I need to be educated on the subject. What exactly determines the impedance of a particular type of connector ? For example, BNC is 75ohm, RCA is 200(?), etc...
I am curious to know what could be the impedances of typical 1/4" and 1/8" stereo plugs ?
I am curious to know what could be the impedances of typical 1/4" and 1/8" stereo plugs ?
This impedance is of interest only at RF where the signal doesn't travel down the cable per se, but traverses the inductance and capacitance of the cable/connectors. At audio frequencies we would refer to these inductances and capacitances of the cable to be stray reactances, and we'd probably wish they didn't exist. At RF they are just a fact of life, and have to be worked with, but at AF they normally are not significant.
To sum it up, at audio frequencies, we are normally only concerned with just the resistance of a cable (if even). There is no 'impedance' to worry about.
Apart from this, phono cable capacitance and capacitance from long runs of professional cable, can be a concern but these are different to cable impedance.
To sum it up, at audio frequencies, we are normally only concerned with just the resistance of a cable (if even). There is no 'impedance' to worry about.
Apart from this, phono cable capacitance and capacitance from long runs of professional cable, can be a concern but these are different to cable impedance.
Hi,
The impedance of any transmission line is determined by the relation between inductance and capacitance at any point along the line. The definition of characteristic impedance is SQR(L/C) for a lossless line.
The relation of L and C in a coaxial connector is determined by the mechanical dimensions, As an example a thin inner conductor gives a lower impedance than a thicker one and so on.
In order to determine a characteristic impedance of a connector, (if the value should have any meaning) it should ideally have the same impedance in any point which is what you are trying to achieve in connectors for RF, the higher frequency the more important it is that this requirement is fulfilled, as an example it is more difficult to use angled connectors at higher frequencies as it is very difficult to make an angled connector that have the same impedance at any point.
For Audio it is not that critical but I think it is in principal wrong to say that a RCA phono connector is 200ohm as in reality the impedance varies along the connector and between manufacturers, it is different with BNC which are made for a certain impedance, (either 50 or 75 ohm, both types exist). For 1/4" or 1/8" stereo plugs I don't even want to guess.
However is it really important which impedance a connector have in an typical audio system? The answer is no for 2 reasons:
1, the typical Audio connection is where the signal source have low impedance, (ideally a voltage source) and where the load is high impedance, in this kind of system characteristic impedance has no meaning.
2, For an impedance matched system for Audio any fault in characterstic impedance introduced by a connector, (which will be seen as an extra minute amount of inductance or capacitance at a ceratin point) will in reality be so small that the effect is negligible.
Best Regards Hans
The impedance of any transmission line is determined by the relation between inductance and capacitance at any point along the line. The definition of characteristic impedance is SQR(L/C) for a lossless line.
The relation of L and C in a coaxial connector is determined by the mechanical dimensions, As an example a thin inner conductor gives a lower impedance than a thicker one and so on.
In order to determine a characteristic impedance of a connector, (if the value should have any meaning) it should ideally have the same impedance in any point which is what you are trying to achieve in connectors for RF, the higher frequency the more important it is that this requirement is fulfilled, as an example it is more difficult to use angled connectors at higher frequencies as it is very difficult to make an angled connector that have the same impedance at any point.
For Audio it is not that critical but I think it is in principal wrong to say that a RCA phono connector is 200ohm as in reality the impedance varies along the connector and between manufacturers, it is different with BNC which are made for a certain impedance, (either 50 or 75 ohm, both types exist). For 1/4" or 1/8" stereo plugs I don't even want to guess.
However is it really important which impedance a connector have in an typical audio system? The answer is no for 2 reasons:
1, the typical Audio connection is where the signal source have low impedance, (ideally a voltage source) and where the load is high impedance, in this kind of system characteristic impedance has no meaning.
2, For an impedance matched system for Audio any fault in characterstic impedance introduced by a connector, (which will be seen as an extra minute amount of inductance or capacitance at a ceratin point) will in reality be so small that the effect is negligible.
Best Regards Hans
hi guys!
i've never got to understand this impedance concept and i'm extremely curious now..
tubetvr,
am i right to say that the only the connectors have a say in this characteristic impedance?
i've never got to understand this impedance concept and i'm extremely curious now..
tubetvr,
am i right to say that the only the connectors have a say in this characteristic impedance?
Here is a reasonable explanation of the concept.
http://en.wikipedia.org/wiki/Impedance_matching
I_F
http://en.wikipedia.org/wiki/Impedance_matching
I_F
Tube gear made in the old days was designed for 47kOhm impedance. Hence the high-impedance, low-capacitance coax cable. (Back then they called it diode cable.) And because of that we got the RCA connector. So it's the cable, not the connector, that primarily matters.
