RCA interconnects; why coax construction and not simple 2 wire?

[flightops]

The ideal approach is to minimise noise entering into your system at every "porthole". It's helpful to understand that there is noise heading towards your portholes that you can hear such as hums and buzzes (usually to do with mains electricity); there are "splats" from switches, refrigerators etc; there is also noise heading towards the portholes that you can't hear, such as radio frequency noises from WiFi, radio/TV towers etc. Because you can't hear these noises, you may assume that everything is OK, but it's not. The radio frequency noises can cause the input circuitry just inside the porthole to be overloaded or to resonate, which can reduce the quality of the performance. In these cases, your ears may not be sufficient to check what's happening and you may need test equipment.

Well, there's currently no "audible" noise in my subjective perception of the sound at the speakers. I'm not sure I'd want to worry chasing something I can measure but not hear.

Here's a question though: RCA shorting plugs. I've never quite understood the idea behind those cutting out RF as some claim. Nor why you woudl want to complete a circuit on an otherwise unused channel.

Well, the claim that they "keep dust out" of the RCA input is one I can see having some validity, but useful application? Not so much.

Just musing now....

 

[solderdude]

RCA shorting plugs.

The reason they exist is to lower hum and noise for not used inputs so these inputs are more silent when selected than without those. Usually the difference is non existant in practice but could be audible with phono inputs.

 

[flightops]

The reason they exist is to lower hum and noise for not used inputs so these inputs are more silent when selected than without those. Usually the difference is non existant in practice but could be audible with phono inputs.

So, if those inputs are never selected, it's never an issue?

I run everything through an aux in and switch with an RCA selector switch. Phono 1 and 2 sit unused (I use a preamp so internal phono stage is not used) but are never selected....same with the tape loops.

Is there not a concern with "shorting plugs" that they create a dead short on that input? or am I mixing up the speaker out concerns with the inputs?

 

[solderdude]

A short on an input is always preferable over an open input.
Shorting inputs is rarely ever needed though.
You just don't want that on any output.

 

[DonH56]

The only times I have seen (heard) any benefit from shorting caps was in a very high noise environment (think house near a radio transmission tower, or stereo on a wall near a furnace blower motor). The biggest difference I recall, from many years ago, was a HAM friend who's stereo would go nuts when he keyed his transmitter in the same room. Turns out the RF signal was causing an unused phono input to oscillate, and that was in turn modulating the (unregulated) power supply leading to both motorboating (LF pulsing) and squealing (was not sure if the squealing was via the supply or coupled from the oscillating stage). Putting shorting caps on the phono input solved the problem.

That said, RFI can be insidious, as it can "sneak" in and be modulated into the audio band as low-level "hash", or worse cause ultrasonic oscillations and ringing that is inaudible and yet suck power (headroom) from the signal chain and potentially heats up tweeters.

Aside: I used to make "shorting" caps with 50~100 ohm resistors in addition to a hard short as sometimes a dead short was not desirable.

 

[MaxwellsEq]

Well, there's currently no "audible" noise in my subjective perception of the sound at the speakers. I'm not sure I'd want to worry chasing something I can measure but not hear

You CAN hear the impact, BUT it won't sound like noise such as buzz or hiss. It will impact dynamic range or maximum power etc.

 

[sergeauckland]

Aside: I used to make "shorting" caps with 50~100 ohm resistors in addition to a hard short as sometimes a dead short was not desirable.

I have a number of terminations, short, 10 ohms, 75 ohms, 110 ohms, 600 ohms to be used according to circumstances.

S.

 

[flightops]

I've just ordered some shorting plugs off amazon:

If nothing else, the center post can be unscrewed and they'll just become "caps".

Looked through the manual for my Sansui 8080DB again and it does indeed stipulate shorting plugs for the unused phono inputs. Apparently, thy were delivered from Sansui with shorting plugs from the get go:

Can't hurt to try I guess. There is a bit of a hum/buzz in the unit, but it sound like a transformer/power supply inside the case itself. Not through the speakers. Still, the plugs are cheap and easy so why not try them?

Worse thing that can happen is I've wasted the cost of a cup of coffee or two (because even coffee is expensive these days!)

 

[mhardy6647]

Here's a question though: RCA shorting plugs. I've never quite understood the idea behind those cutting out RF as some claim. Nor why you woudl want to complete a circuit on an otherwise unused channel.

Well, the claim that they "keep dust out" of the RCA input is one I can see having some validity, but useful application? Not so much.

I believe that the bandwidth of dust is very different than that of RFI.

Seriously, the notion of grounding is, I think, quite simple. Any 'floating' piece of wire is an antenna. As such, it can pick up "RF", and, of course, the frequency of RF can range over many, many orders of magnitude. The 'hot' side of an "open" unbalanced connector is an antenna. DC ground and RF ground are not (necessarily) the same thing, but shorting the hot wire of the unbalanced input to ground will make it, at least, a less good antenna.

 

[flightops]

Well, Sansui put a “dead short” style cap on the 8080BD as delivered and I’m certainly not qualified to dispute them, so it’ll get shorting caps on the phono inputs.

The rest of the open rca inputs will just get covers. No one will see them anyways, but my minds eye will see missing caps everytime I look at the receiver if I don’t….

 

[Chrispy]

Well, Sansui put a “dead short” style cap on the 8080BD as delivered and I’m certainly not qualified to dispute them, so it’ll get shorting caps on the phono inputs.

The rest of the open rca inputs will just get covers. No one will see them anyways, but my minds eye will see missing caps everytime I look at the receiver if I don’t….

Just curious, how do you think you will benefit particularly from this implementation?

 

[flightops]

Just curious, how do you think you will benefit particularly from this implementation?

