Measuring Cable LCR Parameters

Mudcat

Mudcat

Senior Audioholic
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?
 
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J

jneutron

Guest
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
 
Mudcat

Mudcat

Senior Audioholic
jneutron said:
Did you use the 4 wire test lead set, or the Kelvin set?

Thanks for the reply.

I used the Kelvin set.

I'll recheck the RAC @ 10K, but I'm pretty sure I read it correctly, and I'll also remeasure with the unmeasured conductors shorted. and post the results here.

Later on, I'll be posting my other cable, made from Alpha Wire 85104CY, which is a 10 awg, 4 conductor, extremely flexible shielded cable. Each conductor is 658 strands of 38 awg wire. The total OD is 3/4 inches. Wrap it up in a mylar braid, and a polyester braid on top, it will end up looking real nice.
 
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gene

gene

Audioholics Master Chief
Administrator
Lets step back for the moment and verify a few basics:

1) Did you calibrate your LCR meter before doing this test?
2) Did you calibrate for full frequency range? If so, what is the bandwidth of this device? If your device is bandwidth limited to 10kHz, check to see what the accuracy is at its frequency extreme.
3) How are you making contact on the cables at the shorted end to measure Rs and Ls? Are the wires bare? crimping, using aligator clip, compression with your hands, etc?


BTW, if you are measuring a 26ft speaker cable, then shorting one end and dividing by 26 will give you Ls (as total loop inductance /ft) and Rs (AC loop resistance /ft), RDC (DC loop resistance per foot). This is typically how cables are speced. As John points out, you don't want to measure Ls of each conductor separately as it doesn't factor in mutual inductance terms.

Give us the conductor spacing, diameter of each conductor, and dielectric material and we can calculate expected RLC values. Dielectic constant is a bit tricky since part of the E field is in air and thus lowers the effective dielectric constant as I noted in my Dielectric Absorption Article.

Its cool to see folks doing these tests, keep it up!
 
Mudcat

Mudcat

Senior Audioholic
gene said:
1) Did you calibrate your LCR meter before doing this test?
Yes


gene said:
2) Did you calibrate for full frequency range? If so, what is the bandwidth of this device? If your device is bandwidth limited to 10kHz, check to see what the accuracy is at its frequency extreme.
Yes
10K hz
and I donno

gene said:
3) How are you making contact on the cables at the shorted end to measure Rs and Ls? Are the wires bare? crimping, using aligator clip, compression with your hands, etc?
The cable ends (away from where I'm measuring) are were fitted with WBT bananna plugs and inserted into a four jack wall plate (not mounted on a wall). When shorting the ends, I used two pieces of 10 awg wire twisted together (effectively a 7 awg jumper) and fitted with AR bananna plugs inserted into the back side of the wall plates jacks



gene said:
Give us the conductor spacing, diameter of each conductor, and dielectric material and we can calculate expected RLC values.
I can measure this with verniers, but there would be too much guess work, but it is 12 awg wire therefore dia is 0.08081 inches. The conductor insulation is 0.56mm thick lubricated PVC (at least it feel slimey - alot like snake oil ;) ).
 
J

jneutron

Guest
gene said:
Lets step back for the moment and verify a few basics:

1) Did you calibrate your LCR meter before doing this test?!
Geeze, Gene....leave him alone!!!for goodness sakes..(I crack myself up)

The L looks really close to what I'd expect, C, don't know, R looked ok, except 10K..

gene said:
2) Did you calibrate for full frequency range? If so, what is the bandwidth of this device? If your device is bandwidth limited to 10kHz, check to see what the accuracy is at its frequency extreme.
I don't think B&K gives the user any of those options, and I'd expect he'd have to send to factory for cal anyway. I found only base specs and 2 year warranty, no mention of a cal cycle on the web..

gene said:
3) How are you making contact on the cables at the shorted end to measure Rs and Ls? Are the wires bare? crimping, using aligator clip, compression with your hands, etc?
Watch it, Gene..you're beginning to sound like me now.. from his data, I don't see that he had any issue with that.

gene said:
As John points out, you don't want to measure Ls of each conductor separately as it doesn't factor in mutual inductance terms.
Actually, I was pointing out that measuring one wire for inductance was about the same as one hand clapping..

gene said:
Dielectic constant is a bit tricky since part of the E field is in air and thus lowers the effective dielectric constant
Ummm..make sure there's no PVC in the building either...you know how it always wants to get "involved"...


gene said:
Its cool to see folks doing these tests, keep it up!
Concur..

