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Old 19th July 2004, 03:47 PM   #1
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Default Diy spdif cable.

I can't find many diy designs on the net (many reference a certain brand/model of cable)...so last night I just took 1...tp from cat5 cable..and some 3m shielding tape around it soldered it to bnc on dac side and rca on transport side.

Good enough? Any suggestions for improvement?

I will listen to it soon! (Don't have coax dig out on my cd player)

Also ......brace yourself..this may sound very stupid.....how do you measure that it is 75 ohm?

Regards,
Bas
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Old 19th July 2004, 03:53 PM   #2
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Lightbulb Re: Diy spdif cable.

Quote:
Originally posted by Bas Horneman
I can't find my diy designs on the net (many reference a certain brand/model of cable)...so last night I just took 1...tp from cat5 cable..and some 3m shielding tape around it soldered it to bnc on dac side and rca on transport side.

Good enough? Any suggestions for improvement?

I will listen to it soon! (Don't have coax dig out on my cd player)

Also ......brace yourself..this may sound very stupid.....how do you measure that it is 75 ohm?

Regards,
Bas
Hi Bas,
You can simply use double shielded TV/video/satellite 75 Ohm coax. But in order to avoid reflections you have to use 75 Ohm connectors and chassis parts. F-type connectors for satelliet TV would work but are totally hopeless in construction. The most readily available is 75 Ohm BNC.
Just ask for 75 Ohm coax, double shielded in the shop.

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Old 19th July 2004, 03:57 PM   #3
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Quote:
Just ask for 75 Ohm coax, double shielded in the shop.
Thanks Elso...I will...

Don't you think it is funny they use RCA though for co-ax out....when it is not a 75ohm part?

Still that leaves me with the question...how the hell do you measure it to be 75ohm? Either the cable or a RCA plug? (Or both)

Cheers,
Bas
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Old 19th July 2004, 04:09 PM   #4
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Post Diy SPDIF Cable

Quote:
Originally posted by Bas Horneman


Thanks Elso...I will...

Don't you think it is funny they use RCA though for co-ax out....when it is not a 75ohm part?

Still that leaves me with the question...how the hell do you measure it to be 75ohm? Either the cable or a RCA plug? (Or both)

Cheers,
Bas
Hi Bas,
A 75 Ohm RCA plug does not excist to my knowledge. And even if the plug is claimed to be 75 Ohm the chassis part is not.
For the cable you have to trust the vendor, but 75 Ohm TV coax is fairly common.
It is extremely curious that a RCA phono connector is the standard as described in table IV of IEC 268-11 for the consumer interface.
Ask Jocko or Fred if you are suspicious of my answer. They are the expert on this subject.

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Old 19th July 2004, 07:27 PM   #5
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Default Re: Diy SPDIF Cable

Quote:
Originally posted by Elso Kwak

Hi Bas,
A 75 Ohm RCA plug does not excist to my knowledge. And even if the plug is claimed to be 75 Ohm the chassis part is not.
These guys claims to have one !

http://www.bluejeanscable.com/articles/75ohmrca.htm
http://www.bluejeanscable.com/pages/...canarercap.pdf

Click the image to open in full size.
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Old 19th July 2004, 08:42 PM   #6
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AFAIK, the previous suggestions and considerations about RCA are absolutely correct.

I would just like to add a piece to the picture. The problem is that apparently it is the wrong piece.... it does not fit in the frame!!!!

Just to summarize: at tnt-audio we are testing a bunch of digital interconnects. We tested them under different aspects including audio jitter at the output of a DAC outout, smoothness of the transition in time ("eye" in the oscilloscope without neither too much rounding nor spikes), correct impedence, etc. and obviously sound.

What apparently results at present time is that the best cables (which are MUCH better then all the others) from the sound point of view are the ones with the most regular eye pattern, as expected.

The problem is that they are also the ones causing more jitter and with the impedence more different from 75ohm (one far above and one far below)... And this AFAIK is both in contrast with theory (and eye pattern results too).

The impedence test was performed with a MFJ-259, an antenna analyzer which is able to give the impedence module and phase of any complex system, and the measure sounds correct (the measures perfectly fit technical data for all the cables for which these are available...).

I could think there might be a problem in jitter measures, but even them have been performed so many times and sound so stables that I am not so sure about this explanation. The difefrences in jitter are anyway rather reduced.

Has anyone any idea ?


Next steps could be to measure the wire isolation from external disturbs, but the previous tests leavme me rather upset. Has anyone seen anything similar??

Thank you in advance

Giorgio
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Old 20th July 2004, 03:10 AM   #7
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Default IT IS NOT 75 OHMS!!!!!!!!!!!!!!!!

How many times do I have to say that???

I don't want to have to 'splain this again.

So Bas.........what you wanted to know.....

2 ways:

Measure it on a TDR,

Or better yet.........

Pay someone like me who has one to do it for you.

Hint, hint....have TDR, will travel.

Jocko
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Old 21st July 2004, 02:55 AM   #8
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Why do you want 75Ohm?!? connection between 2 SPDIF components could be anything(until voltage remains in min-max range). A 75Ohm cable is not. VHF antennas are told 300Ohm but are infinite ohms, one wire goes to an antenna and an other one to an other.
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Old 21st July 2004, 06:59 AM   #9
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Quote:
Pay someone like me who has one to do it for you.
What is your hourly rate?
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Old 21st July 2004, 07:54 AM   #10
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Default Depends who asks!!!!!

Why 75 ohms..........?

Because that is the spec for SPDIF..........0.5 Vp-p, into a 75 ohm load.

Yes....if you are making your own.........you can make it anything that you want. As long as the TX, RXC, and cable impedances are all the same.

As for antennas.............don't get me off on that tangent.

Antennas are not designed to "look" like a specifed impedance over a few decades, as transmission lines are.

Jocko
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