Watts; the worst measurement unit ever for amp output?

[seamonkey]

"Sound Power"
http://en.wikipedia.org/wiki/Acoustic_power

Is what we should all be using.
Read there that a rock concert is 100w sound power.
This would need more zeros to be believable - 100,000,000uWSP
Now that is more impressive.

Now if all manufacturers could just agree on a rating and actually use it.
It took the FTC to step in and enforce this for consumer applications - like home theater. SAE stepped in for Horsepower. I don't think any standard is going to hold unless some government steps in.

And of course for the tube believers if there was a difference it would come out in the measurements. Measuring sound, perceived or real, is not complicated these days. Hasn't been for a long time.

 

[i_got_a_mohawk]

The output power of tube amps can vary a bit more than SS amps, depending on what tubes are loaded in and how the bias is set. You can also play tube amps closer to their rated output beause the soft clipping from a tube amp isn't avoided by most loud tube amp players, where a SS amp will generally be played further from it's rated output to avoid clipping the power section. That's about as far as the 'magic' in terms of tube amp volume goes (and I think most tube amp users, at least here, would agree on that).

 

[ThisBass]

I'd rather point out the following.

With many tube amps it is awfully pretty possible to run dynamic transients beyond the rated output. The rms (root mean square) however does not reach the rated output even in this case.

With SS amps it is also possible to drive dynamic transients beyond the rated output.
But the reachable rms is below the reachable rms of tube amps and therefore remains much more below the rated output.

The dynamics are beyond the rated output in both cases, and in both cases the rms does never reach the rated output.

Everything all right?


BTW not every tube amp provides a soft clipping. Some tube amps clip behavior equals rather SS amps.

 

[danielfnj96]

A multi meter measures DC resistance, not impedance and that 5.73 Ohms reading is right about where it is when measuring DC resistance on a lot of 8 Ohm speakers/cabinets. The meter is probably fine but I would recommend that you re-test, making sure your meter is set to the X1 scale, not Auto or a higher scale, like X100, etc because some meters round up and remove significant digits.

Well it wasn't set to x1000 or anything it was set to the 200 ohms max setting and measured down to a hundredth of a decimal. I also tested it with a 4 ohm resistor my uncle gave me before testing my cabs and it read 4 ohms (yes i'm sure it was a 4 ohm resistor, it was a large gray resistor with 4 ohms printed on the side of it).

 

[EpiRipper]

Calm down princess, you can spell OUTPUT, good for you, have a gold star.

As I said before, output power ≠ draw from wall, in your case you have a newer more efficient amplifier, hence the output isn't that much greater than the pull from the wall.

The output power for an amplifier is the maximum RMS output through the output jacks, generally at ~0.5% or so THD. It doesn't tell you much about your energy bill either, as you aren't going to be continually pulling 1200 watts from the wall.

It's a perfectly fine unit of measurement if you have an idea of the application in which you'll be using the amplifier, will you figure out the exact SPL using your exact instrument, amp settings and speaker setup? No, but then neither will an SPL measurement which isn't performed using exactly what you're going to be using.

PS - Drummers can't think, so you're not validating the loudness of your setup.

Thanks for the gold star!
My point is this, look at the fuse size in your amp. If you have a 3 amp fuse and you have 110 volt power supply, you can't have a 500 watt amp. It will be 300 watts tops. Some manufacturers do pull random numbers out of the air, and some amp designs are just very inefficient, but, as far as I know, even the best amps can't output more than they draw.
Most of the folks on here are not electrical engineers, and don't have much of an idea what a watt is. They know that 300 watts is louder than 100 watts... but... that's not necessarily true, is it?
An inefficient 300 watts SS amp into a 12" speaker versus 100 watt tube amp into 8 x 10" speakers or 2 x 15" speakers is probably going to lose the volume battle, but, a 300 watt amp is certainly capable of drawing more power from the wall than a 100 watt amp (to back my energy bill comment).
I'll take the hit on the drummer comment. It was late and I was tired. What I should've said was, it'll make the first five rows of the audience soil themselves.

 

[seamonkey]

With SS amps it is also possible to drive dynamic transients beyond the rated output.
But the reachable rms is below the reachable rms of tube amps and therefore remains much more below the rated output.

Not sure what you're saying? Usability beyond rated power is a fringe case. And in any case it's up to the designer what happens when you reach the fringes. Every self respecting amp designer is going to take this into account. There is no fundamental flaw or generalization about SS designs here.

BTW not every tube amp provides a soft clipping. Some tube amps clip behavior equals rather SS amps.

+1 Class-AB is Class-AB, feedback is always used - Class-AB doesn't care about the output devices. And sure enough - measurements of amps say the same.

