Simple onboard Electret mic Preamp for acoustic guitar / changing omni pattern

[parrosa]

Hi All,

It's my first post here, so i'll try to explain myself as best as i can. If this is off topic or unclear, please bear with me.... All the juice is after TOPIC

Introduction:
  I'm a guitar player for some 15 years now, that has some high school electronics knowledge, looking to understand, learn and build audio electronic devices, so i can be more fullfilled while doing what i do best... I have started DIY with cables, guitar electronics, speaker cabinets, then pedal effects, full amp Kit (OTS) and now on to preamps. All this for maybe 3 years now. I'm always on a budget, and i confess if i had the money i would spend more time playing.... anyway, i'm very pleased and proud of the things i've built and learned so far! As for the electric guitar, i've built everything from the pickup to the speaker( i don't build components, you know what i mean...) Just the fact that i can say something over what is going on between the vibration of the string and the sound reproduced on a speaker gives me great joy 
  But besides the electric guitar, i play acoustic too. I have 5 different guitar types, and i often need to play with them in live concerts. None has a built in preamp. I don't have the money to buy high quality onboard preamps for all my guitars, and i'm completly turned down with the quality of the average ones... So i have searched over the last year or so for info, clones, schematics and best practices on the subject... I've managed to get piezos preamps working, like this one http://www.zachpoff.com/site2/wp-content/uploads/Alex-Rice-preamp-on-perfboard-Greg-Perrin.png but wasn't satisfied with the quality. It seems to me that the golden rule for acoustic guitar preamps is the blend between piezo and mic. but it seems to far out of reach for me, mainly because i find no info and difficult part sourcing... 

TOPIC:
1 - Electret mic preamp     

I've buit this circuit http://www.minidisc.org/mic_preamp/Simple%20Stereo%20Electret%20Mic%20Preamp.htm , just one rail (mono). I only tweaked the high pass filter and gain ratio values. Also i'm using a TL072 and panasonic electret WM type (i'm trying with 3 different ones). Powered with 9v battery. I'm quite pleased with the quality of it, and besides being a bit noisy it's fair enough for live concerts. The only problem of course is FEEDBACK!!
-------- How can i incorporate a notch filter in this circuit?
-------- Is there any other circuit or schematic, better for this purpose, and still in these simple lines?

2 - Changing Omni pattern

I'm trying to improve the gain/feedback ratio with the electrets. So far i've managed to change the pattern by taking the capsule into a small tube and pouring hot glue to the back of the capsule. It improved the ratio without degrading the quality, but not enough to avoid feedback at voice leves.
-------Is there any other(s) technique(s) to reduce further the omni pattern?
-------Is it possible to use phase cancellation with more omni mics, to suit this purpose? 



With all the "preampless" acoustic guitars that exist out there, this would be a huge contribution for the DIY guitar world! Adding to this, i have been developing a concept as to have this system installed on any guitar, only drilling the input jack. So can you help me out? Thanks

And finally i would like to praise this forum and all people involved, for sharing, creating and breaking ground on audio electronics! I think great things are born this way! 

...Steve Jobs once said that the greatest improvement on computers came from a group of hobbyists messing with calculators ICs...

 

[G-Sun]

I haven't seen much like this being done around here.
So, you've got you electret-mic mounted in your guitar, have a hole for the jack, and then the preamp in a box somewhere outside?
Omni-condensers are very problematic in situations like this, yet, somewhere along the lines of what you're doing is probably what's inside pro guitar-systems.
Can't really help you much.

 

[Harpo]

...The only problem of course is FEEDBACK!!

You already tried something like this
available for various soundhole diameters for most accoustic guitars ?

 

[parrosa]

That solution prevents feedback, but also the natural acoustics of the instrument. Even more for the nylon type. I intend to have the mic inside the body, so covering the soundhole is not an option for me, so far...

I actually modded one of those, just to have the ring. I suspend all the components of the preamp on it, and therefore have an easy way to install it on different guitars...

