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I would like to ask you about the significance and quantity of electromagnetic interference (EMI) that produce classical mobile phone charger (transformer).

I would like to install one in a turntable as an addition for the control of the added electromagnetic relay, and I am not sure if charger transformer EMI is going to compromise or interfere with the quality of sound.

I would be also grateful if you can explain me how to reduce unwanted EMI in this case.

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  • \$\begingroup\$ Your question is not clear: "classical mobile phone charger (transformer)" can refer to different items. 1) Do mean the common smart phone chargers used today, that feel like an empty plastic block? 2) Or do you mean chargers that were used 15 years ago. They feel like a plastic block with a big piece of metal inside? \$\endgroup\$
    – Pat
    Commented Sep 26, 2023 at 18:48
  • \$\begingroup\$ Yes, I mean the common smart phone chargers used today: input: AC:100-240V/50-60Hz/0.5A; output 5-9V/2.0-1.67A. Thanks!! \$\endgroup\$
    – Jhon
    Commented Sep 26, 2023 at 18:58

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I would be also grateful if you can explain me how to reduce unwanted EMI in this case.

Usually chargers operate a Switch Mode Power Supply (SMPS) that has a control circuit that operates in the kHz range, mainly 50 to 500kHz but doesn't have to be in that range. That signal with higher harmonics can conduct up the cable and couple via electrical fields to other sensors such as pickups or mics.

One way to stop it is with a ferrite that can block signals in the kHz range. Another way is to short the high frequency signals with a capacitor or both. Another way is to use a linear supply.

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  • \$\begingroup\$ Many thanks, you helped me a lot! \$\endgroup\$
    – Jhon
    Commented Sep 26, 2023 at 23:40
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Probably terrible, but it depends on your charger and the relay you're connecting it to.

Will you be connecting anything to the actual phono signal wires? Rather don't do that...

Try it out laying the equipment on top of the turntable, near the tone arm, and see if you can hear it. Especially when the relay clicks.

It might be better to use a linear power supply, not a regular usb phone charger. You can buy them new; for a one-off I would hunt in thrift stores, either for loose wall warts or old cordless telephones, bedside lights, radios etc. The one you want feels like it's full of iron, not light weight like a modern phone charger. Be aware that the voltage output will be higher than it says on the power supply.

Suppression and shielding of RFI is a whole big subject. Try to keep the interfering circuit far away from the audio signal wires. If you can do the relay switching down at the power plug, for example, not in the turntable, that would help. Avoid any loops of wire, keep the supply and return wires twisted.
Grounding and shielding are complicated - the best would be to come back and ask another question with photos etc, when you have a specific interference problem.

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  • \$\begingroup\$ Thanks a million. I just want to install styli protection on the manual turntable, using perfectly fitted and light microswitch built in near tonearm. In addition I planned to use small PCB 6V DC/100Ω/400mW electromagnetic relay for main board DC current control, and I can not use main transformer of the device as a current source for the relay. So I planned to built in small smartphone charger (input: AC:100-240V/50-60Hz/0.5A; output 5-9V/2.0-1.67A), anything bigger would be an issue. All around 30cm away from the audio signal cables. Thanks for help and suggestion! \$\endgroup\$
    – Jhon
    Commented Sep 26, 2023 at 19:09

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