Watts; the worst measurement unit ever for amp output?

[agedhorse]

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.

Let me comment on this a little bit, as it's a VERY important aspect of REAL WORLD amplifier design.

Yes, some portion of the impedance curve may fall below the nominal impedance rating of the speaker. What's really important is what percentage of the useable bandwidth is "sub-nominal" and where is it located. It's also important to look at the driver within the box it's desegned to be used in, as the box may also affect the impedance curve. For linear amps, subnomimal impedance can increase the current at a particular voltage so the overcurrent limiting circuits must be capable of handling this for at least short time periods, and it also increases the thermal load on the amplifier. If the impedance drops for only a small percentage of the bandwidth and by only 10-20% of the nominal value, it's probably not going to be a big deal BUT I have seen a few speakers where somebody must have been smoking crack when coming up with the nominal impedance values because paralleling 2 nominal 4 ohm cabinets should not get down to 1.25 ohms over a full octave. Crazy stuff, but they always assume it's an amp problem when it may not be.

 

[JimmyM]

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.

I have run tube amps at impedances higher and lower than the impedance I was supposed to use. All have been without incident. However, I never crank tube amps hard except for my B-15, which always runs at its correct impedance, because that's when you start having the issues with mismatched impedances, and wear and tear on your output transformer and tubes is increased. And I never run wrong impedances with my own tube amps, except by accident when learning how these new-fangled 810's plug in Only with rentals, and if they're dumb enough to bring me an SVT with an 8 ohm cab instead of the 810e I request, then they can assume the risk.

 

[agedhorse]

Tube amps are (generally) more foregiving, a byproduct of higher impedances and more lossy coupling. This is also part of what gives a tube amp it's feel.
The real issue with tube amps is (much) higher impedance mismatches, where the output circuit can become underdamped and can not absorb the back-emf of any ringing that may occur. The voltage may rise to a point that compromises the insulating system inside the transformer of output tube circuitry causing arcing and then catastrophic (and costly) failure.

 

[Vince Klortho]

Let me comment on this a little bit, as it's a VERY important aspect of REAL WORLD amplifier design.

Yes, some portion of the impedance curve may fall below the nominal impedance rating of the speaker. What's really important is what percentage of the useable bandwidth is "sub-nominal" and where is it located. It's also important to look at the driver within the box it's desegned to be used in, as the box may also affect the impedance curve. For linear amps, subnomimal impedance can increase the current at a particular voltage so the overcurrent limiting circuits must be capable of handling this for at least short time periods, and it also increases the thermal load on the amplifier. If the impedance drops for only a small percentage of the bandwidth and by only 10-20% of the nominal value, it's probably not going to be a big deal BUT I have seen a few speakers where somebody must have been smoking crack when coming up with the nominal impedance values because paralleling 2 nominal 4 ohm cabinets should not get down to 1.25 ohms over a full octave. Crazy stuff, but they always assume it's an amp problem when it may not be.

Thanks for the elaboration. I have read about the situation you are describing. It's strange that they expect 2Ω amps to handle 1.25Ω without an issue. Is reaching an impedance of 2.5Ω from a nominally 4Ω speaker a misrating of the speaker or a result of the cabinet loading or both ?

 

[megafiddle]

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...

If the amp can't deliver the current, then the voltage isn't going to be there either.
No one measures the open circuit votage and then calculates what the power would be
if a load were connected. The voltage is measured across the load, with the load present.

If you have 40 V across 8 ohm, there will be 5 amp flowing. If the amp couldn't deliver 5 amp,
then the 40 V would not be there to begin with. The output would have sagged. If the 40 V
is there, then so is the current (the 5 amp).

You can express the output in watts, or voltage squared over resistance. It's the same value.
If you know one, you know the other.

.

 

[nukes_da_bass]

Im the OP, i didnt disappear- i have nothing scientific to contribute.

I have read every post in my thread. Even the engineers seem to agree (or politely disagree) that power rating systems could be simplified or clarified for layman digestion (I.E. dudes like me!).

In the meantime all this math makes me hungry- ive laid out a nice schmear on the buffet table dig in! Sorry, onion bagels went fast...

 

[1958Bassman]

My point exactly! My amp is a Hartke LH1000, which I run in bridge mode, so I have 2 power cores, each rated at 500 watts in standard configuration. Yet in bridge mode these 2 500 watt amps are rated at 1100 watts (heat generated by running one core into the other may account for the extra 100 watts of rated power). My power outlet supplies 240 volts (Australia) and the fuse in my amp is rated at 5 amps, so if my amp pulls more than 1200 watts of power, or voltage drops below 220 volts... boom, fuse gone!
By the way, I run the full 1100 watt monster into 800 watts (rated power handling) of speakers and haven't cooked a voice coil in the two years I've had this setup.
Absolutely nothing to do with how much noise the amp will make, it's all about power consumption. Try the maths yourself... as 1958Bassman said... fuse rating x line voltage should get you somewhere near (+/- 10%) the true rated power consumption of your amp. If it's rated way higher by the manufacturer... well, don't use the guy that came up with those figures as your tax accountant.

