Mudcat said:
I recently made up a 26 foot speaker cable from 12/4 Carol power cable. This cable carries both the left and right speaker signals to a subwoofer (the left and rightr speakers are connected to the sub). I used the two opposite conductors for each speaker, see attached figure. I measured them with a B&K 885 LCR meter. This meter can measure AC resistance, Capacitance, and Inductance at 100 Hz, 120 Hz, 1000 Hz, and 10000 Hz. When I made the measurments, I only measured the conductors for each speaker.
Here are my results ( resistance and inductance were measured with one end shorted, capacitance was measured with open end):
RDC = 0.059 Ohms
RAC @ 100 HZ = 0.089
RAC @ 120 Hz = 0.087
RAC @ 1000 Hz = 0.098
RAC @ 10000 Hz = 0.422
Cp @ 100 Hz = 730 pF
Cp @ 120 Hz = 723 pF
Cp @ 1000 Hz = 670 pF
Cp @ 10000 Hz = 641 pF
Ls @ 100 Hz = 6.7 uH
Ls @ 120 Hz = 6.66 uH
Ls @ 1000 Hz = 6.6 uH
Ls @ 10000 Hz = 6.52 uH
Now for my question.
To get per foot ratings I'm confident that capacitance should be divided by 26 feet, but are the Resistance and Inductance divided by 26 feet or 52 feet? How would the conductors for one speaker affect the measurments of the other speakers conductors? The above measurements were made one set of conductors at a time. Should I have used two LCR meters (of which I only have one), and measured both sets of conductors at the same time?
Re-try the 10K Rac, it looks too high. The other data looks consistent. The inductance measure is usually where people mess up, various lead dressing issues...but your numbers, like 6.52 uH, div by 26, gives .25 uH per foot, dead nuts on for the cable construction.
You may find that how you deal with the other conductors will affect the meter somehow..if you short the other pair and see a change, voila..
R divided by 26 is the typical measure..you want to know how the cable resistance per foot is, not the individual conductors..
L also, divide by 26, same reason..in addition, a single wire inductance number is a nonsensical entity, there always has to be two wires involved.
Two meters at one time could wreak havoc by coupling, and won't be very useful.
The capacitance will be higher as a result of the other wire pair, but the numbers you got are what each amp channel would see anyway, I don't think loading the other pair will change the numbers, but it would be neat if you verified that.
Rac should not be affected by the other wires, unless there is a capacitive involvement fooling the meter...I wouldn't think B&K would design a test instrument that couldn't see the capacitance reaction and remove it.
Inductive coupling between the two pairs should also be very low, as the geometry is one of orthogonality, the field from one pair is balanced w/r to the other.
Did you use the 4 wire test lead set, or the Kelvin set?
Thanks for the info..nice job..
Cheers, John