[yo2tup]

I'd like to switch out the cheap RCA input in my dac for a 75ohm BNC jank.  what hook up wire should i use to maintain 75ohm thoughout?

[Occam]

Ideally, your 75ohm bnc's internal termination should be to crimped on small diameter, flexible 75ohm coax, such as RG179 (.159"diam) or RG187 (.103"diam). In other words, really small diameter 75ohm coax, that would minimize potential impedance discontinuities and leakage of RFI within your DAC/Transport...  But gaining access to the proper crimping tools for the specific bnc connector and coax can be problematic.
Hopefully, GBB (Gary) or someone else, with current practical experience http comment.

[speedcenter]

I'd like to switch out the cheap RCA input in my dac for a 75ohm BNC jank.  what hook up wire should i use to maintain 75ohm thoughout?

if possible, use 75 ohm coax from the jack straight to the receiver chip input (or transformer) on the PCB. Tough to do with the usually stiff 75 ohm cable, but that's really the way to do it right.   Changing out the RCA to BNC is much more important though, especially if you only have a few inches of wiring to do inside the unit.


Peter

[GBB]

Hopefully, GBB (Gary) or someone else, with current practical experience http comment.

I agree with everything that Paul says.  I've played around a lot with different digital cables and connectors and have concluded that 75 ohm BNC connections are the only way to go.  The typical BNC connector is actually 50ohm characteristic impedance, so you need to look and make sure you are buying 75 ohm connectors.  When modifying different DACs and squeezeboxes, I've used 75 ohm connectors from Canare, Trompeter, and Amp.  The ones from Amp are available through Digikey and seem to work well.  The BNC jacks are mostly solder connections.  If I'm being very thorough then I use miniature 75ohm coax for the internal wiring - my favorite being RG187.  I have to confess that when the runs are very short then I sometimes just use twisted pair wiring internally in the DAC.  But I only do this when the run from the BNC jack to the receiver chip or transformer is less than a couple of inches.  Longer than that and I use coax.
And while you're inside your DAC or CD player you should check that the input and output actually use 75 ohms impedance.  You'd be surprised how many players don't do this right.  And you might think about adding a digital transformer - I've found that it is a nice addition.

I also use RG187 as my digital cable and have experimented with different BNC plugs.  I found that the crimp on BNC plugs sounded best.  You can buy a decent crimp tool on ebay for $25 or so and I recommend that investment since you can make cables for all your friends and get really popular  .
Two places I've bought connectors from are Markertek and L-com.  Both have good websites and ship quickly.

The crimped on BNC connectors are much better than RCA plugs and I've found that a crimped on BNC connector + BNC-RCA adapter is actually better than a soldered on RCA plug.  But the best is to get rid of the RCA connectors entirely.

---Gary

[Occam]

Gary,

As a DIYer with disregard for commercial practicality, your comments have left me pondering to what to do. I'm getting a new dac, largely as a result of listening to your, far better dacs. The transport will, for the time being, be that modded, borrowed transport. (LoanWolf is a Saint!, but I don't think he wants me tearing up his transport) I've already sussed out what initial component mods I'll be doing. But upon reflection, I realized I'm unsure as to the charateristics/measures I should be attempting to optimize on the digital connections.
My initial plan is to remove the extant, non digital specific pc mount rca that serves as the SPDIF input, and hardwire 6' RG187 to the DAC's board and terminate with a 75 ohm charateristic impedance. On the other end outside the dac, crimp to an appropriate 75ohm BNC. My thinking is my major discontinuity is going to now be at the source side. I realize I'm thinking voltage, as opposed to power delivery, but thats a very rf, rather than digital perspective. On which end do I want to lump any potential discontiuities and why? If I add a galvanic isolation via a transformer on the to be chosen digital source side, what are the benefits of also doing so on the (load) dac side? Am I planning on doing this arse backwards?

TIA,
Paul

[yo2tup]

Wow lots of helpful response, thanks!

I'm a DIY newbie and I a few questions, so bare with me!    

