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Old 29th July 2005, 09:48 AM   #21
AuroraB is offline AuroraB  Norway
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Location: Norway, -north of the moral circle..
Coaxial Impedance :

Z0= (138/ Sqr(Er) )* log(b/a)

Er is relative dielectric constant of isolation material in use
b=dist between inner and outer conductor, - a= dia. of inner conductor.

This is the ONLY parameters that matters.- If you connect all or parts of the screen the distance between the conductors is still the same.
Hence -- a phono can NEVER be 75 ohms - No matter what obscure explanations are constructed!!!

Want constant correct impedance ?? Go BNC, N, TNC or any good quality coaxial plug... and good quality coax cable..BTW- sofar I haven't seen any "snake oil" cable being correctly made for constant impedance.

PS- the 75 ohm comes from being 1/4 of 300 ohms- the average impedance most Yagi-Uda antennas ( - i.e normal TV antennas)
A single foldeed dipole is appx 240 ohms- and the 60 ohm coax used in the 50-60's being 1/4 of 240.....

Don't get it????
Go get a book on transmission line theory - and maybe we can get rid of some of the pointless discussions and threads on cable impedance
( and very hopefully also some of the snake oil....)

EDIT:
Good quality coaxial cables cost just a fraction of the snake oil....
So does BNC bulk heads...........
But it is all of no avail if your drive and receive ends are not properly made to use this impedance... i.e properly terminated!
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Old 29th July 2005, 10:08 AM   #22
tekman is offline tekman  Germany
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@ commstech, @ gasho:

well, RCA is used for historical reasons - a pure legacy connector.

and of course it's a matter ofthe price, just as Gasho has mentioned it: Easier to produce, a littel bit cheaper, and easier to work on when you have make a complete cable without professionol tools.

I prefer BNC also because it "locks" on the connector (not so easy slipping of unintentionaly.

My Tek 1730 has BNC connections, but of course adapters to RCA are included too.

hth,
Andreas
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Old 29th July 2005, 03:00 PM   #23
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Quote:
Originally posted by AuroraB
Hence -- a phono can NEVER be 75 ohms - No matter what obscure explanations are constructed!!!
Of course not.

But some have the creativity to change the equations to their taste.

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Old 29th July 2005, 03:44 PM   #24
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Quote:
Originally posted by Jocko Homo
Equations............?

How 'bout you have to increase the inductance by about a factor of 8 to make it work.
Good, at least you seem to understand what the problem is..(that will be my last quip in response to your silly profanity. Perhaps it can remain professional and mature now?? thanks..)

I'll give some explanation for all, I assume you already know this..

A coaxial structure defines it's characteristic impedance by seveal things:
1. The inner pin OD. (outer diameter)
2. The outer shell ID. (inner diameter)
3. The dielectric constant of the material between the two.

Jocko points out (correctly) that for the RCA connector, the pin and shell size conspire to create a characteristic impedance that is LOWER that that required. To raise the impedance to 75 ohms, one must either raise inductance (which is locked by the coaxial structure dimensions), or lower capacitance (which is also defined by the coaxial structure AND by the DC of the insulator.

Jocko is also correct in the relative magnitude that the inductance must be increased {Z = sqr(L/C)}, or the capacitance must be decreased...

The capacitance cannot be decreased the amount needed, even if the insulator is removed leaving vacuum. This is a physical reality, vacuum still has a DC of 1. Again, a constraint of the coaxial design.

So, how to get around it? Simple, break the coaxial design.

The coaxial design works by constraining the magnetic field to the volume between the pin and shell, following the equation:

L = {mu/2 pi} Ln {Ro/Ri}

Note the pin and shell dimensions define the inductance.

inductance is by definition, the relation between the energy stored within the magnetic field as a result of the current.

A coaxial structure constrains the field outside the shell by simple cancellation....the inner wire produces a field strength which falls off as 1/R...the outer one produces an opposite field which also falls off as 1/R, but since the current density of the outer shell is lower due to surface length, its field strength at the shell is exactly the same but opposite to that of the inner wire. I had some pictures showing how this works, but I can't find them on my desktop..windows, sheesh..

Now, the fun part....STOP CONSIDERING THIS AS ONLY A COAXIAL PROBLEM!!! That is where you, Jocko, have not looked..

Once you break the symmetry of the outer shell, you no longer constrain all the field to the volume within the shell..because it is no longer fully cancelled outside the shield volume.

Break the shield at 12:00, break it at 6:00. Then keep backing it off, towards a final destination of a wire at 3:00 and a wire at 9:00. (analog clock, thank you... )

While this "morphing" is occurring, the inner wire field gets farther reaching as the shield current no longer cancels it...and the shield field gets bigger and bigger..THIS IS AN INCREASE IN INDUCTANCE!

