Is it a regen?

 The question is can you build a regen around an opamp?

 Things to consider:

How do we modulate and demodulate an AM signal.

What is the number one requirement for mod / demod?

I produced some screen shot that help answer the question.

I have some wheres to be shortly so am just posting the screen shots now 

and will comment further later. 

The shots are in reverse order. Reading from the bottom up makes more since. 

Using one input create an audio output.

Adding a filter separates the signals.

Driving with an AM signal you can see the distortion created. 

Here I tie the inputs together and we see distortion (ripple). This is the non linear action required to demodulate.

 The plus and minus inputs are inverting and non-inverting. Here we have the same phase out as in. The shot above inverts.

Add low Z output

 Note R10.

 

3 stage JFET amp

 Plenty of voltage gain just needs an output stage or transformer.

 It could drive a high Z phone but I want to drive an earbud.

 

Why is it motorboating?

It is motor boating but stops when the meter ground lead is connected to the chassis?

Could it be lead dressing or interstage coupling through the power supply (battery)? 

 

 

2SK2539 SPICE MODEL

 .MODEL 2SK2539 NJF(VTO=-3.1621 BETA=5.01519m BETATCE=-0.5 LAMBDA=2.40112E-2 RD=1.48485 RS=1.48485 CGS=1.05000E-11 CGD=1.40000E-11 PB=5.68050E-1 IS=9.11151E-16 XTI=3 AF=1 FC=0.5 N=1 NR=2 MFG=PHILIPS)

MOSFET Slider review

 If you build this one using a variable resistor for R2 you can use the MOSFET you have on hand and adjust the bias to suit the circuit. R8 and R9 represent a high or low Z load. The screen shots display the output with different MOSFET as R2 is adjusted from 10K - 20K.

 

9 volt version of AF amp

 When I built this circuit I had a 6 volt lantern battery that was needing using. I tried a 9 volt battery to compare results. My earbud is rated at 1ma and it puts out 1ma with 100uv signal. Looks like this one is a keeper?