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Grounding Issue from Connecting my Class D Hypex Amp to Subwoofer's Speaker Level Inputs?

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I'm having a (what I think is a Grounding) issue with my Audiophonics MPA-S250NC XLR amp, connecting to my active sub's speaker level inputs (RSL Speedwoofer 10s).

Setup: Source = PC > Duok U2 PRO XMOS XU208 Digital Interface > Schiit Modi DAC > Schiit Loki preamp > Schiit Vali2+ headphone/preamp > Audiophonics MPA-S250NX XLR Amp > Speakers (RCA/Banana Plug from Amp's Output)
> RSL Sub, Speaker Level In (Spade Connectors also coming from the Amp's same Output)

  • When I make the connection to any of the Sub's Speaker Level Inputs I get a significant, constant humming from the subwoofer. I am reasonably sure this is a grounding issue.
  • It occurs even when the amplifier is Audiophonics amp is powered off
  • Both the sub and amp are connected to the same outlet (power strip)
    • I tried connecting the sub to an entirely different outlet, issue remained
  • I was using a different amp recently that has a dedicated sub out (RSL ia255.1) without any hum, I just tried upgrading to the Audiophonics and from my research this spade connection to sub speaker level in seemed like the simplest way to connect my sub. I don't have any other available connections on my preamps in this setup.
Since discovering the issue and doing some research, I'm reading that some Class D amp's black speaker terminals are not actually grounded, rather both terminals of each output are active. Is this the case with my Audiophonics amp? If so this means I have a grounding issue, is there an easier/cleaner solution than buying a Line Out Converter?

Spade connectors connecting around the 4 outputs
2024-03-19 19_46_15-AUDIOPHONICS MPA-S250NC XLR Power Amplifier Class D Stereo Ncore NC252MP 2...png


into the Speaker Level Inputs (bottom middle)
2024-03-19 19_50_23-.png


Thanks in advance!
 

pma

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@TheElementOfSurprise , I can help you if you post the simplified drawIng of the whole system. Audiophonics MPA-S250NX amplifier has one speaker output terminal grounded, as it is based on NC252MP. Ground loop issues are severly underestimated at ASR, for unknown reason.
 

JeremyFife

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What happens if you just connect your Sub directly from your preamp's line out? I don't see the need to use your Amp. I may have misunderstood though.
 

JeremyFife

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What happens if you just connect your Sub directly from your preamp's line out? I don't see the need to use your Amp. I may have misunderstood though.
Update: I just realised that the Loki has only one line out. I think, and can be corrected, that you can still split the line out signal and send it to both your amp and your Sub. I think a simple Y-adapter will work fine for you.
 
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Update: I just realised that the Loki has only one line out. I think, and can be corrected, that you can still split the line out signal and send it to both your amp and your Sub. I think a simple Y-adapter will work fine for you.
So something like below; plug the male end into the Loki line outs, send 1/2 to Amp and 2/2 to Sub?

 

DonH56

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I am not a fan of running the main speakers full-range and using high-level subwoofer inputs but the topic has been beaten to death here and elsewhere.

One fairly simple solution I have used in the past is to stick resistors of 100 to 1k ohms in series with all four wires to the sub. That is usually high enough to break a ground loop and low enough to not cause too much voltage loss. The speaker-level inputs are usually high impedance but I do not know how high for RSL. I think I used 470 ohms last time for a friend because I had them and it was not a value I used often so didn't mind losing them.
 

JeremyFife

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@TheElementOfSurprise , I can help you if you post the simplified drawIng of the whole system. Audiophonics MPA-S250NX amplifier has one speaker output terminal grounded, as it is based on NC252MP. Ground loop issues are severly underestimated at ASR, for unknown reason.
will do when I get home from work, don't want to get something wrong from memory alone
 
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