Hearing aid for elderly, looking back to an earlier older circuit, with its good properties

Please read the description first. Schematics can be found via links in the textbox. Both the old circuit of (say) 10 years ago and circuits from now, say 15 July 2024.
Results from 14 July 2024 (2 microphone amplifiers made with electret capsules, working on say 3 Volt) are in this video • Microphone amplifier f...
This video (7 July 2024) is about an older circuit, that I made 10 years ago and published it, in those days, on UA-cam.
Correction: on 5.29 in the video I say "an electrodynamic microphone", of course it is "electrodynamic", but the better word for this is "an electrostatic microphone".
Now a kind of looking back, 7 July 2024.
It triggered me again because of these things:
a) it was elaborated during many hours/days, getting it to the lowest noise level as possible via experiments with different (cheap) transistors, resistors, the transistors lowest base currents, that means in general the lowest (shot) noise problems
b) The base currents to each transistor in the circuit had to differ somewhat, compared to the bias that each transistor needs in such a circuit. It can differ from transistor 1 to transistor 2 or transistor 3
c) It was developed for 3 Volts (=2 Carbon Zinc batteries). I found out, during the past years, that it could even work on one (1) Carbon zinc battery of 1.5 Volts
d) A problem: the circuit started (sometimes) to oscillate (“motorboat”) when the batteries got depleted
e) Whatever that all may be: I still now (July 2024) think that it is a valuable circuit, with which you can help elderly for a tiny mini fraction of costs
f) I found (say 4 years ago) that that the prices of hearing aids for elderly in the Netherlands went up to 2000 Euro’s (…it is true….). That is and was absurd (!). It was (and is) a big “earning model” for enterprises in this field of elderly care or persons with hearing problems and it is still an enourmous “money machine” nowadays in the Netherlands.
g) We see constantly advertisements on social media and on the radio of enterprises like “Beter Horen”, litt. translation: “better hearing”. To get a “free hearing test”. Reason: many health ensurancies in the Netherlands pay the total cost, even when it is 2000 Euro or so. It of course depends on the contract, but anyway.
Whatever that may be: this is a properly working general circuit to help elderly. As long as the batteries are fresh, it works very good. In the future I want to take a deep dive on supplying this hearing aid out of only 1 battery of 1.5 Volt and/or, in a better way (lower depletion), via 2 carbon-zinc batteries of 1.5 Volt = 3 Volt out.
ORIGINAL schematic (many years ago, 2014) • Hearing aid for (elder...
All properties of the circuit are explained there
REVISITED schematic/circuit (20 June 2019) • Hearing aid for elderl...
In that video I am talking about supply voltages
My You Tube channel trailer is here: • Radiofun232 on UA-cam...
Type there the keywords that you like (e.g. radio/audio/amplifiers/test/filter/) in the “looking glass” = search function” and give “enter”. Via that you can find specific video’s (under the say 1500 published).
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My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there. www.lulu.com/s....
I keep all my YT videos constant actual, so the original video’s with the most recent information are always on UA-cam. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. Upload 7 July 2024