muragavino Posted August 10, 2015 Report Share Posted August 10, 2015 Hi There!Could anybody help me to find a specific transistor for a preamplifier for Piezo pickups? The preamp works with the 3.3V feed out of e.g. a PC mircophone input (Phantom).In the attachment I prepared a schematic of the preamp. Please help me to understand what transitor is used here.Thanks! Unknown FET.pdf Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 10, 2015 Report Share Posted August 10, 2015 You forgot to tell us what the pickup is used for. A guitar? A piano? A drum? The horrible circuit will work with an N-channel Jfet or NPN transistor if the piezo pickup is in a drum. The circuit will not be a linear amplifier with a high input impedance if a Jfet is used. The circuit is designed to use an NPN transistor but will work poorly when a piezo pickup or magnetic guitar is used because its input impedance is too low.If an N-channel Jfet is used then its gate must have a 2.7M resistor to 0V, not to its output. Use a 2N5457 or 2N5484. Quote Link to comment Share on other sites More sharing options...
muragavino Posted August 10, 2015 Author Report Share Posted August 10, 2015 Hi audioguru,thanks for the fast reply!The pickup is a simple piezo-disc (buzzer) used in a very cheap throat microphone. Since it wasn´t doing its job - sound was heard from all directions, not only from the throat - I have meliorated the throat-mic by acoustic means using silicon gel as sound conductor and expanded rubber as isolator. Now it works just lovely!As a matter of fact, it works that well, that I wanted to make some more on my own account, but I can´t figure out which transistor to use. The "horrible circuit" works actually very well.Searching the internet I now found as possible candidate the 2SC1623-L6, which has "L6" printed on the SOT23 housing, and which is a NPN. Do you think this transistor is possible? Thanks again,Michael Quote Link to comment Share on other sites More sharing options...
Hero999 Posted August 10, 2015 Report Share Posted August 10, 2015 Where did you get the schematic from? Quote Link to comment Share on other sites More sharing options...
muragavino Posted August 10, 2015 Author Report Share Posted August 10, 2015 I just copied it from the PCB that was in the throat-mic housing. It is a single layer PCB and simple enough not to make any error. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 10, 2015 Report Share Posted August 10, 2015 The circuit you found will not work with a Jfet but will work with an NPN transistor and a low impedance signal source (not a piezo).A piezo throat mic is a contact mic. Go to http://makezine.com/2011/12/20/collins-lab-diy-contact-mic/ to see the different circuit used for a Jfet preamp for a piezo mic.There is a similar circuit at http://www.homemade-circuits.com/2014/07/diy-contact-mic-circuit.html but it has the drain and source pins of the Jfet connected backwards. Quote Link to comment Share on other sites More sharing options...
muragavino Posted August 10, 2015 Author Report Share Posted August 10, 2015 Thank you very much, audioguru, for your advise and detailed information.These two schematics posted by audioguru, work both with 9V. Could they work with 3.3V?What I need is a circuit to work with the 3.3V you can get out of a PC-mic input, where the tip of the phone jack caries the audio signal, the ring is connected via a resistor of 2.2k to 3.3V, and the sleeve goes to GND. Since the preamps in the PCs do always have a decoupling cap, ring and tip can safely be shorted to supply the 3.3V as phantom power.Thanks again!, Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 10, 2015 Report Share Posted August 10, 2015 The circuit you found works with any little NPN transistor including a 2SC1623, it is not a Jfet. It has a low input impedance which loads down the signal level from the high output impedance of a piezo mic and the transistor has low voltage gain so if you speak loudly you will make an output. Quote Link to comment Share on other sites More sharing options...
Hero999 Posted August 11, 2015 Report Share Posted August 11, 2015 The sound card needs to form part of the circuit.Try this. Quote Link to comment Share on other sites More sharing options...
muragavino Posted August 11, 2015 Author Report Share Posted August 11, 2015 Thanks Hero999, I will order some FETs like the above mentioned 2N5457 or 2N5484 and give it a try as soon as I have the parts. The results I will publish here. Thank you all for your help, it is great to meet you guys! Quote Link to comment Share on other sites More sharing options...
Ryan Parks Posted January 3, 2017 Report Share Posted January 3, 2017 Awesome... Keep it up !!!!! Quote Link to comment Share on other sites More sharing options...
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