are clearly stated in the specifications. I know we are not alone, but I agree that it is confusing (and frustrating for both the consumer as well as the manufacturers who do use RMS units) when the units are not stated and the figures appear inflated.

Hope this helps
 
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#13
BogeyBass said:
so let's say the tc electronics is rated at 250watts total power.
1/8 is 31 watts so it draws 70 watts
of power to produce 31

is that correct
Click to expand...
It COULD be correct if the efficiency was less than 50% and/or the stated duty cycle was greater than 0.125 (1/8-power). It depends.
 
#16
Excellent assessment on your part. I believe a lot of companies have strayed from industry standards are are getting quite "creative" in their watt rating. Your own experience with your old Peavey compared to the TC is practical demonstration.

Back in the 80's when RMS ruled, 300 watts was considered enormous. I used a lot of Made in USA Peaveys, Sunns, Fenders, and Ampegs that pale as far as represented "watts" today. But were they loud? You bet! One year I became interested in shrinking down my 215 rig and went with a Peavey Combo 300. Although we had FOH that combo held its own onstage for a small combo.

On the flip side, there are the honest manufacturers like Mesa and Gallien Krueger who use real world numbers to represent their equipment. I've never owned a Mesa head but if it says it has 800 watts I trust that number because of the source. I do own a couple GK's and they are properly represented IMHO. The MB500 can get stupid loud and the MB200 is nice and loud.

I agree, it would be amazing if all the manufacturers got together and adapted a universal power rating. But, that might eliminate some of them from competition because they're using compromised engineering and economics because they don't meet the higher standards companies like Mesa hold themselves to.
 
#17
alaskaleftybass said:
Back in the 80's when RMS ruled, 300 watts was considered enormous.
Click to expand...

I get this exact impression, we consider hundreds of watts few because of so many manufacturers' disproportionate power claims. Can't get rid of my old HA3500 (would love to because of weight) but every class D amp I try holds me back (unfortunately no Mesas nor GKs).

alaskaleftybass said:
went with a Peavey Combo 300. Although we had FOH that combo held its own onstage for a small combo.
Click to expand...

Still regularly browsing the used market for one of those and/or a Databass combo. Used a shared Combo 300 for a gig 15 years ago and have yet to find a combo with more authority.
 
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#18
Coolhandjjl said:
Dispensing with all the tech talk, you simply need a more powerful rig. Even a head rated at 250 watts RMS into a multi driver cab with an efficiency rating around 98~100 is not enough for what you play.
Click to expand...

Not really the question. I don't need a new amp, nor any more power. Was asking why many class D amps I play on sound way less powerful than they claim to be.
 

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