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True power output?

#141
Different power amp (and also preamp) topologies and design choices have completely different distortion curves, from what you describe all the way to straight line smoothly increasing. There are very different feels to the different curves based on how the properties are derived.
 
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#142
I have very little to add to these discussions except my point of view: I'd rather be told no information about power output rather than misleading or even false information. With no information, the buyer understands it's a "caveat emptor" situation, will research user reports and try to audition the gear. It's like the way Rolls Royce used to advertise its vehicles' horsepower as "adequate." You got the real numbers from real world testing by journalists. I prefer that to being misled by marketing-oriented numbers. An example I've cited in the past is how Avatar doesn't advertise a frequency response for its bass cabs. That's fine with me since it avoids the usual misleading "30Hz to 14 KHz" type spec that fails to say that the speaker is down 20 decibels and virtually inaudible at 30 Hz. If power amp specs are that difficult to agree upon and more difficult for a layperson to understand, they are subject to too much manipulation by marketing departments and I'd rather be told that an amplifier comes in three power categories, practice/small gig, medium/loud gig and outdoor/very loud gig ratings, just to make up an example. Then if users reported that the amps indeed functioned well in those situations, I'd know which one was likely to meet my needs and try to audition it.

But to paraphrase Jimmy, it ain't gonna happen based on my input! :D
 
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#143
agedhorse said:
How do we specify power if the power must be specified by a standard at less than 1% (or 0.1%) when you test players "specified" the higher THD as ideal?
Click to expand...

This seems to be the problem to me. Consumers want a value that gives them a comparison figure for different amps based on how they sound or how loud they are. Power is poor proxy for this because it is only part of a very large and complicated series of changeable and dynamic parts when the system is taken as a whole.
 
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#144
agedhorse said:
Different power amp (and also preamp) topologies and design choices have completely different distortion curves, from what you describe all the way to straight line smoothly increasing. There are very different feels to the different curves based on how the properties are derived.
Click to expand...

Ah, OK, I can see how that would make it a bit of a minefield to come up with a comparable rating between amps. I think I've only seen those graphs from a couple of manufacturers of class-D amps, but it makes sense that other topologies or design choices like feedback would behave differently around clipping.
 
#145
Another thought of an example that indeed might happen in practice.

Some players prefere a rough fat sound processed by pedal board or tube preamplifier.
I suppose there is a lot of personal composed MI gear out there where a tube pre or pedal board drives a pro audio Class-D power amp exclusively with those fat signals in rather excessively manner.

In this case the user himself is responsible to take care not to overfill the power amp with too much average power than the amp is designed for. On board limiters can't protect from overheating.
In this case the user himself has to figure out how much power margin must not be touched in long term considertion.
That undertaking might escalate into a very elaborate expense because there a very lot of variables to be considered in relation. How long lasts a set, how long lasts a break, ambient temperature, solar radiation at outdoor gigs.
That's work for engineers to figure out but, the gear is personal composed and therefore is user's responsibility how much power to get out of the power amp with the "fat" processed signal.

If the user does not take care on this because limiter LED is off all the time (an "indicator" for the average user everything is fine) then it may happen on stage the amplifier shuts down due to thermal overload.
What happened? The amplifier produced too much of average power at output terminals over time.

The user asks a question at forum for help. He mentions that he has got this power amp in use for several years now but never experienced issues like that (even with Gain At Full). The issues started with the tube pre cause his current band plays crunch stuff. He also mentions that he surprisingly gets more noise out of the power amp but clipping LED is now steady off, no slightly random flushing like obtained in his previous bands with different sound he played.

How do you guys would help him with "common wisdom" because amplifier ratings regard a common standard?

Compressor that helps to reduce peak levels?
HPF that helps to reduce useless sub lows?
Turn down the bass knob and give more mids to the sound. That helps to cut ....?
Must be a crappy power amp, sell that junk and get with a power amp that ...?

Wouldn't it be better to tell him the truth what really happens?



.
 
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#146
So, noting that consumers like the bigger number when it comes to power ratings, and don't care about the THD much, making it not advantageous to rate at 1% versus 5%, does that mean that Sunn back in the day shot themselves in the foot by doing so?

As far as a better standard for "how loud does this amp get," the ideal thing to test would be loudness... via seeing just how much air it can push with a speaker. The problem with that, obviously, is that you'd need to hook it up to a speaker, and everyone makes their own different speaker cabs with different frequency response curves and sensitivities, so you would need to have some sort of "industry standard amplifier test speaker cab" or something. Which seems like a very unlikely prospect.
 
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#147
I asked this in another thread but this is probably the best place. There's Firebass 700 for sale in the local ads. The head's features indicates it generates "700W @ 2 ohms, 475W @ 4 ohms, 275W @ 8 ohms". The seller has a Peavey can who's impedance is 8 ohm. I never quite understood this impedance thing: am I to understand that I'll never get more than 275w from that head with that cab ?
 
#148
How loud an amp goes depends on the average the amp can savely produce over time.
In the pro audio world the answer is very easy. There is no need to expect more then 1/4 of "rated" power because it makes no sense to expect larger numbers for average. In rough words, average is the thing that makes loudness.
Once a number for average is known THEN there is need to consider the program material that shall amplified to THIS average of power (the loudness) an amplifier can savely produce over time.
If I had an amplifier that savely can produce 1/3 of the rated sine/burst power over time then I will fail to achieve this "loudness" with jazz music If do the same thing with an instrumentaL power trio (no vocals) then a would get much closer to this average the amplifier can savely provide. And if I had a electric guitar signal that is processed by several tube stages (brown sound or overdrive) THEN I easliy can exceed the save operation of the amplifier.

