muragavino

"L6" - what kind of transistor?

Recommended Posts

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

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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

 

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

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.

Share this post


Link to post
Share on other sites

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!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now