Why do RCA cables have a 75 Ohm impedance?

[AnalogSteph]

So I'll be ok with a bundle of cheap/simple cables going from the pre-amp to the amplifier? No interference/crosstalk/whatever?

The parameters you are (potentially) interested in at audio frequencies are:
* shield resistance
* shield coverage
* parallel capacitance

The longer the cable, the more critical it is. For a short run that doesn't involve any ground loops, most any half-decent cable will generally get the job done.

Cheap generic RCA cables can be pretty pathetic garbage in both cable stock and connector quality, so don't buy the cheapest Chinesium jobs you can find... not like price necessarily is much of any indication, mind you. Looks can be deceiving in both directions (some generic-looking cables shipped with hi-fi gear actually are quite decent, while a fancy-looking cable up on Amazon may turn out to be a bust).

It just so happens that proper coax tends to be above average in the above parameters, so it generally isn't wrong. When I needed some decent 10 m long audio cables back in my student days, the most cost-effective solution - short of whipping out the soldering iron - turned out to be buying composite video cable (using actual RG-59 coax).

 

[FriedChicken]

I just got a sony amplifier (STR-DB930) hooked up and tested.

I used the cheapest shittiest cables, and I'm getting an audible hum. It didn't do this when I had just the fronts connected (with monster cables that were gifted to me), but does this with the surround speakers connected.

In retrospect, it's possible it's not humming on the front channel but only the center and surround channels. I'm not sure I didn't test it.

It's a clear buzz that my subwoofer also makes on occasion.

Do I need better cables?

 

[SIY]

Do I need better cables?

More likely, you have a ground loop and will need to determine what needs to be isolated to break it.

 

[FriedChicken]

More likely, you have a ground loop and will need to determine what needs to be isolated to break it.

sdakhals;kjdfhj


How can there be a ground loop?

 

[SIY]

sdakhals;kjdfhj


How can there be a ground loop?

Not uncommon. But easy to fix. Try different outlets for different components as a start.

 

[FriedChicken]

Not uncommon. But easy to fix. Try different outlets for different components as a start.

The receiver is has a two-prong outlet. Maybe one of the RCA cables is touching the chassis? Or are they designed to do that?

Or maybe one of the cables i'm using isn't actually a coaxial cable, just an RCA cable?

 

[DonH56]

Turn off and then unplug the sub when the system is humming and see if that makes a difference. Then try other cables, making sure to turn down the volume and mute the amp before touching anything.

 

[FriedChicken]

making sure to turn down the volume and mute the amp before touching anything.

o_o

 

[egellings]

Standard analog audio small signal cables are not designed to have a 75 ohm characteristic impedance, as that has no bearing at all at audio frequencies for these cables.

 

[Scoox]

For analog audio signals, characteristic impedance of a cable has no bearing at all.

And that's why we can use AES cable for carrying analog signals. The question I'm trying to find an answer to is: does it cost more money to make, say, 110 Ω cable than regular microphone cable? Because if it doesn't, it'd be a lot easier to use 110 Ω cable for both, and then we'd be able to use any microphone cable for carrying AES too.

 

[Speedskater]

The newer AES 3id standard has very close tolerance for the 110 Ω (and 75 Ω) Radio Frequency Characteristic Impedance limits.
Actually mic cables are a sub-set of Shielded Twisted Pair (STP) cables. Probably not the best choice for AES interconnect cables.
And bulk cable manufactures have been making lots of model number balanced interconnect and mic cables for decades, so demand may remain constant.