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Thread: 50Hz Noise on Microphone input (Teensy 3.2 + Audio Adapter + Amplifier)

  1. #1

    50Hz Noise on Microphone input (Teensy 3.2 + Audio Adapter + Amplifier)

    Hello Guys!

    We're busy with a project including audio and recording, we are using the Teensy 3.2 and the Audio Board with a 20W amplifier. We are using the microphone input (not de line in). to record voices. The microphone we use is a Electret Microphone from Adafruit, also we can connect (by-pass) this microphone by using the microphone on a headphone with a jack-plug.

    important to know is that we designed a new PCB including the Teensy, audio, SD and Amplifier.

    At first we were using a power supply (tenma 72-10495) connecting to the Board. While recording we didn't hear any noise at all. As soon we connected a seperate voltage supply (just a simple wall adapter), we get this 50Hz noise when we are recording.

    Now we are wondering what causes this, we've seen information about ground loops but i don't know if that's the problem.
    I have to say, we tested several adapters and with some of them the noise was reduced quite a bit.

    Are we doing something wrong with the grounding? When we just play simple audio files we don't hear any noise btw.

    Hope you could help us.

    Last edited by Chopsticks; 01-17-2017 at 09:20 AM.

  2. #2
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    The quick answer is to try one of these.

  3. #3
    That should work I guess? But i'm trying to solve this on board and/or making use of the software libraries to create filters. Don't know if it's possible to filter the noise away by software?

    I think it's a strange behavior because on the power supply I don't see any noise. Only when we use a wall adapter.

  4. #4
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Ground loops are analog communication problems. If you could precisely predict the ground currents, perhaps you could filter or subtract the noise on the signals you're receiving. But predicting ground currents is probably not very realistic in most practical systems.

  5. #5
    Join Date
    Dec 2016
    How much did you spend on the wall adaptor? The tenma 72-10495 is ~150, if you swap it for a 15 wall adaptor you'll notice the difference!
    Particularly you've noticed that different supplies lead to different noise levels, meaning it's most likely the supply injecting the noise.

    The wall adapter will probably be a "half wave rectifier" which are very cheap and dirty (AC speaking) supplies, maybe with some kind of DC-DC converter. Probe (if you've a scope) the supply rails with different adaptors and you should see that the supplies that give you less noise when recording supply a "cleaner" rail. You can get round it by either tuning the power supply decoupling you use, or changing how you supply power (there's a number of options).

  6. #6
    Senior Member+ Frank B's Avatar
    Join Date
    Apr 2014
    Try an other adapter - i use these little usb-phone-chargers, they work well. (if not a cheap lowest quality china part)

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