> mic pre to record my singing lessons; mic is a dynamic one, PSU a +5 V from an old HD.
Hard to believe you want a fixed gain of 1,000 for that. I would expect levels over 1mV from any adult singer through a dynamic mike. Your output level is at best 5V/2.8= 1.78V, and probably more like 1V; this implies input overload at 1mV.
Yes, R9 is an obvious place to put a gain control, giving input overload ~10mV. I'd want more.
> adapt it to 5V
Working at low voltage is tough. All the "negligible approximations" trip you up. At a glance, R5 has "small" voltage drop, about 0.7V*10K/6K8. "Only a volt". But one volt is big in 5V system.
Assuming infinite Beta: Q1 Base is 2.7V, Q1 Emitter is 2V, R5 drops 1V, Q2 Collector wants to sit at 2V-1V= 1V to satisfy the bias. For realistic Beta, it sits lower.
Also the maximum voltage gain of a resistor loaded BJT stage is Vs/30mV, and half that if you want some voltage swing. For 5V, max gain is 160 to 80, less in real life.
Also note that the current gain around the feedback loop is just Q2's Beta. If that is 100, and R5 taps 10% of R6 current, and Q1 signal current is 10% of R5 R4 signal current, you have no feedback.
If you already built it: reduce R5 to 3K. That lessens its DC drop, and also gets the AC gain more in line, R13 and R14 should be like 100 and 3K for the same reason. Yes, the feedback is a heavy load in the stage: but this is a tight design.
Then reduce R1 (and R10) to bring the collector voltages up. Considering the heavy load, 2/3rd of Vp or about 3V may be optimum.
And put a gain pot for R9.