I forgot to ask in the first post if its also possible to "choke" or reduce the response of the capsule by changing the voltage

Maybe the scope of this topic is not suited for this Forum.... I have been searching on other forums, like gearslutz, Acoustic guitar forum, talkguitar, guitarscanada etc

It's been dificult to filter searches, because it's a specific matter and results vary from none to thousands.... haven't found anything yet. If you know where, please forward me there.

 

[bruce0]

Not really sure what you are searching for but here is a couple of ideas:

1)  If you want the electret capsule inside the body, it cannot really matter whether it is omni or directional.  Many many mic's use the panasonic electret capsule (there are several, some are omni some are directional, they don't all have the same response).
http://www.panasonic.com/industrial/components/pdf/em06_wm61_a_b_dne.pdf

2) For inside the body I would choose omni otherwise you have proximity effect issues and I think will get a bass emphasis.  For outside the body it is different, cardiod or hypercardiod raking across the surface of the soundboard can be nice.

3) The linkwitz mod can improve the mics sound level handling.  Not sure if this matters inside an accoustic, but I think it might be pretty loud in there.  Here is a nice description of how to do it <https://www.youtube.com/watch?v=dSPQf42JgbI> and which also might give you a pretty nice soft mic mount for inside the body.

4) Linkwitz has a circuit for a measurement microphone using modified capsules, which might be a good start.  Powered by 2 9V batteries.  It is here http://www.linkwitzlab.com/sys_test.htm

5) I did make some boundary microphones using linkwitze modified panasonic capsules and being phantom powered, circuit and board here, this could also for you. http://groupdiy.com/index.php?topic=43085.msg546752#msg546752

The panasonic type capsules are pretty cool, and can be modified and wired various ways.  Rip one apart sometime and you can see the little surface mount fet transistor and the plate and diaphragm, a very elegant little design. Here is a great description of the various ways of wiring them up (linkwitz etc).

Questions:

How will you power the mic?
How will you cable the mic?
Where will you locate the mic?
What type of amp will it be plugged into (guitar amp? Soundboard?) so we know what kind of interface is needed.

 

[parrosa]

I have been using the Panasonic WM-034CY, and now i found the Panasonic WM-55D103, wich is noise cancelling. Also i have tried the BESTAR BCM9767.

A hypercardioid electret would be good, but i don't know any or where to source...

Inside the body seems less prone to feedback than at the surface or even clamped. Whatever works best. As for sound quality i would go for sounboard surface.

Tried the source follower linkwitz mod, but i think i dont really need to have more spl handling... I'm focusing on feedback issues - I will try your circuit with a linkwitz modded capsule, though.

How will you power the mic?
How will you cable the mic?
Where will you locate the mic?
What type of amp will it be plugged into?

- Powered by 9V Battery
- Line level mono signal (jack) feeding a Di to a mixer...  Balanced signal would be great if i could keep the circuit dimensions, and if helps lowering noise floor...
- inside the body, or soundboard surface, depending on best feedback rejection
- usually the typical Club mixer or a Mackie type mixer(VLZ model). if line level, first through a DI to mixer.

 

[bruce0]

The patterns formed by these panasonic style capsules are created by porting the capsule, allowing interference of the pressure waves hitting from the front and from the rear.  Take a look at pictures online of the "unidirectional" vs "omnidirectional" capsules, you will see little holes drilled.

Holes - Cardiod

http://datasheet.octopart.com/WM-55A103-Panasonic-datasheet-65748.pdf


No Holes Omni

http://www.panasonic.com/industrial/components/pdf/em06_wm61_a_b_dne.pdf

I think they get hypercardiod by porting differently and possibly using the mounting structure of the capsule to do the interference.

(You can do your own holes, the science is hard for doing it in a controlled way, but porting the back or sides always adds some directionality.  Sometimes a linkwitz mod will accidentally create a directional cartridge by unsealing the back.)

A hypercardioid electret would be good, but i don't know any or where to source...

A good inexpensive source for cardiod and supercardiod (and sometimes hypercardiod, I mis-spoke) capsules is old junked desk boundary microphones.  They all seem to have the panasonic style capule in them, and the Crown are particularly well documented and high quality.

