Understanding amplifier wattage

ethrbunny

ethrbunny

Inspired
Can someone help me to understand what's *truly* being discussed when Im looking at wattage for a power amp?

I have the reactor 50/50 - its plenty loud at low settings. Now Im considering the QSC 12 - which advertises 1000 watts. I don't really want 200x but I assume that's not what Im getting.

Throw me a clue here.
 
i am not an expert, but i believe what you're referring to is the fact that tube watts seem louder than solid state watts. below is a link to an interesting paper on the tube vs transistor comparison, and below that an excerpt which explains why tubes seem louder.

Butler Audio: Tubes vs. Transistors

Vacuum-tube amplifiers differ from transistor and operational amplifiers because they can be operated in the overload region without adding objectionable distortion. The combination of the slow rising edge and the open harmonic structure of the overload characteristics form an almost ideal sound-recording compressor. Within the 15-20-dB "safe" overload range, the electrical output of the tube amplifier increases by only 2-4 dB, acting like a limiter. However, since the edge is increasing within this range, the subjective loudness remains uncompressed to the ear. This effect causes tube-amplified signals to have a high apparent level which is not indicated on a volume indicator (VU meter). Tubes sound louder and have a better signal-to-noise ratio because of this extra subjective head room that transistor amplifiers do not have. Tubes get punch from their naturally brassy overload characteristics. Since the loud signals can be recorded at higher levels, the softer signals are also louder, so they are not lost in tape hiss and they effectively give the tube sound greater clarity. The feeling of more bass response is directly related to the strong second and third harmonic components which reinforce the "natural"' bass with "synthetic" bass [5]. In the context of a limited dynamic range system like the phonograph, recordings made with vacuum tube preamplifiers will have more apparent level and a greater signal to system noise ratio than recordings made with transistors or operational amplifiers.
 
+1 on the tube watts. Tube watts are louder, I'd say 50 tubewatts are like 75 solidstatewatts.

Now, that can not explain why a 200 w FRFR monitor with a 15" speaker is not enough to go live with the axe-fx.

It depends on the speakers how much watts they need to get really loud.
Guitarspeakers are usually very efficient and do not need much power to get extremly loud.
They have their own frequency curve and are creating a sound, not reproducing a given sound.

Speakers for FRFR use must be constructed to reproduce a given sound, wich is more complicated. Efficient cabs exist, but these are high pricing. The cheaper way is to have a less efficient cab and a bigger poweramp and that's how most activ monitors are like.

Another FRFR problem is that lower frequencies need x-times the power than highers. That becomes important when you want to use a 15" speaker. When you feed the low frequencies they suck away much power, even when there is no usual signal and only deep noises. Not to forget: poweramps can produce low frequencies that the speaker can not handle, power is wasted although you can not hear it. It's always a good idea to cut the lowest range off.

A 12" monitor should have more than 250 watts for me and a 15" more then 350 watts to survive a whole gig.
Before I got my axe-fx I usually played Vintage 30s and a 2x50W tubeamp and that has been plenty.
But frfrs are different animals.
 
The simple answer is to try it out and see if it's loud enough for what you want.

QSC and JBL and possibly a few others now use the same power amp for lows and highs simply because it has cost/manufacturing benefits to use the same amp module. It doesn't mean they're pumping 500W through the high-end. One review I read estimated only about 50W in the highs. Whatever it is, I'd use the low-end power as a guide, so treat the QSC as a 500W box. Our sound guys use the K10s for vocal foldback and I think they're excellent for that purpose (light and small as well).

There are other considerations with powered monitors. They use limiting to make them virtually indestructable (that's good), which means you can get some compression on signal peaks and they suffer rather unnatural dynamics issues if you try to push them too hard. Some use autmatic bass reduction as they near their maximum output. Of course, all equipment behaves differently when trying to get more than it's designed for; the main point is that tube gear sounds good going into overdrive.

Clean and semi-dirty guitar signals have a strong pick attack, so percieved volume is affected by the short term capability to handle signal peaks as well as the long term ability to maintain sustained volume. Tube amps do this well, and *some* SS amps can also do this well. Spealker efficiency also affects volume. A 3dB less efficient speaker is the same as have an amplifier with half the power. Some FRFR speakers reduce speaker efficiency to achieve a flatter response.

I recall Jay estimating that SS needs to be about 5 times tube wattage for a fair fight. Of course a five-fold power increase is not 5 times as loud. Generally, you'd need 10 times the power for only double the volume.

Hope this helps :)
 
One watt is always one watt. In the case of an amplifier the wattage ratings are reported by how much clean power they give before starting to distort. For solid state it practically is the full power but for tubes it is not. Tube amps can be cranked way past its wattage rating for that nice power tube saturation.
 
