Michael Kellett's Blog - All Comments

RE: Audio Power Amplifier Silliness

shabaz — Sat, 30 May 2026 19:49:44 GMT

I probably will never use a LM386 ever again, because the discrete design was compact at 40x40mm, especially compared to the speaker that it usually drives, and would be fast to assemble with SMD resistors and transistors, plus it’s far better quality that the LM386.

Although I don’t have a need for 5W, personally I’d still build it to try out if you did get enough interest to want to continue with it. I’ll think about it more, the only suggestion I currently had is that the design could be a SMD/through-hole hybrid, i.e. resistor and jellybean transistor footprints could be 0805/SOT-23, and the power transistors could be whatever is needed, and then it’s still easy-to-assemble and quick-to-assemble. I suspect it will be compact, since there are not many capacitors.

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.

RE: PCB micro surgery.

DAB — Tue, 03 Feb 2026 20:02:28 GMT

Nice work.

Your eyesight and skills are beyond mine.

RE: PCB micro surgery.

shabaz — Mon, 02 Feb 2026 15:27:46 GMT

Nice surgery : ) C120 must be extremely tiny.. that's what is in the tweezers I presume?

I've got T210 and T245 handles, and love the range of tips.

RE: PCB micro surgery.

rsjawale24 — Sun, 01 Feb 2026 12:32:47 GMT

That's some nice SMD work!

I have been planning to switch to C245/210 style soldering station for a long time now. But not sure which station to buy. I have my old Tenma 65W station that uses the 900M style tips, a multicomp pro 120W with same style of tips and an ifixit 100W USB soldering iron. But none of them are good for SMD work.

RE: Measuring DC-DC converter Efficiency

supercachai — Fri, 07 Nov 2025 20:34:17 GMT

Great post, thanks for the write up.

Did you consider using the Riedon PCS series? They are in a similar price range as RSN, but THT and can be mounted on a heat sink. So you can achieve a much better thermal coupling, which will probably eliminate any temp drift.

I am building something similar for currents up to 40 A so heat can lead to significant drift, especially when current ratios are very different.

RE: FNIRSI SAG-55 Hot Air Gun

DAB — Thu, 10 Jul 2025 21:41:35 GMT

Nice review.

I doubt I will buy one, but it is nice to keep up with the current technology.

RE: FNIRSI SAG-55 Hot Air Gun

shabaz — Thu, 10 Jul 2025 17:24:35 GMT

It looks really nice. And as you say the Atten is much larger, it takes up a lot of desk space due to the thick hose, so the SAG-55 is more attractive in that respect as well as the price. I have nozzles for a different hot air station which are commonly available from AliExpress, so if anyone wants me to measure the dimensions in case they might fit the SAG-55, let me know.

RE: FNIRSI SAG-55 Hot Air Gun

genebren — Thu, 10 Jul 2025 15:07:23 GMT

This sounds very promising. I am still awaiting the arrival of mine, but it will interesting to see how I feel about it when it gets here.

I have been looking around for more nozzles, but I have decided to wait until I use it a bit before I start accessorizing it. Thanks for the update!

RE: Two rather different communications testers.

DAB — Thu, 22 May 2025 19:16:04 GMT

Nice find.

It will be interesting to see if they still work and what you do with them.

RE: Two rather different communications testers.

JWx — Tue, 20 May 2025 17:05:31 GMT

I wonder what clocking sources are used in them? Typical quartz oscillators or something more advanced/expensive (especially for clock quality measurements)?

RE: Two rather different communications testers.

Jan Cumps — Tue, 20 May 2025 17:03:01 GMT

Some good data points on running a business here.

We probably made between 150 and 250 of them and I think they were sold for about £350. It might have been a little more but certainly no more than £500.

If we take the best case in those days: 250 units sold, £500 each.
Did this product generate a viable profit? Enough to cover R&D, BOM, production, and expected margin?

RE: RT1-M Digital Audio Analyser

DAB — Sat, 10 May 2025 18:57:53 GMT

Nice find, they should give you lots of fun just checking them out.