Brach here, product designer.
Thanks for the post and the excellent build blog on your site.
It’s true: the instructions are not very clear about which path to follow at that point. In our next revision of the manual I’ll make sure to clarify. Thank you for your honest feedback.
To be clear now: Step 1 on Page 13 applies ONLY if you intend to mount the PCB and the piezo together into a grounded shield container, like the Tin Can Microphone, or a garbage can. Step 1 directs you to wrap the piezo in electrical tape, then add the double-stick tape for mounting. AND YOU ARE DONE! SKIP STEP 2. If you are making the standard contact mic in the Altoids can, SKIP STEP 1; GO TO STEP 2, which instructs you to wrap the Piezo in electrical tape, THEN copper tape to shield it, THEN double-stick as one possible mounting method.
As far as the low-end issues you are having, coupling is everything (as you’ve found out). The painters tape method works for some applications, like a banjo or art installation or something that doesn’t have much low end anyway…but like you, I’m discovering that it usually just does not provide adequate coupling to pick up enough bass. (On our video when I mic’ed the bass and acoustic with the Cortados, I had to eq the low end up.) When we fitted a customer’s acoustic guitar with a Cortado pickup, we fitted the truss with a custom clamp to press the piezo against the inside of the guitar top. I also adjusted some component values in the circuitry to emphasize the low end a bit more. You can simply change C1 and C3 to 22nF to allow more low end to pass through. (In some applications, increasing the low end can really make the signal muddy and unclear, but it doesn’t sound like that’s a problem you’ll have.)
The bias mod you performed has virtually no effect on the frequency response. Please enjoy that high s/n in the best of health.
Let me know if you have more questions/comments.