Won’t know until I try. As I mentioned, sansui shipped them that way, so there must be a reason. I can fix the circuits, but they designed them, so stands to reason they’re not going to spend money on parts they don’t feel they need. They are/were in the business to make money after all.

The plugs weren’t particularly expensive, so no harm if they don’t make any noticeable difference.

If they do nothing, it still might be fun when freinds cmne over and do the “ooooo, aaaahhh” thing at the new shiny bits on the receiver. I’ll tell them it blocks out the cosmic background radiation effects that interfer with the flux capacitor…..lol! Some freinds get my deadpan delivery of bs like that and we have a good laugh, some try the “oh yeah, oh yeah, I heard of that”. Both types always end up with a good chuckle when I give up the truth….lol!

 

[mhardy6647]

Won’t know until I try. As I mentioned, sansui shipped them that way, so there must be a reason. I can fix the circuits, but they designed them, so stands to reason they’re not going to spend money on parts they don’t feel they need. They are/were in the business to make money after all.

The plugs weren’t particularly expensive, so no harm if they don’t make any noticeable difference.

If they do nothing, it still might be fun when freinds cmne over and do the “ooooo, aaaahhh” thing at the new shiny bits on the receiver. I’ll tell them it blocks out the cosmic background radiation effects that interfer with the flux capacitor…..lol! Some freinds get my deadpan delivery of bs like that and we have a good laugh, some try the “oh yeah, oh yeah, I heard of that”. Both types always end up with a good chuckle when I give up the truth….lol!

When you do, you might also go for the gusto and invoke the name of Ted Denny III, of Synergistics Research fame.

 

[mhardy6647]

Oh, fwiw: With an RCA plug*, an itty-bitty bit of wire*, some solder* and a soldering iron, one may construct one's own shorting plug(s). Make as many as you want! Give 'em to friends as party favors or hostess gifts! Fun for the whole family!

______________
* Audiophile grade, of course, is always a tasteful option!

 

[MaxwellsEq]

I'm not trying to start a heated discussion on cables, I'm just trying to ask an honest "i don't know why" question because I'm not educated/trained in electronics. At least not to the level of understanding the theory/principles behind it in audio systems. Repair, replace and troubleshoot? I'm not too bad at that. Theory/principles? Pretty sketchy there....I'm more technician than engineer when it comes to electronic stuff.

Why do RCA interconnect cables all seem to be somewhat of a coax construction and not a simple 2 wire type?

I get that line levels are low strength signal and the shielding of an outer braid helps reject external "noise" sources like RF and EMI, but is it really that much of a problem if a simple two wire cord is used? Would they really be at risk of picking up RF/EMI in a home audio scenario?

I have cut open a set of the really cheapo rca cable before and they are even core and shield type construction, even if they are very (very) thin wires.

There must be a good reason for it because the cheapies would be even cheaper (IE: to construct) if they were simple two wire cords.

Again, not trying to start something, I honestly just want to understand the "why" of it.....

Hi @flightops you asked about some of the theory...

Are you aware that if you have a perfect hollow metal sphere, there can be no charge at any point inside the sphere? You may have heard of Faraday Cages, which work in the same manner. If you were inside such a hollow metal sphere of reasonable thickness and the sphere were struck by lightning or an EMP pulse, you would feel nothing. The charge hits the surface and is distributed evenly around it.

So if you set up a battery operated listening system inside a grounded metal sphere, you would experience no electrical or radio interference whatsoever, even next to a very powerful radio mast.

What's this got to do with RCA connectors? Well electronics gear is often inside a metal box! Radio and other noise impinging on the surface (assuming good joints) will be distributed evenly over the surface and will NOT penetrate! But there need to be holes for the connectors and noise penetrates through these holes. So, we extend the metal box along the wire. THIS IS THE KEY POINT! How do we do this? We put a SCREEN around the connector and the cable and electrically bond it to the box and also to the next box in line. If we are able to, we can include everything inside this one end-to-end hollow metal structure from the source until the end of line. We could screen the loudspeaker cables, but the voltage there is so high that it considerably drowns any electrical noise.

So the screen on the outside of the RCA cable forms part of the overall "seal" between the sealed boxes, that creates a virtual faraday cage for the installation.

 

[flightops]

Not to be a contrarian, but my receivers are "vintage" and have wood cases over a metal "tray-like" lower structure......so I would assume there's some type of additional shielding scheme to just a metal box. Beyond my pay grade, but as far as I can tell they reject any ambient noise sources.....

 

[flightops]

Oh, fwiw: With an RCA plug*, an itty-bitty bit of wire*, some solder* and a soldering iron, one may construct one's own shorting plug(s). Make as many as you want! Give 'em to friends as party favors or hostess gifts! Fun for the whole family!

______________
* Audiophile grade, of course, is always a tasteful option!

Yep, aware of that.

But the dedicated "plugs" weren't much money and they "look the business", even if they're functionally no different than diy.

 

[MaxwellsEq]

wood cases

Do not reject electrical and radio noise.

 

[flightops]

Do not reject electrical and radio noise.

Ah, yeah...they're obviously for appearance as much as anything else. Was the style of the day back then for the more expensive units.

I do understand some theory....was there more you were trying to say?

 

[Cbdb2]

Are you aware that if you have a perfect hollow metal sphere, there can be no charge at any point inside the sphere?

That should say there can be no electric field at any point inside the charged sphere (unless theres electric charge in the sphere).
RFI is caused by the fields so yea a metal enclosure prevents RFI getting in or out.
Think about a (perfect) conductor. If theres a field inside you have voltage across it so you will have current. If the conductor is floating the current (free electrons) move toward the positive voltage and pile up at the end of the conductor creating an electric field that opposes the applied field. Once the fields cancel each other out the field in the conductor becomes zero and no current flows.