[philodox]

Hi,

I made a cable that I am testing out in my setup by braiding 4 solid silver wires [from the group buy] together and used a vampire RCA at each end. It seems to be working fine.

I keep hearing about true 75ohm cable and such. Do I need some special kind of cable or do I need to do anything differently when making a digital cable?

Also I was wordering about directional cables. All of my Zu Cable is directional. How exactly is this done and are there any benifits to it?

I will probably end up re-terminating the cable with some standard RCA connectors of some kind as I hate these vampire RCA's... so if anything can be done to improve my digital cable I could do it at that point.

I used a square sinnet braid and attached two wires to ground and two wires to signal.

Thanks,

 

[HiGHFLYiN9]

I think it may have something to do with the terminations as well www.takefiveaudio.com/ canare_75_ohm_rca.htm it seems there are special RCAs for 75 ohm applications.

 

[philodox]

Quote:

Originally Posted by highflyin9 I think it may have something to do with the terminations

Well that works out well since I am planning on re-terminating them anyways. Thanks!

 

[Moskau]

Standard RCA terminations aren't 75 OHM, at least that's what I've always been lead to believe.

Directional cable I'd assume is cable that has one end grounded with shielding (usually component) and the other end with the shielding ungrounded.

It could also be cables with a zobel at one end (usually component again) and a box over the zobel. Many deem this useless as the place for a zobel is inside the actual amp circutry.

 

[philodox]

I'll have to make sure I get some 75 ohm RCA's then... thanks

What is a zobel?

 

[philodox]

Question... does the fact that the cable that I already made sounds fine without any problems suggest that I already have 75 ohm terminations? If so, I might just try to figure out a way to make these work better... maybe a few layers of heatshrink on the ends to give them some more rigidity.

 

[ooheadsoo]

Talking to the sound engineers at diyaudio, they say that they have NEVER measured a 75ohm RCA. According to them, all of them are around 30-35 ohms at best, including the expensive 75 ohm WBT RCAs - as determined by PHYSICS. Only BNC and F connectors are true 75 ohms. Therefore, the best solution is to get gear with bnc on both ends

Here's an article on it wrt video, but it's the same for digital: http://www.bluejeanscable.com/articles/75ohmrca.htm

Oh, there's a problem with the article though. For digital data, dealing with jitter demands that the cable be as long as possible so that the inevitable reflections arrive late enough to be discarded as garbage, and are attenuated enough to be detected as garbage...or something to that effect.

 

[hubcaps]

Quote:

Originally Posted by philodox Question... does the fact that the cable that I already made sounds fine without any problems suggest that I already have 75 ohm terminations?

Nope, a digital cable still works even if it isn't 75 ohms. As ooheadsoo kinda mentioned, the impedance has to do with jitter and reflections. Whether you can here it or not depends on your system (i.e. source, amp, cans, speakers, etc.). Jitter is a complicated thing, and there is much debate on how to deal with it. I would guess that if the impedance is very far off from 75 ohms then the dac would have problems locking on.

If I were you I'd compare a 75 ohm cable with your diy silver cables. Going true 75 ohm with bnc and such is a bit of a hassel, some sources and dacs aren't even terminated at 75 ohms, like the art di/o. Some people changed the resistor so that it was 75 ohms and some liked, it some didn't. I can't comment much on sound difference since my art di/o has been sitting in the corner collecting dust. It's a pita trying to fit in the bnc jack.

 

[philodox]

Well lets put it this way... there is only a very small difference between the cable I made and my Zu Ash.

 

[ooheadsoo]

Was the Ash RCA terminated?

I would give using a long cable a serious shot if there are serious reflection issues. Several engineers on diyaudio who are in the pro audio field swear by long digital cables for these reasons.

Yeah, bnc is a pain, but that's the way it goes. I'm considering trying to mod my emu for a bnc jack. The dac I'm buying is getting a bnc jack fitted on there for sure. Possibly an xlr 110 ohm jack as well for aes3.

 

[philodox]

My zu ash is terminated with rca's... but I would imagine that zu would have taken any steps possible to make their digital cable 75 ohm.

In any case, it looks like it might be a good idea to move away from RCA's when I upgrade my DAC... not going to happen for a while, but good to keep in mind.

 

[ooheadsoo]

It's a rather stupid contradiction. The SPDIF format calls for 75 ohm RCA, which literally does not exist! BNC is actually for the AES3id format, which is also 75 ohms but the data headers are different from SPDIF. It works better than RCA though because its impedance is as advertised. That's messed up. AES/EBU should always be 110ohm XLR. Man, are things messed up in the low end consumer and pro world or what? The SPDIF format itself is FUBAR since RCA isn't 75 ohm by definition. BNC is the only way.

That's why I've been considering optical out of my emu1212m and to hell with coax

 

[Prune]

ooheadsoo, BNC and F connectors are not the only 75 ohm ones. Some MHV and SHV coax HV connectors are also 75 ohm, and I'm sure there are others.

philodox, an RCA jack is the least of your worries. You are probably getting more of an impedance mismatch for not having an ultra-precie input impedance on the receiving side (and output impedance on the transmitting side). 75 ohm coax (the cheap cable used for cable TV) with the Canare RCA is good enough, and much better than the hugely varying impedance of some braided cable (though depending on equipment and distance it may not make an easily noticeable difference). If you are making your own DAC, you can do as I did and add BNC jacks so you can at least have proper termination on one side. However, other things like transformer coupling on at least one side makes more of a difference. You can go to extreme and use a quality Belden coax and add a second shield grounded at the transmitting end, as per a well known Jon Risch recipe.

As has been discussed at diyaudio by user Jocko Homo, who has worked with optical systems in industry, the standard optical Toslink (polymer fiber and non-laser transmitter) used for S/PDIF is worse than coax because the optical receivers internally are very jittery, and create much more jitter than a non-75 ohm RCA plug would give you. Some very high end transports and DACs use an AT&T optical transmission system (glass fiber and laser) which may compare well to coax, but it would be expensive to DIY as the parts aren't cheap.

AES3 (AES/EBU is an old name) doesn't even deserve a mention since the 110 ohm balanced connection is so lossy it's useless for long cables. Most pro setups using that interface end up having to use impedance matching transformers, 110-75 on transmitting side and 75-110 on receiving, so coaxial cable can be used, which is much lower loss. For example, quoting from here,
"Their use also allows for longer cable runs via unbalanced coaxial lines rather than twisted pair cables that present high attenuation values at the relevant frequencies."

 

[ooheadsoo]

Quote:

ooheadsoo, BNC and F connectors are not the only 75 ohm ones. Some MHV and SHV coax HV connectors are also 75 ohm, and I'm sure there are others.

Of course that may be true, but BNC is the audio alternative. Who uses mhv or shv on their source?

 

[Prune]

Who uses F connectors in audio? Yet he mentioned BNC and F together. I was merely correcting the statement that BNC and F are the only 75 Ohm connectors.