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TOPIC: Hardware Fix for ADC noise?

Hardware Fix for ADC noise? 8 years 2 months ago #135

  • jruddy
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Like so many, I am underwhelmed by the Due's noisy ADC.

I'm wondering if anyone has tweaked the hardware to run more like the old dolby NR for audio tape hiss?

Basically the input would need to be biased to high frequencies (Reducing C3 would do the trick), then the highs would be clipped on the output stage (Maybe by increasing C9 & C12 or by simply jumping a cap across the output.)

I think it might be a good option for clean fx, (delay etc) but it would complicate things for OD/Distortion as the input bias would throw the pre-processing levels off and the pedal would suddenly need to be frequency aware unless you want your highs to clip earlier.

Before I start unsoldering and soldering in sockets for the caps, I wonder if anyone here has tried this approach?
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Hardware Fix for ADC noise? 8 years 1 month ago #143

  • JR
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Hi, the micro-usb power supply has a big influence in the ADC noise, I have found big differences between them, so If you have some around give it a try.

Tweaking the caps may help, the low pass cut frequency is around 5.8KHz (100K resistor + 270pF cap), several effects like the Boss CE2 Chorus use similar frequency. I remember trying some caps in the output stage, after the DACs in different configurations and the hiss could be reduced but also the signal suffers.

The through hole pins are easily available so you can try placing new caps or modifying the existing ones in the input or output stage.
note: Take care with the DAC output pins, if you short them to ground the DAC may be damaged. There are test points placed after the caps C6 and C7 to avoid this issue.

A radical approach like including a third order Sallen-Key after the DACs will improve the signal noise, but I did not include it due to the circuit complexity/friendliness.

Let us know about your progress!
keep it simple
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