"L6" - what kind of transistor?

[muragavino]

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!

View attachment Unknown FET.pdf

 

[audioguru2]

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.

 

[muragavino]

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

 

[Hero999]

Where did you get the schematic from?

 

[muragavino]

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.

 

[audioguru2]

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.

 

[muragavino]

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!

, 

 

[audioguru2]

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.

 

[Hero999]

The sound card needs to form part of the circuit.

Try this.

 

[muragavino]

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!

 

[Ryan Parks]

Awesome... Keep it up !!!!!