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I've just been experimenting. Picture attached tells 1000 words. I brought a small Chinese amplifier off ebay for £20. It is sold under the guise of using it to perve on your neighbours and listen through walls. This particular one was called the HY929 and came with two different microphones. I cut off one of the microphones and wired a knock sensor in its place (a knock sensor is of course a type of microphone). Result is that it filters out background noise extremely well. If I put headphones on I can talk to the knock sensor, blow on it, shake it and it doesn't make a noise. Tap it on the table and it makes a loud thump. Put it on the engine and I can hear mechanical noise pretty well although I'm unsure if it's picking up a bit of electrical noise too. I've not actually induced knock to test it yet as I'm not sure I have the experience to differentiate it from other mechanical noise.
Nice work! Let us know how it works once you've got some testing on it. One thing I will mention is that often the professional knock detection products will incorporate a digital signal processor (DSP) that allows them to focus on the frequencies that knock occurs, improving the signal to noise ratio and making it easier to separate knock from background noise.
Would you be interested in testing it Andre if I sent it your way? Letting everyone know if it is actually a useful tool or a waste of time and effort.