Comments: Proto Pedal Example: Programmable Digital Pedal

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  • Hello? Is anyone paying attention to this page any more?

  • Regarding the noise floor:

    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.

    • Hi AlienRelics,

      Interesting suggestions! I am planning to build one of these, but with lower noise floor. I think you can hear the noise pretty well in this video:

      https://www.youtube.com/watch?v=f2dl8F0xoJc

      At 1:28 the noise is very prominent I think.

      Have you tried building an noise optimized version of the circuit? I would be interested in learning which of the different optimizations had the greatest effect.

      Thanks!

      • Thanks! No, I’ve got about 100 projects going at one time.

        I started a thread in the forum, I’d love to talk about it there. https://forum.sparkfun.com/viewtopic.php?f=14&t=45056

        I noticed they don’t give much time to hear the noise. I hear some digital leakage in there. Put an AM radio near a computer, that’s what you’ll hear.

  • I’d really like to hear the noise. Is there a video clip of this pedal somewhere?

  • I’m at the part where it’s time to perform the following test:

    “Apply power to the circuit, then connect the input and output. We used a signal generator and an oscilloscope. If you don’t have them handy, you can use a guitar and amplifier – just be careful to keep the volume low until you’re certain that the circuit is well behaved.”

    I don’t have an oscilloscope, so I’m using a guitar amplifier. The signal is very low and you really have to crank the amp to hear anything. Is this normal?

    • If it’s working properly, the volume shouldn’t change as you turn the stomp switch on and off.

      I’d start by doublchecking that the temporary wire from 25A to 27I is solidly connected.

      If the temp wire is there, I’d fall back and check everything up to that point.

      • The volume doesn’t change it’s just very very faint in the amplifier.

        • If the pedal is bypassed, it should be as if it weren’t even there. The input should flow directly to the output, unchanged. It should do this even if it’s not powered up.

          If that’s not the situation, check the solder joints on the jacks and switch, and also that the ¼" cables are good (and that they’re TS cables, not TRS).

          • I checked everything and all the joints look good and I’m using the right cables. Can I send you a photograph of the board?

            • You’re welcome to post or link photos here.

              It would be most helpful if you had sharp photos of both sides of the board.

              Could you also confirm whether the pedal passes signal when its bypassed?

              • https://dl.dropboxusercontent.com/u/24478809/BackOfBoard.JPG https://dl.dropboxusercontent.com/u/24478809/FrontOfBoard.JPG

                The pedal appears to work regardless of the state of the foot switch, it is just very faint in the amplifier.

                • Thanks for posting these: I’ll take a closer look later today.

                  But your statement:

                  The pedal appears to work regardless of the state of the foot switch, it is just very faint in the amplifier.

                  Indicates that something more fundamental is wrong.

                  Without powering up, can you test continuity from the IN-TIP test point to the OUT-TIP test point? In one position of the stomp switch, they should be shorted together. In the other position, they should be open.

                  • No response in either position.

                    • If the tips aren’t connected when bypassed, the only components involved are the jacks and stomp switch. I’d inspect them carefully, and maybe reflow the solder points to make sure they’re solid.

                      Again, a bad cable would give very similar results.

                      • I’ve tested the cables - they are in good shape.

                        How do I test the jacks with a multimeter to make sure they are working correctly?

                        • Byron J. / last year * / 1

                          Plug a known good TS cable into the input jack. Put one multimeter probe on the tip of the far end of the cable, and the other probe on the IN-TIP test point. They should read continuous.

                          Then measure from the sleeve of the far end of the cable, to any of the GND pads on the pedal. They should be continuous. Likewise, continuous from the sleeve of the cable to the IN-RING test point. Finally, measure continuity from the tip of the cable to the sleeve of the cable. They should be an open circuit.

                          Then plug the cable into the output jack, and repeat, using the OUT_TIP and OUT_RING test points. Tip->tip should be continuous, Sleeve->ground should be continuous, and OUT-RING->sleeve should be continuous. Cable tip to cable sleeve should be open.

                          Finally, take two cables. Plug one into the input, one into the output. Measure continuity between the tips of the opposite ends of the cable, then actuate the switch. When the pedal is bypassed, one tip to the other should be continuous. At no time should either tip be continuous with the sleeve.

                          • The last test fails - the tips of the two cables are not continuous.

                            • At this point, I’d recommend two things:

                              First, reflow the solder joints on the TRS jacks and switch. From that bottom photo you posted, it’s hard to tell how clean your solder fillets are – some of the look like they could be cold joints.

                              Second, download the board file, and use eagle to inspect the nets that lead between the jacks and switch. Select the eyeball tool, then click on the trace that leads to the IN-TIP test point. It will highlight all the segments of that trace. Then measure continuity between all of the pads that are highlighted. Then repeat with the OUT-TIP test point.

                              One other thing I see in your photo is that some of your leads are a bit long. They could cause problems if they reached adjoining components, or if you put the board in a metal box, and they shorted to the box. In particular, that long wire loop at the VAUX pad looks like it wants to flop around and cause shorts.

                      • The stomp switch turns on/off the LED when I click it. Does this indicate that it’s in working order?

                        • Byron J. / last year * / 1

                          The switch is actually three switches that actuate together when you press the button. One section switches the LED, one section joins input-to-output when you bypass, and the third connects the FX circuit input.

                          If the LED switches, then one of the three sections is working.

                          You can test the other sections with a multimeter.

                          If you look at the board from the solder side, each section of the switch makes a vertical column of three pads, like this.

                          1

                          2

                          3

                          Measure from the center pad of the column to the pad above (1 to 2), and the pad below (2 to 3). One of them should read continuous, the other open. Then operate the switch, and open/continuous measurements should trade places.

                          Repeat for each column, and report back with the results..

  • OK, so… using “Send us your feedback” and “What’s on your mind?” both bounce.

  • A few typos: “resistorsl” -> “resistors”, “regulator is dropping to voltage” => “regulator is dropping the voltage”, in the optional teensy modification section you should probably note the size of the SMD resistor, and include it in the parts list, “Doublecheck” => “double check”, “missle terminal” -> “middle terminal”. The “final incremental test” section contains the text “…cap…” which presumably was meant to be a screen capture. It would be nice to have a clearer shot of the hole you drilled in the side for the USB programming jack, as well as the exact size of the hole. “AuroRoller” => “AutoRoller” (twice!), “sutoff” => “cutoff”.

    • I’ve played so much guitar in the last few days that I can no longer control my fingers. I think I’ve fixed all those.

  • This is awesome! Any idea when the repositories will be up? Showing a 404 error at the moment.


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