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[Russell Ault]

On mic lines over 1000ft sure. Speaker lines?? I am going to have to ask you to show your work.

Speaker lines are typically much worse for this sort of thing, though.

I'm using the E-V Cableloss Calculator. Speaker cable capacitance can be a difficult spec to find from some manufacturers, so for reference I'm using the specs from page 35 of the BRTB Bulk Cable catalogue (they're a reasonably-reputable Canadian cable manufacturer).

According to the E-V calculator, a 1000W-into-8Ω amplifier driving a nominally 8-ohm speaker over 100' of 12AWG "BRTB Contour" speaker cable with a capacitance of 27.4 pF/foot will produce an LPF with it's -3dB point just under 7 kHz (and, I believe, a -6dB/octave slope).

At the distances for the low pass to come into play you would put the amp closer. At line level I've run analog STP (multicable or 8451) 3000' with no ill effects.

I would too, but there are plenty of installers (at least around here) that think nothing of putting the amps for the PA in a booth that's over 100' of conduit away.

-Russ

[Mac Kerr]

According to the E-V calculator, a 1000W-into-8Ω amplifier driving a nominally 8-ohm speaker over 100' of 12AWG "BRTB Contour" speaker cable with a capacitance of 27.4 pF/foot will produce an LPF with it's -3dB point just under 7 kHz (and, I believe, a -6dB/octave slope).

I would like to see some measurement traces for a full range speaker with a 10' and a 200' cable. I have measured many hundreds of speakers with over 200' of cable and have not seen this phenomenon.

Mac

[Russell Ault]

I would like to see some measurement traces for a full range speaker with a 10' and a 200' cable. I have measured many hundreds of speakers with over 200' of cable and have not seen this phenomenon.

That's a fair request. I have seen this effect in a measurement once. It was an installed Nexo PS15 with at least 100' of cable between it and the booth; if I remember correctly the cable is only 16AWG. Unfortunately I don't think I have a copy of the trace (I believe we were using the venue's computer that afternoon). I don't get out to the venue very often any more, but if I get another chance to take a measurement (before their imminent PA replacement) I will.

-Russ

[John Roberts {JR}]

I would like to see some measurement traces for a full range speaker with a 10' and a 200' cable. I have measured many hundreds of speakers with over 200' of cable and have not seen this phenomenon.

Mac

need a like button...

That much capacitance at 8 ohms generates a real pole at 7 Mhz, but I might be wrong...

It seems like a 7kHz half power point would be easily audible and not secret for this long.

JR

[Matt Vivlamore]

buy a spool of 12/4... you can build NL4 and L14-20 power cables. 

[David Sturzenbecher]

Speaker lines are typically much worse for this sort of thing, though.

I'm using the E-V Cableloss Calculator. Speaker cable capacitance can be a difficult spec to find from some manufacturers, so for reference I'm using the specs from page 35 of the BRTB Bulk Cable catalogue (they're a reasonably-reputable Canadian cable manufacturer).

According to the E-V calculator, a 1000W-into-8Ω amplifier driving a nominally 8-ohm speaker over 100' of 12AWG "BRTB Contour" speaker cable with a capacitance of 27.4 pF/foot will produce an LPF with it's -3dB point just under 7 kHz (and, I believe, a -6dB/octave slope).

I would too, but there are plenty of installers (at least around here) that think nothing of putting the amps for the PA in a booth that's over 100' of conduit away.

-Russ

Might want to check your units...  If you can hear that 3dB down point...then damn...

[Russell Ault]

Might want to check your units...  If you can hear that 3dB down point...then damn...

Okay, so, let me start off by eating a healthy serving of crow, with an acknowledgement that more crow might be in my near future.

David nailed the specifics of my error, and to everyone else who pointed out that cable capacitance is immaterial for any typical speaker-level audio run, you are all absolutely correct and I am absolutely wrong.

As I mentioned in my last post, my adamance about the frequency-related effects of long runs of thin speaker cable stems from trying to figure out why there appeared to be a first-order low-pass filter passively inserted into a local venue's sound system (and yes, we checked the DSP). My memory of the explanation we came up with is clearly broken, but I'm pretty sure we figured that it had something to do with a long run of skinny wire inside a conduit. Even so, I spent a chunk of time this evening trying to re-trace my steps, research-wise, and came up with mostly blanks. It seems like inductance (not capacitance, mea culpa) might have been what I was thinking of, but even this now sounds far-fetched.

So, at the risk of damaging my reputation even further, is there a chance that a long, skinny speaker cable inside metal conduit could exhibit enough inductance to cause a noticeable low-pass filter?

Sorry, all!

-Russ

[Steve-White]

^^^ Don't feel alone at the crow table Russ.  We've all dined there. 

[Mike Caldwell]

I like to order 12/4 SJOOW and build my own, but I currently have new speakers in need of jumpers and don't have new 12/4 yet.

What I do have is 100's of feet of 18/2 solid core install cable, so I was considering making short NL4 jumpers using two 18 AWG conductors for each pole of the cable, which I calculated using a website to equal 15 AWG per pole.

So it would be four separate 18/2 cables kind of like a loom. But this is all seeming much to ghetto now so I'll probably just wait until I get more 12/4 and do this properly. I was in a mindset of "how can I use what I already have rather than spend more money".

Most electrical supply shops, Home Depot, Lowes, will have 12X4 SJ. buy what you need to get started and order a roll to make up the rest of your cables.

You would not like dealing with the cable you talked about making for anything more than to test your speakers with out in the drive way. 

[Mal Brown]

^^^ Don't feel alone at the crow table Russ.  We've all dined there. 

Is there another table ?   Why didn't I know that ?

;-)

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