8" run in chassis before terminating SPDIF Signal... 75R Resistor Placement

[PaulHilgeman]

Should I leave the 75R resistor on the board, or is it better to move it right up to the RCA jack? and then run two strands back to the board?

-Paul Hilgeman

[dhaen]

Paul,

leave the 75 ohm as close to the driving device as possible. Then try to maintain the characterstic impedance (parallel, twisted, or screened from that point on.

Cheers,

[Fred Dieckmann]

WRONG! You have 8" of unterminated coax that will bounce reflections back. Since the RCA is also an impedance mismatch (now worse since it has 75 ohms in parallel with it) you will have reflections back in the other direction as well.

Keep the coax shield as close to the RCA jack ground as possible and terminate it at the end by the receiver. You terminate a transmission line at the end or beginning (back termination) preferably both. You don't terminate a line in the middle. Where do you guys come up with this stuff? I wish people would do some research and try to give helpful advice instead of guessing.


http://www.circuitsage.com/tline.html

http://www.mwoffice.com/products/txline.html

http://fermi.la.asu.edu/w9cf/tran/

[jwb]

Calm down smart guy. You ought to point out that both-ends termination will halve the signal level, which may cause a problem.

But of course you should run the coax as near to the receiving device as possible. dhaen's advice is just flat-out misinformation. Twisted-pair won't have the same impedance as coax, and it is unsheilded, so don't bother.

Didn't you forget <a href="http://www.sigcon.com/">your favorite link</a>?

[dhaen]

Mis-information?

Although not ideal, a twisted or parallel pair can have any impedance. Have you measured the impedance of a slow twist in some 7 strand hookup wire? It's not too far off - thank you very much.

Of course coax is the ideal. And who said anything about "unterminated"? Presumably when the cable gets connected the equipment at the other end will terminate the line. That's how transmission lines work - we have a source resistor in series with the source, and the same value across the end of the line - with cable of a suitable characteristic impedance inbetween!

So presumably you would put the resistor at the RCA?
What happens to the driving stage then? It has a cap slapped across it's output pin

[Fred Dieckmann]

Originally posted by dhaen
Mis-information?

Although not ideal, a twisted or parallel pair can have any impedance. Have you measured the impedance of a slow twist in some 7 strand hookup wire? It's not too far off - thank you very much.

Of course coax is the ideal. And who said anything about &quot;unterminated&quot;? Presumably when the cable gets connected the equipment at the other end will terminate the line. That's how transmission lines work - we have a source resistor in series with the source, and the same value across the end of the line - with cable of a suitable characteristic impedance inbetween!

So presumably you would put the resistor at the RCA?
What happens to the driving stage then? It has a cap slapped across it's output pin

I have not made myself clear I fear. If you put the resistance on the RCA jack you will have a unterminated 8" piece of line from the jack to the receiver. It will cause significant reflections. Good transports have 75 ohm output impedance to provide source termination many don't and use twisted pair wires to the jack or poorly terminated pulse transformers. The Crystal receivers don't like less than 0.5 volt signals but a competantly designed front end circuit for the DAC input can take less than half this. BTW if your input SPDIF circuit is within a couple of inches of input jack you can mount the resistor on the jack and use a pair ofwidely spaced untwisted wires to the receiver. Most twisted wires have about a 100 ohms characteristic impedance, this is a serious mismatch for 75 ohms. I can count the number of designers that do this interface right on one hand. I designed external digital interface devices for DACs for about three years. Doing this interface right gives big sonic returns.


The best book on transmission lines (yes, I have a copy):

Transmission Line Design Handbook (Artech House Microwave Library) by Brian C. Wadell

http://www.amazon.com/exec/obidos/tg...glance&s=books

[davesaudio]

whats the impedance of the RCA
I thought it was reasonably close?

[PMiczek]

The impedance of the RCA/phono connector was never 75 ohms, and assuming that 75-ohm BNC really _IS_ 75, the dielectric of the RCA connector looks nothing like it.

Better question would be, how is the typical 2-inch PCB trace 75-ohm, or a postage-size piece of ground plane that the coupling transformer sits on, or that hairpin shape trace in one of my old "digital-ready" components, etc...

...when that expensive interconnect cable makes contact with the chassis, I suspect you may as well kiss 75-ohm impedance good-bye.

[Hisatugo]

interesting discussion. I see many people talking the same thing by diferent ways. Since I do understand a small part of the electromagnetic theory, I also do my most confident audition about the electronics I have. What I can say is: even with all that traces, mismaches, terminations and etc.... there always exist something to do to get the equipment better. If you change something and you can really, honestly hear that change and it made the sound better, you are in the right way. If you can not hear the change, consider the undo possibility. Anyway, you can get wrong things or you can do work simply useless.
"There was a man that spent 20K us dollars to get a high end stereo system like his neighbor. But this man had a serious illness in the ears and a loss of 30dB SPL at right ear at 4Khz, 12 Khz and total loss above 14Khz both sides. But the pleasure of spent so much money conviced him that could hear details, ambience, etc... but in fact, he could not."
So, please guys, don't fight for nothing, the final statement for equipment in audio is music. Let's hear music, as real as it can be and not forget that.
Best regards to all.

[Jocko Homo]

I missed another rock throwing contest............darn.....

Anyway......no you do not want to run 8" of mystery wire from the connector to the PCB.

But if it is an RCA jack to start with, and no attention to detail has been paid further upstream..................

Yeah, it will work, and sound lousy. Passing data is one thing, getting it right requires careful attention to detail.

On both ends.

Of course, some people think that the Bose Wave Radio sounds good, so to each his own.

Jocko