The Piano Tuners Ear started with the discovery of the MFJ-751B.
This was a unit from MFJ industries that was used by ham radio operators. It allowed them to filter out one frequency (notch filter) or filter out all BUT one frequency (bandpass filter).
A bandpass filter is exactly what piano tuners do with their ears when tuning a piano by ear.
The problem was, the unit came with no amplification.
Also, it was decided to use a contact microphone which does not pick up signals from the air, to reduce feedback. This meant that the signal from the mic was very low. So low in fact that the unit needed three amplifiers to boost the signal enough so the the MFJ-751B could filter out.
Also, once filtered, the signal was again so weak that it needed another amplifier after the filter so the filtered signal could be heard.
Below are some of the early Youtube videos I produced describing how the unit works and how to put it together.
Describing a primitive prototype of the Piano Tuners Ear.
Description of how to put the Piano Tuners Ear together from off the shelf components.