Charge Amplifier Guitar Preamp questions

[ddietz]

Hi folks,

I'm working on an acoustic guitar preamp using a charge amplifier first stage. Right now I have it working OK but I'm wondering what I can do to take this further and make it more universal and useful with reasonable work.

The charge amplifier basic schematic i am using is here and uses only the first half of this schematic. I am using a 10M and 1000p in the feedback loop compared to what they have shown. The 1/2Vcc bias resistors are 100K. Single 9vdc battery and I'm using a passive Fishman Sonicore undersaddle coax piezo transducer. I unfortunately cannot find the impedance/specs of the transducer.




I am currently using this into a Fishman Loudbox mini with a 10M instrument input and it works/sounds well. The passive piezo sounds OK directly into the Loudbox, but warmer and more realistic with the charge amp.

Questions:
1. I tried using a 0.047uF in the feedback and got no sound. Does that make sense. I don't have a reference on gain levels for this thing. I know gain is 1/Cf but those numbers are always huge so I don't know what is reasonable.

2. What would the output impedance of the charge amp alone be? Very low and only whatever the output resistor is? If so, in theory this should work fine, loading wise, in any instrument input I use (1M-10M ususally)?

3. Any idea what to expect for output voltage with this, say on a hard strum? I have no scope or anything.

4. Would using the 2nd half of the TL072(actually using an 082) for some more and adjustable gain be a good idea and make it more universal for gigging using various house PA systems? What is a good range of gain to use if this is a good idea?

5. I may incorporate an equalizer (or make a separate eq). I've read that the Baxandall (spelling) is great for clean acoustic, would people generally agree with that?

6. Finally, if I add the gain, would I then end up at line or stay at instrument level, for PA systems.

Thanks all. I've read a bit about this and seen lots of Teleman's posts here, but never seen final schematics with the charge amplifier included.