DRC Posted June 4, 2018 Share Posted June 4, 2018 I'm having some issue with a bit of subwoofer hum. The sub is quiet when powered on, and only develops a hum once it is connected to a Receiver (RCA to RCA). I don't have a spare receiver to check with, but have had similar hum when connecting the subwoofer to other RCA outputs (eg Left channel out) on the receiver. On quiet passages the hum is noticeable from the seating position, but not really noticeable otherwise. it isn't a transformer type of hum from DC. I've placed a DC blocker (PS Audio Humbuster) on the power side of things as a precaution, and moved around the power cabling just in case of a ground loop issue with no effect. the receiver is single ended (RCA) output, but the sub will accept XLR or RCA. Using an adaptor and running RCA out of the receiver then XLR into the sub does help reduce the hum a little. The 4M rca sub cable I have been using is not expensive, could a cable with better shielding help? any troubleshooting advice would be appreciated. Link to comment Share on other sites More sharing options...
warweary Posted June 4, 2018 Share Posted June 4, 2018 Have you tried swapping the rca cables around? Some of those single ended cables have the shielding/grounding at one end but not the other. Subwoofer hum sucks, good luck. Link to comment Share on other sites More sharing options...
davewantsmoore Posted June 4, 2018 Share Posted June 4, 2018 41 minutes ago, DRC said: any troubleshooting advice would be appreciated. Step 1. Disconnect everything from the receiver, except the power cord (you can also leave all speaker wires connected). Now, reconnect the subwoofer (RCA cable). Do you have hum? Link to comment Share on other sites More sharing options...
audible1 Posted June 5, 2018 Share Posted June 5, 2018 probably a ground loop. Try powering sub from same powerpoint as receiver. Link to comment Share on other sites More sharing options...
DRC Posted June 5, 2018 Author Share Posted June 5, 2018 On 4/06/2018 at 3:49 PM, davewantsmoore said: Step 1. Disconnect everything from the receiver, except the power cord (you can also leave all speaker wires connected). Now, reconnect the subwoofer (RCA cable). Do you have hum? Still have low level hum. Link to comment Share on other sites More sharing options...
DRC Posted June 5, 2018 Author Share Posted June 5, 2018 6 hours ago, audible1 said: probably a ground loop. Try powering sub from same powerpoint as receiver. I've tried same/different powerpoints and run an extension from another room just as a trial. Still low level hum. Link to comment Share on other sites More sharing options...
davewantsmoore Posted June 5, 2018 Share Posted June 5, 2018 10 minutes ago, DRC said: Still have low level hum. OK.... Ensure that the AVR and the subwoofer are connected to the same power point. Ensure that there is nothing else plugged into the subwoofer except it's power cord and the RCA from the AVR Ensure nothing is connected to the AVR except power cord, speaker wires, and subwoofer RCA Ensure everything else in the room is disconnected from power. Once you are sure of this, and you still have hum .... Try moving the RCA cord connecting the subwoofer and AVR. Does the hum change when the cord is moved? If so, try a cable with better shielding. How high is the volume dial on the subwoofer? How high is the subwoofer volume in the AVR settings? .... If they are high and low respectively .... try turning up the sub volume in the AVR, and turning down the volume control on the subwoofer. Still got hum? .... Buy one of these and see what happens: https://www.jaycar.com.au/ground-loop-isolator-stereo-rca/p/AA3084 Link to comment Share on other sites More sharing options...
VanArn Posted June 5, 2018 Share Posted June 5, 2018 If both the receiver and the subwoofer amplifier have standard 3 wire earthed mains connections , then when the RCA leads are fitted, a hum loop is created. The remedy is to isolate the ground/ shielding returns of the cables with capacitors (or less conveniently,with isolating transformers). It is also possible to use an IEC mains cable of the two wire variety to solve the problem. Link to comment Share on other sites More sharing options...
DdDesign Posted June 5, 2018 Share Posted June 5, 2018 I have no idea if this helps? I was setting up a subwoofer yesterday. It was all good then I moved my gear around and reconnected the sub and there was hum! What I discovered is that when I put my am/fm tuner back into the amp via rca the sub hum disappeared. I don't know why but I assume the tuner acts as a ground? Link to comment Share on other sites More sharing options...
davewantsmoore Posted June 7, 2018 Share Posted June 7, 2018 On 06/06/2018 at 9:30 AM, DdDesign said: I don't know why but I assume the tuner acts as a ground? Yes, the current is flowing through your tuner ... rather than over your subwoofer signal cable. Link to comment Share on other sites More sharing options...
DRC Posted June 13, 2018 Author Share Posted June 13, 2018 Thank for the input and giving me some ideas to try. I've got the problem under control with the purchase of a new cable. 4m RCA to XLR microphone cable from a music shop on eBay. The hum has now stopped. 1 Link to comment Share on other sites More sharing options...
andyr Posted June 14, 2018 Share Posted June 14, 2018 On 05/06/2018 at 6:49 PM, VanArn said: If both the receiver and the subwoofer amplifier have standard 3 wire earthed mains connections , then when the RCA leads are fitted, a hum loop is created. The remedy is to isolate the ground/ shielding returns of the cables with capacitors (or less conveniently,with isolating transformers). It is also possible to use an IEC mains cable of the two wire variety to solve the problem. But, Barry, all my components - except possibly the DAB+ tuner (as I didn't make this) - have "standard 3 wire earthed mains connections" ... and I don't have any hum! I suspect this is because all my RCA sockets are isolated from their cases. IOW, cases are earthed to mains-earth ... but signal earth is never connected to mains earth. Andy Link to comment Share on other sites More sharing options...
andyr Posted June 14, 2018 Share Posted June 14, 2018 13 hours ago, DRC said: Thank for the input and giving me some ideas to try. I've got the problem under control with the purchase of a new cable. 4m RCA to XLR microphone cable from a music shop on eBay. The hum has now stopped. So is this interconnect shielded - whereas your previous cable wasn't? Andy Link to comment Share on other sites More sharing options...
VanArn Posted June 14, 2018 Share Posted June 14, 2018 (edited) 11 hours ago, andyr said: But, Barry, all my components - except possibly the DAB+ tuner (as I didn't make this) - have "standard 3 wire earthed mains connections" ... and I don't have any hum! I suspect this is because all my RCA sockets are isolated from their cases. IOW, cases are earthed to mains-earth ... but signal earth is never connected to mains earth. Andy Andy all that you need to know concerning the avoidance of hum loops can be found via http://en.wikipedia.org/wiki/Ground-loop-%28electricity%29 Interconnecting cables carrying the audio signals should not be connected directly to a mains ground and it is common practice for RCA sockets to be insulated from a metal chassis . I do have a preference for cables of the twin (Figure 8) shielded per section variety as they allow a pseudo balanced technique to be employed. For safety reasons any equipment connected to the mains supply should have an 'earth' to avoid the possibility of electric shock if say the insulation of a power transformer fails. Note, there is an exception for double insulated equipment. Edited June 14, 2018 by VanArn typo and additions Link to comment Share on other sites More sharing options...
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