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RE: Discrete difference amplifier - DC analysis

Jan Cumps — Sun, 05 Jul 2026 10:13:29 GMT

first signal measurements.

I used a 100 mVpp, 1 kHz sinus. Output loaded with a 27K resistor.

signal at in1, in2 to ground
signal at in2, in1 to ground
same signal at both

the amp does what it needs to do. Amplifies differences, reject the same signal

AI says the gain is 7.96 dB.

> what is the gain in dB if I input 100 mV peak-to-peak and I get 250 mV peak-to-peak out?

RE: Discrete difference amplifier - DC analysis

DAB — Sat, 04 Jul 2026 20:52:47 GMT

Very interesting.

Well done Jan.

RE: Discrete difference amplifier - DC analysis

shabaz — Sat, 04 Jul 2026 15:41:19 GMT

Nice setup! Love that triple-output power supply, it looks like a very useful beast.

If anyone is interested in simulating things with KiCad, this is a suitable example project for that I think, and more useful now that people can see the values in reality too for comparison.

The screenshot shows how to get to the simulator in KiCad:

And then one button-click to run the simulation, and the values for each node in the circuit will be displayed directly on the schematic:

RE: GitHub for Professional Beginners: From First Repository to First Release

shabaz — Tue, 16 Jun 2026 17:59:39 GMT

Even if I don't have a proper strategy, and am just working on a small personal project, bits of the process can be followed minus the development branch. For example, I have a small project in my own repository, I know no-one else is working on it, and I only have a main branch. This would be the situation at the start of a lot of simple projects I imagine.

On the PC, I had done a git clone to obtain the repo contents, and directly made and tested some code changes which have essentially ended up implementing a new feature. How can I get those changes into the repository?

I decided to create a feature branch after the fact that I had already worked on the feature, and merge that into main. You'd never do this for a real project where you'd want the ability to test multiple features before putting them into main; for that, you'd need the development branch to test an entire combination of new features thoroughly before merging into main. But this is my small project, I'm the only coder and tester, and I only want to merge one feature that I have tested (both feature and integration, since there are no other features to be integrated), and I'm not creating releases (so far!).

I did this:

git switch -c feature-acmeas  # create and switch to a branch called feature-acmeas

git add .  # prepare/stage the files for commit

git commit -m "Add AC measurement functionality"

git push -u origin feature-acmeas  # publish your feature branch to the remote (origin)

Then, I went to the GitHub repo web page, and clicked on Pull request, and created that as described in the blog post, requesting a merge into main, from the feature branch. I clicked through to perform the merge.

Finally, to delete the feature branch:

git checkout main  # get away from the branch that is to be deleted

git pull  # update the view with the latest from the origin

git branch -d feature-acmeas  # delete the local feature branch

git push origin --delete feature-acmeas  # delete the branch at the origin

All done!

RE: GitHub for Professional Beginners: From First Repository to First Release

balajivan1995 — Mon, 08 Jun 2026 16:47:17 GMT

Nice write up. But something seem to be missing and it took me a while to realise the part where git asks for password to push local commits to remote branch is missing.

RE: GitHub for Professional Beginners: From First Repository to First Release

DAB — Sun, 07 Jun 2026 19:58:56 GMT

Very good post.

RE: GitHub for Professional Beginners: From First Repository to First Release

shabaz — Sun, 07 Jun 2026 17:11:59 GMT

Just added the following diagram to help follow things.

I hope it's correct, but if anyone spots any error, please let me know!

RE: GitHub for Professional Beginners: From First Repository to First Release

genebren — Sun, 07 Jun 2026 15:35:49 GMT

Thanks for the great explanation! I have used Git in the past, but I am still rusty on all the ins and outs of the process, so this was a great review. Having started my software development on Unix environments, I have had a lot of experience using CVS. I have also used SVN (tortoise) on PCs, but I was never very impressed with that environment. I can see a lot of value in starting to use Git for my internal software development, so I have bookmarked this post to refer back to it when I am ready use it. Thanks for sharing this with the community!

RE: GitHub for Professional Beginners: From First Repository to First Release

kmikemoo — Sun, 07 Jun 2026 02:31:31 GMT

Great information! I may never get to this level, but it's noce to know that this guide is here if/when I need it. Thanks!

