Re: Mains conditioners and power cables


"Roderick Stewart" <rjfs@xxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:VA.00000b7b.008d7fa2@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> In article <4da0274e98dave@xxxxxxxxxxxxxxx>, Dave Plowman (News) wrote:
>> > > No - the induced signals are not in opposite directions in the two
>> > > halves of the circuit - they are in the same direction.[...]
>> > How can two currents oppose each other if they run in the same
>> > direction?
>>
>> > I suppose it depends on what frame of reference you adopt when
>> > referring
>> > to "direction", i.e. physical direction in space, or the direction of
>> > currents in a circuit.[...]
>> Much easier to consider a sine wave.
>
> Thanks for all the explanations of how signals can either add or cancel
> out,
> but this isn't the source of the misunderstanding. It's about currents in
> circuits and the question of how we define the direction of flow.
>
> I've known about balanced signal circuits, and had practical experience of
> their use, for the best part of forty years. The first ones I encountered
> used
> audio transformers, and maybe some of them still do, though it is possible
> to
> emulate the behaviour of a balancing transformer using active
> semiconductor
> circuitry.
>
> A transformer-balanced audio circuit will work with only the two signal
> wires,
> and though it may benefit from screening in areas of strong interference,
> it
> does not need to use the screen as an earth return. (In fact, other
> problems
> can sometimes be caused if it does, and it is not uncommon to connect the
> screen only at one end).
>
> If source and load both use transformers and the only connections between
> them
> are the two signal wires, then that is the circuit, and the current flows
> around it. ONE closed circuit loop with ONE current, flowing in the SAME
> direction everywhere around it, because we are of course using the wire
> itself, including the two transformer windings, as the frame of reference
> with
> which to define direction of current flow. It would be difficult to
> explain
> the behaviour of currents in circuits in a manner that would be understood
> by
> Mr Kirchoff if we used any other convention.
>
The signal currents are not flowing in the same direction on each of the
wires. If you consider the circuit to be a loop, via your send and receive
transformers, or whatever, you could liken this to an oval racetrack.
Although all the cars are going round the track clockwise ( perhaps ),
viewed from a fixed point, cars on the two ' straights ' ( our wires ) will
be going in opposite directions. The differential speed of the cars is
eqivalent to the differential signal voltage in our balanced circuit. If we
now induce an interference signal onto the two wires ( put some cars on the
two straights , going in the SAME direction from our fixed point ie some
going clockwise and some anticlockwise ) there will be no speed differential
between them. This is the equivalent of no interference signal appearing
across the input transformer - hence no interference signal. It doesn't even
really cancel - it simply doesn't exist once it gets onto a balanced sytem,
because it only existed with RESPECT to GROUND in the first place - and as
we all know, there is no ground in a balanced system ...


> Maybe it's a consequence of being taught about balanced circuits only in
> terms
> of their modern practical implementaion that makes some people want to
> analyse
> them as if each signal wire were a separate unbalanced circuit, each with
> its
> own definition of current direction, as if they were completley unrelated
> until the two separate signals arrived at the subtracting circuits in the
> input of the load equipment. However, this leaves a question. If each
> signal
> leg is a separate circuit, how does it work with only one wire and no
> return?
> Easy to explain if each signal wire is the return wire for the other, and
> no
> problem seeing where the circuit loop is, but if they are completely
> separate,
> how do two separate voltages appear across two separate pairs of input
> terminals if there is no ground or return connection from the ground of
> the
> source equipment? Where are the two circuit loops around which the two
> currents flow?
>
> Rod.
>


.

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