Two pre-amplifiers and a power amplifier.

RE: Two pre-amplifiers and a power amplifier.

Jan Cumps — Wed, 10 Jun 2026 14:09:09 GMT

(I do wish that people wouldn't deface beautiful instruments with silly calibration stickers - this one is so well stuck that I think removing it will bring the lettering off too.)

Heating it some time with a hair dryer may make the glue gummy again, while the ink won't.

RE: Two pre-amplifiers and a power amplifier.

shabaz — Mon, 25 May 2026 18:07:47 GMT

(Just noticed a typo, the line

K1 pre-amp distortion 2.35Vp-p out, 1kHz, 8V supply = -65.6dB = 0.05%

should read

K2 pre-amp distortion 2.35Vp-p out, 1kHz, 8V supply = -65.6dB = 0.05%

RE: Two pre-amplifiers and a power amplifier.

shabaz — Sun, 24 May 2026 16:54:01 GMT

If anyone wants to build these, the KiCad files and PCB Gerbers are on Github in a discrete_amps repo.

RE: Two pre-amplifiers and a power amplifier.

shabaz — Sun, 24 May 2026 16:47:03 GMT

Hi Michael,

Thank you for this, and all the measurement! I think that near unnoticeable distortion is worth it, for the increased power output, so I'll be making that modification too, plus the 120pF capacitor. I've soldered up the K2 preamp, and tried it with the MXO 4 frequency response app, I'll post up the results in the next day or so. I did solder the K1 Power Amp, but have not tried it so far. Also, I tried the K2 preamp with a electret microphone element, and the results were excellent (gain is a bit high for some electret elements, but that's easy to adjust with the feedback circuit R9 and C4 I noticed in the simulation.

By coincidence I downloaded the Pico software too today, and was using it (to troubleshoot a UART issue). I have the bottom-of-the-range 2000 series, but it's still so convenient for working anywhere with a laptop.