Balanced interconnect with 2 x 75 ohm coax cables?

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#41
judderod said:
Sorry about that, I made a couple of confusing mistakes in that post. I meant to say connectors and wire but I had found some affordable solid silver twisted pair, so now the only issue is the lack of affordable solid silver XLR connectors and sockets.


Regarding silver oxide, if it's really a concern that the metals on both connector and socket will oxidise over time and drop in conductivity, you could coat the whole lot in silicone grease before connecting to prevent oxidisation over most if not all of the contact surfaces.
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Silver plating on XLR connectors is common, but gold is preferred for low power connections like audio.

Dielectric grease is the preferred material, which may be a type of silicone grease.
 
#42
judderod said:
I assume you're suggesting that screens aren't as bad as I fear, because screens do in fact work very well for RF. Do you have any reference to show how RF over coax compares to over RF over twisted pair shielded to substantiate this?
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No. RF generally uses coax, and any serious non-linearity would severely affect intermodulation performance. I can't remember ever seeing a paper or article on cable IM, except for connector corrosion ('rusty bolt' effect). Twisted pair is rarely used for RF, except for short runs within equipment.
 
#43
judderod said:
Perhaps I've misinterpreted the picture. To me that looks like at the balanced outputs and inputs, consisiting of 3 connections (+, -, and G): G, at both ends is connected to chassis ground and shield.

Does it in fact show (as the only reference to signal ground is on the input side) that the signal ground for the output is deliberately omitted and hence left completely unconnected?
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The connection marked "G" is really not a ground connection, it is a shield connection. It needs to be connected to the chassis at the send end, it may or may not be connected to the chassis at the receive end. Some suggest using a small capacitor at the receive end in series to the chassis. This shield is not part of the audio circuit, although the audio circuit ground is probably connected to the chassis also.
 
#46
Chris Hornbeck said:
The black stuff on silver connectors isn't silver oxide, it's silver sulphate or sulphite. Can't remember which.

Thanks,
Chris
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Right, and only one of those possible forms is electrically conductive:

"Three forms are known: monoclinic acanthite, stable below 500 °C, body centered cubic so-called argentite, stable above 176 °C, and a high temperature face-centred cubic form stable above 586 °C.[3] The higher temperature forms are electrical conductors.[3] It is found in nature as relatively low temperature mineral acanthite. The name argentite refers to a cubic form, which, due to instability in "normal" temperatures, is found in form of the pseudomorphosis of acanthite after argentite. Acanthite is an important ore of silver. Ag2S is used as a photosensitizer in photography."
 
#48
+1.

Where on earth does the sulphur come from? I don't know of any compound of it that regularly just floats around.

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Could someone plz confirm Signal Ground at the source side of a balanced collection should be left disconnected according to Rane's diagram?
 
#53
judderod said:
Perhaps I've misinterpreted the picture. To me that looks like at the balanced outputs and inputs, consisiting of 3 connections (+, -, and G): G, at both ends is connected to chassis ground and shield.

Does it in fact show (as the only reference to signal ground is on the input side) that the signal ground for the output is deliberately omitted and hence left completely unconnected?
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Sorry for the late reply.
Here's the picture again:
An externally hosted image should be here but it was not working when we last tested it.

As you can see the shield is connected to the chassis at both ends, so now the groundloop currents can not influence the audio signal.
But in order to have maximum input and output levels without clipping, the signal ground must be right in the middle of the power supply voltage. Iaw it has to be 0V.
You can do this by connecting chassis ground to signal ground. This can only happen in 1 place because if it's connected in 2 places, a groundloop affects the audio signal. The picture shows this smack in the bottom right corner of the picture.
 
#57
That why surface finish on PCBs is so critical, and why ENIG is now probably the worlds number 1 surface finish for PCB's. Immesrsion silver while providing the superior solder joint termination in terms of resistance (and here this is only critical in very very high speed RF, way out of the audio spectrum before it becomes a problem) makes boards very problematic to handle, and any deviation from strict handling procedures can cause the surface to go off and the boards are unsolderable.
Any connection that is open to the atmosphere and is not soldered is open to the elements and likely to fail depending on the conditions at some time, that why mil and areospace connectors are hermeticaly sealed. We have tried a variety of connection options with a number of connector protection greases etc through rigerous testing, one of the best is Hypertac. Of couse with audio systems the envoironment isn't that harsh (one would hope) in a normal house, and disconnecting the connectors and cleaning with ISO does the job.

Great paper.
I miss reading scientific journal articles as part of my job
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Sometimes I wonder how I manage to do my job (PCB design) with all the papers etc coming out, but over the last several years it is a discipline that is changing in both complexity of signals (super fast rise times) and complexity of bare board fabrication and component assembly🙂
 
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