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John Curl's Blowtorch preamplifier part III
John Curl's Blowtorch preamplifier part III
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Old 11th October 2018, 07:56 AM   #8541
chris719 is offline chris719  United States
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John Curl's Blowtorch preamplifier part III
With a reciever like WM8804 and an ASRC following it, you could probably run SPDIF through a long segment of chicken wire and get no performance degradation at the DAC output. Assuming you can maintain lock on the receiver anyway.
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Old 11th October 2018, 11:02 AM   #8542
DF96 is offline DF96  England
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Quote:
Originally Posted by anatech
I see RCA connectors inside consumer equipment, like old TVs for the UHF tuner signal output. They are often soldered to the UHF tuner chassis. That is an application where the impedance is supposed to be 75R.
For some reason which baffles Europeans, North American practice seems to be to use RCA for all sorts of inappropriate purposes such as UHF tuner inputs and VHF transmitter outputs. This does not mean that RCA can be considered to be 75R, just that the mismatch was regarded as being acceptable - perhaps low cost was a consideration?

Of course, over here we used the Belling-Lee connector almost everywhere and that might not be too good for UHF.

Quote:
Originally Posted by jneutron
The nextgen design actually does give a 75 ohm connector through impedance. It does so by design of the shield path. So, no it is not a 30'ish connector with a matching network, but a true match. Unfortunately, using a standard female ruins the through impedance.

As a side note, by splitting the shield path as they did, they produce an entry path for rf.
OK, another way to change the problem in order to make the solution easier. So to achieve 75R they added inductance to the shield and reduced shielding effectiveness i.e. lose full coaxial construction. That seems a poor compromise. It seems that in order to achieve something which can be sold as '75R RCA' (but why bother?) you have to lose something, and so create problems which may be worse than a little connector mismatch.
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Old 11th October 2018, 11:08 AM   #8543
DF96 is offline DF96  England
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Quote:
Originally Posted by anatech
I was wondering if the characteristic impedance of your garden variety RCA connector wasn't close to 75R.
No, nowhere near. I estimated around 30R; I have seen others estimate around 25R.

Quote:
Originally Posted by john curl
I have always been confused with the 'smooth' F connector and an RCA.
Your picture appears to show an F connector and a Belling-Lee. Belling-Lee is supposed to be 75R, but I don't know how accurate this is.
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Old 11th October 2018, 12:37 PM   #8544
hhoyt is offline hhoyt  United States
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John Curl's Blowtorch preamplifier part III
Quote:
Originally Posted by DF96 View Post
No, nowhere near. I estimated around 30R; I have seen others estimate around 25R.
The connector used does represent a short section of transmission line so it will have a characteristic impedance just as the coax cable attached to it does. Here is what I figure for the precision RCA plug system we all know and love.

Given a fixed center pin diameter of 0.125" (3.18 mm), and outer diameter of .325" (8.26 mm) the metal thickness of the outer jack shell and center female insert needs to be 0.010" minimum even if made from beryllium copper, thinner metal would make the contact easily permanently deformed and the contact pressure too low. This results in a basic geometry for the RCA plug/jack system of 0.145" (3.68 mm) center conductor 0.305" (7.75 mm).

These dimensions would apply best case to the basic RCA plug/jack system, thicker metals would lower the impedance further. Here is a table of the characteristic impedance of this system with different dielectrics I have seen used over the years:

Air - 44.6 Ω
Teflon - 30.8 Ω
PP - 30.0 Ω
PE - 29.7 Ω
Paper - 25.7 Ω
PVC - 25.0 Ω
Nylon - 22.3
Porcelain - 19.0 Ω
Phenolic - <18 Ω

Despite claims by WBT and others like Pasternack that they carry 75 Ω RCA connectors, since they embed no impedance transformation network they cannot be more than ~45 Ω or so, or as JN suggested, a dielectric constant less than 1. Even if they had zero metal thickness and air dielectric they would be ~57 Ω.

Since the RCA plug/jack system represents a very short (1"/25mm) transmission line, issues caused by the mismatch between an RCA plug/jack and 75 Ω cable only apply to signals where this represents 1/8 wavelength or shorter, above 1.5 GHz or thereabouts. In my experience, since the resulting pulse distortion and impedance discontinuity presents is a stable, fixed effect, I have seen zero issues using a decent RCA cable to transmit SPDIF or other low frequency digital audio signals.

Cheers,
Howie

(sorry all, I had nothing better to do this AM, I'm hunkered down for a hurricane, awaiting that call from a station off the air...)
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Old 11th October 2018, 02:25 PM   #8545
jneutron is offline jneutron  United States
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Quote:
Originally Posted by hhoyt View Post
The connector used does represent a short section of transmission line so it will have a characteristic impedance just as the coax cable attached to it does. Here is what I figure for the precision RCA plug system we all know and love.

Given a fixed center pin diameter of 0.125" (3.18 mm), and outer diameter of .325" (8.26 mm) the metal thickness of the outer jack shell and center female insert needs to be 0.010" minimum even if made from beryllium copper, thinner metal would make the contact easily permanently deformed and the contact pressure too low. This results in a basic geometry for the RCA plug/jack system of 0.145" (3.68 mm) center conductor 0.305" (7.75 mm).

