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I don't know if this is the correct forum to ask this, but I have a problem with my audio set up.

The setup goes as follows: PC -> external audio card (via usb from pc) -> stereo equalizer mixer (via headphone output from audio card) -> speakers (via rca audio outputs from equalizer).

I want to add my guitar amp as a speaker and the only available output in the equalizerbis the one for headphones (the equalizer allows to play the sound from rca output and headphones at the same time).

But my problem is that when I connect the amp, the speakers start a high frequency humming sound (which is also heard from the amp).

Details

To be clear, the setup works with no problems when the amp isn't connected.

I tried to put actual headphones in the equlaizer and there was no humming.

When the humming is heard and I move the cursor around, the humming sounds a bit louder.

The amp that I am trying to connect uses a 6.5mm jack

The humming is heard whether the amp is on or off.

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  • \$\begingroup\$ stereo headphone output to mono guitar amp input would take more than just any random cable, but why on earth would you want to do this? \$\endgroup\$ – Tyler Stone Jan 6 at 21:15
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It's probably an earth loop via the amp and the PC both going too mains earth. There isn't an easy cure though you could try some form of transformer isolation. Why are you doing this though, you just want to listen to music louder?

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  • \$\begingroup\$ I think this also could be the case. One wire of the speaker coils is most likely grounded, and thus most likely earthed, so that might give you a humming sound. I guess you could try and insulate the earth connection with tape on the plug (temporarily of course!) to verify this is the cause. If it is, i don't think there's much you could do unless you want to completely disregard safety. \$\endgroup\$ – QuickishFM Jan 6 at 21:22
  • \$\begingroup\$ Some transformers have electro-static shields between primary winding and secondary; you could tie that shield to earth ground. Those transformers are expensive. Topaz makes them. \$\endgroup\$ – analogsystemsrf Jan 6 at 21:27
  • \$\begingroup\$ there are lots of flavours of audio isolation tx, from the very expensive (Lundahl, Jensen) to cheapo ones housed in a jack plug. Of course you get what you pay for but I doubt it is a big deal going into a guitar amp. \$\endgroup\$ – danmcb Jan 6 at 22:29
  • \$\begingroup\$ QQuickishFM the ground loop is probably just from signal ground to mains earth and the two mains earths are connected via the house wiring. You can check it with a multimeter. People used to lift the earth on the power amp but bad bad bad don't do it ... \$\endgroup\$ – danmcb Jan 6 at 22:31
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First: use a multimeter to see if there is a DC voltage between the 3.5mm sleeve and the shell of the RCA jacks on the output of the EQ. If there IS a significant DC voltage (more than 1.5 Vdc), you are best off using an audio isolation transformer between the headphone jack and the guitar amp.

The next thing to try is to disconnect the ring lead from the 3.5mm plug. There is a good chance that the guitar amp is grounding the ring terminal on it's 1/4" input jack.

Please try both of those things and modify your question with your results.

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