Here is a simple circuit that serves to increase the sensitivity of high impedance headphones (crystal or magnetic over 2000 ohm). Experimental radios, intercoms and various experiences using headphones will be facilitated with greater input sensitivity.
This amplifier is extremely simple because it uses only 1 transistor and can be powered with voltages from 3 to 9 V. The ideal is 6 V coming from 4 small batteries that will have great durability given the low consumption of the unit.
We can connect weak signal sources such as crystal radios (galena), experimental radio detectors, microphones at inputs E and T and obtain the signal at the output to the handset, with amplifications ranging from 20 to 200 times, depending on the transistor used and the values of R2 and R3.
These components can be changed. R2 can have values between 47k and 470k being chosen the largest that results in the signal without distortion.
R3 will be between 4k7 and 100k, also being chosen by experience.
The electrolytic capacitors are for 9 V or more and the resistors are 1/8 E. In figure 1 we have the complete diagram of the amplifier.
In figure 2 we have the assembly made on a terminal strip.
Mounted on a plastic box, the E and T inlet can be obtained by a pair of claw wires for ease of use. The output is a jack according to the phone jack.
There is no gain control but a potentiometer from 10k to 100k can be adapted to the input.
Material list
Q1 - BC548 or equivalent - any general purpose NPN transistors
R1 - 1k - resistor - (brown, black, red)
R2 - 100k - resistor (brown, black, yellow)
R3 - 33k - resistor (orange, orange, orange)
C1- 10 uF - electrolytic capacitor
C2 - 100 uF - electrolytic capacitor
S1 - Single on/off switch
B1 - 6 V - 4 small batteries
Miscellaneous:
Support for 4 small batteries, output jack, terminal bridge, wires, alligator clips, etc.