These dimensions would apply best case to the basic RCA plug/jack system, thicker metals would lower the impedance further. Here is a table of the characteristic impedance of this system with different dielectrics I have seen used over the years:

Air - 44.6 Ω
Teflon - 30.8 Ω
PP - 30.0 Ω
PE - 29.7 Ω
Paper - 25.7 Ω
PVC - 25.0 Ω
Nylon - 22.3
Porcelain - 19.0 Ω
Phenolic - <18 Ω

Despite claims by WBT and others like Pasternack that they carry 75 Ω RCA connectors, since they embed no impedance transformation network they cannot be more than ~45 Ω or so, or as JN suggested, a dielectric constant less than 1. Even if they had zero metal thickness and air dielectric they would be ~57 Ω.

Since the RCA plug/jack system represents a very short (1"/25mm) transmission line, issues caused by the mismatch between an RCA plug/jack and 75 Ω cable only apply to signals where this represents 1/8 wavelength or shorter, above 1.5 GHz or thereabouts. In my experience, since the resulting pulse distortion and impedance discontinuity presents is a stable, fixed effect, I have seen zero issues using a decent RCA cable to transmit SPDIF or other low frequency digital audio signals.

Cheers,
Howie

(sorry all, I had nothing better to do this AM, I'm hunkered down for a hurricane, awaiting that call from a station off the air...)
Thanks for the work, it's all good.

The nextgen rca PAIR is indeed 75 ohm. I emphasize pair, as if a nextgen male is plugged into a normal rca female, the z will be exactly as you calculate. The RCA female is what dominates the z. If the female shield metal is cut away into two blades, it can be made into a higher z, but requires a suitable male with shield "blades" as well.

But, the nextgens are definitely 75 ohm as a pair.

jn
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Old 11th October 2018, 03:28 PM   #8546
anatech is offline anatech  Canada
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John Curl's Blowtorch preamplifier part III
Hi 20to20,
Quote:
That was my point, a 75 ohm F connector for typical VHF coax antenna input isn't called an RCA jack. In your earlier post you seem to be confusing UHF impedance matching with VHF impedance matching with RCA connector impedance matching. Your reference to UHF tuner output via RCA plug connectors as being 75R, doesn't take into consideration that the tuner has done the demodulation and is outputing composite signal that can be carried OK with standard cable with an RCA plug and jack. The connector itself is not the determinant of the impedance rating of the circuit, the input to the following amps is. You can put an RCA connector on a 75R coax cable but its not the connector that makes it 75R. So there really is no 75R RCA to ponder.
I was only observing that UHF tuners did use an RCA connector that was soldered to the mating jack in most cases. It was referred to as a 75 ohm connection. These were stripped down versions without a gripping surface, only an area where the twisted shield was looped around and soldered. The centre conductor was soldered in the end of the male part in the normal fashion. The other end was typically soldered directly to a PCB. More recently, I've seen component video delivered on RCA connectors, supposedly a 75R level as well.

Since I don't work in consumer television or video, I only have whatever knowledge I picked up over the years. But again, that's why I have been very clear on the fact that I'm only reporting on things I have witnessed and heard from people in those industries. I am not confusing two impedance levels, nor am I confusing two connectors.

On my bench everything is on a 50R standard for RF except for FM tuner inputs which are on either "F" connectors or a European type connector I don't know the name for. But that's it. The only other RF connectors I have are "N", "BNC", "SMA" and "TNC", and I'm good to 500 MHz with some counters exceeding 1 GHz. RCA connectors are only used for audio unless I get a strange device that uses RCA connectors, then adapters are used to bring it onto a 50R system. I also have two FM generators that use 75R BNC connectors, which requires impedance adapters unless they plug directly into an FM tuner.

So that is my environment, and I am happy to say that I'm learning a great deal on this now that it is being discussed. Trusted information comes from the usual suspects. Some others are generating noise.

Shields up Scotty!

-Chris
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Old 11th October 2018, 03:53 PM   #8547
20to20 is offline 20to20  United States
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Quote:
Originally Posted by jneutron View Post
The nextgen rca PAIR is indeed 75 ohm. I emphasize pair, as if a nextgen male is plugged into a normal rca female, the z will be exactly as you calculate. The RCA female is what dominates the z. But, the nextgens are definitely 75 ohm as a pair.

jn
When the plug and jack are a specialized construction for the coax to maintain the integrity of the dielectric across the connection then it would have a rating for the cable. So I renounce my previous assertion that there is no real 75R RCA plug to ponder... never say never.
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Old 11th October 2018, 04:03 PM   #8548
20to20 is offline 20to20  United States
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Originally Posted by anatech View Post
More recently, I've seen component video delivered on RCA connectors, supposedly a 75R level as well.
I had to go dig out my my RGB component cables to see if they were marked for specs/impedance but, nope. No printing at all. But at the frequencies they carry I'm not surprised there isn't much need for specs to be prominent.

Last edited by 20to20; 11th October 2018 at 04:09 PM.
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Old 11th October 2018, 04:10 PM   #8549
dimitri is offline dimitri  United States
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There are plenty of them:
75 Ohm Rca Connector | Markertek
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Old 11th October 2018, 04:42 PM   #8550
20to20 is offline 20to20  United States
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Originally Posted by dimitri View Post
There are plenty of them:
75 Ohm Rca Connector | Markertek
What sorts of equipment are using 75R RCA inputs? Close coupled computer/digital stuff?

Last edited by 20to20; 11th October 2018 at 04:47 PM.
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