Value for money, best value spdif cables ?

[Mr.Ian]

As I understand it the best route is to have one transformer and hard wire a 1.5m 75 ohm cable. Don't use connectirs

[Barry]

Whats the rhumpus on a good BNC - RCA adaptor (like the Canare) vs soldering a 75 Ohm BNC connector in to replace an RCA coax Spidif out?

In the situation where the digital out socket is an RCA phono socket, you may as well use an RCA plug on the cable end: using a converter will make very little difference.

And if going down the socket refit route, is there any additional head sting as with XLR swap outs, in needing to fit a baluns involved?

Again the use of an impedance transformer is an additional complication as it will need to pass signals of 240 MHz (for a typical bit rate of 480 Mbps).
Unless both the CDP and DAC are fitted with XLR connectors I wouldn't bother with XLR terminated leads. And if you do use an XLR terminated lead, make sure the cable has a characteristic impedance of 110 Ohm.

[Theadmans]

I remember quite a few years ago reading on here that you needed a longer cable. I have a Belden cable of about 1.5 metres on my Beresford Caiman SEG in my second system in my bedroom (sounds very good).

In my lounge system with my Creek 50CD DAC / CD player I use a short 0.5 metre Chord C-Digital - to be fair this sounds good also.

Sadly my wife prefers the aesthetics of the Creek 50A amp and Creek 50CD in the lounge...

...weirdly she rejects the Science Laboratory aesthetics of the Caiman SEG / Dorado blue LEDs (but maybe just maybe it would sound better).

At least when the wife is watching sh*t on TV I can take my Sennheiser HD650s and listen to some quality sounds in the bedroom.

[Barry]

Early CDTs and DACs were a bit sloppy in maintaining an input and output impedance of 75 Ohm. This meant that in order to prevent reflected and re-reflected signals occuring during the pulse rise time and thus confusing matters, the length of the digital cable needed to be long enough so the reflected signal was delayed sufficiently.
Given the propagation velocity of a typical RF cable is 2. 10⁸ metre/second, so for a typical 480Mbps digital signal (corresponding to 240MHz), the necessary length for a half-period at 240Hz is about 0.8metre. Thus a delay of three half wavelengths is 1.2m, which is where the suggested length of 1.5m comes from.

These days manufacturers ensure the in/out impedance is very close to 75 Ohm, so the length of the cable now no longer matters.

[Qwin]

Whats the rhumpus on a good BNC - RCA adaptor (like the Canare) vs soldering a 75 Ohm BNC connector in to replace an RCA coax Spidif out?

And if going down the socket refit route, is there any additional head sting as with XLR swap outs, in needing to fit a baluns involved?

Sorry for the late reply Gaz been on me Hols.
Don't use an RCA adaptor or adaptor lead, it loses any advantage. Stick to all RCA or all BNC.
When changing an RCA panel socket to BNC, its just a straight swap.

[Barry]

It is not necessarily "just a straight swap": it is if the phono socket is a panel-mounting design and is wired to the PCB. But these days it is usual for PCB mounted connectors to be used instead, and these are often difficult to remove. And having removed the socket a replacement BNC socket will not necessarily have the same contact footprint, so a panel mounting type will need to be used and wired directly to the PCB.

Also it must be remembered if the item is still within warranty any alteration would make the warranty null and void.

But if you can replace the phono socket for a genuine 75 Ohm BNC connector then that is as good as you can get.


Given that, you may wonder why phono sockets were specified for SPDIF, especially as the dimensions of an RCA phono connector mean its characteristic impedance can never be greater than 57.2 Ohm, and depending on the support dielectric used, is likely to be reduced to 40 Ohm.
The reason is, a mated RCA plug and socket is electrically 'short', and the two impedance mismatches: the first where the cable joins the plug, the second where the rear of the socket is terminated by a 75Ohm resistor, in effect tend to cancel each other out.

To give some numbers, let us assume the dielectric in a mated plug and socket has a physical length of 18mm, and the dielectric used is either polyethylene or PTFE.
For a digital signal rate of 480Mbps, the corresponding RF is 240MHz. The wavelength in the dielectric will be between 832 to 873mm, depending on the dielectric used, so the 18mm length corresponds to 0.02 of a wavelength.
Now for the first mismatch of 75 to 40 Ohm, the reflection coefficient is -0.3, and that for the second mismatch of 40 to 75 Ohm is +0.3. The two mismatches are separated by about 0.02 wavelengths, so the overall mismatch as seen by the cable is reduced to 0.084 (or an impedance mismatch signal reflection of - 21.5dB), which in practice is a sufficiently low enough to provide a good match.

That is why you can manage to get away with using the much cheaper RCA connector.

[Qwin]

It is not necessarily "just a straight swap": it is if the phono socket is a panel-mounting design and is wired to the PCB. But these days it is usual for PCB mounted connectors to be used instead, and these are often difficult to remove. And having removed the socket a replacement BNC socket will not necessarily have the same contact footprint, so a panel mounting type will need to be used and wired directly to the PCB.

All true Barry, but still a straight swap, no additional circuitry needed. I've replaced PCB mounted RCA's several times, either for freestanding better quality RCA's or BNC's, its usually just a soldering job, though I did have to remove the PCB from its case on the Schiit Freya+ to get better access.
On my Eversolo A6 the RCA's were already on flying leads, which you could unplug from the PCB, replace the connectors away from the unit, then plug the leads back in. I did have to enlarge the holes in the rear panel by 1mm dia. to take the BNC sockets.
So yes, there are a variety of scenarios that could be encountered, but nothing complicated.

If you can't handle that amount of complexity, you should refrain from modifying, or even taking the lid off your equipment, which in most cases is enough to invalidate the warranty.

[Barry]

Fair enough. I could do it as I have the tools required, but not everyone is willing or able to make the changes.

Agreed, a tapered reamer is needed to enlarge the hole for a panel-mounting BNC socket.

[Gazjam]

Sorry for the late reply Gaz been on me Hols.
Don't use an RCA adaptor or adaptor lead, it loses any advantage. Stick to all RCA or all BNC.
When changing an RCA panel socket to BNC, it’s heers Ken,
just a straight swap.

Cheers Ken,
hope the holibags hit the spot.