Amp wattage explained please!

[Guest HRB853370]

Why is it that a 30 watt or even lower rated tube amp can (and does) sound louder than a rated 65 watt SS amp? Is there a difference between SS watts and tube watts?

[tbonesullivan]

it's all to do with how much of the rated power you can use. Solid State power sections sound HORRIBLE when they clip/overdrive, and also they can go into square wave distortion which can nuke speakers. So, most solid state amplifiers are made with lots of limiters and such to avoid overdriving the power section at all costs. So, in the end you can't utilize the full 65 watts.

 

With tubes, they go into soft clipping, so you can use more of the full power output without worrying about horrible sounding clipping.

[Steiner]

The wattage of an amp is rated at the point it starts to break up. Since SS amps don't have that saturation point, the wattage is when it's dimed. Most analog amps break up early on the volume dial killing the watage rating.

[mars_hall]

Why is it that a 30 watt or even lower rated tube amp can (and does) sound louder than a rated 65 watt SS amp? Is there a difference between SS watts and tube watts?

 

The difference is a matter of efficiency, power out divided by power in.

 

There is a conversion of the available power from electrical power to mechanical power which eventually moves the air. There are losses all along that path. If the system, which includes the speaker and enclosure, as well as the electronics, is not efficient in the delivery of the input power during the conversion from electrical power to the air pressure waves, the losses will make the lesser power amp sound louder.

[Trouble]

Tube amps rock!

[blackjack]

Power is defined as the area under the curve. Look at a pure sine waveform (imagine an oscilloscope trace). A solid state amp waveform clips the top cleanly off, when clipping begins. A tube amp waveform widens out with no sharp edges, when clipping begins. So speaker and cabinet being equal, the widened waveform has more area under the curve. More Power. This is compression, and it's going to cut through the mix easier.

 

An even more intersting view is via spectral analysis. Any shape waveform, can be represented as a composite sum of sine waves of different frequencies, each at a different amplitude. While your tube amp is doing soft clipping (the wider, rounded waveform), it can be decomposed into the fundamental sine wave and all of the harmonics. For the shape we're considering, the harmonics include a measureable second and sometimes even a measureable fourth harmonic. Those even harmonics are easier to generate with a tube amp. Once again, take the area under each of the parts and you get the total power, but you also can compute the THD (total harmonic distortion). That's the ratio of the harmonics area under the curve to the fundamental. This is easier to see using pictures and formulas.

 

Odd harmonics; the third, fifth, and seventh are easy to get. When a SS or tube amp is heavily clipped, they're sharp edges and nasty ringing present in the waveform.

 

There's info on my website, specifically in terms of guitar amps...

 

http://bitaramps.com/tone

[GuitArtMan]

Power is defined as the area under the curve...

 

http://bitaramps.com/tone

Area under the curve of what? Voltage? I can have a BIG voltage curve with tons of area under it but with no current flowing there is no power being dissipated. Similarly, I can have a lot of current flow but if there's not much voltage behind it, there isn't much power being dissipated. Power is voltage times current; voltage squared over resistance; current squared times resistance. Now if you're talking AC power, which we are since we're talking the output of guitar amps, then the phase angle between the voltage and the current comes into play in ways I haven't looked at in years. I've never heard of power described as the area under the curve.

[GuitArtMan]

We don't hear things in a linear way. A 60 watt amp is not twice as loud as a 30 watt amp. It requires considerably more than doubling the power to double the apparent volume. Ever wonder why linear pots suck for volume pots? This is why. You turn and turn and turn and nothing happens, and then wham the volume comes up between 9.5 - 10.0. This is why audio taper pots are used for volume pots - they smooth this out to the way we hear it.

 

As others have mentioned, when a tube amp starts to clip its a smoother/rounder clipping, more of a compression of the waveform . It's 30 watt power rating is before clipping, but it can put out considerably more than that when pushed. When a solid state amp starts to clip, it's a hard clipping - the signal is simply clipped off. It just can't give out much, if any more power and just start to distort in a way we find unpleasant.

