With these parameters in mind I input 5v and 4ohms into the ohm's law calculator, and I get:http://www.the12volt.com/ohm/ohmslawcalculators.asp6.25W and 1.25A Now look at the charts on page 5 of the amp's datasheet. The numbers I see there don't match up. In fact, there's two sets of numbers for different total harmonic distortions. How can this be? If I have a 5v supply and a 4 ohm impedance, how can the output, according to these graphs, be either 2.75W or 2.25W? And why is it these values are so low compared with what ohms law seems to say the power should be for 5v flowing through 4 ohms of impedance?
To keep it simple you are comparing a DC power calculation to a AC (audio is a ac signal) power calculation. As the total supply voltage avalible is only 5vdc, the theoretical maximum audio power available where the whole peak to peak waveform can 'fit' within a 5vdc limit is: 5vpp = 2.5v peak = 1.767 volts RMS
and then audio watts would be (ExE)/R or (1.767X1.767)/4 ohms = 3.12 watts rms. As the chip's amplifier is not 100% efficient the 2.75 watt figure says it's running at 88% efficiency, not bad at all.
Does that have to do with how much noise is getting into the amp or what?
Furthermore, are you saying that at 5v with a 4ohm speaker, if I output a square wave then I will in fact be outputting 6.25W and 1.25A as I originally calculated?
So are you saying that if I output a square wave, that would be the worst case?
That datasheet gives two operating conditions that they user can decide to operate at, 1% or 10% distortion. Hi-Fi amplifiers were typically those that would add no more then .1% total distortion or less, often way less.
My concerns are probably unfounded, but if the amp will actually be putting out 2.5W into a 4 ohm speaker, I'd like to know. That could be both good and bad. I mean more power = more volume, but not if it'll end up blowing most small speakers.
Yeah, I know I can allow the user to adjust the volume, but I also know the kind of folks who'll be using this, and they'll do things like max out the volume and connect 4 ohm speakers in series thinking two speakers will be louder than one. Or worse yet connect them in parallel across their 7w amp rated for 4 ohm speakers and either blow the amp or overheat it or cause distortion, I'm not really sure which.I just don't want them to either blow their speaker or think it sounds crappy and blame it on me. :-) And if they do blame it on me I want to understand what's going on well enough to explain to them why it's not working the way they want it to.