Here is a quick summary of Crown Products http://www.crownaudio.com/usa/pcc.htm they call them phase coherent mics (aka boundary microphones).  You want a PCC170 or a PCC170SW (switch).  Both have the capsule you want inside, and both are often sold in junk boxes when conference rooms get torn out.

Also search audio technica boundary on ebay, those were sold in packs, and are surplus in packs.


The little PCB in the crown mics is phantom powered and would do well for your application, you could even wire up the mute switch on a SW version.  You would need to build a new case.  The case would need to be metal.  The manuals are downloadable and have exploded views. 

I don't know whether you could order the cartridge for a PCC170 from Crown (or AKG seems to sell Crown now) or what it would cost.

Inside the body seems less prone to feedback than at the surface or even clamped. Whatever works best. As for sound quality i would go for sounboard surface.

I love the sound of a raking angles on the fronts of accoustic soundboards.  Very natural. 

Not surprised that you get no feedback inside the case, wouldn't imagine that would happen (the speakers are outside).  What type of feedback problem are you dealing with exactly, and in what situation?

Tried the source follower linkwitz mod, but i think i dont really need to have more spl handling... I'm focusing on feedback issues - I will try your circuit with a linkwitz modded capsule, though.

Note sure it matters how you power the capsule, you might want to use the Linkwitz head amp as it is already set up for battery power.

- Line level mono signal (jack) feeding a Di to a mixer...  Balanced signal would be great if i could keep the circuit dimensions, and if helps lowering noise floor...

With all of these microphone designs you can run direct to the board as a mic level balanced signal.  Phantom powered or battery powered.  No DI would be needed.

- inside the body, or soundboard surface, depending on best feedback rejection

I suggest thinking sound quality before feedback, and address feedback in other ways.  For instance:  say you like a cardiod mic aimed at the start of the fret board, or raking the surface of the soundboard, then why put the mic inside the instrument?  Yes you won't get feedback in there, but it will also not get the same sound.

If you want to mic the instrument tape a small cardiod to the instrument or a clip mounted/stand mounted mic pointing where you want it.  Mic the instrument CLOSE so that you don't need much gain to get the signal, and pay attention to where the directional mic points AFTER your instrument. ( If you rake the surface from the bridge toward the neck, and the PA speakers are off your neck, the mic is pointing at the PA! So either stand differently or rake in another direction.)

Another great mic for on instrument micing is the countryman GLM (also manufactured by crown - see Crown GLM, countryman glm, etc). Many folks sticking these to accoustic instruments (they come in omni and hypercardiod and are the size of a pinky fingernail. )  Very useful.

Tell us about your feedback, bad feedback in an acoustic situation is usually pretty manageable.  Is it stage monitors for that cause the problem?

 

[gfr]

Check US patent 4,748,886 by Franz Debyl. The idea is to mount the mic(s) on a foam ring inside the guitar, and the foam will help isolate the mics from external noise (like the soundhole covers). I think it also dampens the soundboard a little (that also helps fighting feedback).

I think they are out of business, but it's extremelly easy to DIY a work-alike. Just cut a ring of foam (the "acoustic" kind used in home studio room treatment, if possible) and stick the mic to the outer side of the ring. Then stuff it throug the soundhole. You can bring the mic wire to the outside through the sound hole, so no modifications to the guitar - no drilling, no jacks - so if you don like it, it's imediately and completely reversible.

The sound inside the body of the guitar is very boomy and you also don't have some higher harmonics that are directly radiated by the strings (and not by the soundboard), so you may need to cut a lot of low end and mid-lows (that helps fighting feedback too) and add a little of presence to make the sound more natural. With proper EQ it can sound way more natural than any piezo pickup (at least with nylon strings, that is what concerns me ) and still allow you to play with a drums and bass in a band (at reasonable levels).

 

[parrosa]

Thanks for the feedback, this is a lot of good stuff in short amount of time!

