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I explained this problem in a video so you can see with your own eyes the problem... touching the GND wire makes the problem go away, however touching the signal wire creates the problem. Even touching the signal wire from the plastic it still creates the problem.

https://photos.app.goo.gl/dK87rBUf9LJme1dWA

I am using the circuit below (instead of 1k resistors I am using 10k resistors) to bring the speaker audio level to mic level. The speaker is from a digital piano where I play musics and I want to record them on my smartphone. This circuit was kindly provided by @JRE.

enter image description here

However today I discovered a weird behaviour with it: if I connect it to my smartphone (to record the sound on the mic line - TRRS) I hear a hum/hiss. However when I touch with my fingers the gnd or signal wire, it disappears. It took me hours to realize this was the problem. And it's 100% reproducible: if I touch any of the wires (signal or gnd) the noise goes away completely, if I remove my finger from any of the wires, the noise comes back.

I am using my smartphone on battery (not connected to any external power supply). I read on Google about ground issues, but how this simple circuit could cause it? And how can I fix it?

As requested on the comments: my digital piano is powered from the outlet (220V AC -> 12V DC -> input of piano). The smartphone is not connected to anything else (not usb, no charger... just its own battery). The only connection between the piano and the smartphone is the GND pin and MIC pin (of the TRRS connector inside the smartphone).

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  • \$\begingroup\$ Please edit your question to include information about how your devices are powered. Whether or not the power supply/supplies is/are grounded is important information to include. \$\endgroup\$ Jun 19, 2022 at 2:27
  • \$\begingroup\$ @MathKeepsMeBusy I added an EDIT to the question. Hope it helps. \$\endgroup\$
    – user314164
    Jun 19, 2022 at 2:40
  • \$\begingroup\$ The complete circuit is far from simple. You probably have a switchmode supply and maybe even a class D audio amp in the piano. Touching the wiring is shunting the noise via your body back to earth. Using an audio transformer to isolate the audio might go a long way to curing the problem. \$\endgroup\$
    – Kartman
    Jun 19, 2022 at 2:52
  • \$\begingroup\$ @Kartman Thank you. Just to make sure I was clear (I read my post again and I thought it confusing, sorry) I decided to record a video to you. Would you mind taking a look at it? photos.app.goo.gl/dK87rBUf9LJme1dWA If you think the easiest solution is the transformer, would you mind telling me where I connect the transformer in the circuit? \$\endgroup\$
    – user314164
    Jun 19, 2022 at 3:05
  • \$\begingroup\$ The transformer would connect to the headphone on the primary and your circuit on the otherside. \$\endgroup\$
    – Kartman
    Jun 19, 2022 at 4:48

1 Answer 1

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It appears that hum is caused by lack of shielding in the interface between the piano and the smartphone.

enter image description here

The high impedance microphone input of the smartphone will pick up hum / noise from the unshielded interface.

A metallic enclosure and shielded cables are a must to avoid unwanted noise pick-up.

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  • \$\begingroup\$ Thank you, I used the metalic shield and it worked! But anytime I touch the smartphone the noise comes back. I tried with my wife's smartphone, and same problem. Weird, right? \$\endgroup\$
    – user314164
    Jun 19, 2022 at 18:17
  • \$\begingroup\$ Anytime, Jonathan. Please confirm that the metallic enclosure and the cable shields are interconnected as shown. \$\endgroup\$
    – vu2nan
    Jun 20, 2022 at 2:33

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