Shielded against 60Hz speaker cable - Page 1
Forum section: electronics / projects

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 15, 2019, 11:13:12 pm
content: I am having a heck of a time trying to eliminate a 60Hz hum from one of my speakers. So, I was wondering if someone makes a 60Hz shielded cable that I could use.Or could I possibly wire some kind of a filter at the end of the speaker cable that would filter out the 60Hz hum.thanks

author: Cliff Matthews
author_rank: Unknown
timestamp: March 15, 2019, 11:43:26 pm
content: Look for hum in the gain stages of audio amp and not the speaker cables. Amp outputs are low impedance 4-16 ohm, so it's an unlikely source. Perhaps you have an amp with separate bridges and caps for each channel? (one leaky filter cap maybe?)It's also possible the biasing on that channel is very close to frying an output transistor, so with no signal input you should have almost no AC or DC offset voltage on that channels output terminals. You'll need to provide a bit more info..

author: bson
author_rank: Unknown
timestamp: March 15, 2019, 11:44:14 pm
content: I'd go look for a ground loop, bad filter cap in the amplifier/source, or other causes.

author: soldar
author_rank: Super Contributor
timestamp: March 15, 2019, 11:50:06 pm
content: Quote from: Hextejas on March 15, 2019, 11:13:12 pmHave you tried switching the speakers one for the other and see if the hum stays with the speaker or with the amp channel?

author: DimitriP
author_rank: Super Contributor
timestamp: March 16, 2019, 12:25:14 am
content: (tapping impatiently)

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 16, 2019, 02:34:49 am
content: Yes I have switched them and the hum stays with the channel. Good idea though. Thanks.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 16, 2019, 03:08:36 am
content: More setup info req'd... are these powered or bi-amped speakers? Make model of amp etc.. move the problem channels cable away from in-floor / in-wall mains wiring (on the off-chance amp has some weird bootstrap cap feeding back something from the cable..)

author: soldar
author_rank: Super Contributor
timestamp: March 16, 2019, 10:59:37 am
content: Quote from: Hextejas on March 16, 2019, 02:34:49 amSo the problem is with that amplifier channel and not with the speaker or its cable (as was 99.9% likely). Why are you looking for shielded speaker cable? Why are you not troubleshooting that amplifier channel?Are both channels powered from a common supply? Is the supply stable? Have you checked all the electrolytic caps on that channel?

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 16, 2019, 12:30:46 pm
content: I think that it has to do with the position of the speaker cable. Because when i elevate the offending cables the hum goes away. I have tried to move them as far away from power cords and that helps. There are 2 speaker cables to the right speaker due to the use of a sub woofer. And the sub woofer is about 20’ feet from the amp and the speaker cables have to avoid 3 power cords. If i hold the cables about 4 feet above the floor the hum disappears.The idea of rebuilding the amp does not appeal since i have already done it 4 times due to another mistake.There is a separate power supply feeding both amps,

author: Cliff Matthews
author_rank: Unknown
timestamp: March 16, 2019, 01:00:42 pm
content: Looks like you're getting closer to solve this.. So we see the offending speaker cable runs from an amp with its own power supply is running parallel with the other speaker cables. It may be that amps power supply is floating and the other channels amp and the sub-w's amp are at the same ground potential. Time to breakout a DMM and measure grounding disparity.

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 16, 2019, 02:24:04 pm
content: Quote from: Cliff Matthews on March 16, 2019, 01:00:42 pmCliff, thank you. I would love to do this if I knew how. I am new to this electronic "stuff" and it's mostly a mystery. Can you explain in simple terms how to do this ? I will be forever in your debt.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 16, 2019, 03:52:57 pm
content: You must have a volt-meter, since you've already rebuilt the amp 4 times?1) Power-on both amps (minimum volume and nothing connected to inputs) measure and report DC and AC differencesbetween both amps output terminals.2) Report any DC voltage present (however small) at the output terminals on each amp.3) Power-off both amps, disconnect from mains and wait a few minutes for amp PSU caps to discharge. Report resistance between positive and negative output terminalsto chassis groundon each amp.4) Still disconnected from mains, report resistance from the chassis of each amp to its wall plug connector (to see if open).There may be other checks others may want to add, but these are just a few.