So it seems the boundary type microphone has the right properties for this job to start with, and has you said the crown PCC170 is maybe the first in line.
I also found the CM-311A http://www.crownaudio.com/media/pdf/mics/102546.pdf
and as you said the GLM-100E seems to be on top of the list http://www.crownaudio.com/media/pdf/mics/125491.pdf
But i found them difficult to find, and the price tag is a bit high, or am i missing something?

I didn't know this boundary type of mic and it seems a perfect mic to attach to the soundboard of the instrument.  I've looked for audio technica boundary mics as well, but couldnt find a good match for this purpose(too big enclosure). Is there any other brand, widely available, i should look for?

If not, is the Panasonic WM-55A a reasonable mic to start with?

And assuming i use a balanced signal, how could i add a volume and a simple eq(notch filter with narrow Q would be great!) right at the instrument?  any sketch i could start with? 

I usually need to amplify the guitar to balance stage volume with the rest of the band, so yes feedback from monitors in the form of squeals, 2khz and upwards are most common...

 

[parrosa]

The Foam Ring (the description makes it unclear for me to understand, in this patent the foam pad is described as cilindrical, so does it covers the full depth of the soundhole area?) concept also looks great for feedback issues, and has a friendly installation on the instrument, but they change its natural acoustics. covering the soundhole prevents sustain and that might take the dinamic response.
I think this kind of compromise would reflect on my playing style, because i always hear my acoustic sound on stage. the louder instrument that i need to match on stage is a brush drumset;
or double bass with clarinet or flute.

So, for a pop, rock or big venues this would be the best solution to avoid feedback. I definitly will give this a try, it seems possible for my skills how could i blend both mics? 

 

[ricardo]

You've not really told us what you want.

Is there any commercial product you've tried which you like?  If 'yes', how about a link?  How does it work?

Presently, which is your favourite Panasonic capsule?  How is it mounted?  Is this the setup that has the Feedback problems?
_______________

The most successful and least feedback prone pickups for acoustic guitars are the piezo strips.
_______________

If you put the mike inside the guitar, the type of capsule will have little to do with the directivity.  It will be determined solely by the guitar.  Blocking the sound hole will change directivity MUCH more than the type of capsule.

However, the type of capsules WILL change the sound.  Cardioids like WM55 have proximity effect so hugely boosted at LF.  Omnis like WM61 are flatter & hence easier to EQ.
_______________

The type of filter you are after to combat feedback is a full featured Parametric so NOT simple.  You can probably do as well with the EQ on your PA.
_______________

I'm not even mentioning the preamp cos there's lots that will do .. but they won't help your problem.

Do you have P48V phantom power?

 

[parrosa]

Yes, i have changed my initial objective(topic) with all this info...

I'm looking to build

-electret microphone preamp with gain control and any kind of effective manipulation for reducing feedback (notch filter with variable frequency, phase inversion etc.) I know most mixers have all this, but i need it on the instrument to fine tune.... and most of the time there is no one at the mixer during the concert....
-Phantom powered or battery - whatever suits best the circuit. I'll connect it mostly to a mixer, so i always have 48v...
-balanced signal out
-external surface mount - from the discussion above, it seems the boundary type microphones are able to stand outside the instrument and have good gain before feedback.

For a commercial product, something like this http://www.lrbaggs.com/pickups/lyric-acoustic-guitar-microphone

I haven't used the Panasonic WM55 as it is designed to be used(as you said, too much LF inside the body) so i have no favorite yet. Besides, i was using this simple circuit with line level out, http://www.minidisc.org/mic_preamp/Simple%20Stereo%20Electret%20Mic%20Preamp.htm
                  so maybe i would build a new one with characteristics mentioned above, and then shootout some capsules.

----
As for piezo, i know they are the best for feedback, but i would like to avoid them for now. I don't like hearing them, even less playing with that sound.
I need high fidelity acoustic reproduction just to match volume on stage, with other instruments. most mics i've listened to so far surpass any piezo...
Check this videos, for type of band settings i work with
https://www.youtube.com/watch?v=c5NGOcNyF4g
https://www.youtube.com/watch?v=WjVZbMDGHKE

 

[bruce0]

The Boundary Mic on the Body

Sorry, I miscommunicated.

