400wattRMS@1%THD...What does this mean??

[KMS]

I have assumed things for years about amps, but now I find myself needing to know for sure what some of this stuff means.

I know RMS means the "root mean square" and is RMS=.707X (x=peak power).

What I don't know is does this refer to amp's power consumption or to the amp's power output?

What does THD mean?  

Also, does anyone know how I can find out what my amp's power output is at 4ohms and 2ohms?

[Karmasound]

THD=total harmonic distortion

[KMS]

That's what I thought.  And it would not make since to reference that for power consumption.  Harmonic distortion would be in the output.

[LP Hovercraft]

LOUD!!!  

[LP Hovercraft]

Sorry, RMS is an accurate measure of wattage at an amplifier output.  That's interesting, because if the manufacturer goes to the trouble of giving an RMS figure, they do it per selected ohmage at the output unless it's specifically set up to put that much out regardless of ohmage selection.

[vanhansen]

Sorry, RMS is an accurate measure of wattage at an amplifier output.  That's interesting, because if the manufacturer goes to the trouble of giving an RMS figure, they do it per selected ohmage at the output unless it's specifically set up to put that much out regardless of ohmage selection.

If my memory serves me right, that is how tube amps are.  Regardless of output impedance, the RMS wattage remains the same.  It differs in solid state amps though I know for sure.

[R.G.]

RMS comes out of the early experimentation with electricity a few centuries back.

The early experimenters had batteries and alternators. Batteries produced a DC voltage, and alternators produced AC voltages. They were different, but ... how...??

They decided to make the AC voltage that produced the same amount of heat in a resistor be nominally the same voltage. For instance, say you have a 12V battery and a 12 ohm resistor. Connected across the battery, the resistor pulls 1A (12v/12ohms = 1A) and gets heated by 12V * 1A = 12W of power.

If you used a 12V peak sine wave to drive the resistor, you'd get less than 12W because the instantaneous voltage is less than 12 almost all the time in a 12V peak sine wave.

Without dragging you through the math, a sine wave with a peak voltage of 12V*1.414 = 16.968V pk produces the same heat in the resistor. The 1.414 is actually the square root of two, and it comes out of the math that computes the square root of the mean square of the instantaneous value of a sine wave.

The resistor doesn't care if you drive it with a sine with positive and negative peaks of +/-16.968V from an alternator or an amplifier. IF you did this with an amplifier that could... just barely... do +/-16.968Vpeak without any distortion (i.e. no clipping), then the amp is producing 12WRMS in the resistor, and if it's doing less than 1% total harmonic distortion when it does that, it's a 12Wrms, 1% THD amplifier.

If you allow distortion, then you can feed the load a square wave that peaks at 16.968V, so the resistor has 16.968 positive or negative across it all the time, so the power is 16.968V * (16.968V/12 ohms) = 23.99W, or about twice the power. So if you were an unscrupulous salesdroid, you could say it was a 24W amplifier.

You can't really specify amplifier power without specifying the load impedance and the distortion. 100Wrms sine wave into 8 ohms is 28.28Vrms, or +/-40V peaks. That same 28.28Vrms into 4 ohms (if the amplifier can do it) is 200W. That same dispicable salesdroid ...could... say that your 100Wrms/8 ohm amplifier could do 28.28*28.28/2 = 400Wrms, or 800W peak without mentioning that the amp can only do that for milliseconds, not continuously.

That's why US FTC regulations make amplifier sellers rate their amps by the undistorted sine wave power they can put out for a significant time, even though you don't usually listen to 100W sine waves.

Always read the fine print.

[vanhansen]

Daaaaaaaamn, R.G.  Great explanation.    I knew those calculations once but have since forgotten about them.  Thanks for the details.

Sticky anyone?  or FAQ Forum entry.

[KMS]

Thanks guys, this forum always comes through with detail like R.G.s explanation and the history behind it.  I never knew that but now I won’t forget it.

I know that a solid state amp’s power output is dependent on the load and this whole topic is focused on a 9 channel Peavey PA900 mixer power board which is identical to the PA600 mixer power board except it has three more input channels and evidently a higher power output.

Why did they limit the load specifications to the writing on the back panel and not mention it in the owner’s manual?

I think I might have an answer to that but first you must put yourself into the year 1973 or 72 and think about what you would want in a system during an era where tube amps were still king of the road. And put your self into the salesman shoes trying to sell a solid state unit.

It made no difference what the load was on a tube amp, a good selling feature and such amps would work with any speaker combos.

Not true for the NEW solid state equipment and I’ll bet that the sails droids that were selling solid state were doing everything they could to avoid discussing the load issue with solid state.  So to keep things simple and “hidden”, no mention of the load in the owner’s manual and they simply solved the problem by writing on the back panel 4ohm/4ohm right over the two speaker outputs. This put Peavey in the safety zone because if someone knew they could run two 8ohm speakers (internal parallel wiring makes that 4ohm) then everything would be OK but as soon as some accidental discontinuity occurred with one of the speakers then the load would be too high.  Also, if some users had two 8ohm speakers hooked up it would be tempting to try just one speaker (because we could always do that with a tube amp and the owner’s manual doesn’t say we can’t).  So the load issue would become a liability for Peavey and sales disadvantage if they addressed the problem out in the open.   It was just easier to dummy proof the back panel and tell the sales people to claim ignorance to why the back panel stated 4ohm/4ohm.

My best guess after discussing this with everyone is that this amp is 400watRMS@1%THD under a 2 ohm load and probably sees peaks at about 565 maybe 600 watts.

And 200watts RMS@1%THD under a 4ohm load with peaks around 280 maybe 300 watts.

Am I in the ball park here?

I just want to make some new speaker cabs that will handle the power and give this amp the correct load.

I’m thinking two 4ohm cabs that will handle 450watts Rms each ought to keep the neighbors up a couple block over for as many hours as I can get away with it.

[vanhansen]

You can always run a speaker(s) with higher impedance than the amp, such as an 8 ohm speaker with a 4 ohm load setting.  The problems come up when lowering the speaker below the rated load, such as a 2 ohm speaker in with a 4 ohm load setting.  The amp now sees 2 ohms and that could fry it.

[KMS]

Yea I just decided to do that. Better safe than sorry, and then I can get back to practice on my parts and not have to work on this thing again.

Thanks a lot.

[barret77]

well, I just love these RG posts where everything becomes clear  
thanks for more this one!