Acoustic guitar piezo pre-amp

[Aura]

Hi All,
I've recently started on this project,building an acoustic guitar piezo/mic blender pre-amp.I lifted this schematic from http://www.redcircuits.com/Page69.htm for a piezo disc preamp, which I've already built on a proto/bread board & it works! However I've encountered one problem, which is.....

The preamp doesn't have any gain & stays at the same level even when the the amp is turned up full on my Roland AC60, yet gives ample volume when I tap on the p.disc itself! (I've only the R4=470K feedback resistor for testing in the feedback loop & have removed the other 2 gain selector resistors).
The bass control has barely any response, although both mid & treble have some effect.

Mod:
I removed R12 & to run it on +/-9 volts (2 PP3) wishfully for better dynamic response.

Q.
Could it be that the input buffer lacks enough current drive? Would it work if I include a high impedence buffer for the input stage?

Do any one of you wizards out there have any idea why it's acting like this? Any help or advice would be gratefully taken onboard as I want to learn more on modifications/circuit functions etc.

Thanks.

PS:Piezo discs don't have very high impedence contra p.film etc..so I gather.

[oc disorder]

Hi Aura and welcome to the forum.
I think its to do with impedance matching and sensitivity.
I think a high impedence fet buffer will make a dramatic difference.
http://www.till.com/articles/GuitarPreamp/
The above could be used with your circuit ..have you tried a "normal"
guitar into the pre-amp ?

Some info here too
http://sci.tech-archive.net/Archive/...4-11/6512.html

You could try 27k for R3 and see if that improves things ie bridge it across the 470k.

Check the polarity of capacitors too.

A lot of commercial units use the TLO62 for its low current drain on the batteries but I gather its a trade off with noise.

That is the TLO62 is noiser than the TL072.

Let us know how you go as there is some interest in this area....

some more reading:-
http://www.harmony-central.com/Guitar/piezo-pickups.txt

[Aura]

Hi OC Disorder,
Thank you very much indeed for the info:..as I too was actually thinking on the same lines!
It may not come as a surprise to you to hear that in an another forum,one suggested/sent me an entire new schematic,when all I wanted was to learn more/modify & so on.My Ex;even suggested that an extra wire would help!!!he..he..

No,I've not tried it with an electric,but hey, it's a (genious/clever) thought!

I also found that TL062 has a fairly pronounced hi frq;response than TL072 & sounds brighter/clearer & both on this & the mic pre-amp(Not sure if it's due the GBP/ or lower IiB or both)It was quite musical sounding too!

I also tried first changing the source R1 to first 1M & then 4M & shounted the "cap" to ground & it sounded just like typical UST pu's I've heard before,"quacky" & harsh (sorry... all you ducks out there)

At the moment I'm only using one 20mm piezo disc..but will try with 1 more in parallel/ & 2 series + 2 parallel combination to get more voltage/signal out of it.I've also seen that on one Taylor acc.they have wired 8 of them in p&s combination!

By the way,Do you think I should have 2 100k or 33k R's from the +/-V to 0 ref. I'm running this on 2 9v PP3's but planning on a +/-15p.supply unit.

I recently came across this excellent article on biasing & swapping op-amps,called "Working wth Cranky Op-Amps" on this link:
http://tangentsoft.net/hs-opamp.html This is a must read for anyone venturing into/want to design or swap op-amps,which is written in an easy "layman" form.It was an eye opener for me!

Yes,of course, I'll keep this forum posted of my progress.

[Aura]

First,compliments of the seasons to you all!Now continuing this saga ...well after even more reading I decided to build an non-inverting input buffer using a TL071 for the above the P.disc pre,rather than taking the singe FET trannie path,since I don't have enough knowledge to "select"/grade thes FET's!(I gather they are quite noisy as well?

This buffer/preamp actually works,but quite noisy,since I've not biased it properly I think!

Changed the Op-amps from TL062 to TL072 on the preamp & it did make a huge difference.I am thinking of trying this with a LF351/3 later.

The schematic below is quite similer for a non-inverting buffer,but with a 1uF cap & the feedback resistor values are 2k2/1k for a gain of 3 & a 1M resistor across the + to ground.The output from the buffer is coupled to the original 1st. stage by a 10uF elec.cap!! So these are the values at the moment;

R1/R2=2k2/1k (feedback R)...should I increase to say 10M/4M7
Load R = 1M.....increase this to 4M/7 ?
I/P cap = 1uF (non pol
o/p cap = 33uf what value should it be?

Although it's quite noisy yet all the controls are responding now at last & it has ample volume/drive.but rather reduced lo freq. Seems to have enough gain though.I don't longer hear/get this"sucking" effect on the amp.

Q:
1.Have I biased the i/p buffer(TL071) with very low value resistors? What should be the optimal values.The Data sheet gives the Ib-i/p bias current @ 20-200 pA for TL071

2.Should I change the i/p & o/p caps to different values?

3.I measured -10v dc offset at both pins ie:1&5.Do I have to null this offset
using preset/resistor?

Any help or suggestions would be most welcome..as I feel I am at last getting somewher with this!

[Aura]

Schematic for the P.Sisc buffer preamp

soooory!here it is

[Aura]

I've managed get the i/p buffer & the preamp to work!This buffer works quite well with the preamp without any "loading effects".I've calculated the following values,but not sure how close these are for optimal performance!I worked out these values to give a gain of 2
=3dB.

Buffer
R=4k7
Rf=10k Rf.R
Rs=3k3 used this formula for Rs=--------
Rl= 4M7 instead of 10M (load resistor) Rf+R
IC=TL071

Also not shown;
C1=i/p cap =1uf

PREAMP
Input cap C1=0.33uF since I didn't have any 0.22

Observations:The preamp is a little unstable...may be it's not biased properly?
though sounds quite musical with just 1 p.disc.The bass response isn't much.I'm not sure how to calculate the C1 value for-3dB point for the lowest bass frequency!

"Null setting" the TL071 wasn't easy...since DC offset kept rising with the temperature! I used a 100k lin "pot" + a 1k5 to do this. I should have perhaps used a dual device instead,since dual ones are internally d.c offset bias adjusted I gather.

There's still quite a amount of noise & hum.This may be due to that it's still on a breadboard & has no proper grounding-only via the o/p jack.(would "Star" grounding give better earthing?)

Anyway,it works & has a rather pleasent musical quality to it although lacking a bit in bass.Any fine tuning/optimising of this circuit will only make it an excellent preamp I'm sure!The Piezo Disc sounds way better than all the UST pu's I have used & heard so far!

Question:
1.Have I biased the opamp correctly?
2.Can any of you wizards see any design errors/suggest any improvements for optimal performance? I feel this preamp is well worth persuing!

PS:changed the C1s (input caps) on both to 0.47uF...more bass & quite impressive sound!

[nbtone]

Do you guys think either of these would work better?:
http://secure.oatleyelectronics.com/...fa4a0c920c102a

http://secure.oatleyelectronics.com/...fa4a0c920c102a