Full text of post which is all I can go by so make of it what you will.
"A bigger capacitor and smaller resistor would simply increase the output voltage even more! Since a magazine suggests it, seems that someone has already tried it and it *should* be safe for the Zeddy! But as the 50mV seems to be enough for a good quality playback or recording of the sound produced, I'd say *not* to push this ancient machine's tolerances to their limits...
Many have tried:
On an old Italian electronics magazine, there is a modification that uses a JFET for amplifying the output signal of the MIC jack.
An old issue of "EchosSinclair" suggests 100pF for C12, 470Kohm for R29 and 4,7Kohm for R33 (to increase the EAR jack sensitivity)
There's something for every taste.
As for higher EAR input sensitivity, may I ask "what for?"
You can already load anything flawlessly, I even use OTLA-converted turbo loading stuff, which does not allow input mistakes at all, and it works like a dream!!
I also use O.T.L.A. (in the past: QSAVE, FastLoadSave, etc.) to load programs in the ZX81. But to do that, I have to keep the volume high enough (compared to the ZX Spectrum). If you use an MP3 player (like I do), it must be preceded by an audio amplifier.
I simply say that a little more sensitivity in the EAR jack does not hurt..."