Noise in the Samples -

Hi,
I just finished building my first WTPA2 Unit - and everthing works fine except one thing
there is a lot of high noise as soon as a recorded sample is in playback mode  :-(
this also appears when i use the "realtime-mode"
i tried different cables, sound sources, and gain setups on the unit - if i record silence - there is no noise ,
but as soon that there is some sound sampled, it always comes with harsh bitcrushed heights ...
The normal input signal sounds good and clean.

I also re-soldered the first part of the unit: the R2R-ladder as well as the op-amps.
I recorded a sound file where you can hear whats going on:
https://www.dropbox.com/s/ubehql9vnxq09bw/WTPA%202%20Noise.mp3?dl=0

does anybody know what i´ve done wrong?
Thanks for your help!
Ludwig

Comments

  • Hi Ludwig,
    I think you're looking in the right direction.

    If the normal audio path is clean, but the sampled output sounds bad, as does realtime, then the problem is somewhere in your sampling path.
    Since realtime sounds bad as well, it's not RAM related.  That leaves basically your DAC, which is one of the latches, and the R2R ladder, or possibly one of the data lines.

    However, listening to your audio sample, the problem isn't as bad as lots of  WTPA2 problems I've heard.

    So:

    1.)  Make sure you are sampling at the highest possible input volume, or you won't be using all your bits effectively.  This means, turn up the gain knob (the first one) until you hear clipping, then turn it down just a hair.  You can adjust output volume as much as you want to make it quiet, but the gain should be as high as possible without clipping for max resolution.

    2.)  Make sure your sample rate is as high as possible.

    If you have done this and it still sounds bad, you should check your DAC again.  If you have a multimeter you can look for AC voltage at the output pins of the DAC latch, IC4.
    Easiest way to do this might be to put WTPA2 into diagnostic mode, make sure the sawtooth is running, then probe the right sides of the R2R ladder's right most resistors (the even numbered ones from R20 to R34).  You should see a healthy AC voltage on all of them when the sawtooth is going; probably something near 5v and just about the same value for all pins.

    You might also try counting frequency if your meter does that.  You should see a frequency doubling at each bit position (with a sawtooth).  You could also listen for this if you have a "ghetto oscilloscope".
    Or you have a scope, you should just use that.

    You may also want to verify the values of all the resistors in the R2R are correct.  Remember the bottom most resistor is 51k (I messed that up on one of my builds I think).

    Let me know and good luck!
    Todd

  • Thanks for the help again!
    i tried different gain settings and unfortunately i found that its not the gain setting that makes the bad noise-

    today i checked my DAC the way you told me and i found out something interesting -
    i used the ghettooscilloscope and listened tho the electronics in diagnostic mode...

    again i made a sund file that captured my dicoveries -
    the sound in the r2r ladder is "good" (or as good as it could be in this use/moment)
    but as i was going on to IC4 the return paths (almost every 2nd pins) got that high noise that i was searching for
    >>https://www.dropbox.com/s/tq27dh9diyd6p64/Noise_spectrum.mp3?dl=0
    the soundfile is starting at R34 going on to R20 an then moving to IC4

    So maybe i just exchange the chip at IC4 an then everything is fine ?!

    i also disovered that my WTPA is accepting only 12V DC - not less ... not that this is a problem for me but maybe this might be part of the noise problem of my unit...

    What do you think about it?
    Thanks!!

  • The "not less than 12v" seems wrong.  WTPA2 should really work at 7v or above.  Let's focus on that first.

    Is anything on the board getting warm to the touch?
    If this true whether or not you have other cables plugged into the WTPA2; IE -- does the 12v threshold change when there is nothing plugged into the audio jacks?

    Does the WTPA2 power off under 12v?  Or does it stop working in some other way?  How exactly does it "not work"?
    What power source are you using?  Is the polarity correct?

    Does the voltage regulator get warm?

    Best,
    Todd
  • ... you were right, todd: i had the polarity wired the wrong way - didn´t thought about this, because it was "working" ...
    Now its running on 9 Volts as well - the high noise produced by the ics is gone ,
    but now there are a lot of ground loops and ac voltage sounds - its changing if i turn the knobs.
    maybe the twisted polarity damaged some parts?!

    I´ll keep searching to solve these problems and keep you updated...
    Thank you very much!

  • Glad you got it fixed!

    Doubtful you damaged anything, but WTPA2 can be sensitive to noise pickup depending on how it's wired.
    Easiest way to tell what's going on is to use a 9v battery for power, and after that then try some kind of floating audio source (like a phone or iPod).  Try them both.

    If the noise clears up with the 9v battery, the AC adapter may be a bit noisy.
    If it cleans up using the floating source, then you either have a noisy source or possibly a ground loop with the source and output of WTPA2.

    NOTE:  you can unplug the output and sample with just the input connected, then disconnect the input, reconnect the output and play your sample that way.  If there's no AC hum (but there was when input and output are connected) then you had a loop.

    Lastly, sometimes people wire up the polarity of the audio jacks incorrectly.  This also causes all kinds of audio mayhem.  Make sure that isn't the case.

    Nice job and good luck.



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