Build Guide CascadiA. GeFet Preamp

Transcription

1 Build Guide CascadiA GeFet Preamp Disclaimery stuff: This project is meant to be assembled by fellow DIYers from the Madbean forum and should only be used for the forces of good. Any other uses prohibited under the penalty of ironic Mummy curses. It s not a complicated project (no manual biasing or parts matching), but it s a big un, meaning there s more opportunities for placing wrong-value parts or bad solder work. Being a DIY project, I can t guarantee a working unit, but if you follow this guide, take your time and are methodical, I bet you ll be rocking out with it. I ve built this up times as of this writing and it s worked as expected every time and been extremely consistent unit-by-unit. The SMD parts will come pre-installed, so you have that going for you! It takes me about 20 minutes per PCB, but I really want my fellow Beaners to have success with this. OK, what is it? The Cascadia is a JFET-based guitar preamp pedal that uses Germanium diodes to add harmonics and then distortion as the gain is increased. It is designed to mimic the response of a tube amplifier operating in its sweet spot, where playing dynamics heavily influence the amount of harmonics and distortion, as well as transitioning into and out of distortion very smoothly to avoid any unnatural or fizzy artifacts. Unlike typical amp-in-a-box JFET designs that seek to mimic a specific amplifier, the Cascadia uses a tone stack with two switches following the clipping stages to allow a great deal of flexibility. While an original ciruit, the inspiration came from concepts laid out by the team at runoffgroove, as seen in their Azabache and Tonemender designs. Partial kit includes PCB with SMD components installed, ribbon connector and fuse.

2 Important stuff Let s start with the Germanium diodes These are the one somewhat tricky part of the build. You need 7 of them, and like all Germanium parts, they aren t as consistent as silicon. Measuring the FV is good, but the results are actually somewhat different depending on what DMM you have. The good news is that this isn t too critical. You just need to avoid anything outside the normal Germanium range of about.25 to.34. Use the basic NOS glass diodes like you find on Smallbear or Pedalhacker: 1N695, 1N34, etc. DO NOT get funky with black glass, weird shapes, brown submarine diodes. I wanted those to work, but they will not sound right. Also, and I don t have good answer for this, no Russian diode of any make will work, regardless of measurements. Don t use D9B, D9G, D9whatever. I know they re cheap and you can source a lot of them, which makes this a sad fact of life. Putting them on the PCB: They re fragile little glass do-dads and come in somewhat varying sizes, so the footprints for these on the PCB have them < Cathode (-) standing up. You will see on the silkscreen that one side is just a round pad and the other is a square pad with a circle around it. The square pad is the < Anode (+) cathode (-). There s asymmetric clipping, so getting the polarity right matters. The enclosure The standard, short 1590BB will not work. It s too short and the jacks won t fit (No, not even if you use mini, tiny jacks i tried). I know people love making pedals as small as possible, so it will go in a 125BB or 1590BB medium or tall. I prefer a 1790-ish size to keep the footswitches farther away from the knobs, and it makes for a roomier build inside. There are drill guides at the end of this document for both sizes. Print them at 300 dpi with no scaling and match it with the PCB before drilling to make sure the print settings didn t get off. The footprint for the potentiometers matches perfectly with the typical Alpha/Mammoth PCB mount 16mm pots, so no worries there. The other parts I highly recommend using 1% metal film resistors. They re less noisy, and the tight tolerance is better for getting the gains and biasing correct on the JFETs. Caps are less critical, so 10% is OK here. Also, I never use ceramic caps. I doubt they d make much difference, but I never tried any in the build, so I can t confirm they won t add noise. This project uses a LT1054 charge pump chip to create a negative voltage. You have to use the LT1054. There is no option for any other charge pump chip because they don t supply enough current.

3 Shopping list Mouser BOM includes: Resistors* Capacitors Zener diode BC549C transistors LT1054 charge pump IC LED fresnel lenses and retainers *For the resistors, I added 10 of each value whether the build requires them or not, because it s a lot cheaper per unit. It s a case of if you have to buy one, you might as well buy 10. Mouser BOM DOES NOT include: Potentiometers & pot covers (You need pot covers or foam or tape to keep the backs of the pots from shorting out against the PCB) Jacks (Audio & DC) Ge diodes Footswitches Toggle switches Enclosure Mouser Shared BOM code: c0dc6976cf Notes: 1% metal film 1/4watt resistors recommended LT1054 required Footswitches must be latching variety Pots are 16mm Alpha/Mammoth with right-angle solder legs Enclosure: 1790ns or other similar or 1590bbs or similar (short 1590BB will not work) Resistors 9 10R 5 100R 1 680R 1 820R 5 1K 1 2K4 1 2K7 8 3K3 1 4K7 1 5K1 1 6K8 1 8K2 8 10K 1 15K 1 22K 1 27K 4 100K 1 200K 1 1M 6 2M2 Capacitors 1 100pF 1 150pF 6 220pF 1 2n2 1 3n3 1 10n 5 22n 1 33n 2 47n 1 68n 3 220n 1 1uF (film) 1 10uF 2 22uF 11 47uF 2 100uF Diodes 1 1N4742A 7 Germanium Potentiometers 1 100KA 4 500KA Quantity IC Value 1 LT1054 Transistors 5 BC549C Switches 1 SPDT toggle 1 DPDT toggle 2 DPDT foot OTHER 2 5mm LEDs 2 Audio jacks 1 DC jack

4 Populating the PCB Stage 1: Resistors & 12V Zener Diode R1: 2M2 R11: 3K3 R21: 100R R31: 1K R41: 10R R51: 10R R2: 100K R12: 10K R22: 10R R32: 5K1 R42: 3K3 R52: 100K R3: 2M2 R13: 10K R23: 3K3 R33: 10R R43: 1K R53: 4K7 R4: 100R R14: 100R R24: 680R R34: 3K3 R44: 3K3 R54: 10K R5: 2K4 R15: 10R R25: 3K3 R35: 200K R45: 10K R55: 10K R6: 22K R16: 3K3 R26: 27K R36: 100K R46: 8K2 R56: 10K R7: 10R R17: 820R R27: 2M2 R37: 6K8 R47: 1M R57: 1K R8: 2K7 R18: 3K3 R28: 100R R38: 100R R48: 10K R58: 1K R9: 1K R19: 15K R29: 2M2 R39: 2M2 R49: 10R R10: 10R R20: 2M2 R30: 10R R40: 100K R50: 10K D1: 1N4742A Stage 2: Capacitors, Fuse, transistors & LT1054 Charge pump (IC2) C1: 47uF C9: 220pF C17: 47n C25: 2n2 C33: 47uF C41: SMD C2: 10n C10: 47uF C18: 47uF C26: 47uF C34: 47uF C42: SMD C3: 220pF C11: 22n C19: 47uF C27: 100pF C35: 1uF C43: 22uF C4: 10uF C12: 220pF C20: 220pF C28: 47uF C36: 220n C44: 22uF C5: 47uF C13: 47uF C21: 33n C29: 220n C37: 100uF PTC1: Fuse C6: 47uF C14: 22n C22: 47n C30: 3n3 C38: SMD Q2, Q4, Q6, C7: 22n C15: 220n C23: 22n C31: 22n C39: 100uF C8: 220pF C16: 220pF C24: 150pF C32: 68n C40: SMD Q9, Q11 BC549C Stage 3: Germanium Diodes D2-8: Germanium diodes, 1N695 or similar, FV, No Russkies allowed! See note on Page 2 OK, at this point, your PCB should look like this:

5 Stage 4: Potentiometers, Ribbon connector & Footswitches Gain: 500KA BASS: 500KA MID: 100KA TREB: 500KA VOL: 500KA For the potentiometers, the PCB is designed for Alpha/Mammoth 16MM, PCBmount pots. Flip the PCB over so you re looking at the outline of the pots. Place each one and tack solder one leg of each. This allows you to position each so it s centered within the silk screen footprint. Once aligned, flip the PCB over and solder them all permanently. Be sure not to start with the leg you tack soldered or it will fall out. Once the pots are soldered on, install the ribbon connector on the same side so the hump is on the same side as the pots. Then, solder the DPDT footswitches on the same side as the pots and ribbon cable. Again, I like to tack solder one leg and adjust if necessary to get it flat and fitting in the footprint before soldering the rest of the lugs. I use Alpha 107-SF12020-L DPDT from Smallbear. Make sure you get a latching DPDT (Not momentary). Now your PCB should look like this:

6 Final assembly Get the enclosure ready One your enclosure is decorated and drilled, install the toggle switches and LED bezels/retainers. (drill guides later on in this document) Doing it this way ensures that the toggle switch height won t put any stress on the PCB. (Picture at right is a Hammond 1590BBS enclosure) And the fun part: Snap the footswitch section off from the rest of the PCB. It might take a little force, but it will give a clean break. Just be careful not to smoosh the Germanium diodes. They re fragile little buggars! You can see a video of it here: Prepare PCB for install: Add wires for jacks Take off all the nuts and washers, except for any locking washers you want to use Insert LEDs into their pads BUT DON T SOLDER YET! Just bend the leads so they won t fall out when you flip it over. This will allow you to guide them into the bezels before soldering them into place. (closeup below)

7 Guide the PCB into the enclosure. If it doesn t slide right in, check your holes and if they aren t straight, you may need to file them out a bit to get it in. The washers can cover a lot of slop. The lugs for the toggle switches should line up through the pads for them in the PCB, as seen in the pic to the right. Once the PCB is in and aligned, add some washers and nuts to keep it in place while you set the LEDs into their bezels and solder them and the toggle switches. Now, just install the jacks and solder the wires to them. The round pads are the ground connections. I don t use a ground wire on the output jack (as seen on right side of pic) because it s grounded through the case. You must use a ground wire on the input jack, though (left one in the pic). Make sure the nuts are all tight and it should be ready to rock. It loves boosts and compressors in front of it, so give those a whirl.

8 Schematic Apetone FX 2017

9 1590BBS drill pattern Print 300 DPI at 100%

10 4S6500 drill pattern From Mammoth. Similar to 1590XX/1790ns Print 300 DPI at 100%