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Re: In a grounded metal chassis, no shielding is needed for TWISTED pair.

>>A simple input transformer constitutes a differential input provided neither end of the primary is grounded.<<

I thought I went over this already. But I will do it again and expound on it further.

There is no differential input if there is no center tapped ground reference. If the primary has two terminals only, it is only differential if the source feeding it is referenced to ground at a center point between the two transmission wires, and that has to be somewhere in the circuit. If ground isn't defined, it is neither single ended nor differential. In fact, it would be a lousy circuit subject to electrostatic noise all over the place. You could have 30 KV static build-up between the floating transmission wires and the secondary that it tries to feed. Not real good. There will be arcing noises on occasion.

The circuit defines whether or not it is single-ended or differential, it is not the fact that the transformer just floats and is two-terminal that makes it differential. With it floating, there is no reference and there is no determination of the type of circuit it is.

A phono cartridge output will be differential or single-ended defined ONLY by the circuit it feeds. Otherwise it is undefined and another bad circuit.

And you can use twisted pairs on unbalanced cables for lowering noise. I do it all over my system. HOWEVER, if the impedance of the circuit is too high at BOTH ends of the transmission line, like between a volume control and a triode grid, the poor shielding properties of the unshielded, unbalanced, twisted pair will show up as a bad design. There will probably be needed some extra shielding to reduce the noise to an acceptable level. (People all the time wonder why the volume control in the middle is causing their amp to hum when it does not at min and max settings.) If the impedance is low, like even between a low output MC cartridge and the phono stage input, a twisted pair can be very low noise because it will then be close to acting "differential" even when it is not. Why the difference?

It's because in a low impedance circuit both wires of the twisted pair are not far from ground potential, an approximation to the condition for a good balanced twisted pair. In the high impedance circuit, one side is at ground potential and the other is floating way away from it. This is where the need for shielding that twisted pair comes from, or just going to coax.

In my system, with a 0.5 mV low output MC cartridge, I have a simple well-twisted twisted pair from the cartridge to the SUT. You can grab it, move it around, put it anywhere and it doesn't pick up audible hum (but to make this work well like this the gauge has to be small and have many tight turns per inch). And it is in an UNbalanced circuit. HOWEVER, the MC pickup has only 4 ohms resistance across it and so the "center RCA pin" is only 4 ohms from the outer pin, which is at ground. It's therefore almost as good as a differential input, but it's not differential. If the resistance were down to 0 ohms, obviously a short, then you might as well call it differential after all.

Kurt

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