Yeh. It really irritates me when peple say tube watts are louder than SS watts. Its complete rubbish. A watt is a Watt. A given speker has NO way of knowing what is generating the watts, and will respond exactly the same.

As per the article, a valve amp is actually capable of up to 15-20db more clean SOUNDING (though it is not actually clean) APPARENT (as in its not actually louder just appears it to the human ear - cos there funny things ears). As 3db = double power, and 10db = 10 times power, even a conservatice 10db apparent increase in volume (which in volume terms is about double) applied to a 50W valve amp, would need 500W to actual power to produce.

As the actual power is only 2-4db more, your speakers are safe. a 3bd actual increase id double - so the amp might actually be pushing 100W, which isnt a speaker killer (the 500W a SS would need to produce for the same apparent loudness might be).

Anyway - the upshot is that to get the same volume from a SS amp that you get from a valve power amp, you need between 5 and 10 times as much power. Thats going to hurt speakers (and why my 250W per side Matrix isnt QUITE as loud as my Old VHT 2:50:2).

That said, once you enter this "compressed overload" region of a valve amp - your actually not hearing CLEAN power. There is some distortion - but its so low the ear doesnt HEAR it as distortion. It appears clean (measure it with a scope though and you'll see its not). Its also why a valve amp generates harmonnics even at apparently clean settings (its not clean, and the distortion provides the harmionics).

So whats the upshot? Well if you want LOUD without hurting speaker - then valve amps are probably the best way of getting it. That said, a 50W valve amp CLEAN will still hurt ears, and irritate soundmen so why would you actually NEED to be that loud. Also, If you want those harmionics your beter off with a valve amp. Now the AFX is supposed to generate ALL the aspects of a real amp - including the harmonics. So - by using a valve amp - while it may SOUNd better the the ear, you actually generating low level distortion and harmonics the real amp would't produce. Its actually moer of what we like but actually not natural.

I had this fight with myself when I got the Matrix. I struggled to get controlled harmonics with the Matrix - but with the VHT they were just there. Now the thing was - when I went into a real amp I had the same struggle as I did with the Matrix. I had to have some volume, and btand in exactly the right position with both the AFX/Matrix and the real amp. With the VHT I didnt need to be anywhere spacific - they were just there. Whether this is good or bad is personal. Using the VHT made it artificially easy to get those harmonics - but could that lead to having them when you dont want them?

Anyway - Im wafflin ga little here.

Upshot -

1. for more apparent volume use a valve power amp
2. for an accurate representation of whats goin in - use a SS amp

persoannly - with the AFX, find the volume you need and if a SS amp produces it - then go that way. If you cant - use a valve Power Amp. A SS PA is actually more like a real amp even though in an A/B with a valve PA its not as pleasing to the ear. A Valve Power Amp with the AXF is artificial ear candy. Its like too much sugar in coffee - its sweat but not as it actually should be.
 
By all means be irritated. Of course a watt is a watt. Well explained in the following text, Paul.

I think the difference comes from the ability to handle dynamic peaks, where solid state clipping needs to be avoided at all costs, while tube clipping is desirable (it's why we're all here talking about modelling). SS clipping can be avoided with limiting which is common in powered monitors, or by having a clean SS amp many times more powerful than needed for sustained passages.
 
Agreed. By irritated I mean that it shows a complete mis-understanding fo what a "watt" IS, and how valve v SS amps work in the "overload" zone.

Its fine to ask questions and be educated (and there are lost much more knowledgeable than me) to to just state "valve watts are louder than SS watts" Grrrrrr

For volume, its perfectly reasonable for a SS amp to match a valve one - its just needs more power so it stays in its opporating zone. The fact si though - that to get the same "apparent" volume from Ss amps requires more actual volume. a 50W valve amp may push out 100W to get its apparent volume - but the SS amp needs to push 250-300 to do the same. If your using a 120W cab, then the SS amp will dio harm. You wont get the same apparent volume safely.

the dynamics/harmonics is another issue. a valve power section adds them because it opporates in its non linear zone - a lot. Even at relatively low levels pick attack may psh the peaks into theis zone. For most guitar applications thats desirable - whether it is with an AFX, or other high quality modellers is questionable. As I said - it adds MORE of what we like that actually isnt there in the original. In an A/B it sounds nicer - but that doesnt mean its nearer the original - in fact its further away. Whether you actually want it is personal. i think its a "growing up" thing. I dont mean that negatively, I just mean a valve power sectio sounds better until you realise whats going on. I did it myself for 15 months so ???? Its only when I started playing real amps for a while I realised what was good about the valve power section wasnt actally there in the originals. It made my lplaying life easier - and because of that I was getting lazy. I switched back to SS amps - which had less "nice harmonics" that the valve one, but was closer to the all valve amp. My playing has improved as a result.
 
