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Thread: Teensy 3.6 Audio: LED overload protection & GND/AGND & 78L05 Power Supply

  1. #1
    Junior Member
    Join Date
    Jul 2017

    Teensy 3.6 Audio: LED overload protection & GND/AGND & 78L05 Power Supply

    Hello there,
    I have 3 questions regarding a setup with the Teensy 3.6 mounted on top of the SGTL5000 audio adaptor. i built a small amplifier/filter board with some op amps that buffer and scale a guitar input signal for the Audio Adaptor board (with a gain and a highpass filter before the ADC and a dry/wet mixer/attenuator after the DAC).
    please excuse my beginner skills, as this is the first circuit i designed.

    this is the circuit:

    1. as you can see in the schematic i put two LEDs as a voltage overload protection to save the ADC pins from the SGTL5000 from any damage. i already tested the circuit and everytime my signal is too strong (which is probably > 1.7 V or < -1.7V) the LEDs start glowing and the sound distorts, but the thing doesn't stop anymore, it instead does a strange oscillation, sounding like a rectangle oscillator at 3 or 4 Hz. i can only stop this by pulling the power and reconnecting the power again. it happens when playing the guitar too heavily (i know i can fix this by adjusting the gain in the circuit), but it also happens when plugging in the guitar cable.
    is there another, better way to protect the ADC from too high voltages? or is it safe to not even protect it at all?
    this solution is far from good, because when clipping, the oscillation is 3 times louder than the guitar signal was, and it sounds terrible.

    2. How should i handle the whole GND/AGND thing? on the Teensy 3.6 pinout diagram, i can find "GND" and "Analog GND" pins, whereas on the Audio Adaptor, i find "GND" and "G", which seem to be connected to each other. so does it matter which Ground i use for the ADC/DAC audio processing, for the power supply and for the analog and digital inputs (e.q. buttons, potentiometers)? if not, i would just choose any Ground pin on the Audio Adaptor Board for all the Ground connections? i'm asking because i read somewhere that you should keep certain Grounds separated.

    3. this is really the main problem: the Teensy 3.6 doesn't seem to power up when using an external power supply. when using USB power, it works fine (with LED blinking), but when using 5V generated from a 78L05 voltage regulator, it doesn't power up (and no LED blinking). i checked with a multimeter: i have 5V from 5V pin to GND (both on Audio Adaptor and on Teensy, as both are connected), so it seems the voltage regulator is working. i also get the 3.3V from the Teensy's internal voltage regulator.

    i know that this power supply problem has been discussed in a thread already, some people said it was a matter of a slow power supply or a large parallel output capacitor on the voltage regulator's output.
    im using 10uF on the output of the 78L05, this should be okay, right?
    also, i tried to connect the Teensy's "Reset" pin to "Ground" while already connected to the external power supply/voltage regulator - would this be the right solution if the power supply was too slow? well, it didn't power up this way, either.

    is the Teensy damaged already? is there a quick and easy way to find out? i have a multimeter, but no oscilloscope.

    Thanks a lot! Best regards, Jan

  2. #2
    Senior Member
    Join Date
    May 2017
    For problem #1, I would try removing the lower end of your clipping circuit from the 4.5 volt virtual ground and connecting it to ground via a large value capacitor much like the circuit diagram suggests before one realizes the battery symbol is not a capacitor.

  3. #3
    Junior Member
    Join Date
    Jul 2017
    thanks, rcarr!
    for the other problems, especially #3, nobody has any suggestions?
    best regards

  4. #4
    Senior Member
    Join Date
    Apr 2013
    Haven't checked the T3.6 but would have expected reset to be active low, so correct manual start would be hold low, power up, release to go high and off it goes. Certainly holding it in one state isn't the way ahead here.

    Also, you don't have any code that's waiting for USB to init in your setup loop? while(!serial()); is the common one.

  5. #5
    Join Date
    Jun 2014

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