RCA noise - Page 1
Forum section: electronics / projects

author: pree
author_rank: Contributor
timestamp: October 21, 2024, 12:37:48 pm
content: Hi,I have a speaker which has different modes such as Bluetooth, AUX and RCA. Whenever I put the speaker in RCA mode and connect the RCA cable without connecting to a device, I could hear this buzzing sound. This sound only comes when the speaker is in RCA mode and the noise varies if the RCA cable long or short.I tried to short the chasis ground to the analog ground. But nothing works.On HW the structure is like that:1. AC-DC module which provide 24V to the main board which has the AMPS, DSP and RCA connectors.2. I tried to cover the SMPS in a farday cage too, nothing helped.3. I put ferrite clamp on the DC input to the main board. that did not help either.Can somebody help with this?attached is the block diagram and rca input schemaitcs

author: CaptDon
author_rank: Super Contributor
timestamp: October 21, 2024, 12:53:10 pm
content: An unterminated cable is an antenna!! Plug the cable into something as is the normal use. A guitar cable not plugged into a guitar will produce all sorts of noise through the guitar amplifier. The answer to the question is obvious, we don't listen to unterminated RCA cables....unless you want to hear local AM radio stations without a radio!

author: pree
author_rank: Contributor
timestamp: October 22, 2024, 07:55:28 am
content: @CaptDon thanks for the input. However, even if the the RCA cable is connected to the load such as vinyl player. I could hear some buzzing noise and that can be heard.

author: moffy
author_rank: Super Contributor
timestamp: October 22, 2024, 08:48:25 am
content: If the noise increases with RCA cable length, then the cable is picking up stray fields as the noise source. Try inserting the cable and shorting the unconnected end of the RCA cable i.e. signal to shield, and see if the noise disappears or changes. Make sure you are not touching the shorted end as the human body is also a fair antenna.

author: ftg
author_rank: Frequent Contributor
timestamp: October 22, 2024, 09:15:03 am
content: Some of the cheaper RCA cables just use two wires, instead of proper shielded wire. So that might also contribute to the noise heard.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 22, 2024, 09:19:52 am
content: Quote from: pree on October 22, 2024, 07:55:28 amUnless your record player has line level outputs, or your speaker has phono-level inputs, that's normal. It would help if you gave model numbers here.

author: pree
author_rank: Contributor
timestamp: October 22, 2024, 11:57:12 am
content: the vinyl player model is Rega rp6.

author: pree
author_rank: Contributor
timestamp: October 22, 2024, 12:36:50 pm
content: @moffyI insert the cable in and unconnected end i short with the shield or chasis ground. This I understand as it is just closing the antenna loop or shorting the end.

author: pree
author_rank: Contributor
timestamp: October 22, 2024, 12:38:56 pm
content: I tried with good qlality Shielded cables, but still when i connect them and can hear the noise and this noise level I can reduce it with volume knob.

author: moffy
author_rank: Super Contributor
timestamp: October 22, 2024, 01:05:46 pm
content: Quote from: pree on October 22, 2024, 12:36:50 pmYou have a signal, you can control its level by the length of cable or the volume control it might be induced through the shield of the cable to ground of the unit.

