Interesting phenomena I'm not sue I can explain about cable behavior

[PHD]

Hi

I have two Amphenol RCA cables of the same length but from different models/colors. Both seem to work fine when used to hook AVR subwoofer out to the subwoofers.

But, when I connect RCA one cable from the subwoofer's LFE input and then touch the signal lead on the other end, I hear a loud 100Hz hum, which is expected.

But, when I replace the RCA cable with the second one (same length) and repeat the same test, the hum is much weaker and at a higher frequency.

What could explain such behavior? Better shielding on the low hum cable? Or perhaps the low hub is faulty with some sort of attenuation?

Thanks

 

[Speedskater]

How about trying some other brands of RCA cables.

 

[Doodski]

Hi

I have two Amphenol RCA cables of the same length but from different models/colors. Both seem to work fine when used to hook AVR subwoofer out to the subwoofers.

But, when I connect RCA one cable from the subwoofer's LFE input and then touch the signal lead on the other end, I hear a loud 100Hz hum, which is expected.

But, when I replace the RCA cable with the second one (same length) and repeat the same test, the hum is much weaker and at a higher frequency.

What could explain such behavior? Better shielding on the low hum cable? Or perhaps the low hub is faulty with some sort of attenuation?

Thanks

The hum is either 50Hz or 60Hz dependent on your location in North America or maybe Europe etc. It is possible that you have a cable that is more prone to picking up the EMF from the air. Don't worry about it this is normal and use the cable that works.

 

[jmdesignz2]

Not sure I understand what you’re doing

Are you using two different rca cables
One touching the other makes a hum?

 

[Sokel]

Do you have a DMM to measure them?
Cross measure at resistance mode at all possible ways and you may find the cause.

 

[WXfreak]

The difference you are hearing can come from how the two cables are built, even if they are the same length. Some RCA cables use a full braided shield, while others use a spiral wrap or foil layer. A braided shield usually blocks more of the 50 or 60 Hz hum your body introduces when you touch the pin. A cable with lighter shielding may let more of that noise through, so it sounds louder and lower in tone. Internal layout and capacitance can also change how the noise comes across.

If you want to test further, you can swap the ends of each cable and see if anything changes. You can also check continuity on the tip and shield with a meter to make sure both are solid. Another option is to send an actual signal through both cables and compare the output instead of using the touch test. A weaker hum does not always mean a bad cable. It may just have different shielding or construction. If both work fine with the subwoofer, you probably do not have anything to worry about.

 

[DVDdoug]

Be careful when doing that! You can sometimes get a high-voltage static discharge from your body directly into the electronics. And in some cases you could get a full-power signal and blow a speaker! Turning-down the volume can help but that might not protect from static.

I'm not sure what would cause that difference... But touching ground at the same time will make a difference.

The actual frequency shouldn't change but you can sometimes get harmonics which makes more of a buzz than a hum. But I can't explain why that would be different either...

 

[RCVTech]

The difference you are hearing can come from how the two cables are built, even if they are the same length. Some RCA cables use a full braided shield, while others use a spiral wrap or foil layer. A braided shield usually blocks more of the 50 or 60 Hz hum your body introduces when you touch the pin. A cable with lighter shielding may let more of that noise through, so it sounds louder and lower in tone. Internal layout and capacitance can also change how the noise comes across.

If you want to test further, you can swap the ends of each cable and see if anything changes. You can also check continuity on the tip and shield with a meter to make sure both are solid. Another option is to send an actual signal through both cables and compare the output instead of using the touch test. A weaker hum does not always mean a bad cable. It may just have different shielding or construction. If both work fine with the subwoofer, you probably do not have anything to worry about.

I agree. Extra foil and thicker shield seem to make a difference in blocking 60Hz from the mains.

 

[Speedskater]

Yep, a heavy braided shield can reduce AC power line related Common Impedance Coupling Noise current. By its low end-to-end resistance. While a foil shield or extra foil won't.

 

[WXfreak]

Dave Rat released this video yesterday. While it is only somewhat related, since the cable lengths discussed are different, it presents his perspective on balanced cables for professional audio venues. I found it interesting as someone who is also interested in the production and recording side of audio.