Unsure about my PCB

Y

young.javier

Member
Hello!

I've been playing around with teensy for several years now, and I decided that I should make a real complete project for once. I've been playing the guitar more lately so I would like to create a guitar pedal.
Knowing little about electronics (I'm a software developer) I did some cool effects but found out that the line in/out of the teensy audio board is not optimal for a effect pedal board, because they are low impedance.
I decided to follow this tutorial, but given that I don't like to solder (I always make a big mess with the wires) I'm trying to make a PCB. I made the diagram in EasyEDA so I can print it with JLPCB.

I know that it's a lot to ask, but I would really love another pair of eyes on it. I'm 99% sure it's OK (specially the parts that are not in the tutorial schematic), but it takes a lot of time for it to ship to my country, so I would really like to get it right in the first try.

If anyone has a few minutes to spare and would like to take a look I would appreciate it very much :)

schematic.png
 
Audio is not really my area, but here's what jumped out at me

1. are you sure about the pinouts on the jacks? I thought the "ring" (shown as the rectangle) was ground.
2. I would add a 0.1 uf ceramic cap to output of the 7805 and at the power inputs for all devices, and as close to the device as possible on the PCB.
3. It's been a long time since I used the audio board, is a pullup resistor on CS needed?
 
I see a couple things that may or may not be a concern.

If VCC has noise in the audio band, that noise could possibly couple into your signal by those resistors meant to set the DC level.

Teensy uses about 100mA. Than 7805 regulator could become quite hot, depending on your input voltage and whether it has a heatsink that is exposed to moving air or is large enough in free air for natural air convection.
 
Using two 2.5 MOhm resistors (R1,R2) as a voltage divider creates a Thevenin equivalent resistance R of 1.25 MOhms at the non-inverting input. While this consumes very little power, it is generally not recommended for the LM358 due to its bipolar junction transistor (BJT) input stage.

Noise Sensitivity: High-impedance nodes are extremely susceptible to EMI/RFI and crosstalk.
Thermal Noise: Large resistors generate higher Johnson-Nyquist noise, degrading the Signal-to-Noise Ratio (SNR)

Also LM358 is NOT rail-to-rail amp its output is limited to (Vcc-1.5) V

For high-impedance inputs you might consider opamp with JFET input stage such as TL072 that also has much smaller noise, but that would require higher voltage supply >10V, since it is also not rail-to-rail.

If you are limited by 5V power supply MCP6022 would be a better choice (CMOS, rail-to-rail, much lower noise 8.7 nV/sqrt(Hz) - as compared to 55nV/sqrt(Hz) for LM358 , low distortion 0.00053%, works with 5V supply and below)
 
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The confusion might come from the fact that it might not be classic 7805 linear regulator, this part on the schematic has symbol of K7805-500R3LB. When I searched for it it looks like switching regulator that is "replacement" for 7805, but might come in non standard package. For low noise circuits I would suggest using classic LM7805 (linear, not switching) regulator and then indeed pin 3 would be output and pin 2 GND.
 
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Totally agree, K7805-500R3-LB is a DC-DC switcher and a linear regulator would be a better option. From the various manufacturers I looked at for K7805-500R3-LB, they follow the linear 7805 pinout. Maybe @young.javier is using a part from a manufacturer that I didn't review.
 
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