A standard speaker of any size can also be used as a microphone. Only one additional component is required, as we will see below. A loudspeaker can both reproduce the sounds and can capture them by turning them into electrical signals for an amplifier, or other equipment.
However, since the speaker is a low impedance device, to be used conveniently in most equipment that has high impedance input, a transformer must be added.
The recommended transformer is the output transformer for devices with valves, such as those using the 6L6, 6AQ5, 50C5, etc. These transformers have a winding of 2,000 to 10,000 ohm that is connected to the input plug of the amplifier, in our case, and a low impedance winding that is connected to the loudspeaker.
For ease of operation, the speaker should be small, no more than 5cm.
Old transistor radios (output transformers), which have primary windings of 200-2000 ohm, may also be tried, although they do not result in the same sensitivity.
In some cases, in addition to this feature, you will need to use a good add-on preamp, The connection is made at the AUX input of the common amplifiers. On the site we have several good preamplifier circuits.
Material list
T1- Output transformer for 2 500 to 10 000 primary and 4 or 8 ohm secondary
FTE- 4 or 8 ohm common small speaker
Miscellaneous: shielded cable, plug according to the amplifier, wires, solder, etc.