[Blunote]

Area under the curve of what? Voltage? I can have a BIG voltage curve with tons of area under it but with no current flowing there is no power being dissipated. Similarly, I can have a lot of current flow but if there's not much voltage behind it, there isn't much power being dissipated. Power is voltage times current; voltage squared over resistance; current squared times resistance. Now if you're talking AC power, which we are since we're talking the output of guitar amps, then the phase angle between the voltage and the current comes into play in ways I haven't looked at in years. I've never heard of power described as the area under the curve.

I think he was referring to a measure of power...at least that's the way I tried to visualize it. The cumulative power used to produce the various frequencies are the waveform. By the way, the AC power is converted to direct current before any amplification takes place.

[mars_hall]

I think he was referring to a measure of power...at least that's the way I tried to visualize it. The cumulative power used to produce the various frequencies are the waveform. By the way, the AC power is converted to direct current before any amplification takes place.

And AC is just a varying DC relative to some arbitrary reference point which may or may not be between the highest and lowest swings of voltage present. The point is you can speak of amplifiers in terms or voltage or current or power or other standpoints. They are all different ways of looking at the same problem and you look at the problem from the different perspectives depending on what you are trying to explain or solve at the moment. Blackjack was trying to explain from a power standpoint, though he may have generalized and left out quite a bit of detail about the other perspectives in the explanation.

[GuitArtMan]

I think he was referring to a measure of power...at least that's the way I tried to visualize it. The cumulative power used to produce the various frequencies are the waveform. By the way, the AC power is converted to direct current before any amplification takes place.

Yes the AC wall voltage is converted to DC to power your guitar amplifier, but the signal being amplified and signal coming out of your amp IS AC.

[Steiner]

Area under the curve of what? Voltage? I can have a BIG voltage curve with tons of area under it but with no current flowing there is no power being dissipated. Similarly, I can have a lot of current flow but if there's not much voltage behind it, there isn't much power being dissipated. Power is voltage times current; voltage squared over resistance; current squared times resistance. Now if you're talking AC power, which we are since we're talking the output of guitar amps, then the phase angle between the voltage and the current comes into play in ways I haven't looked at in years. I've never heard of power described as the area under the curve.

 

Yes, I felt the same: what curve? What function is integrated to identiify power? In Blackjack's defense, I gave the Bitar link a cursory read. It may say which curve there.

[Kuz]

You guys are ALL missing the boat.......

 

It can be summarized as simple as this........ a tube amp rated at 12 watts or louder IS..... loud enough!!! (period, end of discussion) LOL

[barrymclark]

Agreed, Kuz. My 20w is plenty loud for most things. It really can get as loud as my 80w ss Cube. However, they act, as mentioned, very differently from each other as the volume goes up. Not better or worse. Just different. Just have to determine which amp suits your needs better.

[LK155]

I sometimes have to laugh at guitar amplifier power ratings.

Coming from the audio world, I was used to specifications that you could actually compare from one amp to another.

 

Power ratings for quality audio equipment always used to have 3 elements:

> the number of watts RMS per channel;

> the frequency range over which that power is available; and

> the amount of total harmonic distortion the amplifier generates at full rated power.

For example, 35 watts RMS per channel between 20 and 20,000hz, at 0.1% THD.

With those kinds of specs, it was easy to make comparisons. You could usually count on the fact that more power, wider frequency range, and lower distortion numbers would cost more.

Yeah, there were usually other power ratings, like 'dynamic', 'peak', and 'music power', but everyone knew those were pretty well meaningless.

 

But guitar amps? Good luck making sense of it. Their power ratings seem to be whatever the marketing department thought they could get away with. And you NEVER see frequency response or THD numbers, likely because they're not particularly good.

Yes, I realize that guitar amps don't need to be as good as audiophile amps in terms of frequency response and THD, but I still find the guitar amp manufacturers' disclosure of specifications woefully inadequate.

