Using only RCA Cables with 75 OHM Configuration for Single Ended Analog Inputs

[Albatross]

I saw this video today on YouTube,

. It’s D’Amore Engineering explaining that we should only use 75 Ohm cables with single ended analog inputs, and twisted pairs on differential analog inputs (more rare). This video is short and concise, and the logic makes sense to me. Unfortunately, I’m having some cognitive dissonance, since: 1) I have been in the audio industry for 22 years and never heard about this. 2) most cable companies don’t seem to follow this formula. 3) I’ve got a bunch of expensive cables that are twisted pairs on single ended component. So, if someone would just tell me, “Fuggit about it!”, I’d appreciate it. OK, jokes aside, I’d like more knowledge on this subject. Any cable aficionados out there with some insight on this matter?

 

[JP]

Well, coaxial cable - characteristic impedance isn't going to matter at the lengths people deal with at home. Seems odd to have a differential amplifier that only has an RCA input, but if you did, that'd be an appropriate way to cable it, though I'd still want an overall shield - like on the XLR connection that should've been used for that application.

 

[lini]

Didn't hear or see RF impedance explicitely mentioned in the video. But a 75 Ohm coax cable type is a smart choice, if one likes to make one's cables oneself and want's to be able to use the same, equally well-suited cable type also for electrical SPDIF, analogue video and, at least over here (*), also as radio/TV antenna cable (also including satellite and cable radio/TV) - so one can buy more at once and therefore typically at a lower price per length unit (foot, metre, whatever... ).

And as JP already wrote above, a differential (and/or) balanced input with RCA connection would be quite unusual. Most typical for such inputs would rather be XLR, TRS and screw terminal blocks with three contacts (for hot/+/phase, cold/-/inverted phase and shield ground) - for which one would usually use cable types with two twisted inner conductors plus shield.

Greetings from Munich!

Manfred / lini

 

[Catmanboo]

Just run what ya have, if connectors are compatible. The impedance gestapo wont know-if you don't tell. It's only audio frequency, not RF. If it doesn't work out, call me an idiot & spank my a$$!

 

[JoeESP9]

I saw this video today on YouTube,

. It’s D’Amore Engineering explaining that we should only use 75 Ohm cables with single ended analog inputs, and twisted pairs on differential analog inputs (more rare). This video is short and concise, and the logic makes sense to me. Unfortunately, I’m having some cognitive dissonance, since: 1) I have been in the audio industry for 22 years and never heard about this. 2) most cable companies don’t seem to follow this formula. 3) I’ve got a bunch of expensive cables that are twisted pairs on single ended component. So, if someone would just tell me, “Fuggit about it!”, I’d appreciate it. OK, jokes aside, I’d like more knowledge on this subject. Any cable aficionados out there with some insight on this matter?

After 50+ years with this hobby I've never heard of this before. 75 Ohm cables are designed to match an output that's expecting to see a 75 Ohm load. Many audio cables including some very expensive ones are not considered to be coax. His explanation doesn't work for me.

Besides, he's adding another op-amp stage. If you're concerned about impedance matching switch to balanced inputs and outputs.

The only time I've ever had to be concerned about a cable's Ohmage was years ago when I was called in to add another couple of stations to an old-style Novell network. The specs called for 50 Ohm coax. So, I arrived with a 100' roll of 50 Ohm coax and crimping tools. After wiring up the new workstations I found they wouldn't connect. It turned out whoever did the first installation used 75 Ohm coax (it was much cheaper than 50 Ohm coax back then). I had to replace all the 75 Ohm coax with 50 Ohm. That solved my problem. The client said it cleared up a bunch of small problems the network had been experiencing.