 

[BassmanPaul]

A watt, in regards to musical instrument amplifiers, does not refer to how loud the amp is. It refers to how much electrical power it will suck from a power outlet. An amplifier with a rating of 300 watts will use 300 watts of electricity. The power amp section may be rated at 200 watts, the pre-amp at 10 watts, the EQ may use 20 watts, and all the pretty lights might use 70 (for the sake of this argument).
So you have an amp that may sound the same as a bare bones, no features 200 watt amp, but it's rated at 300 watts because all the add-ons make it a 300 watt amp... because that's how much POWER it uses, not how much volume it makes.

If an amplifier is rated at 300W that is the power that it is capable of delivering into it's load not what it draws from the power line. The power line draw is always larger than the rated output. Power supply efficiency is a factor here. With Class D amps and switch mode power supplies that efficiency can be in the high nineties. With a tube amp with a standard transformer, rectifier capacitor power supply it's closer to 50%.

 

[P Town]

Well it wasn't set to x1000 or anything it was set to the 200 ohms max setting and measured down to a hundredth of a decimal. I also tested it with a 4 ohm resistor my uncle gave me before testing my cabs and it read 4 ohms (yes i'm sure it was a 4 ohm resistor, it was a large gray resistor with 4 ohms printed on the side of it).

Even if you have a good multi-meter, that has been calibrated within the past 12 months, and you zero out the resistance of the test leads, you should not expect the resolution of the measurement to be less than one Ohm. If you have a meter capable of making resistance measurements by what is called four wire resistance measurements, then you can talk about a reading with digits after a decimal point. So for two wire Ohm meter readings, it is best to disregard any digits to the right of the decimal point.

 

[SactoBass]

You know a ballsy bass rig when you feel it.

+1

IMO, it all comes down to this.

 

[Major Softie]

BOY is there a lot of total confusion here amongst a gang that presumably actually knows how to use this equipment.

Some of the "statements as fact" in this thread are just plain goofy, but then, so is the premise introduced by the OP.

Here's some facts that are actual facts:

Amps are rated in watts because that is the useful unit to measure their output. They cannot be rated in SPL or dB or any other measure of sound level because amps don't make sound. Amps "amplify" the strength of an electrical signal. The only SPL or dB rating you could give an amp is the loudness of the fan.

Speakers (transducers) change electrical energy into mechanical motion which, in turn, creates sound waves. Speakers can therefore be rated in units of sound volume, as in: so much volume of such and such a frequency when fed this much power. Yes, IF IF IF you could get the industry to accept a certain test speaker as the "certification speaker," you could use that to give a volume rating to amplifiers, but every example of that speaker would be slightly different (so they'd all be looking for the "Golden Speaker"), and you'd still have to standardize what frequency will be measured, and different settings on the amp could greatly change that, so that it could become more a measure of EQ flexibility than loudness. You'd still have to establish the measuring point, so if it still is a certain level of distortion, then the tube amps would still have a rating far below SS amps that can achieve the same volumes. And, what speaker could be a useful measure of the output of every amplifier from 10 watts to 2000 watts, some putting out their max power at 4 ohms, and some at 2 ohms? Most importantly, such a scenario would then turn into a competition of who can make an amp that best serves the test rather than who can make an amp that best serves the musician.

The amount of power used by an amp has very little to do with its output power. Yes, it is true that the amp cannot put out more power than it takes in . . . continuously . . . but it can, in fact, provide bursts of power (peaks) that far exceed its continuous input. An amp may also produce far far less output than it consumes, which would make it, by definition, an inefficient amplifier. That doesn't mean "bad," it just means it is not electronically efficient. It may well be sonically superior. An Ampeg Heritage SVT-CL is rated at 300w of output and 10A of 110v input. That's 1100 watts to make 300 - hardly efficient, but sonically worthy.

An Ampeg PF-500 has an output rating of 500w, and an input rating of 200w. WHAT?! How can this be????? Is it so efficient that it makes more than it is fed? Genz-Benz Shuttle 6.0: maximum output 600w, maximum 110v input 400w. MarkBass seems to do all they can to avoid actually stating input wattage. So, are Class D's just magical?

I have not actually found a completely clear explanation of this, but from my understanding of Class D amplifiers, it is because it is not creating a continuous signal, but pulses (a PCM signal), it is much more efficient than the other designs, but (and now we are into my edit) agedhorse explains below that it has to do with the fact that none of these amps are rated at max power, and different designs are rated at different percentages of their max power.

Thanks, agedhorse, for fixing my spurious speculation.

