katopan wrote: ↑Mon 05/03/21 5:56 pm
katopan on Wattkins wrote:
I strummed chords on a plugged in guitar with the guitar and/or amp volume set so the output signal viewed on my CRO was just past the point of clipping the peaks. Into a dummy resistive 8 ohm load. Take that peak voltage (not peak to peak, just one side) and work out the equivalent power for a sinewave of the same peak voltage. Difference with this method over actually injecting a sinewave is the real dynamic signal from a guitar allows the power supply to recover and gives a more consistent measurement...
I believe just the opposite is true. The harmonic energy in the waveform from a guitar - not to mention the additional harmonic energy created by the amplifier itself - is extremely variable. This variability contributes to inconsistency regarding the RMS power of the waveform, and it in turn also affects power supply sag. If the intent is to determine the dynamic, "pre-sag" value of the waveform just prior to the onset of clipping
as a function of amplifier output power capability, i.e. the maximum short-term power output, a pulse generator is needed. A burst of a few cycles of sine energy at the frequency of interest, repeated every few seconds, will allow measuring the peak output power using a scope. The result will be consistent from one amplifier to the next without regard to the tonal (spectral) content produced by the pickups or the amplifier's tone control settings.
This type measurement will provide a basis for knowing how the amplifier will initially react to a strong signal, and it is useful in determining whether the physical excursion of the speaker will be within mechanical limits. This aspect of a speaker's power rating becomes more important when it is expected to be used in free air (not a vented or infinite baffle enclosure) with no acoustic dampening.
The other important issue is the power dissipation capability of a speaker's motor and whether the amplifier will cause it to overheat. The variability of the waveform noted above creates a complex and inconsistent relationship in this regard, so in the absence of overriding empirical data ("XYZ amp blew up ZYX speaker), we are left with only the usual long-term RMS measurement for making decisions. As noted by katopan, and as I've said previously, amplifiers typically deliver much less long-term power than data sheets would lead us to believe. For this reason, I believe there is little risk in applying a significant discount to the output ratings of most guitar amplifiers when evaluating whether they will work properly with a given speaker. If the numbers are close, there's little risk unless you're constantly banging the voice coil against the back side of the magnet structure.
Jack