Cheers, John
 
gene

gene

Audioholics Master Chief
Administrator
John;

Just wondering if you have a cable sniffer? You always seem to find a post at any forum that discusses cabling. As soon as I say mudcats first post, I knew you would join the party ;)
 
J

jneutron

Guest
I couldn't find any reference on the website to user calibration. I only found the reference to the direct test function, for testing the lead components..

How do you cal that thing?

John
 
J

jneutron

Guest
gene said:
John;

Just wondering if you have a cable sniffer? You always seem to find a post at any forum that discusses cabling.
Ummmmm...Gene?...Isn't this a cable forum??

Now.....ask me about capacitors...oh baby..

Cheers, John
 
Mudcat

Mudcat

Senior Audioholic
Hey You Two, Keep it down

Gene, John,

You two sound like a married couple.

Since the LCR meter is used is "borrowed" from the plastic pocket protector brigade (read electrical engineers - us mechanical engineers wear Harley Davidson T-shirts and Doc Martins) here at work, the manual probably dissappeared long ago. However, I downloaded the manual from the B&K Precision site.

After reading the manual, I realize I did not calibrate the meter properly, and email B&K asking about how I calibrated would effect readings. When I get the time (you know, wife, kids, soccer, work, sleep - oh excuse me, sleep is for the dead) I'll retest. In addition, I'm making a test rig so that when I test this particular cable, the two unused conductors will be subject to loads such as a light bulb, fan, and normal speaker loads.
 
J

jneutron

Guest
Mudcat said:
Gene, John,

You two sound like a married couple.

Since the LCR meter is used is "borrowed" from the plastic pocket protector brigade (read electrical engineers - us mechanical engineers wear Harley Davidson T-shirts and Doc Martins) here at work, the manual probably dissappeared long ago. However, I downloaded the manual from the B&K Precision site.

After reading the manual, I realize I did not calibrate the meter properly, and email B&K asking about how I calibrated would effect readings. When I get the time (you know, wife, kids, soccer, work, sleep - oh excuse me, sleep is for the dead) I'll retest. In addition, I'm making a test rig so that when I test this particular cable, the two unused conductors will be subject to loads such as a light bulb, fan, and normal speaker loads.
Hi Mudcat

I downloaded the manual..they talk about short and open cal, which appears to be internal functions to help very low and high value tests..

But for actual calibration to standards, which is what I thought Gene was referring to, ya gotta send the unit to B&K..and boy, it costs...

I don't think you need it, btw..your numbers are good..

Mechanical engineer????? Geeze, wish I had known that before...(boy, I could really have some fun with this...but I'll be a good boy today..Gene's bro is one...)

Pocket protector? what is that?? My short sleeve golf work shirts don't have a pocket..Especially the Swizzle Inn "STAFF" shirt I'm wearing now..

Cheers, John
 
Mudcat

Mudcat

Senior Audioholic
jneutron said:
Mechanical engineer????? Geeze, wish I had known that before...(boy, I could really have some fun with this...but I'll be a good boy today..Gene's bro is one...)

it's actually worse. My title is mechanical engineer, my degree is in Marine Engineering (diploma actually say Bachelor of Engineering opposed to Bachelor of Science that others get), which has the all the requirements of mechanical engineering but a lot more thermo and chemistry. No english courses though, we were not expected to be able to write, only to know which end of the pencil to put on the paper.
 
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