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Ohms FAQ

#745
my half-assed mental cheat here is to think "half the average" for two loads in parallel.

8Ω and 16Ω? the average between the two is 12, half that is 6; the right answer is 5.3Ω as stated above, close enough to know off the top of your head if you're gonna blow up your amp or not.
 
#746
walterw said:
my half-assed mental cheat here is to think "half the average" for two loads in parallel.

8Ω and 16Ω? the average between the two is 12, half that is 6; the right answer is 5.3Ω as stated above, close enough to know off the top of your head if you're gonna blow up your amp or not.
Click to expand...

Don't mean to burst your bubble, but your "cheat" only sorta works (close enough) for those two specific ohm loads where one is exactly TWICE the value of the other one----IOW, it is just a numerical coincidence.

case in point:

2ohms in parallel with 8ohms

with your 'cheat'/half-the-average, would be 2.5 ohms. In reality, it would be a 1.6ohm load. Parallel wiring in no way follows averaging techniques, but rather as has already been stated, it is product-over-sum (for two loads that is).

For more than two speakers in parallel, the technique is "the inverse of the sum of the inverses"..


Here is a calculator screenshot of the final impedance of three ohm loads in parallel (4 ohms, 6 ohms, and 16ohms)

ohms_zps584748a9.png
 
#747
OscarJr said:
case in point:

2ohms in parallel with 8ohms

with your 'cheat'/half-the-average, would be 2.5 ohms. In reality, it would be a 1.6ohm load.
Click to expand...
hmm, yeah; that "rule of thumb" would conflict with the other, correct rule of thumb that the total is always lower than the lowest cab.

still, it's at least in the parking lot if not the actual ballpark, as opposed to someone just guessing 5Ω in your example instead! (like happened earlier in this thread)

how about "a little lower than half the average"?
 
#749
I would point out that carrying the impedance calculations out to anything beyond a single decimal place is a bit silly. Since we're dealing with AC impedance rather than DC resistance, the actual impedance is going to vary with frequency, so the result of any calculation is only approximate and could vary by whole ohms. So all of this 5.33333 and 2.66667 stuff is a waste of precious internet bandwidth. You could say 2.7 or even 3, and 5 instead of 5.3 and still have the bases covered.
 
#750
Munjibunga said:
I would point out that carrying the impedance calculations out to anything beyond a single decimal place is a bit silly. Since we're dealing with AC impedance rather than DC resistance, the actual impedance is going to vary with frequency, so the result of any calculation is only approximate and could vary by whole ohms. So all of this 5.33333 and 2.66667 stuff is a waste of precious internet bandwidth. You could say 2.7 or even 3, and 5 instead of 5.3 and still have the bases covered.
Click to expand...

Absolutely. My sole point was there there are proper ways to calculate this nominal impedance very easily.
 
#754
walterw said:
hmm, yeah; that "rule of thumb" would conflict with the other, correct rule of thumb that the total is always lower than the lowest cab.

still, it's at least in the parking lot if not the actual ballpark, as opposed to someone just guessing 5Ω in your example instead! (like happened earlier in this thread)

how about "a little lower than half the average"?
Click to expand...

OscarJr said:
I gave no such example... :confused:
Click to expand...
your example of 2Ω and 8Ω in parallel!

i just meant how someone earlier was thinking "halfway between two loads", which would have been 5Ω in this case.

at least my wrong answer was closer to the right answer, maybe enough (when coupled with the other rule that the answer is always lower than the smallest load) to keep somebody from blowing their gear up, which is all anybody who has to ask this question ever really cares about.
 
#760
lucas vigor said:
What does the word "bridged" actually mean?

I had a Carvin RC210 combo with head. In the back, the outputs were " 4ohms hi-freq, 4 ohms lo-freq" (I understand that these two are the bi-amped outputs) and then there were two separate outputs labeled "bridged only".
Click to expand...

Providing your stereo amplifier is capable of being bridged, when bridging an amplifier, the left+ and right+ output terminals are connected to your speakers positive and negative to produce a more powerful signal. The output bridged power will be very different than left and right output.
(much more power)
A Crown XTI 4002 is rated 1200 watts @8 ohms each side. When bridged it is rated 2400 watts mono. So you are taking the left and right channels, and combining the positive from each to create much more power. The owners manual will show you exactly how it is done.

So the positive from the left channel will go to your positive on your speaker. The right channels positive will connect to your speakers negative.
The Crown amplifier will then need to switched over to the bridged mono operation.

I would need to look at the rear panel of the Carvin and go from there.
 
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