1.   Right now I'm using a Stereovox HDVX.  I wouldn't mind hardwiring the cable directly to the dac board, but I dont really want to cut up an HDVX.  But I wouldn't mind doing it with the RG187 you guys recommend.  If the RG187 sounds just as good or better than the hdvx, I'll switch over to that.   Has anyone compared the RG187 cable with the hdvx?  

2.   If I were to do this, what happens to  ground connection?  The stock rca's looks to have a postive and ground connected the board.  Doesn't the coax only have a signal wire, or do you have to add a ground?

3.  As far as connecting it directly to the reciever chip.    I'm following the trace where the rca jack connectsm and there looks to be a resistor (?) before the rca jack, then another a capacitor or resistor (?)  after it before going into the reciever chip.  You can see it in the pictures below.  Where would be the best place to wire either a BNC jack or cable?  

4.  Also, could i just leave the rca jack where it is and wire the bnc jack from the bottom of the pcb?  I won't be using the RCA jack of course, but i'm just wondering if I could leave it there in addition to the bnc.  Or will that mess up the impedance or dirty up the signal?


Here's the bottom of the dac board, you can see where the rca digital in connects and follow the traces to the receiver chip... i think?



Here's the top of the board:



Can you guys make any recommendations to me?  Thanks!

[viggen]

I rewired my Nixon Dackit with 47 Labs OTA wire with excellent results.

[audioengr]

I have been doing this for 30 years, so I'll share some knowledge.

First, the internal 75 ohm transmission-line can easily be a twisted-pair of 26 gauge with TFE insulation, such as a pair from a CAT5e cable.  This is very close to 75 ohms actually.  If the line is short, the twisted-pair has very little loss.  The mini-coax that is crimped to the connector on the inside of the chassis is better, but not much.  It is usually not possible to get an isolated ground on the coax connector.

The destination of the pair should be right at the pins of the receiver chip, such as the CS8414 for instance.  The ground of the pair should go to either the chip ground pin or the negative input, depending on the chip.  The negative of the line should not be tied to the chassis, it should be isolated.  The 75 ohm termination resistor should be across the positive and negative or positive and ground pins of the chip, using a SMT 0603 chip resistor if possible.  All other traces from the positive and negative input pins of the chip should be cut so there are no "stubs" in the line.  There is usually either a transformer or a capacitor or both in the line.  The series capacitor should be replaced with a low internal resistance .1 uFd cap, with low loss, such as a polystyrene or polypropylene.  The transformer should also be replaced with a high-quality, low-loss version.  If a transformer is used, the capacitor is usually not necessary.  It is only there to prevent saturation of the transformer if there is DC on the line, which should never happen if there is a transformer or series cap at the driving end.

Steve N.

[GBB]

My initial plan is to remove the extant, non digital specific pc mount rca that serves as the SPDIF input, and hardwire 6' RG187 to the DAC's board and terminate with a 75 ohm charateristic impedance. On the other end outside the dac, crimp to an appropriate 75ohm BNC. My thinking is my major discontinuity is going to now be at the source side. I realize I'm thinking voltage, as opposed to power delivery, but thats a very rf, rather than digital perspective. On which end do I want to lump any potential discontiuities and why? If I add a galvanic isolation via a transformer on the to be chosen digital source side, what are the benefits of also doing so on the (load) dac side?

Paul,
You're thinking is reasonably good.  The frequencies of the spdif digital signal are RF so you want to treat the signal chain like you'd treat any RF system.  You get best power transfer and minimal reflection when all impedances are matched.  The output impedance of the transport should be 75 ohms, the connectors should be 75 ohms, the cables should be 75 ohms, and the dac input impedance should be 75 ohms.

Regarding where's the best place to put a digital transformer, I've heard people suggest that the right place is in the transport but I'm not sure I can argue convincingly that one place is better than the other.  Since as a DIYer I can overkill everything, I tend to use transformers in both the transport and the dac.

