rms ratings and volumes

[welly_59]

can anyone tell me the relationship between rms ratings and volume? specifically between 28rms and 30rms. i understand the decibel scale - 10db extra is double the volume, but dont get this rms stuff!

[ESBlonde]

In terms of overall volume the difference between 28 and 30 wrms is insignificant. There is more perceived loudness from some amps because they have spike frequencies in the upper mids (which our ears are more sensitive). Also the sensitivity of the loudspeaker makes a whole lot of difference.
Also the rateings of different makers use different ways to express the RMS at a certain points of total harmonic distortion (THD) so one may measure at 0.1% another at 0.05% and another at 0.01% and another at 1.0%. So there is no fixed accepted measurement of RMS.
And finally valve v transistor amps raises its head, Valves are High Voltage low current whilst SS are High Current low voltage output, this and other things results in valve amps sounding louder that an equivilent powered SS amp (distortion notwithstanding).

Don't worry about 28 v 30 watts then. What does it sound like and does the internal gain structure allow enough clean headroom?

[ecc83]

can anyone tell me the relationship between rms ratings and volume? specifically between 28rms and 30rms. i understand the decibel scale - 10db extra is double the volume, but dont get this rms stuff!

Well Welly, one reason not to "get this rms stuff" is because it is mathematical bollox! Power delivery should be specified as "continuous sinewave power" (into X load R) but the adpuff men are lazy Bs! But I fear that is a battle long lost.

You can calculate a ballpark figure for Sound Pressure Level (at one mtr) by: SPL= 10log W +S. Where W is the power in watts cont' and S is the speaker sensitivity as dB/W/mtr. For 28W and a 100dB S speaker you get 114.47
dBSPL and 30W gives you 114.77 dBSPL. Never gonna tell the diff!

But as ESB has pointed out, published power outputs mean very little in terms of the actual subjective "noise" a guitar amp/speaker system makes. If you want to stay very clean and especially if you want a fair bit of low frequency "whoompf" you need a lot of power. 30W and 100dB speakers being about the minimum even in a smallish venue, 50W would be a safer bet. Then again a decent 15watter driven hard and dirty can make a very serious noise.

You cannot even compare different amps of the same quoted power level and the same speakers! Amps are "voiced" and a slight difference at 2.3 or 3.2 kHz (say) can make one amp shout down another.

Sorry chum, all you can really do is go out and harken unto a few!

Dave.

[ICBM]

'Volume' is only loosely related to power. The difference between 28W and 30W in a guitar-amp context is completely meaningless. It's perfectly possible that the 28W amp could not only be perceptively a lot louder than the 30W one, it cold even put out more measured power - because the rating depends massively on the conditions it's measured under. (Unlike hi-fi amps where there is more of a defined standard - but it can still vary a bit.)

A good example is to compare the Vox AC30 - rated at 33W by Vox - and the Marshall JTM45 'Bluesbreaker' - rated at 30W by Marshall. (I don't actually know whether these power ratings are claimed using the same definition.) Into their own standard speakers - Alnico Blues for the Vox, and Greenbacks for the Marshall - the Vox is louder. Into the *same* speakers, the Marshall is louder. But in a mix, it can actually swing back the other way because the Vox is more 'bouncy' sounding (due to the way the power amp overdrives) and has more of an upper-midrange focus, two things which make a sound very 'present' compared to the background, whereas the Marshall has a tighter, broader sound which tends to get a bit more buried by other sounds. But even then, it probably depends on how you set them, and the context... more bass and less midrange gives a 'louder' sound in isolation but a 'quieter' one in a mix.

[p90fool]

All the maths aside, it's a bit like asking how fast a 2-litre car is - there are a whole load of variables other than engine size which will affect the final figure, but with a little understanding of these variables and a realistic idea of what you need the car to do you won't get any huge surprises either way.

Similarly, as an example, I know that a 15w, EL84-powered amp won't give me the clean headroom I need to play song-X, with drummer-Y, no matter what brand it is or what the speaker sensitivity is, but it may well work for other songs/drummers.

Vague I know, but there it is, the audio spectrum is a funny old thing full of subjectivity and (educated) guesswork. Best bet is just to buy a Marshall Major and a fuzzbox, then you will ALWAYS WIN.

[welly_59]

rms rating is meant to be clean though isnt it?

[Danny1969]

rms just means the measurement is from the root mean square of the constantly changing wave form ....... not the peak to peak value

But yes amp volumes and wattage are deceiving, I gig a 5 watt amp and have enough clean headroom, some people like 30 watts or more

[jpfamps]

rms rating is meant to be clean though isnt it?

The "rms rating" is a bit of a misnomer, as there is no such entity as an rms Watt.

The rms value of a waveform is equal to the DC voltage that will be required to dissipate the same amount of power in a load, ie if you want to calculate the power dissipated by a waveform in a load you need to know the Vrms.