Yes agreed. btw - lovin' my Matrix. I was surprised how great it sounds clean with my backup "rig" (Tech 21 Liverpool pedal). Will love it even more when it finds an Axe-Fx II to mate with ;-)
 
to the OP... clear as mud? :lol

if so, welcome to the debate that has gone on longer than guitar amp modelers have been around.

the first thing is to make sure your amp and speakers are a good match so there will be no equipment damage. in the case of a self-powered PA speaker like the QSC K12, this has already been done for you so you don't need to worry about it. and as you've probably figured out by now, the K12 is not 20 times louder than your reactor 50/50.

the second thing is to get your hands (ears) on both solutions and listen to them in the places you will use them at the volumes you will use them, then let your ears tell you which is best. unfortunately, this is easier said than done for many reasons, not the least of which is the amount of time required to really put both options through the paces. but, if you have the money and the time (and the patience!) to try both together, side-by-side, for a good period of time, that will tell you more than any forum post you will ever read.

both methods of amplification have some good things going for them.
 
ethrbunny said:
Can someone help me to understand what's *truly* being discussed when Im looking at wattage for a power amp?

I have the reactor 50/50 - its plenty loud at low settings. Now Im considering the QSC 12 - which advertises 1000 watts. I don't really want 200x but I assume that's not what Im getting.
OK..I'll take a crack at it.
Watts..1st are "consumption wattage"..how many watts the device requires to run at 120 volts AC/ 60 Hz
Output power watts is used in marketing gear to consumers for most stuff.
Look at the specs.
SPL= Sound Pressure Level,(usually 1 watt @ 1 meter distance is standard).
THD= Total Harmonic Distortion (measured in percentage)
FREQUENCY RESPONSE= For an Full Range, Flat Response (FRFR) system, you want an amp/cab that will reproduce full response.
Sure wattage has meaning. But how is the manufacturer measuring? RMS or P2P? (Root Mean Square) is the most accurate, the other is Peak to Peak method. This method includes noise, high distortion components (THD) and piss poor frequency response. A cheapo advertised as 1000 watts..might actually run about 100 watts clean. U get what cha pay for bro.
SS or valve..your choice. What counts is good clean power, fast recovery of the power supply when high demand requires more power. And of course, high compliance drivers with well designed passive crossover networks.
Don't go too small on the power amp (or powered speakers).
I set the Axe output to full, adjust the power amp for desired level. I don't crank the power amp to full, if I do, then the power amp is too small to begin with. This will preserve the drivers and crossover from damage. High freq. drivers don't do well with clipping at high gain settings on the power amp. Power amps with fast limiters are desired IMHO.
Click to expand...
 
Id go with most of that.

the only small point, is that it doesnt matter is you crank the Axe and not the amp - or crank the amp and not the Axe.

The volume/gain knob on the amp normally attenuates the input signal - the amp itself is always working flat out. As the Axe volume attenuates the output signal which is the last thing before it hits the amp, your actually doing exactly the same thing either way.

Personally, I crank the amp AND crank the Axe - BUT reduce the gain level in the Global out to the max volume I need (and a safe level). That way I (or anyone else) cant turn one up - either accidentally, or be tempted to on purpose - and overload teh speakers.

Its much easier to do this with SS amps than valve amps because they dont need a load attached when working. it means you can use the clip light (or the signal light on the Matrix) and raise that gain level until you get the LED (without a speaker attached - but with yiour loudest patch and any vol boosts engaged). At that point you know what the amp is producing (if the peak light starts to flick you JUST over max power. On teh Matrix the LED flicks at -9db from Max). you then back off the gain until the level is safe for your speakrs (in my case back off by 6db as my speakers are 60W 8 Ohm and teh Matrix deliverws 250W into 8 Ohm Max). THEN connect the speakers. At this point, the system is at its max safe volume. I can then control actual volume by either the AFX output OR the Amps gain safely.
 
BTW, I am also using Greg K's PUPs. I screwed up when ordering them though. My center and neck pick up magnet poles don't line up with my strings. My neck is slimmer than the standard neck. Works OK, but I probably loose a little because of it.
 
Now, I don't believe that all manufacturers meassure wattage at the same THD at the same freqeuncy points and with the same input signal.
Who does?
There are some specifications about soliod states like watts RMS where the parameters are given, tubeamps have their wattage not in rms however and no, they are meassured at a higher point of distortion where you would ever listen to a solid state.

Wattage is voltage x amperes. Right? So does it matter in a musical instrument how fast a signal rises to its wattage? What if an amplifier is slower than another? It matters! Yes! That's important for the pressure that comes out of the cab!

It is a fact that a tube rises it's power faster than a transistor, much faster! And the tube is at the same speed for the whole frequency range. Solid states have the problem that the amplification is falling by frequency, so higher frequencies need to be feeded in louder wich means that they need even more amplification stages wich means that they are even slower at the higher frequecies.

When you say wattage is wattage, you are right in theory. But there are fast and slow watts and the watts that someone prints on his gear are not watts that another one uses for his gear.

I've seen funny things btw, gear that takes less power from supply than what it promises to push out, tubeamps with higher output wattage than what the tubes can produce...

I wonder if the deeper details will help any newbie...maybe it is more important just to get out that we can not trust any specs on any gear. Talk with other musicians, that's a good way.
 
paulmapp8306 said:
... you can use the clip light (or the signal light on the Matrix) and raise that gain level until you get the LED (without a speaker attached - but with yiour loudest patch and any vol boosts engaged). At that point you know what the amp is producing (if the peak light starts to flick you JUST over max power. On teh Matrix the LED flicks at -9db from Max). you then back off the gain until the level is safe for your speakrs (in my case back off by 6db as my speakers are 60W 8 Ohm and teh Matrix deliverws 250W into 8 Ohm Max). THEN connect the speakers. At this point, the system is at its max safe volume.
Click to expand...

Just checking your calcs here, Paul.

Matrix delivers 250W max into 8 ohms
LED flickers at -9dB, that's a gain of 0.125, so 250W x 0.125 = about 31 watts
You then back off by -6dB, that's a gain of 0.25, so 31W x 0.25 = about 8 watts

Seems to me you could add 3dB instead of reducing 6dB on top of the -9dB LEDs for your 60W speakers.

Of course, Jay and others have given general guidelines to the effect that typical guitar speakers under typical conditions can handle about double their rated power.
 
What I mean, is that on the Matrix I reduce the gain UNTIL the LED Flickers - as thats around -9db so yes around 30 Watts. On another amp (like the SLA I used first withteh AFX) I used teh clip light - then backed off the relevant db.

I actually do this WITHOUT my vol boost engaged with teh Matrix. Only because its convenient. Because the LED flicks at 30W - and my vol boost is 3DB, that means if I have it set by the LEd flicker without the boost - when I add the boost Im at around 60W - which is my cab rating :)
 
As far as rise time of valves goes - yes they are fact. In fact in a valve amp its generally the transformer that limits the "speed" of the amplification. Its why you get sag at higher volumes - the transformer cant produce the volts the valves are demanding quick enough.

however, its wrong to say SS amps cant be just as quick. The limiting factor again is the voltage supply. On a lot of amps - and almost all lower priced ones - the voltage rails are "just " good enough to give the specs. This does indeed lead to a slower response time. The Matrix on the other had has been deliberately designed with a HUGH differnce accross its power rails. This enables the amp to be very Fast - so much so that the original XT was much faster than the valve amps, resulting in a far too clinical/sharp front end to notes. the response had to be slowed down to get the amp to respond in a similar way to valve power amps.

I totally agree about manufactureres specs too. some are WAY off. But your also wrong about valve amps being measured at a different THD. Some are - and some arnt I guess - Its why a Matamp at 28W is louder than a Marshal (a modern one anyway) at 50W - and why old Marshalls are LOUD in comparrison to newer ones. Either way you have to be a bit carefull with specs. IF they list a certain power, at a certain freq (or better freq range) and list the THD THEN you can be fairly sure. If any one of those figures is missing - then its a safe bet that there massaging the figures.

Again - the Matrix is measured accross the freq spectrum from 10 hz to 20hz, with a max THD of 0.01% and the power is the minimum RMS at any point in the spectrum. Some other amps are measured at 0.1% THD. Some are measured at 1khz only - some take an average accross the freq range rather than the minimum. Some quote peak power - or PTP power which looks much more impressive. I cant say whether valve power amps are RMS or Peak. Certainly the VHT 2:50:2 is quoted in RMS.

As for the input signal - it really doesnt matter. It should state in the specs what the input signal should be to reach the max O/P (so either in volts like 0.775v or in db like -10, 0 or +4) however that just says if you put this signal ibn youll get this power out. The amp is still capable of generating that max power though even if you feed it a different signal. Its just that if you feed it a smaller one the power will be less, and if you feed it a bigger signal the amp will produce its max rated power before the front knob is fully up (which will lead to clipping). It only really becomes an issue if you input device isnt capable of giving the signal level the amp needs to reach its max O/P. Its also why you cant just switch two amps and say "X is louder than Y" It may be that X IS louder than Y because it needs less of a signal to reach its max power - BUT, if you turn the single power UP, amp X might clip (as it was at its max) but amp Y may get louder - a lot louder, because its more capable but needs a larger signal to give its max O/P. The only way to really measure which amp is "loudest" is to turn up the I/P signal until both amps reach clip as there then at their max O/P (well just over but you get the point).



I only mention the Matrix as Ive had a lot of input so am privy to the info. I cant say which other amps are or are not spec'd correctly.
 
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