 

Fender thinks a Deluxe Reverb with two 6V6 tubes is a 22 watt amp.

If my memory serves me correctly, a 6V6 tube is good for 5 watts RMS. A 6V6GT, 6.5 watts RMS. EL84's....about 7.5 watts RMS.

So to me at least, these artificially inflated power numbers don't mean much.

 

I once was looking at an audio amplifier in a stereo shop, and asked what the rated power was.

The young staff member who was 'helping' me admitted he didn't know, then proceeded to look at the back panel of the amp, found the notation that the unit would draw 150 watts from the AC line it was plugged into, and proudly informed me that it was a 150 watt amp. Jeez.

[Gitfiddler]

Great topic and excellent points!!

 

If tube amp power ratings are all over the board, and possibly exaggerated, are solid state amp power ratings more acurate?

 

For example, the Roland JC120 is rated at 60 watts rms for each channel and 120 in stereo. 60 watts in that amp sounds like 60 watts should sound to my ears...LOUD, CLEAN and toneful. Most of Roland's solid state amps just seem more honestly rated to me.

 

http://www.rolandus.com/products/productdetails.php?ProductId=249

[GuitArtMan]

And AC is just a varying DC relative to some arbitrary reference point which may or may not be between the highest and lowest swings of voltage present.

No, that's simply modulated DC. In AC the current direction changes (alternates). Think about what's applied to your speaker: with 0 volts applied to your speaker it's at rest and sitting in the middle of it's range. As a positive voltage is applied it moves in one direction (say forward). If a negative voltage is applied it moves in the opposite direction (say backward). You can demonstrate this with a nine volt battery. Place the battery across the terminals of your speaker and notice that it will move in one direction. Reveres the battery terminals (and therefore the current flow) and the speaker will move in the opposite direction. This is what AC does.

[mars_hall]

No, that's simply modulated DC. In AC the current direction changes (alternates). Think about what's applied to your speaker: with 0 volts applied to your speaker it's at rest and sitting in the middle of it's range. As a positive voltage is applied it moves in one direction (say forward). If a negative voltage is applied it moves in the opposite direction (say backward). You can demonstrate this with a nine volt battery. Place the battery across the terminals of your speaker and notice that it will move in one direction. Reveres the battery terminals (and therefore the current flow) and the speaker will move in the opposite direction. This is what AC does.

No, wrong and direction is relative to the point of reference. When one declares the speaker node as the point of contact, a reference point has been set. To the same node apply +6 volt then +2 volt. Which way did the speaker move? Certainly not in the same direction.

 

None of this is directly related to the OP, which should be the focus.

[Steiner]

No, that's simply modulated DC. In AC the current direction changes (alternates). Think about what's applied to your speaker: with 0 volts applied to your speaker it's at rest and sitting in the middle of it's range. As a positive voltage is applied it moves in one direction (say forward). If a negative voltage is applied it moves in the opposite direction (say backward). You can demonstrate this with a nine volt battery. Place the battery across the terminals of your speaker and notice that it will move in one direction. Reveres the battery terminals (and therefore the current flow) and the speaker will move in the opposite direction. This is what AC does.

 

That's the trick I use to keep a multitude of speakers in phase.

[GuitArtMan]

No, wrong and direction is relative to the point of reference. When one declares the speaker node as the point of contact, a reference point has been set. To the same node apply +6 volt then +2 volt. Which way did the speaker move? Certainly not in the same direction.

 

None of this is directly related to the OP, which should be the focus.

In your case above the electrical current is flowing in the same direction. There's just less of it, so the speaker cone is deflected less, but the electromagnetic field is still going to deflect the speaker in the same direction, just not as far. Heck you could apply +6 volts, and then remove the the voltage and the speaker will move back to it's quiescent state. In fact, you could produce a sound doing this, but this is NOT how your guitar amp works. Try applying +6 volts to the speaker and see how far it moves. Now apply -6 volts to it and it will move the same amount in the opposite direction. This is how your guitar amp works. The signal applied to the speaker absolutely does change direction. The quiescent state is with no signal applied - the speaker is at rest at it's zero point. Apply a positive voltage and it will move in one direction, apply a negative voltage and it will move in the other direction.