Again, amplifiers aren't 100% efficient, so I would take a good look at the specs- Peak power is often used because it looks better to a potential buyer and since it's not forbidden, it's on the spec sheet. I would be happier if they stated "500W continuous @.5% THD, 20-20KHz+/-.1dB, 1% TIM and .5dB dynamic headroom with .5VAC input. S/N ratio 100dB, A-weighted". THAT'S a useful spec.

 

[1958Bassman]

Most serious manufacturers give detailed specifications for the line socket current consumption which is measured at 1/8 and 1/3 output with Pink Noise.

The 1/8 output power method is very narrow to the real world for any PA amplifier. Whereas the 1/3 output power measurement is most of theoretical nature but not a really practical validity.

Sometimes manufacturers state an additional current consumption for full output power 1% THD. Those specifications are absolutely meaningless for max rms current consumption at the socket.


Ok, so lets calculate the current consumption at 1/8 Pink Noise output for an amp with 500 Watt sinus 1%THD output power by 50% Heating losses.

500 Watt : 8 = 62.5 Watt
62.5 Watt * 2 = 125 Watt Power consumption at the line socket.
The rms current consumption at 110V line socket calculates to appro 1 Ampere.

1/3 Pink Noise calculates to 3 Ampere current consumption.

Whereas full output power 1% THD calculates to appro 9 Ampere at the socket. But this specification is meaningless for all PA amplifiers in every real world applications.


The truth for maximum power consumption is probably somewhere in the range of 1/8 .. 1/3 Pink Noise.

But not knowing the nature of a given Audio program means in effect that it is not valid to state a max. possible consumption at the socket (or max. possible output power).


But it is valid to state the following point:
for any PA amplifier the rms current consumption calculates to significant lower values than it would be supposed at full output sinus 1% THD.



see consideration with Pink Noise 1/8 .. 1/3 vers. full sine output power.


GK states an average Power consumption at the socket for appro 1/3 of full power output.
In addition to this specification GK states as well the Power consumption for full output power (which is IMO absolutely obsolete).

The methods you list are seldom seen on spec sheets for consumer goods because they won't be understood. The fact that people think the Watt is the most useless spec is proof of this.

Let's face it- consumers look at the numbers without understanding what they mean and more is better. Engineers use the context to determine something's usefulness for a particular application.

Actually, 1/3 output power IS a valid rating because it's the average output over a specific time. It's not possible to constantly output 100% unless certain conditions are met- a compressor is ued and set to eliminate all voltage drop at the input, a synth/function generator is providing the signal or the amp/instrument are in a feedback mode. If these occur, the fuse will fail, a breaker will open or the amp/speaker will die.

 

[1958Bassman]

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.

The part I "enhanced" is the problem- too many people buy equipment without thinking, knowing or understanding this.

Re: drummers- "If a bass player turns their amp down in a forest, can the drummer still think?".

 

[1958Bassman]

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).

Wow! I don't think I have ever seen a speaker that measured exactly 4, 8 or 16 Ohms in the ~40 years I have been working with speakers and meters.

 

[beans-on-toast]

…onion bagels went fast.

Bread is so simple: flour, yeast, water, sometimes oil and other ingredients. For such a basic thing, the result is incredibly complex. Textures and taste can vary depending on the raw materials and how everything comes together. People have different tastes but they know what they like.

Tube amps are the same way. At first glance they are so simple, yet the way in which the components interact are incredibly complex. There are so many dynamic attributes. Understanding those complexities is something we haven't fully worked out. There isn't a complete way of characterizing the performance an amp. Intangibles such as tone and knowing what you like are a big part of defining ones taste in amps. More so than specs.

 

[1958Bassman]

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.

Before any resistance measurement, it's best to touch the tips of the probes to see the resistance of the leads and probes. Even the best meter will show something, but they usually have a 'Zero' button, which makes the reading show 0, but ignores the added resistance. This is obviously more of a factor at low resistance because it's a higher percentage of the total.

 

[ThisBass]

Some woofers impedance curve show a very significant valley in the range of 150Hz .. 1.5kHz.
If a 8 Ohm woofers given application is to work only in this frequency range (for example as a midrange driver in a 3 way PA system) means that this woofer impedance can't never be a 8 Ohm.

