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.