[mars_hall]

In your case above the electrical current is flowing in the same direction. There's just less of it, so the speaker cone is deflected less, but the electromagnetic field is still going to deflect the speaker in the same direction, just not as far. Heck you could apply +6 volts, and then remove the the voltage and the speaker will move back to it's quiescent state. In fact, you could produce a sound doing this, but this is NOT how your guitar amp works. Try applying +6 volts to the speaker and see how far it moves. Now apply -6 volts to it and it will move the same amount in the opposite direction. This is how your guitar amp works. The signal applied to the speaker absolutely does change direction. The quiescent state is with no signal applied - the speaker is at rest at it's zero point. Apply a positive voltage and it will move in one direction, apply a negative voltage and it will move in the other direction.

This whole varying DC idea is not my own, but one rendered in college (Purdue) to help us get passed a hurdle in the real world observation. Measurement depends on perspective. Most amps actually work with a DC offset and if a scope is used to view the individual stage voltages, the varying DC will be seen. So it's pretty much semantics.

 

That said, the point previously made about a reference has been missed or ignored. I'm not saying that applying a voltage of the opposite polarity won't move the speaker in the other direction, but rather making the point that the idea of positive and negative and direction for that matter is relative to an arbitrary reference point just as up and down is relative to who is making the observation. It's a yin-yang zen moment.

 

When looking close to where one stands, up is easy to call but it's only relative to the one making the observation because the observer usually chooses to call the earth's surface down. Now go into space, say to the moon, and looking back at the same point on Earth the judgement of what is up and down changes. Such is the idea of the center of a voltage swing and its direction being arbitrary. It's a matter of perspective and relativity.

 

Thinking about individual tube amplifier stages might help to understand the concept. Each stage is an isolated DC stage with a common plate voltage of say 300-400 Vdc. Excite the front-end of the stage and the DC at the output of the stage varies. Does it go to zero? No. Now consider that the plate supply voltage may be momentarily limited by the current available at the rectifier. The DC plate voltage sags and the individual stages go closer to saturation, which may or may not cause overdrive clipping and oscillation. Is the sag AC or DC? What if the sag is periodic? Is the frequency of the change in DC plate voltage AC?

 

Another example:

When a car comes speeding up on a vehicle preparing to pass it, would the driver of the passing vehicle say the vehicle about to be passed is coming closer to him? This is a relative direction. Now the guy being passed decides he is not going to be passed and accelerates. Does he not think I'm going away from the guy behind? Again a change of relative direction, yet both continue in the same direction. Now the guy in front says "Enough!", and locks them up. Right before they collide, do they not both think, he's coming right at me? The direction is relative and arbitrary.

 

Now consider how phone lines work. The AC component is actually derived from the varying -48 Vdc supply present. Does the DC voltage ever go to zero?

 

Working with real system applications in hardware, particularly applications where there are multiple modules separated by long wire harnesses with inherent resistances, such as in a vehicle, one finds that there really is no true zero volts point across the entire system but a series of relative zero volts at the connectors of each of the modules and this is continually changing relative to the dynamic currents flowing and voltages dropping in each of the other harness branches of the system. The system floats. So there are offsets and periodic noise that is radiated in this purely DC system. The relative frequency of the noise disturbance determines whether it is perceived as a transient or an oscillation. In either case, the variance in the DC voltage behaves as an AC signal.

 

I agree that an amp speaker receives an AC voltage.

 

We can agree to disagree.