Considering that the energy density is decreasing vers. increasing frequency even in the midrange band means that probably even some very popular 8 Ohm woofers work in a 3-way system in the midrange band rather like 6.5 Ohm or something above Zmin.
That means two 8 Ohm cabs in parallel would rather calculate to something around 3.25Ohm, and four of those in parallel calculates to 1.6Ohm.

I remember the Audio Engineering Society AES recommends to calculate a speakers Power capacity by the use of Zmin. The intention for the use of Zmin is NOT to calculate a higher CAPACITY, it is rather to state a max. power consumption (or current consumption) for worst case scenarios if the speaker is running at the rated continous.
Invalid Link Removed


But in an application for instrumental bass the situation is totally different. In most of all bass cabs the woofer is working as ful lrange across the whole frequency band. The energy density is concentrated on the lower frequencies where the impedance curve shows a significant Zmax.

As Andy already said, the frequency spectrum of a bass guitar is very time sensitive, starting at the first moment of attack during decay up to the sustained note it is impossible to state a steady spectrum.

Every different frequency spectrum causes different Z because Z is frequency sensitive.

The equalizers and tone stacks on preamps force an very interesting appearance in this context.
Extreme boost of bass frequencies onto the spectrum i.e. of notes E1 or B0 pushes nearly all of the energy onto the low band where Z of a bass cab has in general an extreme maximum.
In this special case a given random 4 Ohm cab is rather a 8 Ohm, that means:
Playing low notes on the fretboard the cab shows rather 8 Ohm behavior, whereas playing higher notes on the fretboard the cab behavior shows 4 Ohm.

Considering my own preferred eq settings on my 70s svt means to my own 4 Ohm cab,
Probably I have got something arround 5..6 Ohm whenever I play low notes but I have got a 4 Ohm cab on notes like E3 or above.


Sometimes I ask myself why there is no existing recommendation to measure a cabs Z depending on different application i.e. different frequency bands.

My imagination (referring to the recommendation of measuring Power Capacity)
Pink Noise +6dB Peak limited.
1) measuring rms current in a band limited frequency range of 150Hz..1.5kHz
2) measuring rms current in a band limited frequency range of 50Hz..1.5kHz
in addition to that an additional rms current measuring with a shelf bass boost +6dB / +12dB at 80Hz.

I did similar measurements in the past and I can tell the different results for Z are very impressive.

 

[1958Bassman]

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.

Is there a way that you could post some oscillograms of sine wave, pure & clipped and the wave forms for notes that were plucked, slapped and picked with measured output for each? If this could be done at low output, medium and high output, it would show the changes to the waveforms and it might make the concept easier to comprehend. To remove variation in the signal, it could be recorded and sent to the amp's input.

 

[nukes_da_bass]

Bread is so simple, flour, yeast, water, sometimes oil, etc. and yet the result is incredibly complex. Textures and taste can vary depending on the raw materials and how everything comes together. People have different tastes but they know what they like.

Tube amps are the same way. At first glance they are so simple yet the way in which the components interact are incredibly complex. There are so many dynamic attributes. Understanding those complexities is something we haven't fully worked out. There isn't a complete way of characterizing the performance an amp. Intangibles such as tone and knowing what you like are a big part of defining ones taste in amps. More so than specs.

And yet, that bagel that you characterize as Bread is actually coated in a certain type of sugar and boiled for certain specific amount of time long before it's baked to create that lovely skin that a real bagel has.

So somewhere in there I'm assuming you have some sort of engineering or troubleshooting experience with tube amplifiers and even you admit we don't understand all the implications as to how the components interact...which before science confirms entirely how something works, we can only refer to the results that our ears show us as magic (meta)!

 

[1958Bassman]

Im the OP, i didnt disappear- i have nothing scientific to contribute.

I have read every post in my thread. Even the engineers seem to agree (or politely disagree) that power rating systems could be simplified or clarified for layman digestion (I.E. dudes like me!).

In the meantime all this math makes me hungry- ive laid out a nice schmear on the buffet table dig in! Sorry, onion bagels went fast...

It would be great if a simple spec could be used, but too much information is needed in order to make an informed decision. It's better to get the needed background in what's happening and why, rather than relying on what amounts to a cheat sheet. I don't mean that someone who's not interested should get a degree in electronics (although, it's not a bad field if they have the ability AND can become interested), but a lot of the basic concepts of electricity can be explained in ways that are more easily understood. One problem with understanding this stuff is the fact that it can't be seen unless something foes wrong, or very wrong.

An example that I saw when I went to technical training for certification when I worked as a boat mechanic- people who were considered to be exceptionally good with mechanical devices were completely dumbfounded by something as simple as the small, black relays used in car audio/security, automobile headlights, etc. The diagram is molded into the side of the cover and it's basically a switch that's "flipped" using 12V DC. If you remember grade school science class where someone wrapped wire around a nail and connected it to a battery, it picks up paper clips and other steel/iron. The same concept is what flips this switch. It's also the cause of speaker cone movement and it doesn't matter whether AC or DC is used.