RE: GitHub for Professional Beginners: From First Repository to First Release

manojroy123 — Sat, 06 Jun 2026 17:03:53 GMT

Very Interesting. I was looking for such Beginners guide t GitHub.

RE: Building a Transistor Audio Preamplifier

jc2048 — Sun, 31 May 2026 09:21:04 GMT

You and Michael are way ahead of me, this is all I've done so far - just built the K2 one.

Since it's a bit cooler today, I'll try doing some measurements later.

"I’m no audio expert!" Me neither, but I seem to be slowly learning.

R13/C6 (and R8/C3) is a little more than just filtering supply noise. With ac coupling and multiple time constants clustered together down the low end of the frequency range there's potentially an issue with stability there as well as at the top end (you'd hear it as 'motor- boating', a kind of puttering at a few hertz). That can be dealt with the same way as at the top end - make one of them dominate and position the others more than a decade away - but there's probably some merit also in reducing any unwanted parasitic feedback through the supply rails (later stage affecting the supply voltage feeding to the earlier stage). In my simulation it looks fairly safe (about 40 degrees of phase margin at worst, just below a hertz) but that possibly depends on assumptions about what is driving it (I had a source impedance of 600R for the generator).

From simulation, I'd be a bit uncomfortable with C8 and C4. C8 alone is an integrator and nicely rolls of the response whilst giving an extremely safe phase margin, but C4 is then lifting the response which massively extends the bandwidth and gives an open-loop 0db point up in the multiple MHz area with next to no phase margin. Of course reality can be very different to what's in the simulation. The simulator is modelling C1 as a pure capacitor, but a real (leaded) electrolytic would probably have an srf low enough to be (perhaps massively) affecting things and is possibly what is saving the day. It would be interesting to try and measure the real open loop response, though I suspect it would be difficult to do well.

Final thought - audio designers seem to be sniffy about using ceramics as coupling caps (microphony, I think, though value change with dc voltage may also be a factor). Not sure how much that's a real concern, just thought I'd mention it. There do seem to be SMD film caps now, if the concern is to keep things compact, though you'd need to keep an eye on the voltage ratings for the smaller ones.

RE: Building a Transistor Audio Preamplifier

DAB — Tue, 26 May 2026 19:26:26 GMT

Very interesting.

That is a very good design for basic audio.

RE: Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

shabaz — Fri, 24 Apr 2026 15:33:06 GMT

Here's the complete intercom circuit proposal.. The simulation of the sub-circuits seems OK, but needs constructing!

I scrapped the 30 kHz alert detect circuit, I didn't have a good feeling about it. Instead, now there's DC applied when the user wants to alert the remote end. The wires need to be connected with polarity for that though (future circuit could toggle positive and negative on the line slowly to get around that), but it seemed a minor limitation.

RE: Reflow Micro Table: Compact USB PD Reflow Table with Browser Control

DAB — Sat, 11 Apr 2026 19:35:45 GMT

Great project.

RE: Simple DCR (Direct Conversion Receiver) – Part 2: Common Emitter Amplifier

michaelkellett — Fri, 27 Mar 2026 16:13:31 GMT

That's a really weird pre-amp.

R16 - R19 set up the DC biasing of Q5 with 3.3 mA of collector current.

The AC impedance at the emitter is the impedance of C13//R19, the LF breakpoint is at 0.72Hz - why !!!!!!

At higher frequencies the emitter load impedance tends to zero.

The AC impedance at the collector is more complex at low frequencies 1k//(180R + ZC12).

The LF breakpoint is 19Hz.

The input impedance will be very low (because Q5 emitter is effectively grounded for AC).

The actual gain will depend on the emitter resistance of Q5 .

Since the intention is to drive about 100mV into a 1k load a lower Q5 collector current would be fine.

I tried this (in simulation)

It has good PSU rejection (18dB worst case at 30 Hz and 90dB at 100kHz.

About 2mA current drain.

Much smaller capacitors.

It's not the best way to use two transistors but not bad. It still uses the first transistor open loop so the gain at 1kHz is lower that you would expect from the ratio of R3 to R4 because of emitter resistance.

It manages 28.7 dB of gain (you would expect 33.4) and can drive 1V pk-pk into the 1K load.