Digital gear works different.
Digital gear works different.
No, not at all, it goes for the whole transmission line. i.e connectors and cable.
But as I also said the characteristic impedance doesn't matter for audio as the line is not terminated in a defined impedance but as I said you have a voltage source and a high impedance load. What matters is the total capacitance of the cable and connectors compared to the source impedance, in this case the capacitance of the connector is so small that it can be ignored.
Regards Hans
i'm thinking of this concept when impliement bncs in spdif transmission, i see there are 50ohm and 75ohm connectors, what difference would these have? what is the essense behind the need for these impedances? bandwidth or is it to minimize losses?
You are refering to the load impedance that in tube gear can be anything form 10s of kohms up to Mohm. However the load impedance is not that important but the source impedance is, even in tube gear this can be quite low, say 100s of ohm up to 10s of kohm, (a cathode follower is easy to make with standard tubes and this will give source impedance of 100s of ohms)
The RCA connector is not designed for any specific impedance or for low capcitance, it is only designed to be coaxial and low cost. The capaciatnace of a connector doesn't matter as PHN says, and example a coax cable have 200pF per meter, and RCA connector is approx 4cm, then even if the RCA have double capacitance per length, (it is unlikely that is the case) then the RCA will give approx 8pF more than a cable of same length.
Regards Hans
Hi Hack,
digital is a different ball game.
All the essentially square wave signals passing down the line like to see a constant impedance to very high frequency.
You need to avoid reflections by ensuring correct source and load impedances.
You also need to preserve the squareness of the waveform as best you can to allow easier decoding, with fewer errors, at the load end. This requires a flattish reponse out to about ten times the bit rate.
Since all the above effectively requires a transmission line, you are now into line drivers and line receivers with correctly matched cables, connectors, sources and loads
BTW how does one differentiate between a 50ohm BNC and a 75ohm BNC?
digital is a different ball game.
All the essentially square wave signals passing down the line like to see a constant impedance to very high frequency.
You need to avoid reflections by ensuring correct source and load impedances.
You also need to preserve the squareness of the waveform as best you can to allow easier decoding, with fewer errors, at the load end. This requires a flattish reponse out to about ten times the bit rate.
Since all the above effectively requires a transmission line, you are now into line drivers and line receivers with correctly matched cables, connectors, sources and loads
BTW how does one differentiate between a 50ohm BNC and a 75ohm BNC?
There are essentially two types of 'matching', impedance matching (best power transfer, but efficiency always below 50%) as used in RF circuits. Then there is voltage matching, where a low source impedance feeds a high impedance load, this is used in audio connections - as you're not matching impedances the efficiency is considerably improved.
I found a Web site that have description of the differences here http://www.levitonvoicedata.com/support/technotes/tech_note_detail.asp?tnID=175
Quote "The 75 Ohm plug’s center pin has the same diameter in the rear as in the front mating interface area. Whereas, the 50 Ohm plug’s center pin has a thicker diameter in the rear area where it is crimped.
Both plugs have the same pin size in the mating area. Regarding the dielectric on each, the 75 Ohm connector’s dielectric is made of Teflon which has higher impedance properties than Delrin. The 50 Ohm connector’s dielectric is made of Delrin.
Finally, the main physical difference is that the 75 Ohm plug does not have extended dielectric around its outer spring fingers."
75ohm BNC is becoming rare now as many are using 50hms connectors even in 75ohm systems, if the frequency is reasonably low, like Audio it doesn't matter that much.
Regards Hans
As a non-propeller head I always get lost in the details and engineering as ALL about the details. It's bad, I know.
47kOhms is probably wrong. Back in the 1920s-'30s, which I referred to, it was most likely 50, if it was in that range. 47 came later.
One of the digital guys here (Elso?) says it's the thick center pin on RCA connectors that makes 75Ohms impossible. An RCA connector is around 30Ohms.
47kOhms is probably wrong. Back in the 1920s-'30s, which I referred to, it was most likely 50, if it was in that range. 47 came later.
One of the digital guys here (Elso?) says it's the thick center pin on RCA connectors that makes 75Ohms impossible. An RCA connector is around 30Ohms.
Hi Hans, thanks for your thoughts on the subject.
Is that Squared or Square Root ? (I think its root but I'll just confirm anyway)
So in summary -
its the "capacitance" of the connectors, because of the dielectric(insulation) between the contacts, and not the "impedance". And even that really does not matter for audio because of the very small physical length, right ?