AND, as this morphing occurs, less and less of the dielectric is being used, so the capacitance starts going down..

Increase L, decrease C...voila...impedance goes up..when it hits 75 ohms, stop...

That is what the wbt part does..

Downfall to this method? Glad you asked...

1. Breaking the magnetic symmetry this way makes the local area more susceptible to RFI, both radiated and received.

2. Use of a standard rca male or female with a wbt unit negates entirely the 75 ohm impedance, it goes back down to it's design impedance. This forces the use of pairs to achieve 75 ohms, but at least the design is compatible with existing product physically.

3. It's too darn expensive..


Quote:
Originally posted by Jocko Homo

On paper.
I used electrons, bits, and photons...print it out if you wish..

Quote:
Originally posted by Jocko Homo
An XLR is a 110 ohm connector. It even measures 110 ohms. It is totally useless in the RF world.
Umm, I missed something...who is talking about an XLR?

Quote:
Originally posted by Jocko Homo
Experience trumps your silly equations. Sorry, bub.
Jocko
You are standing in the middle of the road claiming cars don't exist, only to be run over by a tractor trailer.. (i love that smiley)

My experience exceeds (by a small tad), a simple exercise in modification of a coaxial structure for magnetic reasons. You need only examine the wbt part to see how others, by both experience and by shipped product, have done what you claim is impossible..

I have tried to keep the explanation as simple as possible without losing content. If you wish more elaboration, just ask..

Interestingly enough, this is the simpler of the two methods I know of to get an RCA up to 75 ohms. The second opton does not suffer points 1,2, or 3 above...

Cheers, John
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Old 29th July 2005, 04:03 PM   #25
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Default No, I am not standing in the middle of the road.......

I am standing in front of my TDR, and does not lie.

Unlike the people who make up stupid **** like "75 ohm RCAs" to sell to gullible audiophiles. It is a lot cheaper, and far better, to use a real 75 ohm connector. But no money in that for crooks. So they invent garbage like what you describe.

You obviously missed the point about XLRs. A connector that does measure 110 ohms, but is totally useless.

The "75 ohm RCA" may look like 75 ohms, for one small point in time, but will be just as useless as a 110 ohm XLR.

Go put one of your magic RCAs on a TDR, and then try to tell us that it really works. And while you are at it, try to figure out how high the rho can be before it makes the SPDIF sound even worse than it really is. At that point you may understand why it is a stupid idea.

At least to the rest of us. A great idea to the crooks who peddle such nonsense.

Jocko
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Old 29th July 2005, 04:14 PM   #26
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Notice that this kind of marketing nonsense doesn't even worry about the connection of the cable inside the plug.
75ohms already stops there.
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Old 29th July 2005, 04:21 PM   #27
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Default Re: No, I am not standing in the middle of the road.......

Quote:
Originally posted by Jocko Homo
I am standing in front of my TDR, and does not lie.Jocko
Um, Jocko...what are you measuring with your TDR? As I said, normal RCA's are not 75 ohm..did you measure one of the wbt's, or are you generalizing.

If you have actually taken the time to aquire a pair of wbt's and they do not measure as good to 1 GHZ, as is claimed in their ad copy, let us know...

Quote:
Originally posted by Jocko Homo
Unlike the people who make up stupid **** like "75 ohm RCAs" to sell to gullible audiophiles. It is a lot cheaper, and far better, to use a real 75 ohm connector. But no money in that for crooks. So they invent garbage like what you describe.
What I describe is the reality of using maxwells equations to produce a solution you didn't think possible. Unfortunately, the wbt patent was applied for 31 calender days before I posted the solution on AR.

I also agree that a 75 ohm rca is pointless..given the choice between a bnc and an rca, it's a no-brainer..

Quote:
Originally posted by Jocko Homo
You obviously missed the point about XLRs. A connector that does measure 110 ohms, but is totally useless.
I didn't see the point of discussing an audio connector in an rf and video discussion..

Quote:
Originally posted by Jocko Homo
The "75 ohm RCA" may look like 75 ohms, for one small point in time, but will be just as useless as a 110 ohm XLR..
I do not understand the words"one small point in time"..nobody is discussing a time varying impedance...you will have to explain that. A 110 ohm XLR for audio frequencies is a convienience of form, not reflection coefficient.
Quote:
Originally posted by Jocko Homo
Go put one of your magic RCAs on a TDR, and then try to tell us that it really works.
Didn't you just tell us you are standing in front of a TDR, and it doesn't lie??? Are you saying you DON'T have one to measure?? Then, how "is it not lying "...you aren't measuring one, are you?

