analog audio over short CAT5 twisted pair

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Twenty Log

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'twas thinking of a short board-to-board jumper with connectors; connectors for serviceability and debug purposes... there are some pretty high voltages present elsewhere on PCB such that removal of PCB might be desirable for debug / rework...

Thought:  CAT5 twisted pair... for a short (6-inch? ~ 15.24 cm?)  patch cable, and just have simple CAT 5 connectors on each PCB....  About to do a respin anyway....

CAT 5 UTP is typical (unshielded)....  Although STP (shielded) is available....

The 4-twisted pairs would be ideal for a bundled solution, for stereo in and out from board-to-board...

but.... this ain't individually shielded microphone cable.... cross talk at these audio frequencies with CAT5?  I know of NEXT and FEXT (near-end / far-end crosstalk) for ethernet on these (typically long) CAT5 cables at RF frequencies....  is this the same for audio?  I have no idea...

COTS CAT5 type of wire/jumpers would be ideal if there aren't any audio issues...  Thoughts?  Thanks!
 

JohnRoberts

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The benefit of shielding depends not only on level but impedances. If the source impedance is low, unshielded (inside a metal chassis) may be adequate.

JR
 

Bo Deadly

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Shielded CAT5 would probably work pretty well for sending audio. But I don't think it's going to help in your case because the cables are so short. Using conventional wires as a jumper would work equally well.

The important thing would be if you have high voltage AC, you want to twist it with either a very low impedance wire such as a ground or an AC signal that is complementary (like the return of the same signal). And the twist doesn't need to be tight. You just want to minimize the loop area.

Actually coax would be best. A very close second would be microphone wire or snake cable or something like that that has a full shield and a drain wire. That would give you 3 conductors including the shield / drain wire. Have the extra conductors can be very handy.

For example, this is switched input in a guitar amp I did:

OiPSPgd.jpg


That's Redco snake cable. I just removed the jacket and used the sub-cables. But Redco has shielded cables like this that are actually quite a bit thinner. Can't recall the SKU.
 

emrr

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200 foot 4 channel CAT5 line level audio from FOH to powered speakers is standard operating procedure these days, frequently with many feet of power in parallel. 
 

Speedskater

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emrr said:
200 foot 4 channel CAT5 line level audio from FOH to powered speakers is standard operating procedure these days, frequently with many feet of power in parallel.
Is a shielded CAT5 cable or a Plane Jane cable?
 

gyraf

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Our good old Calrec UA-8000 was stuffed full of telecom-type twisted-pair wire - a good 15 meters from e.g. channel 1 to patchbay and return (!!)

Microphone input was shielded though..

Jakob E.
 

[silent:arts]

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You can buy StageTec AD converters with RJ45 connectors.
http://www.stagetec.com/en/audio-routing/nexus/analogue-audio-components/nexus-analogue-input-board-xad.html
 

PRR

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Gentlebeings-- this is a "short (6-inch? ~ 15.24 cm?) patch cable.

Issues that matter across a cow-palace are irrelevant for this job.

> cross talk at these audio frequencies

The RF data is not much use.

The cable is a capacitor. Pencil 30pFd per foot or a better number from the data. So half-foot is 15pFd. This is 530,000 Ohms at 20KHz.

It is driven by some impedance. Let us say 530 Ohms. Say Left channel leaks through 15pFd and hits the 530 Ohms of the Right channel. Crosstalk is roughly 530,000/530 or -60dB.

Amplifiers driving cable "should always" have some built-out resistor so the amp does not have to drive pure capacitance. However with good opamps, 56 Ohms may be ample. Now -80dB.

If the signals are balanced, the crosstalk nominally cancels out. Except this assumes both sides of each pair are equally exposed to both sides of the other pair. Your twist may be 4 inches so a 6 inch run has 1.5 twists. Cancellation is not complete.

The wonderful thing about modular connectors is once you have the crimper and a PCB layout, there is no cheaper good interconnect.
 

