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Thread: Help - Noise caused by TRACO 5V?

  1. #1

    Help - Noise caused by TRACO 5V?

    I'll post an MSPaint schematic or a picture if needed, but here's a simple discussion of my problem:

    Setup: I have a portable (battery powered) Teensy 3.6+Audio Shield. I am converting 12V-->5V with the Adafruit Traco to power the Teensy/Shield/TFT display, and a 12V regulator powering a Class D Amp/speakers.

    Problem: I have a buzzing noise coming out of my speakers when I am powering via Traco.

    Troubleshooting: When I power the Teensy via USB (bypassing the Traco) and only have the 12V regulator powered via battery, the buzzing goes away.

    Help?

    I already have a 47uF capacitor between the Traco PWR IN and GND lines (pins 1 and 2).

    I do not have access to an oscilloscope - only a multimeter.

    Would it be worth trying to add a capacitor between the Teensy 5V power in pin and Gnd? How else can I smooth the 5V power?

    Any other suggestions? Or alternatives to the Traco that might fare better?

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by ragemachine View Post
    Would it be worth trying to add a capacitor between the Teensy 5V power in pin and Gnd?
    Yes, definitely try adding a capacitor.

    How else can I smooth the 5V power?
    Maybe add 2 capacitors, with an inductor between them.

  3. #3
    Well... I don't have inductors, so I guess I'll try a huge cap first. Will report back tomorrow - troubleshooting this too late into the night.

  4. #4
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    Re huge cap, remember that large caps can be inductive so for really drastic action you may need multiple sizes of cap. Have only seen that in high end Audio applications (aka ones not involving a class D amp) though. A huge cap may also draw too much current during startup and cause the Traco to give up so do not go completely overboard.

    If the only high noise source is the Amp may be worth putting the filter cap at Amp input to isolate it's PWM output from the rest of the circuit. If you only have a multimeter, if it has a reasonable low end on the AC scale it may be informative to measure your 5V rail while trying different things. Ideally it will give a 0V AC reading but sounds like that is not the case at the moment.

  5. #5
    Senior Member+ Theremingenieur's Avatar
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    I would continue to power the Teensy and the TFT shield from the TRACO, but use a 78xx linear regulator to power the audio shield separately. I'd also keep care of avoiding double grounding.

  6. #6
    Tried a 470 uF cap between pwr and gnd and still getting buzzing. Biggest cap I have available right now. Should be more than large enough, right?

    How would the caps+inductor design look? Should I grab something like a "Joe Knows" Inductor kit to have available?

    I have another variable voltage stepper (a "DROK" off Amazon) - I might try that instead of the Traco, but don't have high hopes that it will be any different.

  7. #7
    I already have the shield soldered on top of the Teensy so splitting it apart would be quite an effort at this point.

    Can you elaborate on the double ground? The layout is such:

    PWR+GND from battery to pins 1+2 on TRACO (pwr in and common gnd pin)
    pins 3+2 on TRACO (pwr out and common gnd) to Teensy Vin and GND
    Audio shield soldered on top of Teensy
    GND on Adafruit MAX9744 tied back to Teensy GND

    I do not have the MAX9744 AGND pin connected. Should that be connected instead of GND? Or in addition to GND?

  8. #8
    The MAX9744 comes with a 470uF cap for filtering the power input - I have that installed on the 12V power in.

    The 470uF cap on the Teensy input did not seem to stop the TRACO from working, but did not help with the buzzing, either.

  9. #9
    Other info:

    On the Audio Shield to Amp, I'm using the stereo headphone output.

    Is there any other GND on the Audio Shield I should be using? Only the +3.3V and GND pins (per https://www.pjrc.com/store/teensy3_audio.html) are connected.
    Edit: I notice I do not have the Teensy Analog GND pin soldered to the Audio Shield. Suspect that could help? Should that be tied to the amp AGND?

    In code, I have mixer gain for Sdwavplayer1 set to 1.0 - could tweaking that up or down (and using volume pot to compensate) help reduce noise? I suppose if the noise is FROM the stereo output, I want the S:N to be higher coming out instead of amplifying the noise with the amp.
    Last edited by ragemachine; 09-27-2018 at 06:52 PM.

  10. #10
    Paul,

    Should I be using one of these? https://www.pjrc.com/store/audio_ground_isolator.html

    Think that would help?

  11. #11
    Senior Member PaulStoffregen's Avatar
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    Hard to say if it'll help without knowing more about the problem.

    The Traco is obviously using pulsing power. If the noise is due to only the 5V power line having noise and that's getting coupled into the 3.3V power and then to the audio, isolating the grounds would have no effect.

    But if the Traco is consuming huge pulses of power at its 12V input and some of the GND current it's getting happens to come from over the signal GND wires, then yes, it would likely help.

    One way to check is listening with headphones that have no GND connection to anything else, so the Traco can't possibly get any power over the GND to your headphones. Disconnect *EVERYTHING* else, so it's just power into the Traco and sound out to your headphones. If that is noise-free, then the isolater ls likely to help. If you still hear the noise with only headphones and no way for the Traco to get GND current from any other sources, then isolating grounds probably will not help and you need to filter the noise between the Traco and Teensy.

  12. #12
    One more dumb question:

    Should I have diodes (probably Schottky type) on the inputs to the 5V step-down and 12V regulator? Could I have ripples coming back to the battery and crossing into one another?

  13. #13
    So as an update to this thread... I still haven't fixed the buzzing, but I'm working around it a bit via software.

    I've tested some different settings and seem to be able to max out my Audio Shield volume (0.8) and Mixer gains (4.0) without clipping. The mixer setting was surprising - I had it set to 1.0 before and thought I would not be able to go much further than that. The buzzing noise is still present at higher volume levels, but the S:N is much, much, much better.

    I'm going to play around with it a bit more, but this is for the NY Comic Con On Oct 6 so I don't have a whole lot of time - Thx to all who contributed advice!

  14. #14
    Sigh. I'm back already. A new twist:

    I'm using a PWM output to control the TFT display brightness. When I max the brightness [aka analogWrite(x, 255)], the buzzing goes away. But ONLY when it is max - any other setting (25 min, up to 250) still causes buzzing.

  15. #15
    Senior Member PaulStoffregen's Avatar
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    Maybe try analogWriteFrequency() to increase the PWM to a frequency above the audio range.

  16. #16
    That fixed it completely. You're the best, Paul.

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