Charge Amplifier Guitar Preamp questions
 

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.

http://www.geocities.jp/dgb_studio/circuit/for1617.png


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.

Hey DDietz, has anyone ever responded to your piezo charge amp circuit? I am prototyping a guitar with built-in mic and piezo combo and want to know if you had experimented any further with (for instance) other r/c combos for lower corner freqs associated with hi-z inputs? My goal is to utilize strickly 48v phantom power for an on-board active pre with balanced in and outs and let the board equalization do the rest. The way I figure it, the less E.Q. in the gain stage, the less noise. Also, most decent mixer boards have parametric E.Q. anyway. I KNOW this is an old post, but the circuit might help me out.

Well, I've just seen this old post, and since it went unanswered, I can do it now.
Never seen a charge amplifier before, but looks reasonable and Op Amps have been my favorite tool for almost 40 years.

You might use all of it; you have the other 1/2 TL072 there, after all.
Any particular reason to use those values?
You will lower biasing string current from 0.2mA to 0.04mA ; but the big current draw comes from the TL072, some 3mA.
If you *really* want to lower current consumption, use TL062, which in this use is fine.
Forget high end Op Amps here.
Many multimeters let you measure the Piezo capacitance, which is the important parameter here.
Fine amp.
Probably, because with the charge preamp you are capacitively loading it.
Way too high feedback capacitance, it killed your gain.
If Piezo capacitance is Cp and feedback capacitance is Cf , gain is= Cp/Cf .
And what value is Cp?
Either they state it, or you measure it, there's no other way.
As an example, I use typical Piezo disks from clock alarms and such, and often measure them as .05 to .01uF, depending on model and diameter.
Typical commercial Piezo pickups are usually lower.
Its output impedance is very low, in the range of tens of ohms, or less. ... plus, obviously, any series resistance you add ;)
Op Amps driving long lines should have 100 ohms in series anyway, for stability.
With the values you used it probably saturates on a hard strum, a TL072 struggles to provide, say, 2V RMS, with a fresh battery.
A TL061 fares better, but the 9V PP available is the real limit anyway.
Even without a scope, you can *measure* the signal output with any decent multimeter.
Adjust Cf until you do not surpass, say, 200 to 500mV (tops) to have clean headroom.

See above.

You may add a Baxandall using the 2nd half of TL072 .
Not bad to have some extra EQ power at your fingertips.
Anyway, use it flat unless strictly necessary.
And in practice the best use will be *cutting*, rather than boosting, for which you do not have much headroom.
As in: *cutting* some nasty boominess or scratchiness.
Staying at instrument level is safer.
Once you distort, that's it.

And, good luck !! :)

Wow, Thanks for dredging this back up. What is really interesting is that I was searching the web a couple days ago and came across my old post in google! I wonder if that triggered something here at DIYaudio since I re-read my post? I've not touched this project since shortly after my first post.

Anyway, I did go ahead and build a version based on the original schematic I posted. I worked fine until a couple weeks ago when the unit suddenly stopped working. I'm not sure why but after a little trouble shooting I figured it was easier to just make a new one. This time I simply used the charge amp section with 1/2 a TL082 since the gain section was never used and using the second half to invert did not change anything sonically.

I should note that this ended up being used with a cheap peizo buzzer, like are found in clocks, bought at radioshack and taped to the inside of a ruan (a Chineese style guitar). This instrument has a rather boomy tone and so my use of the feedback capacitor and resistor are based on 1) what I had available in my box, and 2) setting a useful corner frequency to compensate for that as well as for the percussion effect of handling the instrument.

Thank you JM for all your feedback. If I make another one of these I will definitely spend some time exploring your ideas.

One thing I did notice is that the output sees 1/2Vcc, which I though might be an issue with some systems. A reversed biased 47u/63v on the output seems to take care of that, and is an idea I pulled from a few mic pre-amps I have built based around INA217, etc.

And if anyone is curious about the Ruan, here isa video of my friend playing the instrument on Youtube. The recording quality is poor, but you get the idea.
http://www.youtube.com/watch?v=4-Z3d7GWvqo

You're welcome.
And ... the original preamp *does* have a 10uF output decoupling cap ;) .
You may pull the 2nd TL072 half ... but keep the cap.
Good luck. :)