The boards I ordered came in in about a week. And the cost (with shipping and my new customer discount) was one dollar per board. Pretty dang cool. Here is a picture of the front and back of the boards:
I populated one of the boards with components and quickly found the first of several problems. The footprint I used for the resistors was too small even for the quarter watt resistors I used. So, I was forced to install the resistors vertically. I guess I really should have measured these footprints more carefully before ordering the boards. I'll try and fix this on the next iteration.
Everything else fit the footprints fine. Even the cheap Amazon 35mm faders I made custom footprints for mounted up fine. However at this point I discovered a few more problems:
- The bargain Amazon faders I got were not such a bargain as about 50% of them were faulty. The rivets connecting the pins to the resistive track seem to be faulty and I was not able to repair them. So, I ordered another batch and managed to find five that mostly work.
- When the slider pots center wiper in near the non-inverting input of the differential amp, it is a cut. Not a boost. Apparently I noticed this while builing the circuit on the breadboard and fixed things by reversing the inputs to the differential op-amp. However I failed to make a note of this and thus designed the pcb backwards. So. what I wanted to be up is a cut and down is a boost. This can be fixed by flipping the thing upside down and having the frequencies run high to low. Oh well, this first board is a prototype
- Debugging pcbs is much more of a PITA than turret board circuits.
First the classis V configuration with the mids cut and lows and highs boosted:
Next a cut at 6k6 and a boost at 240Hz. Note that I forgot to set the breadboard controls for this so the yellow trace still shows a V. So, overall, not a total success. But I'm gonna call it a victory never the less. It is my very first "modern" PCB and it does work. And I can still learn from it. I plan to try and mount it into a pedal enclosure and try my hand at cutting the vertical slots. I have no idea how I'm going to do this. Any advice at all would be appreciated. Right now my plan is to try and cut the slots with a combination of a drill, metal nibbler and a file. I'm not yet brave enough to try this on my one and only chassis. So, truing it on a pedal box with this first prototype seems like the way to go.
The next PCB version I will try will be for a better set of 45mm faders. These taller faders will look better on the control panel. And have nice detents on the center position. Plus they actually work smoothly
. The only downside is they take up more room. So, I will try using the SOIC op-amps suggested above. Mike
P.S. I also worked on the layout a bit. It is not done yet. But I have all the major circuit bits placed and I think I am going to have enough room to build the whole amp using turret boards: