Crow & Breadboarder
Breadboarder
Hey Crow, I’m digging out an old RCA 30C15 amp kit and thinking about re‑building it with modern parts—just want to keep that vintage warmth while tightening the signal path. Got any strategies for mapping the resistor network so it stays clean and balanced?
Crow
First, map every resistor to its exact part number and value from the original schematic. Then, line them up in a table and note the tolerance and power rating each one needs. Next, group them by signal stage—pre‑amp, driver, output—so you can see where any imbalance might creep in. For a clean, balanced path, replace each resistor with a matched pair of metal‑film parts that have the same tolerance and temperature coefficient. Finally, test each stage in isolation, measure the gain and phase, and tweak the values only if you see a clear deviation from the original. Keep the changes minimal; the goal is to preserve the warmth, not overhaul the sound.
Breadboarder
Nice, you’re treating the amp like a time capsule. I’d add a small note on the 4.7 kΩ series pair near the input stage – if you lean too far on the lower value, that little mid‑range bite will start to taste like burnt toast. Keep the tolerance under 1 % for the high‑gain section, and if you can grab a 0207 0.5 W part instead of the bulkier 0805, you’ll save a few centimeters of board space and avoid that annoying “pop” when the board shifts. After you finish the table, run a quick oscilloscope check on the driver output; a phase lag of even 5 % can make that old RCA hum feel more like a hiss. Happy soldering, and remember: the goal is to make the sound feel like it was always meant to be on the board, not just a relic in a case.
Crow
Sounds good. Stick to the plan, keep the tolerances tight, and double‑check the phase on the driver. That should let the amp sound natural, not like a relic that’s been shaken. Good luck.
Breadboarder
Glad you’re on board with the plan—just remember, no shortcuting the resistors with a cheap 0805 set; those will taste like nostalgia with a side of noise. I’ll keep the solder wet, the parts lined up like a librarian’s books, and the phase checked on the driver with a tiny oscilloscope. The amp will sound like it’s been in a museum all along, not like it just escaped the attic. Good luck to you too, and feel free to ask if the resistor table turns into a maze.
Crow
Thanks, I’ll keep an eye on that. Just let me know if the table starts looking like a maze.