problems with building a piezo preamp for undewater mic

[todoke]

Hey,

I've built Alex Rice' piezo preamp, but it seems not to work correctly. I get very poor gain, I only hear something above the noise when I directly prod the surface of the casing.

I'm a newbie and spent quite some time building it.
I think I got the FET-Matching correct (using this approach).
I got values from around 70 to 110 mV and selected two close-by low values for the differential pair and a high value for the power.

I'm pretty sure I soldered everything together correctly and with good connections, but I've no idea why it doesn't work as I hoped.


I've built two circuits so far and one was even quieter. In that second one I used different capacitors of same value, but it could've also been that it was due to the FET's having slightly different values.


What do you suggest to do in order to get this project done with usable results, i.e. a good amplification so it's actually usable?

thanks so much for help, I really don't know what to do next and how to find the fault and I don't know people I could ask for help with knowledge in this field. so please help me. thanks.

 

[AuroraB]

Did you link pin1 and ground, i.e. the gate and source resistor?
The XLR has no internal link for this....

BTW- wehad another discussion about this in this thread
http://www.diyaudio.com/forums/musical-instruments/145898-piezo-disc-preamp.html

I'm about to build this for the same purpose, albeit not inside an XLR. It's supposed to be more quiet than regular preamps - being it's based on charge coupling..........remains to be seen though...

 

[todoke]

Hey and thanks for the answer. Good to hear you're also going to build it.

I did not build it into an XLR plug but with wired components on a small perfboard.

XLR pin1 and ground are linked.
oaaarghh.. I don't know what to do.
I will connect a piezo directly to my recorder to get a clue of the level and be able to compare with the amplified signal.
I estimate a 8x amplification (from the diagram Alex measured), so I guess it should be very loud when I prod it just very softly (which is not the case )

 

[todoke]

Holy Sh¡t!

AARRGH!!!!!!!!!!!!

I presume I ran into an unbelievably annoying pinout mistake by trusting the first found image from a google search. wrong!

the official fairchild semi datasheet depicts it the other way around

will definitely be more careful next time, desoldering parts with bend legs is not fun.

 

[todoke]

Hey Guys, it's me again.

I wrote the webmaster of this page where I found the wrong pinout for the 2N3819 and he educated me with his experience and some references that the source and drain are reversible for these kind of JFETs.

Now again I don't know where there could be a mistake and a reason for the unit not working.

In the meantime I connected a piezo directly to my recorder and it gives a pretty loud signal. MUCH louder then through the "pre amp".
So there's definitely something very wrong with it.

I attached an image. It would be very nice if you could have a look and compare it with the schematic.


One last question: Everything is quite close together, the areas with solder on the backside are closer then 1mm in some places.
Could this result in a capacitance that leads to such extreme malfunction?

Thanks a lot!

 

[amundsen]

Hello todoke,

Have you succeeded with your preamp ? I am planning to build one according to Alex Rice's schematics, so I'd like to know if there are issues about it.