How to define the output impedance of a gainstage?

[Roly]

{I'm still traveling}

There is *very* complex equation which can be reduced to a "magic number": Re=26/Ie (in milliAmperes)

Thanks for that JM, I had forgotten that little gem (and have completely forgotten how it was derived at College many moons ago  )

Re: digital stuff; after many years mixing and recording on purely analog gear ('coz that's all there was) I had to readjust some of my ideas when digital equipment started turning up in racks.  In the days of reel-to-reel tape the rule used to be "just lickin' the red" where occasional, or even frequent peaks into the red above zero dB were quite acceptable to produce a "full" recording, but applying that idea to digital desks and recorders produced some very nasty garbage.  Digital gear seems better these days, but some of the early equipment (which is still floating about) was very unforgiving of even the occasional peak over 0dB and one quickly learned to mix to -3 or even -6dB and avoid digital "overs" like the plague.

And I'll add my voice to the chorus that actually building stuff, then the struggle of getting it going, is the only real shortcut to understanding what you are doing.  I've got to say though that I'm not a fan of protoboards because, in my limited experience of them, I've seen more time wasted struggling with poor connections than with actual circuit problems, and if you build on strip or dab board you end up with an assembly that can be of lasting usefulness and does not have to be disassembled.