On the dac side, I actually like 1:2 step up transformers and typically use the Scientific Conversion 944-05.  A 1:2 transformer converts impedance with a 1:4 ratio, so you need to terminate the transformer with 300 ohms.  I've found that a balanced termination works best as shown here:



---Gary
p.s.  let me know if you need some RG187

[GBB]

1.   Right now I'm using a Stereovox HDVX.   Has anyone compared the RG187 cable with the hdvx?  

2. Doesn't the coax only have a signal wire, or do you have to add a ground?

3. Where would be the best place to wire either a BNC jack or cable?

4. Also, could i just leave the rca jack where it is and wire the bnc jack from the bottom of the pcb? I won't be using the RCA jack of course, but i'm just wondering if I could leave it there in addition to the bnc. Or will that mess up the impedance or dirty up the signal?

1.  I don't know anything about Stereovox HDVX so I can't comment on its quality.  I have found that when properly terminated, RG187 is very good.

2.  Coax has a signal wire and shield which is used as ground.

3.  If you are not using a transformer then the connection to the CS8414 receiver chip should look as follows:



If you can find the capacitors shown in the diagram then wire directly from them to the BNC jack.  Or you can put new caps right on the BNC jack and then wire directly to the RXP and RXN pins.

4. I'd recommend removing the RCA jack since its characteristic impedance will mess things up.

---Gary

[audioengr]

Paul,
You're thinking is reasonably good.  The frequencies of the spdif digital signal are RF so you want to treat the signal chain like you'd treat any RF system.  You get best power transfer and minimal reflection when all impedances are matched.  The output impedance of the transport should be 75 ohms, the connectors should be 75 ohms, the cables should be 75 ohms, and the dac input impedance should be 75 ohms.

Regarding where's the best place to put a digital transformer, I've heard people suggest t ...

What are the 10-22 pF caps for?

Steve N.

[GBB]

What are the 10-22 pF caps for?

Steve N.

Steve,
These were first suggested in some interesting articles on usenet a long time ago by an engineer by the name of Pete Goudreau.  According to his measurements of the Scientific Conversion 944-05, they help damp a slight peaking of the response of the transformer.  More importantly,  they provide a path for common mode noise that couples across the transformer due to winding capacitance.
Give them a try - you might be pleasantly surprised.

---Gary

[yo2tup]

so i did it

replaced the crappy rca with a canare BNC jack, and used 35gauge audioconsulting silver hookup wire.  wired directly to the capacitor before going into the reciever like you guys suggested.

wow!! what a difference!  all the digital hash that i never noticed before is now completely gone...if that makes anysense     totally black background and everything sounds REAL  without any hints of "digitalitis."

thanks for the help everyone.  i knew it would make a difference, but i didnt know the difference would be this significant.   i guess that little crappy rca was bottlenecking the dac and the rest of my system.

[Jocko Homo]

First off, it is not hard to find BNCs with an isolated ground that are actually 75 ohms. You just have to know where to look. (Trompeter.............)

CAT5 cable......75 ohms......? I don't think so. Don't have any one hand to measure, but I am sure it is closer to 100 ohms. Unless the insulation is around.........lemme push some buttons my calculator........around 1.5 mils thick, it is not going to be 75 ohms.

Contrary to what Scientific Conversion will tell you.............low capacitance transformers is not the way to go. They have too much leakage inductance. Only thing worse is the 2:1 versions, that have a 4:1 impedance ratio.

Actually, you can use Pete's technique on a 1:1 transformer, just remember to put 100 ohms on the primary side to make it 75 ohms. A zobel network helps out, as well.

Just get some cheap Belden mini-coax that is 75 ohms. The insulation melts rather easily, and the dimensional stability is not the best, but it will work better than a mystery twisted pair.

Yes, I have been doing this for 35 years or so, but most of that is in the world of RF and microwaves. I may have a rough idea about some of this.

You can read my many rantings on this subject on other forums, including how to make your own  TDR. Which is what you will have to have to make a really good SPDIF interface.

Really good meaning return loss >30 dB.

Jocko

[ctviggen]

TDR = Time Domain Reflectometer?  Now that would be fun.  (And, I'm not being facetious.)  We used to do something of the sort to check out network connections when I worked at Honeywell.

[Jocko Homo]

Well, it can be fun for us RF types.

You would be surprised how bad 99% of the commercial gear looks on one. Actually, as bad as a lot of it is, a simple return loss bridge is enough to see how poorly it is done.

Jocko