50hz mains hum - Page 1
Forum section: electronics / projects

author: m3vuv
author_rank: Super Contributor
timestamp: April 07, 2020, 02:08:39 pm
content: Hi all,im trying to run the audio from my lappy headphone jack thru my amp using the headphone output,ive got a 3.5mm male jack to rca converter/adaptor,as soon as i plug it in the lappy i get a terrible mains hum(loud),if i unplug the lappy power adaptor its fine,also i seem to get a lot of 50hz mains interferance on all my psu's and scopes,could this be faulty mains wireing?,any ideas welcom,73 m3vuv.

author: magic
author_rank: Super Contributor
timestamp: April 07, 2020, 03:13:36 pm
content: Most likely the laptop PSU doesn't connect its secondary side to safety earth so that all the charge injected by capacitance in the PSU stays on the secondary and flows to earth through the audio cable's ground once that is connected.Do you have differential (so-called "balanced") inputs on the amp?

author: m3vuv
author_rank: Super Contributor
timestamp: April 07, 2020, 03:17:25 pm
content: no just normal rca style phono jacks.,also the cable for the lappy psu is a figure of 8 type so has no earth/ground,would it be better to mod it with a clover leaf type with an earth?

author: Gyro
author_rank: Super Contributor
timestamp: April 07, 2020, 04:23:35 pm
content: You may want to check continuity of the screen connection between the 3.5mm jack barrel and the RAC skirts, and also tightness of the RCA skirt contacts. The screen connection should ensure that the Laptop ground and your audio input. At one time you could rely on such things but there's so much crap for sale these days... There should be no (normally) measurable screen connection resistance.

author: m3vuv
author_rank: Super Contributor
timestamp: April 07, 2020, 08:30:44 pm
content: they seem ok,it only does it when the lappy is running from the mains,its ok just on batterys,i need a fix!

author: Gyro
author_rank: Super Contributor
timestamp: April 07, 2020, 08:51:52 pm
content: That's odd. Normally 50Hz hum injection requires alow impedanceground loop between the audio output ground and audio input ground (in order to generate any significant voltage across the cable screen). I can't see how that is possible if the laptop PSU has a 2 core mains lead.Maybe you're getting a buzz rather than hum - high frequency hash generated by the PSU which is modulated at 50Hz (more likely 100Hz in reality).That doesn't really help with a cure though - unless wrapping the lead through a ferrite core helps.

author: m3vuv
author_rank: Super Contributor
timestamp: April 07, 2020, 09:30:50 pm
content: well it sounds deffo like 50hz,it is more of a buzz tho,its like grabbing the center conductors going to the amp,ie mains pickup.

author: Electro Detective
author_rank: Super Contributor
timestamp: April 07, 2020, 10:30:52 pm
content: Have you tried flipping the Figure 8 cord if it's a 2 prong type?Try keeping the laptop power supply far away from the laptop, amp and audio cablesOtherwise you're stuck with 'dirty' SMPS style junk circulating at the mains power neutral to earth and into the audio return path earth etcWell, something loopy nasty like thatYou could buy/try a stereo or two mono audio isolation transformers and play with the earth ground switch/es on thembut keep the receipt and packaging with a firm OK to return them if still noiseFYI: audio transformers can differ in performance for various reasons including price,and some are just plain wired wrong, they pass the audio great, including the offending hum and bonus harmonics and -BUZZzzz- !so try a few out, even a cheapie car audio one might get the job done and sound ok with some EQ tweaks if requiredSometimes those laptop power supplies spew out interference that goes everywhere into everything,usually something inside is cooking, died, dying, or a half baked suppression design, and you can't winso battery power is the only optionBeen there...

author: magic
author_rank: Super Contributor
timestamp: April 08, 2020, 07:11:55 am
content: I forgot about one thing: turn the volume up on the laptop and use the volume knob of the amp.Isolation transformers should work. Alternatively, a few opamps and well-matched resistors can make a solid state differential to single ended converter which will work OK if the laptop is grounded by other means, e.g. a USB cable connected to the amp's chassis on the other end.Quote from: Gyro on April 07, 2020, 08:51:52 pmAs they say, it takes 1mA through 1mΩ to make 1µV of noise.