regarding shielding


My question...some interconnects have the shielding only connected to one end of the cable ( often the pre amp end) .WHY? If both ends are not connected then there would be no circuit, no current flow and so would there be any effect at all?
Is the idea not to send any generated current from interference to ground so it does not interfere with the signal?
pl_guy

Showing 4 responses by nsgarch

You are referring to a "floating shield" (one end 'floats' -- is not connected.) This is exactly to PREVENT any curent flow, which would result in hum and RFI being mixed with the audio signal (and thus winding up in your speakers!)

Having the shield connected at one end only does not prevent intercepted hum and RFI from draining to ground at the connected end.

In the old days ;~) single ended interconnects contained only one (center) conductor (+) and the shield carried the (-) signal, so it had to be connected at both ends. Then Bruce Brisson (owner of MIT cables) invented the "shotgun" single-ended interconnect while working for MonsterCable, which is what put MonsterCable on the map! It is called "shotgun" because it contains TWO signal conductors (double-barreled shotgun ;~) surrounded by a shield -- but since the shield no longer has to carry the audio signal, it can be 'grounded' at just one end, and can do its shielding job even more effectively.

Mr. Brisson provided these shotgun cables with ARROWS, which point to the end of the cable where the shield is connected to ground (no, the arrows have NOTHING TO DO WITH SIGNAL FLOW!) He did this specifically so that all interconnects could be installed with the grounded end of their shields connected to the preamp (yes, EVEN THE ONES BETWEEN THE PREAMP AND THE AMP!) That is because such a configuration is what's called 'Star Grounding' -- with the preamp at the center of the 'star'.

Star Grounding provides the quietest system; and in order to prevent ground loops, the preamp should be the ONLY component actually grounded to the wall. In the old days ;~) we generally put 'cheater plugs' on the (three prong) power cords of all other equipment, to prevent additional grounding points (which will always produce ground loops.)

Unfortunately, installing 'cheater plugs' can produce a, shock hazard if you are working on your system and remove the interconnects (and thus the ground connection) from a piece of equipment that has a cheater plug on it (for example: if you were adjusting the bias on a tube amp, which requires removing the input interconnects ;~) So be careful! Much of today's equipment (especially amplifiers) have switches on the back that allow you to 'float' (disconnect) that piece of equipment's audio circuits from ground, and thus eleminating the need for cheater plugs (the equipment's chassis is still grounded) and preventing you from accidentally electrocuting yourself!
mt10425, could you provide a little more detail:
What is the construction (or brand) of your (single-ended?) interconnects? Just because cables are unshielded doesn't necessarily mean they will be noisy if they don't pass near power cords (or equipment transformers) and if there is no strong RFI coming from your system (usually digital gear) or from fluorescent lights or dimmers in your home, or broadcasting stations in your area.

What is a c-7 power cord? Is that a brand?

Dedicated circuits are good, but you still have to observe the same grounding techniques as with any wall citcuit.

thx
Hi Mt10425:

I checked and found that the Music Meter cables are constructed of TWO wires in a "twisted pair" configuration. This works well for rejecting RFI (with no shield) but not so much for 60Hz hum (although hum is more easily controlled by careful cable placement.) Twisting the + and - conductors together though, creates other issues that might negatively affect performance (or not) depending on the cables' application.

The Vinyl Junkie phonoamp is actually solid state for the first gain stage (first and second gain stages if you're using a MC cartridge.) That's where tube hiss would happen if tubes were used there.

The unit's power supply is a (transformerless) "switching" power supply providing high voltage to the tubes, and the 12V DC is provided by a wall wart transformer. Neither of those devices require a third ground conductor, as would be required for a more conventional power supply which is often housed in a separate chassis away from the sensitive phonoamp circuits.
Pl_guy, you would connect the red to the RCA center pin (as red is customarily +) and the white to the ring (body) of the WBT RCAconnector. The shield braiding can be separated at one end, and about half the wires trimmed back to where the outer jacket ends -- the other half you twist into a little (uninsulated) conductor and solder one end to the body of the RCA connector (or you can do it along with the white conductor when you solder the white conductor to the RCA connector body.) You only need to do this (with the shield) on one end. If the outer jacket has arrows, then you should connect the shield this way on the end of the cable that the arrows will point to. If no arrows on the outer jacket, I use a piece of green heatshrink denoting 'ground' (along with the red or black heatshrink for channel colors) on the end where the shield is connected to ground.

On the other end, only connect the red and white signal conductors. Then after carefully separating the shield wires, spread them out and using a fine scissors or cutter, carefully trim all the shield wires back to where the outer jacket stops. If you plan ahead ;~) you can have a piece of heatshrink waiting 'up-cable' to slide down and shrink over the place where you trimmed the braid -- just to make sure the braid doesn't touch the (ground) barrel of the RCA connector ;~)