Putting a boundary mic on the outside of your guitar would be an exceptionally bad idea.

I looked at the LR Baggs link, and they have a very impressive group of equiptment (I was impressed by a pickup at a recent show, but didn't know what it was, and recognized it on the site (thanks)).  They have one system, Pressure Zone (aka Boundary) mic mounted INSIDE the body on the bottom of the bridge plate. 

Mounting a PZM or Boundary Mic inside a guitar is one thing, outside is a totally different thing.

Why?
Boundary microphones use a surface to enhance the signal to noise ratio of a signal.  But the surface must be a reflecting surface (they don't, or shouldn't, pick up signal from the surface).  The way they work (there are lots of articles if you search for PZM, Pressure Zone Mic, or Boundary Mic) is based on a principle of physics which is that the wave amplitude is doubled when reflecting.  The simple way to think of it is that when recording you are always worried about interference between a wave bouncing off a wall and the "free field wave".... that accepted then the following is true.  That as you move the mic closer to the wall the reflection and the free field wave move closer together in time.  When you are closer than a 20k hz wavelength (1/2 inch) then essentially you are receiving both at the same time (so the amplitude is doubled).

So that is really good, and the possibility for cancellation in a large room is essentially optimized (since any wave that would cancel is coming from the other side of the room, or off an adjacent wall).  However in a small guitar body, high amplitude resonant reflections must abound (that is the PURPOSE of the body after all) and thus LR Baggs refers to "noise canceling technology" in that mic.

I figure a Boundary mic in the body can't be worse than a free field mic in the body, but I have never tried it.

A boundary mic outside the body would act like a mic aimed at the audience.

The surface used for a boundary mic will double any wavelenght up to the surface length (rule of thumb) so you can get a nice response curve off a stage floor, and a nice vocal curve off a small table top.  They are used inside grand piano lids (pasted to the underside) beautifully.  They are used as conference room mics.  They are used in theatre and on stage because they are invisible.  I use them to record informal jam sessions because the get the room (on the floor if nobody is tapping a foot on top of them, or on the wall if it is crowded).

A boundary mic on the outside of a guitar would pick up frequencies just about perfect to record audience chatter (due to the size of the guitar body front).  I emphasize this because I think it would work badly in a very dramatic way.  If you want to try it, tape any omni to the front of a guitar and listen to it while aiming the guitar at a group of people.

The YouTube recordings.

I looked at the You Tubes, and I like the band, and with problems of youtube audio quality it was not particularly revealing except:

1) The sound of the venues suggest that the feedback you are talking about is in the low frequency range.  They sound hollow, like a gym but not as bad.  They can be terrible places to record and to do live sound.  (But note, if you are having feedback problems as you try to get the sound reinforcement up to acceptable levels for the Venue, that is not a mic type or placement problem as much as a PA speaker placement problem. More on that if you confirm that that is the problem.)
2) I understand you want to improve the sound of the guitar, which (maybe due to youtube audio) seems to lack resonance and sparkly high end.
3) You seem to have very flexible mic placement and nice stationary musicians (a godsend).  The mic placement in the second video on the guitar should be great, but the result suffered, so I suggest two adjustments.  1) Mic 1, the mic pointing at the fretboard should be directional (cardiod, or hypercardiod) if you are missing the high end and plectrum sounds (or is it nylon strings?). 2) the guitarist has a fat arm (no offense) and so the effectiveness of the raking mic off his elbow is not the best.  So pull it stage front more to miss his arm, or pull it lower to miss his arm (in which case you need to EQ out more LF room reverberations off the ceiling), or pull it above his shoulder on a boom aiming down.

You could also replace mic 2 with an "in the body" mic, which would avoid the arm problem.  You would get the high end off Mic 1, and could mix the two as before.


Lastly, I was not suggesting any of the crown mics for use un-altered (although the PCC 170 on the floor in front of the guitar wouldn't be too bad.  A PCC 160 is a better, tougher and more typical choice).  Both I think would lack the isolation you want for the guitar (get too much of the other instruments).  I was simply suggesting them as a cheap source of parts.

