Preamp to Audio amplifier issue

Thread Starter

Pinkfruity345

Joined Feb 6, 2021
16
Hello everyone,

I built a jfet preamp on my breadboard and I’m trying to connect to adafruit stereo audio amplifier MAX98306 to view the speaker output on the oscilloscope. I don’t really know what I’m doing wrong since I’m not familiar with audio signals but if anyone who’s familiar with audio signals can help I would appreciate it. Thank you.

This is my preamp
0B732686-C8EB-4C84-9482-6FAF06DBC9BB.jpeg
 

AlbertHall

Joined Jun 4, 2014
11,309
What FET are you using?
The second FET has an unmarked resistor in the drain connection - what value is it.
That FET also has a 1MΩ source resistor. 270Ω like the first FET would be more like it.
 

Thread Starter

Pinkfruity345

Joined Feb 6, 2021
16
What FET are you using?
The second FET has an unmarked resistor in the drain connection - what value is it.
That FET also has a 1MΩ source resistor. 270Ω like the first FET would be more like it.
I’m using 2n5457 jfet transistor and the drain is 1.5kohms resistor for the second jfet.
 

bertus

Joined Apr 5, 2008
21,364
Hello,

Here is the schematic again:
Jfet preamp.jpeg
I would expect a 1M resistor on the second fets gate.
The source resistor would be around 270 Ohms in stead of the given 1 M Ohms.

Bertus
 

Thread Starter

Pinkfruity345

Joined Feb 6, 2021
16
Where did you get that design?
I drew it after making my calculations for using 3.3V and values on the jfet datasheet ID = 3mA, Vdd = 3.3v, vgs = 0(off) = - 2.5v. but I had make some adjust due my lack of capacitors. It’s a common source jfet design.
 

Thread Starter

Pinkfruity345

Joined Feb 6, 2021
16
Probably doesn't need two stages but have you tried troubleshooting the circuit with your oscilloscope?
Yeah I tried viewing it using the oscilloscope while talking with the piezo and it picks up my voice signal without any much disturbance that why I feel so stuck. I also checked the audio amplifier using a multimeter if it was getting voltage and it was but whenever I check the output of the amplifier it’s nothing. I read somewhere about impedance but I don’t know how to apply that.
 

Thread Starter

Pinkfruity345

Joined Feb 6, 2021
16
Yes on the oscilloscope but at the speaker output which I connected to the second channel of the oscilloscope it doesn’t pick up my voice or movement.
 

Thread Starter

Pinkfruity345

Joined Feb 6, 2021
16
So you can see a signal going into the MAX98306?
How did you connect the preamp to the MAX98306?
Yes on the oscilloscope but at the speaker output which I connected to the second channel of the oscilloscope it doesn’t pick up my voice or movement.
 
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