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

 

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.

 

What type of mic are you using?

 

I’m using 2n5457 jfet transistor and the drain is 1.5kohms resistor for the second jfet.

 

What type of mic are you using?

I’m using a piezo because I’m trying to make a contact microphone

 

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.

 

Hello,

Here is the schematic again:

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

 

Like this.

 

Hello,

Here is the schematic again:
View attachment 229684
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

I drew it wrongly because I was in a hurry but this is how it actually looks

 

Where did you get that design?

 

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.

 

Probably doesn't need two stages but have you tried troubleshooting the circuit with your oscilloscope?

 

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.

 

Have you tested the MAX98306 ?

 

Have you tested the MAX98306 ?

The only test I have done with the MAX98306 was checking if it was on with 3.3v and grounded properly and if my preamp signal was going in and yes to both

 

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.

 

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.

 

Appears the MAX98306 is not working.
Again how did you connect the preamp to the MAX?

 

Appears the MAX98306 is not working.
Again how did you connect the preamp to the MAX?

I connected the output of the preamp to L+ and L- to ground and connected Lout to oscilloscope using the probe, connected to my voltage supply