Figures..

Cheers, John
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Old 29th July 2005, 04:22 PM   #28
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Yes, you will have some discontinuities breaking out the centre conductor from the shield. Maybe they figure that into their increase in inductance.

Of course, all of this nonsense is negated the moment you plug it into a standard female RCA. I wonder if they ever thought of that? You have to measure the connector pair to see if it really does hold its characteristic impedance.

Which, of course, this silly thing will not.

BTW..........let us see a show of hands of folks who think that they can hear the effect of a 50 ohm barrel used to connect two 75 ohm SPDIF cables together. Asssumes that the rho of the rest of the signal path is already low. While experience says that below 300 MHz, it should not matter, it does on SPDIF. Go figure.

Jocko
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Old 29th July 2005, 04:31 PM   #29
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Quote:
Originally posted by AuroraB
Coaxial Impedance :

Z0= (138/ Sqr(Er) )* log(b/a)

Er is relative dielectric constant of isolation material in use
b=dist between inner and outer conductor, - a= dia. of inner conductor.

This is the ONLY parameters that matters.- If you connect all or parts of the screen the distance between the conductors is still the same.
Hence -- a phono can NEVER be 75 ohms - No matter what obscure explanations are constructed!!!
Please, read the explanation and understand it..

Quote:
Originally posted by AuroraB
Want constant correct impedance ?? Go BNC, N, TNC or any good quality coaxial plug
I already do..I do not consider RCA's as viable. I hate them..

Quote:
Originally posted by AuroraB
PS- the 75 ohm comes from being 1/4 of 300 ohms- the average impedance most Yagi-Uda antennas ( - i.e normal TV antennas)
A single foldeed dipole is appx 240 ohms- and the 60 ohm coax used in the 50-60's being 1/4 of 240.....
Why is the 1/4 of 300 important? I cant help but recall from '74, some kind of 1/4 wave transformer thing...but when I run the numbers, I get 112 ohms...Zo=sqr(ZrZi),

Quote:
Originally posted by AuroraB
Don't get it????Go get a book on transmission line theory - and maybe we can get rid of some of the pointless discussions and threads on cable impedance.....
I have three of them 2 feet away, next to Jackson, Becker, Shadowitz, Rojansky, and Ragan. Plus, I can walk 100 feet down the hall to the people who write the books..

Quote:
Originally posted by AuroraB
But it is all of no avail if your drive and receive ends are not properly made to use this impedance... i.e properly terminated!
I currently use a 160 foot run of rg-6 as a video feed, bnc at the receiver, rca at the transmitter..works very well..thank goodness the receiver had a bnc connector, as the rca one woulda reflected all to heck.I don't recommend rca at transmit end, but the dvd player didn't have the option.

Cheers, John
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Old 29th July 2005, 04:39 PM   #30
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Quote:
Originally posted by Jocko Homo
Yes, you will have some discontinuities breaking out the centre conductor from the shield. Maybe they figure that into their increase in inductance.Jocko
They actually don't break the center conductor out, as they retain symmetry for both the jack and plug..

They don't retain cylindrical symmetry of course, but bi-lateral symmetry. And yes, this is how they increase the inductance, and lower the capacitance too.

Quote:
Originally posted by Jocko Homo
Of course, all of this nonsense is negated the moment you plug it into a standard female RCA.
I have stated that twice already within this thread.
Quote:
Originally posted by Jocko Homo
I wonder if they ever thought of that?
The fact that they made a jack that functionally is compatable to existing RCA's proves they indeed did think of that.
Quote:
Originally posted by Jocko Homo
You have to measure the connector pair to see if it really does hold its characteristic impedance.
They actually specify the connector via TDR, out to a ghz..

Haven't you measured a pair?
Quote:
Originally posted by Jocko Homo
Which, of course, this silly thing will not..
Hmmm..it will, just measure a pair. Simple enough with the instrument in front of you..Ask Ray for a freebie pair..
Quote:
Originally posted by Jocko Homo
BTW..........let us see a show of hands of folks who think that they can hear the effect of a 50 ohm barrel used to connect two 75 ohm SPDIF cables together. Asssumes that the rho of the rest of the signal path is already low. While experience says that below 300 MHz, it should not matter, it does on SPDIF. Go figure.

Jocko
Why? I'm discussing characteristic impedance, and the fact that it is indeed possible to achieve 75 ohms by alteration of the connector e/m symmetry..

Cheers, John
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