There is no standardize method of measuring the power of an amplifier.

The most common method is to measure the maximum continuous power output that can be achieved using a defined at a minimum distortion value into a resistive load; for example an amp may be rated a 450W continuous power output with 0.5% THD in to 4 ohm load using 20Hz - 20kHz pink noise. This would be a well specified PA amplifier.

However, to conduct the above measurement requires quite a lot of gear including a spectrum analyser, which until recently was a very expensive piece of equipment.

A more rough and ready method of measuring power output (which is what I use in the workshop), is to use a sine wave into a resistive load and monitor the output waveform on an oscilloscope. You can judge the point on clipping on the 'scope as you tune up the gain, and calculate maximum "clean" power from this.

Although this method is not especially accurate, it is "good enough for rock and roll" as a few Ws here and there are inconsequential, and is a useful diagnostic test.

Undistorted output is an important parameter for a PA or hi-fi amp, however amps can deliver far more power than their continuous undistorted rating when driven into clipping as guitar amps frequently are, thus the "power rating" may not actually be an accurate measure of the power output in real use.

[ecc83]

When reasonably reliable, reasonably high audio quality (tho' that last was hotly debated!) solid state amps started to appear in the hi fi world the industry and the punters became obsessed with power output levels and the meaningful measurement of same.

There was also the relatively new Trades Desciptions Act that made it illegal to claim a performance from a product that it could not be shown to deliver and this meant the less scrupulous amp makers devised very clever ploys to pimp up the advertised power of their products. 10watts measured as a continous sine across 8R* became 20W peak per channel and then maybe 1/2 as much again if "Music Power" was given, the two channels then combined to give a published figure of 60watts "total peak music power" and that from a an amp that could probably be beaten to death by a single EL84, certainly an EL34!

We knew where us was with valves you see. If you were strapped(me!) you had a Rogers stereo amp, 2x 2 EL84s and that was quite loud in a modest lounge. The weller heeled went for 15W Quads or Leak TL12's using KT66 and EL34rspctvly. Top for home grown was probably the Radford 25+25 2x 2 EL34 and I would bet one of those would still give a modern 100Wper ch solid stater a run for its SPL money? (they also LOOKED bloody gorgeous!).

By the time the "good guys" like Quad (and boy, DID they take their sweet time!) came out with solid state designs the hi fi speaker world had moved on from 2cuft+ monsters with 12" bass drivers (still got my Wharfedale RS-DD-12 chassis) to compacts with 8" and smaller in 1/2 cuft cabs. These were far less sensitive than the biggies and so amps had to have that much more muscle. Quads' first transistor amp, the 303 could put out a genuine 45W per channel both channels driven and at vastly lower distortion than any valve amplifier (or indeed ANY amp about at the time almost!).

*Speaker people rushed into changing the standard hi fi and industrial speaker Z to 8 Ohms. Had they taken a bit more time they would have found 6R was a better overall "match" for transistor amps and 6R is found now for quite a few hi fi speakers. That said, tranny amps of yore blew up on 8Ohms pretty regularly, six would have probably depleted the world's supply of 2N3055s!

Bottom line: For valve guitar amps 10, even 20% either way for claimed power output is rarely an issue.

Dave.

[ICBM]

For guitar amps, I go as jpfamps said, on the visible point of clipping on a 1KHz sine wave into a dummy load. It's not super accurate but it's a good real-world value for 'clean power'. Maximum distorted power can be anything up to double this depending on the amp design - and the power rises very sharply as distortion sets in, so many amp manufacturers use this to claim more power, since in a valve amp the onset of distortion is quite soft and difficult to hear precisely in use, especially as to begin with it only applies to the picking transients. And even then the power stage and power supply design, the voicing of the amp and the amount of distortion the preamp can generate make at least as much difference to the final perceived volume... even before it gets to the speakers, which make even more.

Speaker sensitivity really is more important than amplifier power, for rock guitar. For example, a 15W amp driving a 4x12" with 100dB speakers will actually be louder than a 50W amp with a single 97dB speaker. And yet to the layman, 97dB and 100dB sound like they should be nearly the same thing! The main reason a traditional cheap practice amp is so quiet is the single 8" or 10" speaker. Stick the same thing through a decent 4x12" and it will scream - although it still won't have the same big clean tone and dynamics of a more powerful amp, which is where the practical differences really come in these days.

To take it to real extremes, as Dave said - I have an old 50s "hi-fi" (mid-fi is probably closer, but I like it ) speaker cabinet which is 3' high by 2' square (less some angled rear corners) so its internal volume is about 10 cubic feet! The bass driver is a 12" alnico Goodmans, which has a sensitivity of 100 or maybe even 101dB. The cabinet is loud enough to generate good volume and huge tone - louder than you can comfortably talk over, and with room-filling deep bass - when driven from the *headphone* output of a computer or even an iPod... no other amp. 1W would be complete overkill for driving it.