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ddietz 26th December 2011 05:31 PM

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.

marvinkite 2nd December 2012 12:14 AM

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.

JMFahey 2nd December 2012 03:40 PM

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.

Quote:

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.
You might use all of it; you have the other 1/2 TL072 there, after all.
Quote:

I am using a 10M and 1000p in the feedback loop compared to what they have shown.
Any particular reason to use those values?
Quote:

The 1/2Vcc bias resistors are 100K. Single 9vdc battery
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.
Quote:

and I'm using a passive Fishman Sonicore undersaddle coax piezo transducer. I unfortunately cannot find the impedance/specs of the transducer.
Many multimeters let you measure the Piezo capacitance, which is the important parameter here.
Quote:

I am currently using this into a Fishman Loudbox mini with a 10M instrument input and it works/sounds well.
Fine amp.
Quote:

The passive piezo sounds OK directly into the Loudbox, but warmer and more realistic with the charge amp.
Probably, because with the charge preamp you are capacitively loading it.
Quote:

Questions:
1. I tried using a 0.047uF in the feedback and got no sound.
Way too high feedback capacitance, it killed your gain.
Quote:

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.
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.
Quote:

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)?
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.
Quote:

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

Quote:

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?
See above.

Quote:

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?
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.
Quote:

6. Finally, if I add the gain, would I then end up at line or stay at instrument level, for PA systems.
Staying at instrument level is safer.
Once you distort, that's it.

And, good luck !! :)

ddietz 2nd December 2012 08:58 PM

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

JMFahey 3rd December 2012 03:09 AM

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. :)

multi-volti 4th December 2013 12:55 AM

Well, this is interesting...I didn't think people were using charge amps for music...naive on my part, I guess...

A short piezo cable sensor is pretty low capacitance...the two I checked were around 150 pF, including the cable.

They don't have a single impedance value...it's frequency-dependent. Usually you consider the lowest frequency you don't want to have problems amplifying and the input impedance, which for the purposes at hand will be the resistor at the input...the capacitance of the piezo and the input resistor form a high pass filter that is the cause of LF loss with haphazard combinations, or wishful attempts to use a low capacitance piezo and a low-Z preamp or amp, and there's no low end.

You could calculate the piezo impedance at frequencies of interest but it's not meaningful...choosing the low corner freq. is the tool to solve problems.

I want to pursue a charge amp for a variety of other reasons I don't want to ramble on about. Main reason I believe they're used is they eliminate effects of long cables, commonly for vibration analysis piezo accelerometers.

I didn't look at the RC pair you have in the charge amp, but where a very large resistor is used, like with photodiode preamps that have tiny feedback capacitors, the large resistor causes large gain at DC and any op amp offset or bias current can send the output to + or - supply voltage...complicating the tradeoff between charge amp benefits and 'costs'...better op amps can be necessary and choosing an op amp that is good enough but not 'too good' for your budget is frustrating... of course once you know what op amp parameters you want to optimize, you're unhappy with the cost...or how much current the op amp needs...

If none of that came up in your charge amp development, that's good news.

If you don't need electrometer grade op amps, you can spend that op amp money on your instrument instead ;O)!

I'm happy to see this grade of op amp works for you...a charge amp and piezo can bring a whole bunch of special gremlins to the table that voltage amps do not have.

I'm impressed at the same time I'm embarrassed I hadn't uncovered any commercial music preamps that were charge amps...but most don't publish schematics ;O)...

multi-volti 4th December 2013 01:01 AM

The 6-segment ceramic piezo 'sticks' I've measured were roughly 600-650 pF, FWIW. I think using a feedback or 'gain' capacitor equal to the sensor capacitance is the preferred way to assure stability, give you a transimpedance gain of 1 for lowest noise (assuming you don't want loss at the front end), and set the low freq. corner with the RC pair...I didn't say which because I have to go review...there is a way to set the low end with one RC pair and the high end, if desired with another RC pair.

Inverting vs non-inverting have some different characteristics with a charge amp...the virtual ground being useful with a charge sensitive preamp.

shanx 6th December 2013 07:00 PM

Multi-Volti, you are right about charge amps use in accelerometers, they mount the vibrationsensors to a unit under test, and some need to be extremely low mass (so more difficult to integrate a preamp), and tend to use long cables. The charge amp allows use of longer cable without having the signal attenuated by cable impedances.
There is another thread going here on this forum for piezo pickup and I posted a link for a violin project with a charge amp.

multi-volti 7th December 2013 01:15 AM

I will look for the violin article.

I just saw an earlier post in this thread about the INA217.

I just sidelined a hare-brained idea to build a 4-bump DSBGA mic preamp adjacent to the piezo...DSBGA standing for die-scale Ball Grid Array...I was starting to say it meant Damn Small BGA - it took me over an hour trying to solder #42 magnet wire to 3 of the 4 solder balls under a microscope...a 1 mm x 1 mm part with the 4 balls on a 0.5 mm square. I didn't do the 4th wire yet because I don't know what I want to do with the opposite end yet!

I will try conductive silver epoxy next, but the two types I have are TBD for suitability...one is for bonding in thin layers, not structural thick layers...the other I didn't check before buying...I'm hoping that will be easier than dealing with the solder balls that are something like 0.13 mm diameter. Maybe not my brightest idea, but it also hasn't failed either - I just don't think I'm up to the challenge yet.

shanx 7th December 2013 03:16 AM

Here is the link for the e-violin pre.
Electric violin

If you are able to get that mic preamp work done...very forward thinking. Keep us posted please.


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