If you do want to put a mic in the body, you can have it work like a lavallière mic, and have the amp connected to nice thin wire.

I guess before you do anything that my suggestions would be to use a lav mic and masking tape, and try it in different positions.  You will learn a ton.

(PS: two other thoughts (sheesh) ....the whole mix seems to lack high end sparkle (air) was that you tube?  ...also how much of the lack of resonance is that enormous arm sitting on the body, again no offense)


Do you get sounds you love in a studio setting?  If so, what is different?

 

[ricardo]

For a commercial product, something like this http://www.lrbaggs.com/pickups/lyric-acoustic-guitar-microphone

That's a very nice guitar sound .. assuming its using the Lyric 

It looks like a small flat Panasonic omni, WM63, WM64, WME10, WME13 or WME22.  If you dig up datasheets for these, post to let us know where to find them.

It's under the soundboard, pointed towards the sticky base and half exposed to get more HF.

Positioning is VITAL so bruce0's advice of playing with a lavalier (or better still, a Panasonic WM61a if you have one) is good.  I think the half exposed capsule & soundhole mounting is to pick up more Hf.

You can try just sticking the capsule (facing up or sideways) under the soundboard or more difficult ... a 'proper' PZM mounting with the capsule facing the mounting surface but spaced off it by a smidgen.  There's a lot of hype about PZMs so just experiment.

It's likely the electronics has LF cut to stop things sounding boomy.

At first I though it might be a close use, noise cancelling cardioid (as in phones) capsule, where the proximity is compensated with huge LF cut built into the capsule .. but these have to be much thicker.
___________________

Have you actually played with a Lyric?  I'd suggest you go to a shop with one and try.  You'll learn a lot.

Don't forget to report back to us.  Like what diameter & thickness is the capsule and where's the best position.
___________________

The best Parametric on this forum is that from the Calrec that Gyraf posted.  You want just the section centred around 2kHz.  There are simpler ones but I'm ex-Calrec & biased.

But I'd suggest you play with whatever EQ you have on your mixer first.  It'll give you a better idea of how to customize the Parametric to your application.
___________________

If you find some EQ settings on your mixer that seem to work all the time without further fiddling during performance, I've got a very simple P48V circuit for WM61a type capsules (all those I mention above) that's just 2 bits on the MicBuilders Yahoo forum.  You have to join.

I don't think its worth doing the line level preamp unless you want to build the Parametric into the same box .. a much more complicated project.

 

[dai h.]

hi, found these for WM63PR, WM64C/K, WM64MN, WM64PN, WM-E13U, plus some info on different styles of mounting and some (general overview?) datasheet (couldn't find anything on WME-22, unfortunately)  :

http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE3.pdf

http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE6.pdf

http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE8.pdf

http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000CE11.pdf

http://www.panasonic.com/industrial/includes/pdf/39-E13U_datasheet.pdf

http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000PE43.pdf

http://industrial.panasonic.com/www-data/pdf/ABA5000/ABA5000PE44.pdf

-----
(plus a whole page of datasheets for discontinued mics)  :

http://industrial.panasonic.com/www-ctlg/ctlg/eABA5000_EA.html

 

[parrosa]

Just to clear things up, those are not my videos, and that's not me playing.... I just wanted to give an example on the kind of live setting and instruments i would play with. Every other instrument is louder than the guitar, so i need to amplify the guitar to match their levels;  have as much a natural amplification, so i can "feel" and play with them.

As for audio quality it's you tube; as for the lack of some characteristics, it's from the type of guitar- maccaferri guitars. Very popular for jazz manouche style. very strong mid freq and somewhat compressed tone. 

Bruce0 and ricardo, you left precious info here in these last 2 replies, i will post tomorrow about that- I've just arrived from a 50km away gig, and it's 6a.m...

 

[Richard Crowley]

You have a quite challenging situation there. As you say, you need reinforcement to hold up in the ensemble with the other louder instruments.
Directional characteristics of those small electret capsules are modest at best, and using them INSIDE the instrument will yield dubious results.
And frequency shaping isn't going to take you very far in feedback reduction because of the range you need for the fundamentals and overtones.
It seems quite possible that there is NO practical ACOUSTIC solution here.  Implying that a contact pickup may prove more usable.

