Piezo transducers like the one we're using have a very high impedance output and therefore need a high-impedance input preamplifier in order to drive line inputs (or headphones). Also, the sounds we're trying to capture are very quiet so we also need some gain.
I built this basic circuit on veroboard and put it in a metal box. I actually used a TLC271CP opamp, but I don't think it matters much. It has a gain of 1000x, and a high-pass to try to cut mains hum and lower harmonics, without really affecting the 'tick tock' sounds.
I can't stress enough the need for screened cable on the input, and a metal box, to keep out mains hum - nothing else comes close!
I tried connecting this circuit to my Motorola smart phone (via a 4-pin 'TRRS' plug) and it kinda worked, but was defeated by poor software! The phone kept insisting it used the built-in mic every now and then, and I had to keep unplugging and replugging to force it to use the external mic instead. Very annoying, so in the end I gave up and used a 3-pin 'TRS' plug, aka regular headphone jack, to plug into my PC. Besides, the TG software is much better than any of the Android apps I tried. More later...
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