Here are cheap hearing aids that can help people with hearing loss. Commercially available hearing aids are very expensive. This is an inexpensive hearing aid circuit that works using only four transistors and a few passive electronic components.
When switching the switch S to the "on" position, the condenser microphone detects the actual sound signal, which is amplified by transistors T1 and T2. The amplified signal flows to the base of the transistor T3 through the coupling capacitor C3. This signal is additionally amplified by PNP transistor T4 to drive low impedance headphones. Capacitors C4 and C5 are power supply decoupling capacitors.
The circuit can be easily built on small general purpose printed circuit boards as well as Vero boards. It operates from a 3V DC power supply. For this you can use two small 1.5V batteries. Always keep switch S "off" when the circuit is not in use. To increase the sensitivity of a condenser microphone, install it inside a small tube or pipe.