- Thu Dec 08, 2016 3:13 pm
#192492
I've got some suggestions for this which I've placed in the Comments, but had no response. The "Whats on your mind?" and "feedback" email addresses both bounce.
So I'll copy-paste here and see if I can get some discussion going.
This is the project:
https://learn.sparkfun.com/tutorials/pr ... ital-pedal
Regarding the noise floor:
I’d really like to hear the noise. Is there a video clip of this pedal somewhere?
I see a noisy digital circuit suspended millimeters over a sensitive ADC/DAC. Use the header pin lengths and lift the Teensy much higher, and place a grounded shield between them. Make sure it isn’t shorting anything out. Even better, use a socket for the Teensy, that lifts it up and will help keep a ground shield from touching any pins. Tape, copper foil, tape.
Use shielded wire for the Input and Output. Twisted pair works, too.
Put 0.1uF bypass capacitors on the LM7805.
As much as possible, keep analog wiring away from digital circuits. Separate the digital and analog grounds as much as possible. I’d prefer seeing separate voltage regulators for digital and analog circuits.
Make that input buffer into an amplifier with gain control. You can add a resistor and anti-parallel LEDs to the output of that Op Amp to ensure it doesn’t go above 3Vpp. Then you can adjust the gain to just under the point where the LEDs blink while playing. Get rid of that ridiculous attenuator/voltage divider of R3 and R4. You want the maximum signal without clipping going into the ADC.
In fact, I see a microphone input on the Audio Board. Have you tried using that? I’d remove the 2.2k MICBIAS resistor and maybe put a larger capacitor (10uF?) across the 0.1uF capacitor going to pin 15 of the SGTL5000. A 500k audio taper volume control, and adjust to just under clipping. That removes one Op Amp from the circuit entirely.
You should have antialias filtering on the input. Otherwise, any noise above 22kHz gets translated down into the audio band. Use that Op Amp for good - turn it into a dual Op Amp lowpass filter, -3dB about 12 to 16kHz.
Don’t amplify the output, just buffer it. Why did you amplify it? A guitar amp is expecting from 100mV to 1.5V peak to peak, ie, guitar to line level. Not 9Vpp.
So I'll copy-paste here and see if I can get some discussion going.
This is the project:
https://learn.sparkfun.com/tutorials/pr ... ital-pedal
Regarding the noise floor:
I’d really like to hear the noise. Is there a video clip of this pedal somewhere?
I see a noisy digital circuit suspended millimeters over a sensitive ADC/DAC. Use the header pin lengths and lift the Teensy much higher, and place a grounded shield between them. Make sure it isn’t shorting anything out. Even better, use a socket for the Teensy, that lifts it up and will help keep a ground shield from touching any pins. Tape, copper foil, tape.
Use shielded wire for the Input and Output. Twisted pair works, too.
Put 0.1uF bypass capacitors on the LM7805.
As much as possible, keep analog wiring away from digital circuits. Separate the digital and analog grounds as much as possible. I’d prefer seeing separate voltage regulators for digital and analog circuits.
Make that input buffer into an amplifier with gain control. You can add a resistor and anti-parallel LEDs to the output of that Op Amp to ensure it doesn’t go above 3Vpp. Then you can adjust the gain to just under the point where the LEDs blink while playing. Get rid of that ridiculous attenuator/voltage divider of R3 and R4. You want the maximum signal without clipping going into the ADC.
In fact, I see a microphone input on the Audio Board. Have you tried using that? I’d remove the 2.2k MICBIAS resistor and maybe put a larger capacitor (10uF?) across the 0.1uF capacitor going to pin 15 of the SGTL5000. A 500k audio taper volume control, and adjust to just under clipping. That removes one Op Amp from the circuit entirely.
You should have antialias filtering on the input. Otherwise, any noise above 22kHz gets translated down into the audio band. Use that Op Amp for good - turn it into a dual Op Amp lowpass filter, -3dB about 12 to 16kHz.
Don’t amplify the output, just buffer it. Why did you amplify it? A guitar amp is expecting from 100mV to 1.5V peak to peak, ie, guitar to line level. Not 9Vpp.