unshielded SPDIF digital cable?

Hi Chris,

Several questions: What are you connecting? Does the dac have a jitter reduction function, such as the Benchmark's do? How long are the cables? AES/EBU is normally 110 ohms, and uses either BNC or XLR connectors. Is the SPDIF cable connected between different connectors than the Nirvana cable, or the same connectors, and are they BNC, XLR, or RCA?

Twisted pair cable typically has an impedance in the area of 110 to 120 ohms, not very different than the AES/EBU standard. Lack of shielding, I suppose, could in some situations result in a degree of isolation (at high frquencies) between the chassis of the two components that would reduce coupling of digital noise from transport to dac.

Regards,
-- Al

Hi Al, I have a JMF Audio DXC2.2 D/A-converter with jitter reduction feature. My transport is MBL 1621. If the DIY has an impedance of 110 ohms, then it is a gross deparment from the recommended 75 ohms! That would be the same as screwing up digital data transmission. The other possibility is that I like the sound of jitter + phase anomalies!
P.S.: The connectors are WBT locking RCA's.

Chris

Wow! Nice equipment!

It's very conceivable to me that the impedance mismatch would not be significant, given that the transport appears to be redbook only (16/44.1, as opposed to 24/96 or 24/192), and assuming that your cable is approximately the 1.5 meter length that is considered optimal for transport-to-dac connections, and given the fact that the dac has a FIFO buffer to deal with jitter.

A FIFO (first-in first-out) buffer memory clocks the incoming data into a memory based on the clock that is recovered from the incoming signal, but clocks the data out of the memory with the dac's own internal clock. So jitter is totally eliminated unless the incoming waveform is so severely distorted, or jitter is so severe, that the phase-locked loop clock recovery circuit sees extraneous or missing clock edges as a result of the distortion.

Basically, the impedance mismatch results in reflection of a fraction of the incoming energy back to the source, where a fraction of the reflection will be re-reflected back to the load. Most of the re-reflection (the fraction that is not re-reflected once again) will sum together with the original incoming waveform, resulting in distortion. That distortion will only be significant if its timing is such that it arrives coincident with an edge, or if it is EXTREMELY severe such that it causes a high-level (logic 1) or a low-level (logic 0) to be seen as an edge.

A 1.5 meter length (or more) will delay the re-reflection past the original incident edge, and so it most likely won't become a problem until it is delayed to the next edge. That won't occur, in the case of redbook 16/44.1, until the cable length becomes greater than around 10 meters. See my post dated 3-23-09 in this thread:

http://forum.audiogon.com/cgi-bin/fr.pl?ddgtl&1237611683&openflup&13&4#13

I'm still not clear, though, on what you have been comparing. The transport and dac both appear to have AES/EBU and SPDIF interfaces, and xlr as well as rca connectors (as well as bnc's on the transport). Isn't your Nirvana cable a balanced AES/EBU cable, while your diy cable is unbalanced, with rca connectors, and if so doesn't your comparison encompass several variables other than the cable itself and its lack of shielding (including balanced vs. unbalanced signal paths within the two units, different impedances, different voltages, etc.)?

Regards,
-- Al

Hi Al, I compared the DIY with Nirvana AES/EBU and Atlas Opus SPDIF. Both the Nirvana and Atlas sound quite similar. The sound is clear and spacious, but rather "hifi-ish", with some emphasis on the highs. High pitched violin sounds a bit thin through these cables. The DIY brings a bit more warmth into the sound and while it is not as good in the bass department, it sounds more "musical/natural" to me, which is a remarkable thing. I've heard that some fellow audiophiles who own some $$$$ audio rig prefer the very simple 47-Labs OTA for their digital than the expensive boutique cables. Apparently the reason was not because they haven't the money to buy those expensive cables...

Chris

The 47 Labs OTA is very good. Sometimes simpler is better.

Hi Clio, could you explain a bit why in this case simpler would be better? The OTA cable (like the DIY cable) is a non- shielded and non-75 ohms cable, which is against the existing paradigm with regard to a SPDIF cable!
What are the sonic attributes of the OTA when used as a SPDIF in comparison with a "real" (shielded, 75 ohms, 1.5 m length) SPDIF cable?

Chris

IMO simpler is better in this case because of both the lack of shielding and minimal use of dielectric and other materials. As I have become more experienced with different cable designs I have come to appreciate these two attributes. However, they are system and environment dependent. For example, I used to live near a radio station and using unshielded cables on my components yielded a lot of noise. In my current situation that is no longer an issue. I have actually come to appreciate non-shielded designs for the ability to provide more openness and air to the sound.

As for SPDIF, well lets just say I'm not going to provide much of a technical explanation here simply because I'm not qualified. However, from what I've read I think we realize this isn't the best digital interface. So paradigms for me are non-essential in my choice of cable with digital.

As for testing, I made up a 1.5m OTA digital cable using the stock parts in the OTA kit and compared it to my 1.5m Creative Cable Concepts Silver Bullet cable. The differences were minimal to my ears, mainly preference. I thought the Creative Concept Cable was faster with better extension at the frequency extremes, noticeably in the bass. The OTA cable still had nice extension, just a little leaner in the bass IMO. I'm not sure what, if any of this had to do with 50 ohms vs 75 ohms or shielded vs unshielded.

FWIW - I also use unshielded PCs through out my system. I don't notice any additional noise versus my shielded PCs, especially on my digital components.