Gold

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I use Cat5e Belden Mediatwist for AES audio.  I want a shield on analog audio. I like stripping out the bonded twisted pairs for use inside shielded boxes. Steve Lampen from Belden claims the twist on the bonded twisted pair is so good you don't need a shield. I haven't tested it but I'm not going to try.
 

JohnRoberts

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Gold said:
I use Cat5e Belden Mediatwist for AES audio.  I want a shield on analog audio. I like stripping out the bonded twisted pairs for use inside shielded boxes. Steve Lampen from Belden claims the twist on the bonded twisted pair is so good you don't need a shield. I haven't tested it but I'm not going to try.
A bazillion miles of unshielded twisted pair telephone wiring was good enough but for critical noise rejection twist quality can make a measurable difference.

Star-quad, the cable with pairs of audio conductors twisted around a fiber core provide a very symmetrical wrap. Decades ago i did some comparison testing by draping high gain mic cables over some fluorescent fixtures and measured noise floor differences between star quad shielded mic cable vs standard shielded mic cable. 

I don't expect unshielded twisted pair to do well in that test (even Belden).

YMMV

JR.

PS there may be a little more to the star-quad story but not for now.
 

PRR

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> Look up cat 7

??!! The first hit (g) is baseball bats... and it looks AudioPhile....

AV2 Knob - Offers a Better Feel and Less Negative Vibrational Feedback
AZ4X Alloy - Increased Strength With a Higher Response Rate
Optimized Barrel Design - Sweet Spot is Twice as Large as CAT 6
Ring-Free Barrel Technology for Maximum Performance and No Dead Spots

  "New for 2017! This 2017 Marucci CAT 7 BBCOR Baseball Bat: MCBC7 is similar to that of the CAT 5 and CAT 6, only double the power! You are really going to scare some people if you come to the plate with this bat. "

Yowsa! "Double my power"!
 

ricardo

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http://www.etslan.com/products.cgi?cat=9

We've tested this at audio frequencies and would have no hesitation in using it for whatever distance and in the highest quality applications.

Having said that, I agree with those who point out a couple of inches doesn't need supa dupa cabling.  Twisting is just fine as long as you use cable hand carved from solid Unobtainium by Virgins  ;)
 

Gold

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Belden Mediatwist is very good cable. The twisted paris are bonded so the twist is perfect. Each pair sits in its own 'trough' in the cable. The cable jacket is arched. The geometry keeps the pairs at an equal distance from each other even around bends.

The best shielding I've run across in a cable without the star quad penalties is Gotham Audio GAC-2.
 

Twenty Log

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Wow~~~  never would have thought that ethernet cable I used to test in the 1990s would work for FOH and other sundries...  Thanks for the replies; indeed the twists are not there to look pretty!  Thanks everyone....
 

Monte McGuire

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I'm pretty sure that the ethernet cable used for modern FOH is actually running a network protocol carrying digital audio, and not analog at all. People tried to use time domain multiplexed analog decades ago (then over XLR -> STP snakes) but there's really no point anymore with modern digital audio.

So, yes, they run UTP between FOH and the stage / amps, and it's just a local area network, with some extra pairs for bandwidth and redundancy.

To the OP: one nice thing about CAT5 etc. is that, if you get the plenum rated version, you get a really nice FEP insulation, basically the same as Teflon. It's a great dielectric for analog audio.
 

emrr

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Monte McGuire said:
I'm pretty sure that the ethernet cable used for modern FOH is actually running a network protocol carrying digital audio, and not analog at all.

No.  Analog audio.  We do this every day in portable PA installations, as do most for-hire outfits these days.  There are Dante networks / etc that run digital protocol, they seem to need the exact right shielded CAT cable more than analog installs do. 
 

Monte McGuire

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Wow! How many channels? I guess some situations don't need dozens of channels, but I was not aware of this. Thanks for the data point!!
 

emrr

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Speedskater said:
Is a shielded CAT5 cable or a Plane Jane cable?

CAT5e is what I see on today's cable run.  It's a 200 foot 4 channel line level run to powered speakers, and has 100 feet of parallel power and CAT6 video.  I can't tell if the AC cable is shielded from the sheath labeling.  No problems with audio or video.
 

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