I think you can use TL081 and TL082 or 3 X CA3130 that have similar features. The first high impedance stage use bootstrap technique
http://web.telia.com/~u43200663/bloc...tstrapping.htm the bottom stage with 2 OP (TLC272) has a greater impedance than the TLC271 amplifier, and is also greater the RC constant. I think that the differential amplifier only responds to variations of signal input, when you move the instrument in different site, and like a "motion metal detector" the 4K7 potentiometer is the preset "tuning". There is no need of retuning push.