John K Posted June 12, 2017 Posted June 12, 2017 I've got a powered sub woofer on the end of a 12mtr home made cable (RG6 with soldered plugs). If I crack the sub woofer amp to max (it's 250w) there is a very, very quiet 50hz hum - I mean really quiet. You have to put your ear next to the cone to hear it. Definitely from the cable because if I unplug it from the AVR, it's still there, if I totally unplug it from the woofer (so no signal cable to the woofer at all) the hum goes away. There is no leak from the signal core to the screen (that my multi meter can find on 'ohms') and the coax doesn't run anywhere parallel to any mains cables at all. The closest it gets it a perpendicular crossing 4" from a ring mains cable. This is all a dry run before final fixes so I can still change things - and I do intend to move the sub a few feet from the amp and use a dedicated manufacturers cable to see if it still hums. Just wanted to check my expectations with you lot. Again it's quiet, with the sub amp on 50% you can't hear it. But you can't watch "San Andreas" or "John Wick 2" with the sub on 50%... FYI the LFE crossover on the sub is set to 120hz and the AVR amp set to the same. Makes no difference is the amp LFE is set to "LFE Only" or "LFE + Mains" Quote
corsechris Posted June 12, 2017 Posted June 12, 2017 RG6 is really intended for RF applications. It's a 75 Ohm cable so technically an impedance mis-match for your sub input I'd imagine. That said, it is well screened. on a practical level, does it matter that there is a hum you can't hear unless you put your ear to the speaker? I doubt you'll be getting that close with program material going to it? 1 Quote
John K Posted June 12, 2017 Author Posted June 12, 2017 7 minutes ago, corsechris said: RG6 is really intended for RF applications. It's a 75 Ohm cable so technically an impedance mis-match for your sub input I'd imagine. That said, it is well screened. That's interesting, didn't have enough technical knowledge to realize that... I picked the RG6 because it was so well shielded and thought that was all that mattered. As this is a dry run, the cable could be change to something else. 8 minutes ago, corsechris said: on a practical level, does it matter that there is a hum you can't hear unless you put your ear to the speaker? I doubt you'll be getting that close with program material going to it? On every level you are right - but once heard, never unheard... Quote
Wobblyprop Posted June 12, 2017 Posted June 12, 2017 Your cable is acting like an antenna, it's picking up regardless of whether the AVR end is plugged in. What's the input connector on the sub? In fact, let's make it easy - what's the sub? I run twin subs with no hum, so it can be done Quote
Grizzlee Posted June 22, 2017 Posted June 22, 2017 Are you sure it's the sub and not just you tuned into the mains? 1 Quote
John Dolan - Wirral & North Wales AO Posted June 22, 2017 Posted June 22, 2017 Those old style hearing aids can be a problem.. 1 Quote
SootySport Posted June 23, 2017 Posted June 23, 2017 Not surprised there is a slight hum if you turn a 250w sub to max. You can't have it that loud when listening to music or film, can you? Certainly couldn't with mine which is only 120watt, as the house would shake itself to rubble. The 50hz hum is just it's integral power supply, none of them are completely silent unless you are spending big money. Quote
John K Posted June 24, 2017 Author Posted June 24, 2017 21 hours ago, SootySport said: The 50hz hum is just it's integral power supply, none of them are completely silent unless you are spending big money. It's a Dali EF9 so decent but no mega cash. I don't think its the internals because with no cable attached you can max the internal amp and it's silent. I'm failr sure its the cable and I'm getting used to it now. Know what you mean about the volume, mine is set to 50% and it is plenty loud enough..! Quote
John K Posted June 26, 2017 Author Posted June 26, 2017 Looks like I have a faulty plug, also explains why the hum wasn't there from day one. I could have sworn the sub was OK on day one and the hum 'arrived', but we all know the tricks our minds can play Well having had a good fiddle round the back (excuse me vicar) it looks like one of the RCA plugs is faulty, the core is spinning in the insulation and the hum comes and goes. So new plug will go on tomorrow and we'll try again... Quote
Lyonspride Posted June 30, 2017 Posted June 30, 2017 Just a thought..... Do you have powerline adaptors for your home network? These dump massive amounts of noise onto the mains, so much so that it can actually turn your house into a transmitter capable of blocking HAM radio. After installing mine, I found the noise was even setting off my security lights. Quote
John K Posted July 9, 2017 Author Posted July 9, 2017 On 30/06/2017 at 14:41, Lyonspride said: Do you have powerline adaptors for your home network No, but I've heard they can be noisy as hell. As per another thread I'm a saddo and cabled the house with LAN cables. Quote
John K Posted July 9, 2017 Author Posted July 9, 2017 And I solved the problem by getting a proper sub cable. Got a 7.5 metre (just long enough) Cambridge Audio cable and the hum is gone and the cable has the advantage of being thinner and more flexible then the RG6. Still believe I made the RG6 cable up well enough and there was something else in play, possibly like CorseChris said, the impedance. So it's back up to eye watering, bowel loosening volume levels again..! Quote
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