If I was asked a question by a guitar player like this:
"how much power for a class-D amplifier do I need for my modelling preamp. I prefere Vox AC30 sound and want get same loudness on stage with a 212 cab."
Theoretically it would be possibe with an amplifier around 40-50 watt sine continous 1% THD. In practice there is need to challange the desired "average" of 30-40 watt regarding save operation over time. Very likely it's more practical to select an amplifier of 100 watt (at least) to reduce the risk of overheating the amplifier over time.
The guitar player as well needs a clean sound that "feels" the same loudness like the Vox AC30 sound. The modelling preamp provides a Fender Twin Reverb emulation. In this case the power amplifier is forced to produce much more of attacks and tranisents to get the same noise on stage. That means same loudness respectively same average of power.
Both sounds make just same noise/average regarding power. The rms voltage at amplifier output measures just same numbers.

Both sounds on a thought bench test with sine stimulus.
Vox AC 30 sound: 30 watt sine power
Fender Twin sound: 100 watt sine power

Whereas the Fender Twin "clean" sound can reach the 100 watt capability of the amplifier the Vox AC30 "crunch" sound restricts the amplifier to exceed more then 30 watt with sine waves.

Allthough both sounds achieve obviosly different "power ratings" at the bench with sine waves the more "practical" power average ratings (derived from rms voltages) would be the same for both if they where determined with an electric guitar.

In both situations the class-D amp would provide same average (same loudness). If this average capability was 1/3 then the power amp is operating at "full" power on both.
 
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#151
Bassmike62 said:
I asked this in another thread but this is probably the best place. There's Firebass 700 for sale in the local ads. The head's features indicates it generates "700W @ 2 ohms, 475W @ 4 ohms, 275W @ 8 ohms". The seller has a Peavey can who's impedance is 8 ohm. I never quite understood this impedance thing: am I to understand that I'll never get more than 275w from that head with that cab ?
Click to expand...

Yes. A Firebass 700 will only produce 275W into any 8 Ohm cabinet.
 
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#152
There's a thread on this in the FAQ's post: Ohms FAQ | TalkBass.com

Bassmike62 said:
I asked this in another thread but this is probably the best place. There's Firebass 700 for sale in the local ads. The head's features indicates it generates "700W @ 2 ohms, 475W @ 4 ohms, 275W @ 8 ohms". The seller has a Peavey can who's impedance is 8 ohm. I never quite understood this impedance thing: am I to understand that I'll never get more than 275w from that head with that cab ?
Click to expand...
 
#153
I tend to not believe published amp specs. They are good to have when roughly comparing different amps so in that respect they are useful. But I don't take the numbers to the bank.

Audio analyzers allow me to get a handle on how an amp is performing but until I hook up the amp to the test equipment I can only guess. I can't do it by ear in a meaningful way.

As a player, when listening to an amp, rms watts output and thd doesn't mean a lot to me. I know not loud enough, just right, and too loud; distorted vs clean. That's all I need to be concerned with.
 
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#155
agedhorse said:
Thermal capacity, at all rated impedances, based on the number of minutes at 1/8, 1/4, 1/3 and 1/2-power (this would show the thermal performance under different duty cycle conditions, the longer it performs, the more robust the design, but the user can choose how robust they need, or are willing to pay for. 1/2-power is so heavily compressed as to be barely useable, for reference.
Click to expand...

I think that THIS directly applies to the question "how load can an amp go" and indirectly returns question to the user himself if the neccessarily resulting sound is desired.
AND it returns question to the user how long a distinct sound is needed on stage? Playing a dirty solo for 32 bars in length is a different animal then playing just the same sound and loudness level all along the line during a whole lasting gig event.

Although specs like that are more common with pro audio amplifiers. Never the less I'd wish to see more of them and more detailed on MI amplifier data sheets. Pro audio/sound engineers (mostly) know their stuff by default. For musician folks it could be a very helpfull information to compare apples to apples, with respect to personal/individual sound goals.
 
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#156
IMO it would be real pity to let this thread fade out.
Lot's of fine input here to think about.

I think it would be pity due to some (distinct) reason.

At just this point of the discussion there is a very good chance to discuss about different signals and what that means to a power amp capability. IMO the most interesting question to the average player is "power". That of course is often discussed in a more basically manner (the more or the less) but, why not grab a chance to get more into details what is really going on under the hood?
One thing I'd wish to see is an improved discussion based on an "virtual" amplifier power rating in relation to "virtual" different sounds.

IMO that can help to improve our understanding for the way "how it all works".
 
#158
Gaolee said:
Power specs are completely irrelevant, since there's a whole lot more to it than that. It's like horsepower ratings. They are a raw number, but they don't tell you that the motor is anemic down low or has an unusable power curve that looks like a spike.
Click to expand...

Indeed, you bring up a very good point. It's one thing to generate raw horsepower, it's another thing to generate usable horsepower.

Horsepower curves that require you to operate at an unreliable portion of the curve is not very usable (unless you are ok with constant repairs). Horsepower that can not get transmitted to the wheels is also unusable.
 
#160
Exactly. That's why I'm perfectly satisfied with far lower power ratings from the amplifiers I use. It just doesn't matter, since I get the sound I want and have enough volume to play every venue I'll probably ever play.

Truck motors are often not particularly high horsepower relative to the torque they put out. They don't rev very high, relative. A 600 cc motorcycle, especially a full on sportbike, makes an insane amount of horsepower for its displacement, but it won't have much torque, relative, since it makes the power by revving high. A two stroke motorcycle has a light switch power band, and those are a blast if you are prepared for them. Otherwise, they are awful.
 

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