Comparing two pieces of gear is something I end up doing a lot. With music equipment, listening to the circuits with a guitar or other instrumnent plugged in often reveals more than just looking at a sine wave on a scope. Before this build, gear comparisons were frequently a frustrating process involving a lot of plugging and unplugging while balancing a guitar which made it difficult to listen critically. Differences can be subtle and any dead time during the switch can cause your brain to misremember or otherwise trick itself into hearing one thing or another. There's also personal bias to make things more complicated. Working on the
G-2 Project
gave me a reason to build something that would automate signal switching to eliminate these problems. What I ended up with is an AB box that can switch an input and output between two circuits at a variable rate. It also has manual switching options and a bypass switch for sanity checks. It runs on 9V and has three, parallel, pedal-compatible barrel jacks to provide power to the connected devices. Dead time is eliminated and with the indicators covered, there's no way of knowing which circuit is selected so listening tests can be more objective.
After some breadboarding, I ended up with this schematic that I made in EAGLE [RIP :(]. It's a low frequency oscillator using a 555 timer and a pot to vary the speed. The duty cycle isn't exactly 50% but it's close enough for this application. T1 drives the relay when the 555 output is high. Since the current through the relay is just about 18mA which is under the max current for an LED, I put the indicator LED in series with the coil. The 33 ohm resistor adds another voltage drop keep the relay coil at its rated voltage. T2 also conducts when the output is high and shunts current around LED1 to turn it off.
I used a 3018 CNC to route the traces and drill the single sided copper clad board. I don't know what I would do without one of these machines. Fab houses are fast but this little CNC cranked this board out in 15 minutes. (.8mm endmill for routing, drilling, and cutout and FlatCam to generate the gcode)
All tinned up. Used solder wick is great for tinning homemade boards if you aren't put-off by the idea of someone else's solder.
Components installed
My relay footprint was incorrect on that board so I made a new one and wired it up in an old enclosure that was already full of holes
I used shielded cable for all the sensitive signals and made sure to leave enough wire length to allow the enclosure to hinge open in the future.
After labeling the controls and jacks
For the first test I used the box to compare an FZ-1A clone to an FZ-1 clone with good results.
I can think of so many uses for this thing. In the future, I plan to make some improvements like isolated jacks to reduce ground loops and switchable buffers to deal with sensitive pedals but for now, I'm very happy with how it came out. If anyone is interested in the gerber files for this project, you can
download them here.
