|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
216.103.104.218
I put together some phono cables basically on Chris's design, 26 ga mag wire spiraled around 3/8 backer rod. Great, low capacitance and inductance. Also great for attracting RFI, at least during the day. So I wrapped them in aluminum foil and grounded the foil to the ground lug on my preamp and the RFI problem went away. However the capacitance doubled. Next step was to put the cable inside of some 1/2ID foam pipe insulation and wrap that with Al foil. The capacitance went back down. Unfortunately now my cable is 1 1/2 inch india and not very user friendly. Questions before I start experimenting blindly,1) what is the minimum coverage to reject RFI. I have 15 inches of 33 ga Cardas unshielded externally on the tonearm and it does not seem to be a problem.
2) What is the minimum standoff distance so s to not affect the capacitance of the cables.
3) Where do I get the theory behind this to give me some guide lines.
Follow Ups:
1. This depends on the source of the RFI. Since RFI can be from many different things it will naturaly have many different profiles. Electrical length of the "recieving antenna" aka the node in your circuit that is picking up the RFI and the characteristic impedance of the node directly determine how much of a "problem" a particular RFI source will be in a circuit. The "electrical length" is proportianal to antenna gain and thus how much total energy is recieved. The node impedance will allow you to calculate what effect this particular quantity of energy will have in your situation. The problem is that the math to theoretically approach these numbers without having taken actual measurements can be quite the bear.2. Minimum distance would be infinite for no effect according to theory. You see, there is a distance term in the equation. However all hope is not lost since practically there exists a distance where the effect becomes negligible depending on application of course. If you don't have time to learn or do the math you kind of have to play with it to see what sounds good.
3. I think everything you ask can be understood by acquiring a firm understanding of Schroedingers equation, and esecially the subset which contain Maxwells equations. This could take several lifetimes to master though. Unless you have aspirations to become an EE or something it might just be easier to ask questions in a place like this. I'll warn you though, solving RFI problems can bring even the smartest EE's to their knees sometimes.
Hi Jim,"Where do I get the theory behind this to give me some guide lines"?
Jeez, you certainly ask for a lot!! :-))
I've made all my ICs - including a phono IC from arm to phono stage - using a similar scheme to yours (/CvH's) ... it's a modification of one of Allen Wright's recipes from his "Super Cable Cook Book".
Putting a shield on ICs is a necessary evil, AFAIAC. However, this has two effects ... firstly, yes, it increases the capacitance but this shouldn't do too much damage if the Zin of your destination device is at least 50 or 100 times the Zout of the source.
Secondly, the presence of a grounded shield right next to the signal wire seems to have an undesirable sonic effect. As I understand it, this is because the ground interferes with the magnetic field around the signal wires.
So the thing to do to ameliorate this undesirable effect is - as you've figured out - space the shield away from the signal wires. :-))
IMO, yes, the furthest possible spacing is the best but practicality enters into the equation ... as you've said, ICs with a 1 1/2" diam are not very practical.
My own view is that because the current flowing in the ICs is so minute - especially phono ICs - the magnetic field around the signal wires is not very large/extensive. So what I do is enclose the basic IC (the two signal wires wrapped around a 'rod' ... although in my case, I use 4mm diam teflon tube, not 3/8" rod) in two layers of cotton tubing and then thread this through 1/4" diam (nominal) braided shield.
You can buy cotton sleeving in various diameters but I in fact use cotton bootlaces! :-))
I have no idea whether this distance away of the shield is enough ... but I cannot hear any bad sounding effect with my ICs - so I'm sticking with what I've done. I suppose you could use 3 or 4 layers of bootlace if you wanted! :-))
Regards,
I am not sure how low to go with capacitance. The larger the central core, the lower the capacitance. I did discover that the RCA plugs have a large influence. I have some Dayton full metal locking barrel RCA's and they add something like 20pF to the overall value. Eichmann Bullets don't add anything at all. Makes me wonder what the female jacks add. I am using some 1/4 braided ploy rope for the spacer. Does not take much to keep the effect of the shield on the overall capacitance. Right now I am experimenting with magnet wire before I settle on a design and then execute it in silver.
This post is made possible by the generous support of people like you and our sponsors: