The non-inverting input impedance of most modern op-amps tends to be damn near ∞, or at least very high- on the order of megaohms. Input impedance in the non-inverting circuit is set with a resistor.

Here's an example of a really simple amplifier circuit. No filtering, no nothing, just an adjustable input impedance and a fixed gain:

 

Tex; sometimes a conversation has more than 1 participant and the others just listen!

Keep up the good work. I want you to succeed. I just don't have a clue about electronics!

 

Well I sent Boris (the guy who builds the Knocksense) an e-mail explaining what I want to do and asking of there is a point in the unit that can be tapped into; He has seen this thread and is interested in this modification. He can add a tap after the KS knock filter circuit, so that should not only give me the audio signal I am looking for, BUT the engine noise will be filtered as well.

*Spicoli voice* - Boooonuss

The KS uses surface mount components which are a PIA to work on without a good soldering bench (which I do not have) so he has offered to do the mod for a modest fee. I am going to send him my unit to do the mod so I'll update this thread after the mod is in place and I have tested it out. According to Boris, though he has done something like this before, there is some risk of inducing noise back into the KS circuit via the amplifier connection that could cause a false knock trigger. But being the R&D kind of guy that I am, I am going to go for it anyway. Stay tuned...

 

ZX-TEX,
Any updates on this? I just got my car back on the road and the 3" enthuza makes it hard to hear any knocking. I need a way to tune the KS without damaging the motor.

 

Just found this:
THE KNOCK BOX - Knock sound recordings

How does it compare to what you hear?

 

Well I switched over to the Adaptronic and so I do not have the knocksense anymore since the Adaptronic has built-in processing for a knock sensor. I will still be using the Radio Shack amplifier in the glovebox, either tied in to the stock sensor (connected to the Adaptronic) or using a separate knock sensor.

 

another nguyen for det cans.

 

God find. I had not seen that one. That is almost exactly what I hear with this setup; the sound is a little bit different, but that is essentially it. The normal engine sounds are rythmic and go up and down with RPM. The knocking sound is just like in the recordings in that link, a random, high pitched rattling sound, or like BBs (marbles, coins, whatever) in a shaking metal can.

I can hear it with the electronic det can setup well before I hear it in the cab, even with my (former) quiet exhaust. So the knock can be caught much earlier when it light and presumably not doing nearly as much damage.