author: coromonadalix
author_rank: Super Contributor
timestamp: March 16, 2019, 06:13:47 pm
content: you can short your amp input at 0 ohms and see and follow your hum in the circuits ... the short will garanty nothing enter or no antenna effects / noise picking, or defective signal input cables.Even saw long time ago, bad grounding for the frame screws to a ground tab/log on pcb's, even checked main ac sockets for proper wiring ...What is the amp model ??

author: DimitriP
author_rank: Super Contributor
timestamp: March 16, 2019, 06:41:15 pm
content: QuoteDid you end up removing temporarily or routing the speaker wires away from the UPS?

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 17, 2019, 03:26:04 pm
content: Quote from: DimitriP on March 16, 2019, 06:41:15 pmI did this partially which I think helped the left speaker. The amps and power supply are on a mains circuit by themselves. The left speaker is free of hum. But there is a UPS in the path of the right speaker so I will try removing it entirely.Separate question. The UPS has a heavy battery in it. Can the battery contribute to the hum problem ?The sub woofer I think, is part of the problem. It is located near the right speaker with its own power cord plugged into a separate wall receptacle. For a short length, the right speaker cable parallels the sub woofer power cord.I am gonna take the sub woofer out of the chain and see what happens. I will post my findings later.I really appreciate all this excellent help.

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 17, 2019, 08:34:21 pm
content: Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pmPlease be aware that this is new to me so I might be doing things the wrong way. For task 1 I measured the voltages with the probes on the +(Red) and -(Black) of the amp outputs which are what are connected to the speakers.So:Left DC =0Left AC=0Right DC=.003Right AC=0Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pmI am assuming that you want the voltages measured with a reference to chassis ground.So:All are 000 with the exception of the Right DC=.005Quote from: Cliff Matthews on March 16, 2019, 03:52:57 pmAll readings are ohmsLeft --- = 0kLeft +++ = 41.3kRight --- = 0kRight +++ = 41.7kQuote from: Cliff Matthews on March 16, 2019, 03:52:57 pmNo connections are open and they report a small ohm reading.These readings might give you a clue but they are meaningless to me. However I wonder where the small DC voltage on the right channel is coming from.I removed the sub woofer from the chain and it made no difference. I routed the cable many different ways and was able to get the hum to a minimum level though I hear a strange tic, tic, tic sound like the sound at the end of a record being played. There is also a faint signal from a radio station. Weird !The problem I have with the present routing of the cable has to do with an unhappy spouse. Admittedly the routing is ugly.

author: Cliff Matthews
author_rank: Unknown
timestamp: March 17, 2019, 10:40:51 pm
content: All looks OK, but the presence of a UPS I did not know about.. some UPS's run a stepped square-wave through the output transformer even when idling, so this could be the source. Why it's even never near an audio system I cannot guess.

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 20, 2019, 03:34:06 am
content: Well, I saw that one of the input RCA plugs was slightly loose so I decided to fix it. Then I discovered that the grounding wire was not making contact with the chassis, a poor wiring job. When I corrected that, the 60Hz hum pretty much disappeared..So, I owe a hearty thanks to all that helped me and sorry for the shoddy job.

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 20, 2019, 04:32:42 am
content: Quote from: Hextejas on March 17, 2019, 03:26:04 pmNot the battery itself but I used to have a Belkin UPS that used a pulse charging circuit that really likes to induce noise into nearby analog audio amplifiers. It worked by shorting the low voltage side of the transformer for a brief moment and then letting the leakage inductance work as a boost converter. It doesn't help that it was a $10 or so special with a plastic case...

author: DimitriP
author_rank: Super Contributor
timestamp: March 20, 2019, 07:16:51 pm
content: QuoteIt could've been a lot worse: Somone could have posted a part number for shielded speaker cables.Be glad no one did

author: Hextejas
author_rank: Frequent Contributor
timestamp: March 21, 2019, 11:06:04 am
content: Quote from: DimitriP on March 20, 2019, 07:16:51 pmeHahahaha well I deserve to have my wrist slapped.