Impedances...

[Externet]

Hi all.

A normal, typical speaker being used as a microphone has a
- low impedance output or a
- high impedance output
and requires a
- high impedance input preamplifier, or a
- low impedance input preamplifier ?

And; a normal, typical earphone being used as a microphone has a
- low impedance output or a
- high impedance output
and requires a
- high impedance input preamplifier, or a
- low impedance input preamplifier ?

Found conflictive tutorials and search results.

 

[danadak]

A normal, typical speaker being used as a microphone has a
- low impedance output or a
- high impedance output
and requires a
- high impedance input preamplifier, or a
- low impedance input preamplifier ?

LOW
HIGH (typically, no point in altering speaker response or output with LOW)

And; a normal, typical earphone being used as a microphone has a
- low impedance output or a
- high impedance output
and requires a
- high impedance input preamplifier, or a
- low impedance input preamplifier ?

If earphone magnetic
LOW
HIGH

If earphone crystal/ceramic
HIGH
HIGH

If earphone has its own buffer
LOW
HIGH (if further amplification needed)

 

[logandrift]

When a normal speaker is used as a microphone, it typically produces a relatively low-impedance output (since most speakers are in the 4–8 ohm range) and works best into a high-impedance input preamplifier to avoid loading the signal too much. Similarly, an earphone used as a microphone also tends to have a low impedance output—often even lower depending on the design—and likewise should be connected to a high-impedance input preamp for better signal pickup. In both cases, using a high-impedance input helps preserve the weak signal generated when these devices are used in reverse.

 

[AnalogKid]

A normal, typical speaker being used as a microphone has a
- low impedance output

and requires - nope, there is no requirement. The signal from a speaker can be treated as either a voltage or a current, and can be amplified with either a high or low input impedance amplifier circuit. However, the input impedance of the amplifier acts as a load on the speaker output, which is basically an AC generator, and this affects the frequency response of the speaker/mic.

'a normal, typical dynamic earphone'
still is a pretty low impedance source, often around 60 ohms. IIRC the next higher common impedance for a headphone is around 2 K ohms. Now you're pretty solidly in the voltage source range, so a high input impedance amp would be best.

There is an ooooold intercom circuit that has a common-base amplifier as the input stage, with the speaker/mic connected to the emitter.

What is it that you are trying to achieve?

ak