Calling all digital cable experts - SPDIF termination question

[HighFidelityGuy]

Hi, sorry if this has been discussed before.
I've been thinking about digital coax cables today and in particular the argument of proper 75 Ohm termination.

To set the scene, the basic argument is that for the best SPDIF signal transfer, all the plugs, sockets and cables in the signal path should be true 75 Ohm to make sure that reflections in the cable are minimal. I hear that most phono plugs and sockets are not true 75 Ohm and even the ones that claim to be are not likely to actually be true 75 Ohms. To get true 75 Ohms I hear you need to use BNC connectors.

So, if BNC connectors are a fairly certain way of making the best possible connection, then the most productive upgrade to SPDIF equipment would not be buying expensive SPDIF cables with expensive phono connectors, it would be by swapping the phono sockets for BNC and getting a BNC to BNC coax cable. Surely this would be cheaper and better in the long run as it would provide better signal transfer?

I then started to think outside the box a bit. When thinking about this from an electrical conductivity point of view, putting any socket and plug connection in the signal path will degrade the signal slightly. Some are better than others. The best ones are usually expensive. So removing the phono or BNC connection out of the signal path all together would surely be better and cheaper.

So my main question is this; Would de-soldering the sockets from the source and receiver and directly soldering a good quality true 75 Ohm coax cable to the PCB at either end be the best solution or would this not provide a true 75 Ohm connection due to some reason I'm not aware of?
Obviously the results wouldn't be very practical due to the permanent connection but we'll ignore that for now.

I'd also tike to understand how much difference in sound quality there is likely to be between using a Phono connection, BNC, or a directly soldered connection.

I'm hoping to get some solid scientific facts about this to help me understand what's actually best. Hopefully this won't turn into a big argument like I know it would on other forums.

Thanks.

[NRG]

OK I'll chip in my 2pence worth...in answer to your first question, yes, for sure.

The answer to your second question may not be so firm, I'm sure STan or Dave can shed some insight here but without being able to see what the effect would be of a direct cable to PCB track solder connection via a TDR (Time Domain Reflectometer) I think the result would be very hit or miss.

For me the inconvenience of a direct end to end connection would outweigh any benefit...

[Tony Moore]

Well I suppose it depends on how the current phono connectors are wired to the transmitter and receiver at the two ends. If they are pcb mounted and have lengths of pcb track snaking around the place, how can that have similar characteristics to the cable? Is it too short to matter? Are the termination components critically chosen to encompass the effects of the pcb tracks to the socket? (and the socket itself maybe?)

Or do most implementations simply ignore the effects of the link to the socket?

[HighFidelityGuy]

All of the SPDIF gear I own have the phono sockets soldered directly onto the PCB. I'm not sure on track length after that. I'm also not sure whether the length and composition of the conductor inside the socket is factored into the design. I guess this may vary from product to product. From a simple conductor length point of view, I would have thought directly soldering the cable to the PCB would result in a similar conductor length as the cable would need to be slightly longer to get inside the case and this would balance out the loss of conductor from removing the socket. However, the loss of the socket would no doubt reduce the resistance of the circuit despite the extra cable length. I don't know whether that could have a negative effect on the signal.

[electric beach]

Hi Dave,

Try Mike Homar's digital cable, then say if you can even imagine needing anything better! I've used a Trichord Research Pulsewire with BNC connectors into my MF DAC21 before but it's never sounded anything like this!

(Sorry Mike, just one more...)

[HighFidelityGuy]

Thanks, I've read a bit about Mike's cables and from what various people have said they seem like an excellent product. I'd be interested to find out if they sound better with the phono plugs he uses or BNC plugs. It would be rather difficult to do a comparison though. From a scientific point of view any phono plug should be inferior to a BNC in this application so it's difficult to imagine that the cable would sound better with phono plugs. But audio is an odd beast and things that should improve the sound quality don't always and can sometimes make it worse. I'm just trying to find out what approach would be best from a scientific point of view to improve my understanding. I'll then have to decide if it's worthwhile actually trying any different methods.

I'd be interested to get Mikes input on this subject.

[The Grand Wazoo]

From a scientific point of view any phono plug should be inferior to a BNC in this application so it's difficult to imagine that the cable would sound better with phono plugs.

I'd suggest that although we all use them, any phono plug is inferior to almost any connection you can make short of a quick twist of two wires & a bit of insulating tape!

You can put all the locking devices, exotic metallurgy, solder & crimping you like into them, but you're just polishing the turd of a fundamentally bad design.

[alb]

I would use BNC connectors. I could hear a difference when changing from phonos.
Also, be aware that not all BNC connectors are 75 ohm. No point in using 50 ohm connectors.
Keep the cable length shorter than 0.75m or longer than 1.25m.

[NRG]

Keep the cable length shorter than 0.75m or longer than 1.25m.

That only really applies / seems to work if using phono's. With 75R BNC and a good RG6 cable you can choose what length you want...within reason.

[Mike]

Or do most implementations simply ignore the effects of the link to the socket?

It wouldn't surprise me at all, TBH.

Thanks, I've read a bit about Mike's cables and from what various people have said they seem like an excellent product. I'd be interested to find out if they sound better with the phono plugs he uses or BNC plugs. It would be rather difficult to do a comparison though. From a scientific point of view any phono plug should be inferior to a BNC in this application so it's difficult to imagine that the cable would sound better with phono plugs. But audio is an odd beast and things that should improve the sound quality don't always and can sometimes make it worse. I'm just trying to find out what approach would be best from a scientific point of view to improve my understanding. I'll then have to decide if it's worthwhile actually trying any different methods.

I'd be interested to get Mikes input on this subject.

I can make those cables with BNC's, no problem. I agree wholeheartedly that they'd be a superior connector for this sort of application. The RCA phono should have quietly disappeared decades ago!

As for soldering the cables directly... no idea. I'd imagine the results could/would vary depending on the multitude of different DAC and transports kicking around. Almost impossible to predict the outcome if you ask me!

Cheers...