Coax vs. twisted pair for AC wiring?

[Rhodesplyr]

I'm nearing completion of a reproduction outboard power supply/spring reverb for a Farfisa Compact Duo organ, the unit known as the F/AR. (I bought an organ that was missing its power supply.)

The outboard unit supplies 9VAC (~300mA) through a Leslie cable-style umbilical that also carries the audio signal from the organ. Farfisa ran this 9VAC to the organ via a coaxial cable inside the umbilical. One side of the 9VAC secondary is referenced to ground inside the power supply, and this side is connected to the coaxial cable shield. Thus, the shield does carry AC current, being the ground-return path.

What I'm wondering is whether, theoretically, it would be better to send 60Hz AC through an umbilical like this via a twisted pair or a coaxial cable. 'Better' would be defined as having less tendency to couple 60Hz noise into the audio signal.

I suppose the best method would be a twisted pair with a non-current carrying shield around the pair, but I'm not sure that's practical in terms of the thickness of the umbilical.

I'm more interested in seeing how various forum readers would approach the problem :-)

[Enzo]

What carries the audio? if it is a shielded cable itself, then i wouldn;t worry that their coax worked less well than something else.

[Steve Conner]

Since the shield is referenced to ground, the coax is better.

Reasoning: The grounded shield stops electrostatic (capacitive) coupling between the core and anything else in the loom.

And there's no magnetic coupling because the currents in the core and shield are equal and opposite, and the conductors are concentric, so their fields cancel.

If the 9V were centre tapped to ground, then a twisted pair would be better, because it would have more chance of cancelling the capacitive coupling. However, the twisted pair doesn't cancel magnetic coupling nearly so well.

In practice, if the AC were able to find any other ground return paths besides the shield of the coax carrying it, this would be the major source of trouble. Farfisa presumably handled this properly inside the accordion.

[Jag]

There is nothing proper about an accordian!

[Rhodesplyr]

And there's no magnetic coupling because the currents in the core and shield are equal and opposite, and the conductors are concentric, so their fields cancel.

In practice, if the AC were able to find any other ground return paths besides the shield of the coax carrying it, this would be the major source of trouble. Farfisa presumably handled this properly inside the accordion.

Thanks for the thorough explanation, Steve. I think the point on which I was uncertain was the extent to which the core and shield magnetic fields would cancel each other since the two conductors don't have the same physical dimensions, but, from what you say, I gather that, in a case like this, the shield behaves magnetically as if it were another core with equal and opposite currents. Or, at least, it's close enough to provide the best practical solution.

As you predicted, the ground path associated with the 9VAC supply is kept separate from the DC supply ground inside the organ. If you're wondering why it has BOTH an AC and DC supply, I think it has to do with the current capacity of early solid state diodes and the problem of providing a reasonable degree of regulation for the organ's oscillators and transistor audio circuits. With modern high-current rectifiers and easily available voltage regulator ICs, no one would design a power supply like this today. The original actually uses a selenium bridge for the DC supply. The AC supply is mostly for bulbs used in the opto-coupled volume pedal and effects.

And the Compact Duo isn't an accordion! It's what Rick Wright played in Pink Floyd songs like "See Emily Play." See one in action here:

[ame="http://www.youtube.com/watch?v=ww87Thd4YdA"]YouTube- David Gilmour (Royal Albert Hall) - Echoes Part 3[/ame]

[Old Tele man]

There is nothing proper about an accordian!

...samething is often said about bag pipes!

[Enzo]

I am getting nae sustain in me bagpipes.

[Rhodesplyr]

What carries the audio? if it is a shielded cable itself, then i wouldn;t worry that their coax worked less well than something else.

Enzo, the audio is sent via its own shielded cable.

A interesting detail is that Farfisa used a 7-pin Amphenol plug for the umbilical even though it only uses 6 of the contacts. I suspect that they were trying to discourage an adventurous end-user from trying to plug it into a standard 6-pin Leslie socket and causing fireworks.

[Steve Conner]

Thanks for the thorough explanation, Steve. I think the point on which I was uncertain was the extent to which the core and shield magnetic fields would cancel each other since the two conductors don't have the same physical dimensions

As far as I know, if one of the conductors is hollow and the other one is inside it, then the fields always cancel perfectly outside, no matter what the shapes and sizes of the actual conductors. I dimly remember some proof of this that involved a lot of integral signs. But could be mistaken.

Enzo, you need the Les McPaul bagpipes. You can blow them full of air, go and have your lunch, and when you get back they'll still be droning, unless someone has shot them to stop the noise.

[Rhodesplyr]

As far as I know, if one of the conductors is hollow and the other one is inside it, then the fields always cancel perfectly outside, no matter what the shapes and sizes of the actual conductors. I dimly remember some proof of this that involved a lot of integral signs. But could be mistaken.

Steve,

I think I remember the same thing, but it's been a long time since third semester calculus. Off the top of my head, I'd wager that it has to do with a lower current density in the outer, hollow conductor, thus generating a smaller field relative to its size, but equal and opposite to the center conductor.

Time to give a listen to the CD of Arvo Pärt's "Te Deum"?

[J M Fahey]

I am getting nae sustain in me bagpipes.

Easy, replace all yer caps for paper-oil ones, all yer resistors for carbon comp, all yer tubes (sorry, VALVES) for 1945 vintage metallic case Tung-Sols, all solder for non-ROHS compliant leaded one, and you will see Heaven. (At least, some believe so and others profit from that)
Using the correct clan kilt patterned cloth covered wire will be even better.
Of course, a casket or two of 15 years old Glenlivet will replace all of the above, and then some.
Sustain? .... What sustain???? Stop babbling and fill me glass again.

[jrfrond]

Enzo, you need the Les McPaul bagpipes. You can blow them full of air, go and have your lunch, and when you get back they'll still be droning, unless someone has shot them to stop the noise.

Now THAT is friggin' FUNNY, especially coming from a Scotsman like yourself!

For the record, it's the ONLY instrument I really hate, but then again, I've mostly heard them played badly.