Voltage, power, impedance - diyAudio
 Voltage, power, impedance
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 1st April 2009, 02:56 AM #1 diyAudio Member   Join Date: Jan 2004 Location: Toronto, ON, Canada Voltage, power, impedance I wasn't sure where I should put this, so I guess it ends up here. Anyway, I'm a little confused about the relationship between peak voltage and RMS voltage as it relates to power and impedance. Let's say I have an amp with +/- 30VDC rails and it is driving a power resistor of 4 ohms. The Vp is 30V, so the Vrms = 30/1.414=21.2V. This is the value that I use for power calculations, right? So P = V^2/R P = 21.2^2/4 P=112W Is this correct? If so, then I move on to the following calculation: an amp which needs to provide 100W to a 7-ohm load must have +/- 37.4VDC rails (assuming that it can rail), correct? Vrms=sqrt(100*7) Vrms=26.46V Vp=26.46(1.414)=37.4VDC
 1st April 2009, 03:35 AM #2 diyAudio Moderator Emeritus     Join Date: Oct 2007 Location: Santa Cruz, California That's correct. In reality, your OP trannies can't actually swing rail to rail and then the PSU usually sags a bit at full load hence YMMV. edit: oh, and a 7ohm speaker isn't really 7 ohms, well maybe at a couple of frequencies
diyAudio Member

Join Date: Jan 2004
Location: Toronto, ON, Canada
Quote:
 Originally posted by Iain McNeill That's correct. In reality, your OP trannies can't actually swing rail to rail and then the PSU usually sags a bit at full load hence YMMV. edit: oh, and a 7ohm speaker isn't really 7 ohms, well maybe at a couple of frequencies
Thanks for the help.

I can read impedance charts, though.

 1st April 2009, 11:27 PM #4 diyAudio Member     Join Date: Dec 2006 that sounds about right. a while back i made up a "power guesstimation" chart based on rail voltage, peak voltage and p-p voltage so that techs wouldn't have to mess with a calculator when testing an amp. it's mostly "ballparked" values, but it works and saves a bit of time. as they used to say "it's close enough for rock and roll"...... __________________ Vintage Audio and Pro-Audio repair ampz(removethis)@sohonet.net spammer trap: spammers must die

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