[GuitArtMan]

Fun conversation mars_hall, not trying to argue just enjoy a good debate from time to time. Of course in electronics (as in just about everything) you need a reference point. That point is commonly ground. And yes I know that ground floats and that one ground may have a different potential than another ground. I remember one of the first labs I worked in back in the '80s. There were two circuits in the lab. My understanding was that the difference in ground potential between the circuits was nearly two volts! They had to separate the equipment so that systems that needed to be connected together were on the same circuit. Even so, I remember the huge grounding straps that went between racks. These were braided copper, maybe 3 or 4 inches wide and about 1/4 inch thick.

 

Yes I know about DC offsets and that the musical signal in an amp is riding on a DC offset. I know about the coupling caps which are used to remove the DC offset and allow the - gulp - AC component of the signal through. Does not the output transformer do the same thing? If we look at a speaker at rest and apply what we will call ground level to it, it wont move correct? Now if we apply a positive voltage to it the speaker cone will move in one direction, correct? Now if we apply a negative voltage to it the speaker will move in the opposite direction, correct? What I am saying is this is the way the signal applied to your guitar amps speaker works. I choice to call this AC, if you want to call it DC and keep moving the reference point I guess we do have to agree to disagree.

Cheers,

Bob

PS Good debate though. As with many such things it's all relative and a matter of perspective.

PSS Are you a EE or physicist or ???

[mars_hall]

EE

[Blunote]

Interesting explanations. I've always found electrical concepts confusing.

 

Even so, my understanding of AC is that it is used for electirical distribution because it is a vastly more effecient way to transmit over long distances. Furthermore, AC at the wall plug is varies between positive and negative about 60 times/sec. I can't any imagine a speaker producing anything sounding like music if it's getting alternating current 60 times per second.

 

My belief (at least until now) was that the amp modulated the current to the speaker relative to the input from the pre-amp stage, whether that modulation alternated +(positive) - (negative) + (positive) in and strictly repeatable manner or a less structered pattern; ie +++ - - ++ +++ .

 

Is that not correct?

[blackjack]

So the original question was hey, if you make all things equal including the guitar, cab, volume pots, speaker, listener, as much as possible, why do tube amps sound louder? The answer can only be that a compressed waveform is a different timbre and stands out in the crowd. So to answer GuitarArtMan, let's also make the power factor match as well.

 

So, back to our story, why not just get a SS amp and use a digital compressor? Go for it. Tubes have kept their relevance because all of the great SS modeling needs to generate the faux harmonic balance of tubes using lots of extra processing, which means long, long chains of silicon gates, and the side effects associated with that particular method. The tube does it in one big analog mess of a glass bulb with archaic metal gratings and plates.

 

GuitarArtMan makes the point that measuring the power at any instant of time, is a function of voltage and current, which by definition includes the nature of the load. Thanks for keeping it honest. That relationship is termed "Power Factor" and indicates the efficiency of delivering power to the load. But the point is still this; once you've measured the the dueling tube and SS amps in question and published that they are both the same wattage (based on the fundamental frequency at which you are testing), then you have to turn them up, at which point the tube amp distorts differently than the SS. If that tube amp in front of you sounds louder than the SS one you've chosen, the reason is that a compressed waveform is giving you additional power via the harmonics. The power is measured via the fundamental (or clean) response, and the THD rating (the distorted response) indicates the power in the harmonics that accompany the fundamental.

 

Unless you're really into the device physics, why does a guitarist need to be bombarded with questions of (and these are all related to your question of the measure of average and instantaneous power), electron mobility, shot noise, johnson noise, strength of the electric field, etc?

 

A sub-plot in this thread was about watts vs loudness but that's a different question, and a thread that last occurred a few months back.

 

My original post didn't include enough background, regarding the "area under the curve" statement, and thuswas not sufficiently qualified. The explanation above implies the answer, but I'll take a shot; measure the voltage amplitude of the two amps, and -here's the missing info- even if the voltage to current phase angle is the same for both amps at that frequency and instant of time, the whole difference then is found in the waveform (the harmonic balance). The waveform is why a clarinet sounds different than a guitar when emitting the same frequency, why a compressed waveform sounds different than uncompressed, why Aguillera sounds different than Lambert.