- Build Rating
Says the rest of the forum, "Sheesh Matt, haven't we seen this a few times already?"
Yes, but I finally got it right! I started with the Aion Cepheus. Then I made a custom PCB version of the circuit that had all kinds of problems, and though I got it working, it was too nasty to post a build report. I did post my second try, which also had issues, but not nearly as many as the first go.
So here's the most recent version, which worked perfectly on first power up!
...almost. The circuit is perfectly fine, but when I did the enclosure artwork, I forgot that I swapped the Bass and Treble knobs in the most recent PCB revision! I was playing through it, putting it through the paces, and thought, "Where's my treble? Did I make another circuit mistake?" I kept turning up the "Treble" knob, but definitely wasn't getting more treble... then I turned up the "Bass" knob - there's the treble, d'oh! But I consider that a cosmetic error, so no need to revise the PCB.
Anyway, this build has a few features that are either new to me or somewhat different from typical builds:
Now that I have a working PCB, I'm going to build the Cornish GC-1. Per Aion's trace, the GC-1 and CC-1 are the exact same circuit, but have about a dozen component differences. The GC-1 is much higher gain.
I have extra PCBs I'm happy to give away if anyone wants one, just PM me!
Yes, but I finally got it right! I started with the Aion Cepheus. Then I made a custom PCB version of the circuit that had all kinds of problems, and though I got it working, it was too nasty to post a build report. I did post my second try, which also had issues, but not nearly as many as the first go.
So here's the most recent version, which worked perfectly on first power up!
...almost. The circuit is perfectly fine, but when I did the enclosure artwork, I forgot that I swapped the Bass and Treble knobs in the most recent PCB revision! I was playing through it, putting it through the paces, and thought, "Where's my treble? Did I make another circuit mistake?" I kept turning up the "Treble" knob, but definitely wasn't getting more treble... then I turned up the "Bass" knob - there's the treble, d'oh! But I consider that a cosmetic error, so no need to revise the PCB.
Anyway, this build has a few features that are either new to me or somewhat different from typical builds:
- The stock circuit, as traced by Aion is in my opinion kind of weird in that it uses a single transistor buffer, a dual-opamp and a single-opamp. That single transistor buffer is used in many (maybe all) Cornish designs, it's been discussed here and on other forums. I don't have the tech chops to weigh in on the engineering behind its design, but all the other experts seem to agree it's nothing particularly special, just a bootstrapped emitter-follower. It's cheap to implement, but a fair number of parts. So my thought was, why not use a good ol' opamp buffer instead? Then we can use a dual-opamp in place of the single for no change in required PCB space, and an opamp buffer uses fewer support components than the transistor buffer, for a net saving on components and PCB space.
- If you look closely at the top of the PCB, you can see a large-ish transistor with a metal backing. This is the Infineon IRFU5305PBF, a power P-channel MOSFET used to implement the polarity protection described here by RG Keen: Advanced Power Switching and Polarity Protection for Effects. This scheme is indeed more complex than the classic series 1n5817 that we typically use. The downside of the series 1n5817 is the small amount of voltage drop. Using the P-MOS scheme has no fixed voltage drop like a diode, only some resistance-based voltage drop due to RDSon of the transistor. So to minimize voltage drop, we want the lowest possible RDSon. I spent some time looking at P-channel MOSFETs on Digi-Key, and getting past the TO-92 package, into the MOSFETs I assume are mostly intended for high power/high current applications is where we find the best RDSon numbers. This IRFU5305PBF has an RDSon of 65 milliohms! It's capable of handling 30 amps - the metal backing is for mating it to a heatsink. It's in a TO-251 package, which actually isn't that much bigger than TO-92. File under deliciously overkill!
- Since I saved approximately $600 by DIY'ing this circuit, I figured I had room left in the budget to splurge on a fancy footswitch. So when Antique Electronic Supply (aka Tubes and More aka @Amplified Parts) had a sale recently, I grabbed this Lehle Long Life SPST Footswitch. This is a ludicrously overbuilt switch, but I'm a sucker for overkill!
- This is the first time I've used "teardrops" in the PCB design (see here for KiCad docs on teardrops).
- The engage/bypass circuit is integrated on the board. This isn't new, but you can read about it here: Buffered Electrical Bypass Module.
- The "blobs" around the wires coming off the board are bits of hot melt glue I used for strain relief. (For some reason I forgot to apply some to the LED wiring at the bottom.) Also not new, but I think it bumps the professionalism up a notch.
Now that I have a working PCB, I'm going to build the Cornish GC-1. Per Aion's trace, the GC-1 and CC-1 are the exact same circuit, but have about a dozen component differences. The GC-1 is much higher gain.
I have extra PCBs I'm happy to give away if anyone wants one, just PM me!