 

[danielfnj96]

Even if you have a good multi-meter, that has been calibrated within the past 12 months, and you zero out the resistance of the test leads, you should not expect the resolution of the measurement to be less than one Ohm. If you have a meter capable of making resistance measurements by what is called four wire resistance measurements, then you can talk about a reading with digits after a decimal point. So for two wire Ohm meter readings, it is best to disregard any digits to the right of the decimal point.

Ok well i guess i'll just take your word for it then. It is a very cheap meter with two wires and has never been calibrated so it wouldn't surprise me if it gave a bad reading.

 

[JohnMCA72]

Ok well i guess i'll just take your word for it then. It is a very cheap meter with two wires and has never been calibrated so it wouldn't surprise me if it gave a bad reading.

There's a difference between impedance & resistance, even though both are measured in ohms. When you make a static measurement across the terminals of a speaker, you're measure the DC resistance of the voice coil. When you apply an AC signal to that same speaker, it moves the coil back & forth through a magnetic field to produce sound. That same motion, of a coil through a magnetic field, also generates AC power in the reverse direction, sort of an electronic back-pressure. This is impedance. The impedance of a speaker varies depending on the frequency of the signal, which is why you always see speakers' impedances listed as some number of ohms at some particular frequency.

The rig to measure impedance is a little more than just your simple multi-meter. For starters, you've got to generate an AC signal at whatever frequency you want to test at. Often, you'll see a graph of an impedance curve, which shows the impedance across the speaker's operating range. This is produced by sweeping the frequency across this range.

 

[agedhorse]

Wow, other than the usual folks who have a history of understanding this stuff, there's a whole lot of really significant misunderstanding going on here. I will touch on a few items that jump out as particularly challenging...

1. A power amp is indeed measured in watts, since watts is a unit of power. For those of you who say that it's all about voltage, if the amp can not deliver the current required by the load at the particular voltage, it will not deliver power in proportion to (the square) of the voltage. Current is as essential as voltage, and those of us who designed linear amplifiers for a living are particularly sensitive to this as a power amp is both a voltage amp and a current amp. Now add phase shift between voltage and current due to the reactive load of a speaker and the two have a "complex" relationship.

2. Measuring using a resistive load does not indicate power accurately... this is true, but in general it's the most practical, repeatable, and functional test for most folks. It doesn't mean that we don't test (extensively) into reactive loads of all types, but we do this to verify adequate current is available, stability and protection loci. In fact, using real world reative loads can INCRASE the rated output of some class D topologies, provided the amp has been designed to take advantage of recovering reactive energy (like regen. braking in an electric vehicle).

3. Power output can never exceed the input power listed on the amp (or specs)... this appears to be true until you recognize that the rated input power is a minimum of the power at 1/8-output power, or whatever the amp is likely to draw under typical or implied design applications (as dictated by the safety agencies). Since one of our products was mentioned in this comment, let me clarify that the %-output power that we rate our products at varies by the type of product and if it's designed to be operated under overdriven conditions. Acoustic amps will be rated at closer to 1/8-output power because they are generally not intended to be overdriven, but a bass amp like the Streamliner, will be rated at closer to 1/3-output power as it's commonly overdriven as part of the desired sound and feel. The Shuttles will be somewhere inbetween (like 1/4-rated power) whereas electric guitar amps will be closer to 1/3-power. So this, plus the various efficiencies all factor into how this input power number is derived. I recall going into excessive detail on this about a year ago in another thread.

4. The DC resistance of a speaker will ALWAYS be different than the nominal impedance, the nominal impedance is generally a weighted average based on the actual impedance within a specific "standard" box or infinate baffle over a specified bandwidth. The DCR is one point and only one point, and at 0Hz. This is something that a speaker designer must understand in great depth and detail. Unfortunately, I have seen some who do not recognize the significance of this information and can result in "amplifier killer" type speaker products.

5. All that has been discussed to this point is steady state power ratings, but since bass guitar is anything but steady state signal, a whole another aspect comes into play that may in fact be even more important than rated steady state resistive power. This is dynamic power, the ability of an amplifier to deliver greater than steady state power for a short time, and/or to deliver with plesently increasing distortion and without terrible recovery artifacts. This is an area that many of the top MI product engineers (including myself) have studied for many years, and lead to better performing products in the hands of players and out in the real world.

 

[beans-on-toast]

Now add phase shift between voltage and current due to the reactive load of a speaker and the two have a "complex" relationship.

Excellent comments.

I'm sitting here laughing thinking of how one explains that complex relationship where designers work with real and imaginary numbers. Indirectly, when an amp is connected to a speaker cab, real watts and imaginary watts. How confusing is that.

 

[Major Softie]

Thank you, oh aged horse. I thought, when you consider the wasted heat, etc., it was pretty hard to imagine how that could yield the kind of numbers we were seeing listed, even with PCM magic.

I will adjust my post above.

 

[ThisBass]

5. All that has been discussed to this point is steady state power ratings, but since bass guitar is anything but steady state signal, a whole another aspect comes into play that may in fact be even more important than rated steady state resistive power. This is dynamic power, the ability of an amplifier to deliver greater than steady state power for a short time, and/or to deliver with plesently increasing distortion and without terrible recovery artifacts. This is an area that many of the top MI product engineers (including myself) have studied for many years, and lead to better performing products in the hands of players and out in the real world.

IMO you state a very valid point of bass guitar reinforcement.
Probably it is an unknown fact to most of folks that a bass guitar has an excessive dynamical behavior.

 

[danielfnj96]

There's a difference between impedance & resistance, even though both are measured in ohms. When you make a static measurement across the terminals of a speaker, you're measure the DC resistance of the voice coil. When you apply an AC signal to that same speaker, it moves the coil back & forth through a magnetic field to produce sound. That same motion, of a coil through a magnetic field, also generates AC power in the reverse direction, sort of an electronic back-pressure. This is impedance. The impedance of a speaker varies depending on the frequency of the signal, which is why you always see speakers' impedances listed as some number of ohms at some particular frequency.

The rig to measure impedance is a little more than just your simple multi-meter. For starters, you've got to generate an AC signal at whatever frequency you want to test at. Often, you'll see a graph of an impedance curve, which shows the impedance across the speaker's operating range. This is produced by sweeping the frequency across this range.

Interesting, thanks for that tid bit of information.

 

[ThisBass]

Not sure what you're saying? Usability beyond rated power is a fringe case. And in any case it's up to the designer what happens when you reach the fringes. Every self respecting amp designer is going to take this into account. There is no fundamental flaw or generalization about SS designs here.

There is no reason to worry about. Some clipped short term transients are not audible anyway.
Human ears are like a lame dug

 

[Vince Klortho]

... The impedance of a speaker varies depending on the frequency of the signal, which is why you always see speakers' impedances listed as some number of ohms at some particular frequency.

The rig to measure impedance is a little more than just your simple multi-meter. For starters, you've got to generate an AC signal at whatever frequency you want to test at. Often, you'll see a graph of an impedance curve, which shows the impedance across the speaker's operating range. This is produced by sweeping the frequency across this range.

I rarely, if ever, see a speaker's impedance rated at some particular frequency. In fact, I can't remember the last time that I did. What is almost always listed is the "nominal" impedance which is usually a little more than the minimum value on the curve. Here is one example. That one is listed as an 8Ω speaker but you can see the minimum is a bit lower. Eminence does not rate their speakers at a specified frequency. They always use a nominal value.

This brings up an important point. The impedance of a speaker will usually drop below its rated value at certain frequencies. This means your amp will see less than its minimum impedance rating at times. This is rarely if ever an issue though. The fact is many solid state amps can actually drive less than their minimum impedance, especially at less than maximum power. What happens is the amp will attempt to draw more current when the impedence is lower and current is what kills amps. Fortunately, when running at less than full power and the impedance gets too low the current draw will still be less than the limit of the amplifier so it won't be damaged. Don't interpret this to mean it's OK to run a 2Ω load on your 4Ω amp. You can probably get away with it for a short time at a low volume but it's definitely not recommended. I am not certain how safe it is to run tube amps at lower than their selected impedence but I would not assume that it is.

 

[agedhorse]

IMO you state a very valid point of bass guitar reinforcement.
Probably it is an unknown fact to most of folks that a bass guitar has an excessive dynamical behavior.

One of the most eye opening moments in an amp designer's life is when (or should be when) they study the physics of the various string movements and resulting waveforms. It's different for guitar, bass and piano... they share some similarities but all have significant differences as well.

Things like the attack envelope, sustain and decay envelopes, the harmonic structure (varies with frequency, and by string), the effect on using a pick, slapping, bending a note, using different styles of string, weights and tension, pickup type and location. All of this affects the waveform. An amp designer needs to be aware of this and design the amp's thermal and electrical performance envelope around what some form of worst case bass guitar (of the style that fit's with the amp's intended application of course) will deliver. Then, overlapping the amp's performance envelope with the bass guitar's attack, sustain and decay envelopes versus frequency, the designer will have a good idea it the amp will satisfy reasonable performance and reliability expectations based on the above data. This is the sort of stuff that makes a product a top tier type product, one that leads the market, with a product that is for a less demanding performance (and hopefully price) point.

This is really fascinating stuff to me (I hold a patent in this area) and I'm sure it is to some other amp designers as well. This sort of stuff occurs throughout all engineering fields, the processes are similar but also different.