Eliminating Induction / Ground noise

[Piers1989]

Earlier in the year I put a new sound system in my VR4 to replace the dying speakers from the factory audio upgrade.

I made a thread at the time, but it consisted of:

Pumpkin Android KitKat head unit
Focal 6.5" Component fronts
Focal 6.5" coaxial rears
Alpine Multi-Channel Amp
JL Cleansweep (EQ flattener)
Custom enclosure 12" Sub

Due to my lack of time and patience (plus my tendency to break trim panel clips) I asked the garage to run all the wiring - they are a body shop so capable of this.
Again due to lack of time and patience I had them run the main power cable down the passenger side under the carpet / door trim.
I also ran standard cheap-o b&q speaker wire down the side, 4 times over (for each channel) from the stereo's pre-outs to the Cleansweep.

Obviously I now have a significant "hum/whir" with the car running, and even a whine with it off.
Both the CleanSweep and the Alpine are grounded together firmly to the chassis.
I know I need to replace the cable with better quality and ideally not run it parallel with the power, but I had an idea with my IT background - Shielded CAT7 cable.

I could run a single CAT7 cable from the head unit to the boot, 4 pairs, 23AWG, individually shielded and then with aluminium braid before the sheath too. See here https://www.amazon.co.uk/Ethernet-Streaming-Satellite-Receivers-Desktops/dp/B01L4664N0/ref=sr_1_10?ie=UTF8&qid=1483580639&sr=8-10&keywords=cat7
CAT7 also includes a ground wire to earth the braid. I was thinking I could actually tie the head unit ground to the ground in the rear with this too.
This connection would only take the low power pre-out signals, but as it is designed to take 10Gb Ethernet up to 100m the whole design is designed to remove Electro Magnetic Interference and signal degradation - sounds like what I want!

Depending on the results, some of my cars pre existing speaker wiring is starting to fail (drivers door cuts in and out which is very annoying), I was considering replacing some of the speaker wire with CAT7 too. Obviously not for the sub, but again the shielding sounds like it would be great. I could double up (as in like 2 pairs) the connections so run 1 CAT7 for the fronts and 1 for the back to double their current load rating, just to be on the safe side, but I can't imagine door speakers consuming enough current to heat up the wires anyway.

I can't find much info on doing this and when people proposed it with Cat5 on other forums (after a google search) it was generally frowned upon but not really for any good reasons I could see other than low quality cable, CAT7 doesn't have this problem!
Anyone with a background in this stuff have any input?

[Nick Mann]

I'm someone who is interested, not an expert.
I would guess that low level line signals should be fine.
What power is your amp? For example if it is 100W per channel and your speakers are 4 ohm then you can expect 5A current to be flowing. The current goes up as a square of the power. So 200W would be 20A.
100W would be close to the limit for swg 23 cable and potentially not good for music quality anyway! So I would say definitely not for the speakers.

[MarkSanne]

No experience either, but interested to see. One thing you can do right now without spending much time installing is see if there is any significant difference in resistance when comparing a few meters of normal copper (speaker)wire and CAT5 or CAT7 cable. If none or little: for for it! One thing though: if the core or the individual CAT5/7 wires are mono-filament and you'd use them in doors, I'd assume after so many open/closes (bending the wire back and forth) it could break.

[Piers1989]

I'm someone who is interested, not an expert.
I would guess that low level line signals should be fine.
What power is your amp? For example if it is 100W per channel and your speakers are 4 ohm then you can expect 5A current to be flowing. The current goes up as a square of the power. So 200W would be 20A.
100W would be close to the limit for swg 23 cable and potentially not good for music quality anyway! So I would say definitely not for the speakers.

Indeed Nick, thats what I was thinking.
The amp is 100W RMS per (non sub) channel, the speakers are 4 Ohm 80w RMS front and 60w RMS rear.

I don't tend to REALLY crank it, but with the idea of using 2 pairs to feed each speaker, this should double the cross sectional area of wire and double the current carrying capability?
At the moment the speakers are all fed from factory wiring.


No experience either, but interested to see. One thing you can do right now without spending much time installing is see if there is any significant difference in resistance when comparing a few meters of normal copper (speaker)wire and CAT5 or CAT7 cable. If none or little: for for it! One thing though: if the core or the individual CAT5/7 wires are mono-filament and you'd use them in doors, I'd assume after so many open/closes (bending the wire back and forth) it could break.

That's a good shout.
We don't stock CAT7 normally, but I have CAT5e and CAT6 lying around I will test. Looking at some CAT7 specification, it looks like resistance is around 70 Ohm / km. I guess I will need maybe 3m? So thats pretty negligible resistance. Obviously that's subject to change at higher current values.
All the cables are solid (mono filament) so yes I was considering using a small tail of copper speaker wire to go through the doors.

[Tony_T]

Speaker wires operate at high level and low impedance so there's absolutely nothing to be gained from using shielded wiring for them and that breakage problem will surely arise. Line level wiring is a different thing and if your system has facility for using balanced line outputs and inputs then that cat 5 or higher shielded cable will be excellent and unscreened speaker wire totally UNsuitable. For unbalanced circuits (RCA lines for example) then the problem is often more one of ground loop noise entry rather than shielding, although shielding is also necessary of course. The type of screened cable will have little effect on ground loops, you may need to consider one or more of the commercially made ground loop isolators to help with that issue if your amplifier doesn't have a true differential input.

[Piers1989]

Speaker wires operate at high level and low impedance so there's absolutely nothing to be gained from using shielded wiring for them and that breakage problem will surely arise. Line level wiring is a different thing and if your system has facility for using balanced line outputs and inputs then that cat 5 or higher shielded cable will be excellent and unscreened speaker wire totally UNsuitable. For unbalanced circuits (RCA lines for example) then the problem is often more one of ground loop noise entry rather than shielding, although shielding is also necessary of course. The type of screened cable will have little effect on ground loops, you may need to consider one or more of the commercially made ground loop isolators to help with that issue if your amplifier doesn't have a true differential input.

Thanks for this helpful reply.

I don't think I'll bother running this for the speakers after the feedback.

I have used commercial ground loop isolators before, and found there to be a pretty significant reduction in quality. Granted they were very cheap, but I couldn't see much better ones last time I searched for them.
If I were to use the ground wire from the CAT7 to connect to the ground on the stereo, would that help? I don't REALLY understand what ground loops are or how to combat them.
For reference they outputs are unbalanced RCA.

I'm considering trying bypassing the CleanSweep as it it designed to clean a signal if the head unit outputs it with shifted EQ (for defeating factory fit stereos when upgrading sound system). It should not be needed in this case and may actually be making things worse.