author: janoc
author_rank: Super Contributor
timestamp: October 22, 2024, 01:15:13 pm
content: That's common mode noise. What you are likely hearing is mains hum. Your Faraday cage of the power supply or ferrites do nothing with this because this is 50-60Hz low frequency hum conducted over the wire, not radiated from the supply. Never connect signal and chassis ground together! That ensures that you have ruined the star grounding scheme the amplifier likely uses, introducing more hum.If the cable is not connected at both ends that's normal - you have built an antenna. That will hum regardless of shielding of the wire. Hearing hum is completely normal and expected in such case.If the cable is connected at both ends and you still hear this, then (assuming your record player is not producing this hum and outputting it on the connector - did you test that?) you have a ground loop between your two devices. The grounds of the two devices don't have the same potential and you get AC current flowing over the shield of the cable equalizing it. This is what you hear. The solution can be tricky, depending on the state of your house wiring, what options you have, etc.- Connect both devices to the same mains outlet. That will minimize the length of the loop and hopefully do away with the problem.- Make sure both devices have properly earthed mains connections (unless they have only a two prong plug) - then the currents will hopefully flow through the mains wiring and not your signal cable. Assumes your house wiring is in order, obviously.- Use a good quality shielded cable. Don't use longer cable than required. Ideally the shield shouldn't be used for signals (i.e. don't use a wire where the shield is used for signal ground!), shield should be connected to chassis ground at one end only. The signal ground should be connected to one of the internal wires inside the cable. Not the shield! Unfortunately, a lot of devices don't have sockets wired for this, the shielding is often used for signal ground as well - with all the problems that brings.- Use an audio isolation transformer. That will cure the hum for sure but likely introduce distortion. Possibly not what you want.- Use a symmetric (differential) connection between the devices - then there is no common ground and this problem can't occur. Commonly used by recording studios but likely not practical at home, with single-ended devices.- Use a main isolation transformer for one of the devices - will cure the ground loop but depending on how your device is built it may compromise protection against electrocution. That would be a solution of last resort because if you get unlucky and something goes wrong, you could easily get an electric shock and things like GFCI in your house won't trip because of the isolation transformer. Same caution applies to the various external grounding wires used to connect devices together - that's the job of the protective earth pin in the outlet. If you don't have it and do this instead, in some situations you could get shocked because one broken device could make other devices that aren't even powered up live.

author: CaptDon
author_rank: Super Contributor
timestamp: October 22, 2024, 01:16:29 pm
content: This sound like the typical 'ground loop' noise. So the vinyl player has its own mains plug and the stereo sound system has its own mains plug and probably neither one has a ground pin. Turntables (vinyl players) years ago has a dedicated ground wire attached to the turntable chassis that would get connected to a dedicated ground lug on the stereo to reduce this problem. The problem can be so annoying that people resort to using a small isolation transformer (100va to 200va) to power the turntable and eliminate ground loops. This has always been a problem with 'component stereo systems'. Light dimmers, variable speed ceiling fans, nearby SMPS power supplies will all cause this problem and often it is very hard to get the noise down to an acceptable level.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 22, 2024, 11:00:02 pm
content: That turntable has phono-level outputs. Plugging that into a line-level input will just get you noise.You need a phono pre-amp or what Rega calls a "stage" apparently.Most turntable's phono outputs have a ground lead but not Rega apparently.

author: pree
author_rank: Contributor
timestamp: October 23, 2024, 08:58:34 am
content: thanks for the input. This is what I thought of that it is Common mode noise, so my plan is to use CMC on the RCA In to mitigate that. I will check if that helps or not.

author: pree
author_rank: Contributor
timestamp: October 23, 2024, 11:26:01 am
content: the vinyl player has a earth connector which can be connected to the preamp chasis ground. But the speaker and the vinyl player does not have a seperate ground. both the devices have mains plugs with two pins(EU plug)

author: CaptDon
author_rank: Super Contributor
timestamp: October 23, 2024, 12:27:55 pm
content: In the case of the two pin plug, reversing one of the plugs (if possible) may help. There are mains noise and spike filter capacitors from mains to chassis internal to the unit and they are responsible for a huge part of the ground loop noise on equipment using only a two pin mains plug.

author: pree
author_rank: Contributor
timestamp: October 24, 2024, 07:05:37 am
content: if this is the Common mode noise and it is coming from the mains. I think then i need to filter this noise on the PSU. which is a AC-DC module.Probably using a CMC or trying different values of Y capacitors.

author: janoc
author_rank: Super Contributor
timestamp: October 24, 2024, 05:15:06 pm
content: Quote from: pree on October 24, 2024, 07:05:37 amI think you need to re-read the thread ...Neither a common mode choke or a Y capacitor have nothing to do with (and will do nothing about) mains hum. It makes a big difference whether your common mode noise is at 50-60Hz caused by bad grounding - or at 100kHz coming from a noisy switching power supply or mains and where that choke or Y capacitor would be relevant.

author: radiolistener
author_rank: Super Contributor
timestamp: October 24, 2024, 07:27:27 pm
content: Chinese RCA cables don't have shield, so it works like antenna

author: pree
author_rank: Contributor
timestamp: October 25, 2024, 08:46:56 am
content: the noise is disturbing in the range 400Hz to 2Khz.

author: pree
author_rank: Contributor
timestamp: October 28, 2024, 07:53:23 am
content: as i mentioned the noise peaks are ranging between 400Hz to 2KHz.