 

[bruce0]

I think if you could use the under the soundbridge PZM approach it might work.
They samples from LR Baggs have a nice guitar sound.

+1 to Ricardo's suggestions

Look at the LR Baggs site, note the position of the under the bridge plate PZM.  Mount any type of cartridge omni preferably (omni, cardiod, all fine) facing the bridge plate (from the bottom) anywhere from 1 to 5mm from the bridge plate (LR Baggs said 3mm) facing the wood.

Try it. I am pretty sure you won't cause feedback to the mic in there without rattling the windows, the PA system will be attenuated by the body to some extent.

I don't think you will have the sound hole EQ problem in that position (sound hole is an LF pump essentially), but you may have some issues with handling noise, if you drum on the face, etc, (you may be able to control this somewhat with some fast attack fast release compression like a pop filter).  You would need something external to get string plectrum fret sounds, etc. 

I don't know what they sound quality you will get there will be, I am very interested. ( I don't know what kind of noise canceling LR Baggs does and how much it matters).

Let us know how it goes.

 

[parrosa]

Yes challenging, but i still hope to come up with something. After all, i don't need that much amplification to match the ensemble. It gives me a feeling that its feasible, so far.

...The more i search, the more i find! I have been reading other posts, links and related searches already mentioned here. it really takes time to get a hold of my options and limits for this project, but i'll try to make a point of situation right now. I'll repeat myself and other user's posts in some points...

Here are some interesting links i found regarding this kind of device(this is more for me or anyone wanting to gather info):

the user Teleman made something beyond my needs, but ideal, in this discussion(end of page 1) 
http://www.diyaudio.com/forums/analog-line-level/205980-help-circuit-preamp-eq-balanced-output-powered-phantom-power.html

about balanced signals
http://groupdiy.com/index.php?topic=11723.msg136998#msg136998

Calrec type of Eq is far out of reach for me. The linkwitz lab has more than enough info to source filters and other corrections to the signal, i think
http://www.linkwitzlab.com/filters.htm#11

Another high end product of the same type
http://www.appliedmicrophone.com/portfolio/amt-s15g/
 

- Pzm mics would be nice to work with, despite all the possible problems they would have in this situation; specially the application on the surface of the soundhole, wich is moving as it amplifies strings, therefore making its technology impredictable in this case(right?)... I would buy some of these ( not the 100's$ ones, neither the big enclosure ones) just to try them out, if you know where to get them please point me there(i've been searching, couldn't find any so far- im based in Europe).

  So, for now i have unidirectional and "noise rejection" electret capsules to start with. Bruce0 mentioned his Nebish circuit, posted here http://groupdiy.com/index.php?topic=43085.msg546752#msg546752

I found this (i think) somewhat similar circuit http://www.mp3forkidz.com/mic/phantxy.html . It seems easier than bruce0, specially because i don't have etching tools.... I'm going to build as soon as i get all the parts. i'll do the linkwitz mod to both  WM34 and WM55 - Then i'll try to shootout the best position to fix the capsule on the instrument.


As an example, i recorded these tracks (first 5 tracks) https://soundcloud.com/paulorosa-2 with an Rode NT1000 and MXL V69, into a That mic Pre http://electronics.dantimax.dk/Kits/Preamps_-_poweramps/index.html, into a Emu 1212m. This kind of maccaferri guitars have a "rude" type of sound to it....

 

[bruce0]

I think you are on the right path.

One point:
A PZM mic is just a boundary mic with the capsule pointed toward the boundary (wall, floor, or bridge support... all boundaries) very close.  They are not particularly sensitive to vibration (especially if they little omni panasonic capsules). 

A PZM does not belong IN the sound hole, it belongs on a boundary.  Stick it to the bottom of the bridge inside or somewhere. In the soundhole there IS NO BOUNDARY (no wall, no panel, no wood), and the sound hole acoustics are not flat at all.