So there have been some discussions here on the benefit of using a bandpass filter or the like to help out with knock sensing. JasonC even posted a diagram of a filter circuit/amp in this thread.
For those who do not know, the idea is that you use a bandpass to filter out all of the other background noise and just focus in on the frequency range that knock tends to produce. Jason indicated 13 kHz is about right based on his recordings. I run my electric det can through my stereo and used the EQ (16 band IIRC) to EQ out everything but the upper frequency range and IMO it definitely helps with hearing the knock. If you have ever listened to the Miata engine with a det can you know it is a noisy little bitch.
I was looking for a cheap and easy way to build a bandpass circuit to process the signal before it goes into my Adaptronic. The Adaptronic has an analog input for knock (not a digital like the MS) with a engine RPM filter. It works but he log plots show it is a really noisy signal. More discussion on that here http://www.miataturbo.net/forum/t34218/
The hope is that by filtering out all of the other noise that I can do a better job of setting the ECU knock threshold that controls timing retard.
So, rather than build a bandpass circuit from scratch, I wondered if I could just buy a small, cheap bandpass filter/amp that would achieve the same thing. So I started digging and found this, a cheap ($28 shipped) 4-band parametric EQ.
VISONIK HALF DIN 4 BAND EQUALIZER 1/2 CAR PARAMETRIC EQ - eBay (item 200301586749 end time Apr-16-10 11:47:00 PDT)
This seems like a win, since it is cheap, already built and packaged, already set up for 12V DC power, small (1/2 DIN) and the top parametric EQ range adjusts to either side of 13 kHz. Granted this is clearly not an audiophile quality product, but I am not using it to listen to a SACD recording of symphony, just using it to filter a knock sensor. I have run a knock sensor straight into a line input and it works. Plus this unit has input gain adjustment.
It is so cheap and easy to implement I think I might just get one and try it out unless there is some obvious fail mode that I have overlooked. Thoughts?