 

[ThisBass]

Actually, 1/3 output power IS a valid rating because it's the average output over a specific time. It's not possible to constantly output 100% unless certain conditions are met- a compressor is ued and set to eliminate all voltage drop at the input, a synth/function generator is providing the signal or the amp/instrument are in a feedback mode. If these occur, the fuse will fail, a breaker will open or the amp/speaker will die.

Even with compressor it is impossible to get 100% because a compressor cant knock of the envelope of a sustained bass guitar note down to 3dB crest.

In addition to that, it is unknown to most of folks that standard compressors (and limiters) need a little time before they can start to control the signal.
Either way if it is an analogue or digital compressor/limiter, all those "automatic gain control amplifiers" cant process a given signal at once.

That means that initial attack peaks remain unprocessed.

But this is not a problem at all. Because human ears are like a lame dug and cant hear this little amount of unprocessed content of the original signal, by the other hand the clipped content (additional Harmonics in the treble band) is reduced to a minimum and therefore prevents the voice coil in tweeters.

But compressors cant prevent vented cabs for overexcursion in the lows! Or prevent amplifiers from clipping!

 

[beans-on-toast]

So somewhere in there I'm assuming you have some sort of engineering or troubleshooting experience with tube amplifiers and even you admit we don't understand all the implications as to how the components interact...which before science confirms entirely how something works, we can only refer to the results that our ears show us as magic (meta)!

Absolutely. The further research takes us, the scarier it gets as to how little we really know.

We've been baking bread for thousands of years, we've only been building amps for around a hundred. We have a long way to go. Once we have a bit of theory, building things is far easier than fully understanding how they work.

PS: a little honey in the bagel water goes a long way.

 

[ThisBass]

If the amp can't deliver the current, then the voltage isn't going to be there either.
No one measures the open circuit votage and then calculates what the power would be
if a load were connected. The voltage is measured across the load, with the load present.

If you have 40 V across 8 ohm, there will be 5 amp flowing. If the amp couldn't deliver 5 amp,
then the 40 V would not be there to begin with. The output would have sagged. If the 40 V
is there, then so is the current (the 5 amp).

You can express the output in watts, or voltage squared over resistance. It's the same value.
If you know one, you know the other.

.

Hm, yes and no, I agree and I disagree.

If you are really sure you have exactly 8 Ohm all along the line than you are fine.
But if you don't know the Ohm exactly you will have got a problem.

I.E. lets say the Ohm amount is frequency sensitive but the frequency spectrum itself is a random jumble.

So what Z would you give a preference? Znominal or Zmin or something else different to it?

Rated Power capacity of speakers is derived either by Zmin or Znominal, which one of Z is used depends on the individual recommendation paper.

It is right and wrong to use Zmin but is also right and wrong to use Znominal. Both methods are homogeneous but have got advantage and disadvantage as well. Both methods lead to significant different results for the capacity, that is the biggest disadvantage!


Z of any random cab is not as steady or homogeneous as it is most of the time assumed, and no amp manufacturer can predict which cab will be used, which instrument will be played, which stuff of music will be played, fingerstyle or picking or a lot of thumbs etc.

Measuring an amplifiers output on resistive loads 8 Ohm / 4 Ohm / (2 Ohm) is a very practical method because its a unique load and therefore an identical condition for all manufacturers.

Therefore the inevitable major problem is a very simple personal consideration:

What does the amount of rated power mean to me in particular, what is the meaning to my own personal needs.

 

[nukes_da_bass]

Absolutely. The further research takes us, the scarier it gets as to how little we really know.

We've been baking bread for thousands of years, we've only been building amps for around a hundred. We have a long way to go. Once we have a bit of theory, building things is far easier than fully understanding how they work.

PS: a little honey in the bagel water goes a long way.

THEN you have the urban legend that Brooklyn bagels are best due to "Brooklyn water".
So-called "brooklyn water" comes from the same Catskill area aquifers which feed ALL of downstate NY!!! So why not Newburgh water, or Nyack water, its all on the same feed supply chain.
Like amplifier marketing, bagel marketing does just fine on bullsh!t stats that may or may not have basis in truth.

I rember when 8 ohm cabs were the standard in the 80s then Ampeg started rating amps at what they would put out into 4 ohms (as well as building 4 ohm cabs) with the marketing ploy that "if you buy our 4 ohm cab to run your 4 ohm optimized amp, you will be WAY ahead of the game because X model amp and Y model cab are engineered to get the FULL POWER from X model amp!"