Similarly, can you also please explain if capacitance (and/or inductance) of the cable matter in audio-frequency connections ? and how ?
Thanks!
tubetvr said:
Is that Squared or Square Root ? (I think its root but I'll just confirm anyway)
So in summary -
its the "capacitance" of the connectors, because of the dielectric(insulation) between the contacts, and not the "impedance". And even that really does not matter for audio because of the very small physical length, right ?
Similarly, can you also please explain if capacitance (and/or inductance) of the cable matter in audio-frequency connections ? and how ?
Thanks!
It is Square root .
Yes that it is essentially correct, the capacitance of the connector is small compared to the capacitance of the cable.
Capacitance matters in the case where the source impedance is relatively high, an example: A source of 10kohm and a cable capacitance of 200pF will have 3dB point of ~7kHz meaning that frequencies of 7kHz will be attenuated 3dB compared to low frequencies, 14Khz will be 9dB lower.
The solution to this problem is not to find a better cable, although there exist cables that are better it doesn't give a major improvement so the problem that need to be solved is to lower the source impedance. In solid state gear this is usually not a problem but a tube based design could be, check what the manufacturer give as output or source impedance.
Regards Hans
percy said:
In fact, the wavelength of an electrical signal in a conductor at 20kHz is potentially a little less than 10 miles. I'd say you're right 🙂
would it be safe to say then "regular" (and not those pricey high-end)connectors are just fine to use in audio equipment ?
what's so special about the high(er) priced Canare and Cardas connectors ? Some people claim they sound better (??). I believe they have superior conductivity (gold plated), but just like capacitance, that conductivity increase/drop for such a short physical length shouldn't matter.
I have also seen claims about doing away with the connectors and connecting the cables directly. How does one explain that ?
what's so special about the high(er) priced Canare and Cardas connectors ? Some people claim they sound better (??). I believe they have superior conductivity (gold plated), but just like capacitance, that conductivity increase/drop for such a short physical length shouldn't matter.
I have also seen claims about doing away with the connectors and connecting the cables directly. How does one explain that ?
In my opinion the RF shielding properties of a connector, and its ability to make a clean connection are the primary issues. Whilst I am no authority, I chose to use off the shelf connectors and can't complain.
Gold is not the best conductor, it is free from tarnishing which is its strong point, hence gold plating .
Gold is not the best conductor, it is free from tarnishing which is its strong point, hence gold plating .
Hi Percy,
Instead of answering your question directly, I will describe how I have selected phono connectors in my own equipment.
I was living many years in Tokyo where you can find many components of varying quality. When I wanted to select phono chassis connectors for mypreamp I bought one of each of any connector I could find and then I cut it through with a hacksaw and examined the inner parts. What I was looking for was an inner connector for the pin that was designed of good quality phosphor bronze and where the gold plating was reasonably thick, also important is the number of "fingers" the connector consist of. The connector I fiinally selected has an inner connector that consist of 8 fingers and where a circular spring put pressure on the fingers from outside.
My thinking about how to choose connectors is applying the same principle as I would choose when selecting a professional connector in my work, (I have been working with RF and microwave design).
Maybe this doesn't clarify matters but one thing is clear, different brand of phono connectors are not performing better because one is a better impedance match, as I wrote before a RCA phono doesn't have a defined characteristic impedance.
If a certain phono connector performs better it is only because it is better designed, more like a professional connector.
Contact surfaces are important, gold or alloys of gold gives good contact and long life without risk of oxidising, thick plating is important if you need to redo connections many times.
Regards Hans
Instead of answering your question directly, I will describe how I have selected phono connectors in my own equipment.
I was living many years in Tokyo where you can find many components of varying quality. When I wanted to select phono chassis connectors for mypreamp I bought one of each of any connector I could find and then I cut it through with a hacksaw and examined the inner parts. What I was looking for was an inner connector for the pin that was designed of good quality phosphor bronze and where the gold plating was reasonably thick, also important is the number of "fingers" the connector consist of. The connector I fiinally selected has an inner connector that consist of 8 fingers and where a circular spring put pressure on the fingers from outside.
My thinking about how to choose connectors is applying the same principle as I would choose when selecting a professional connector in my work, (I have been working with RF and microwave design).
Maybe this doesn't clarify matters but one thing is clear, different brand of phono connectors are not performing better because one is a better impedance match, as I wrote before a RCA phono doesn't have a defined characteristic impedance.
If a certain phono connector performs better it is only because it is better designed, more like a professional connector.
Contact surfaces are important, gold or alloys of gold gives good contact and long life without risk of oxidising, thick plating is important if you need to redo connections many times.
Regards Hans
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