Help with Forrsell Jfet Opamp

[theoverstayer]

I breadboarded the 'first generation' jfet opamp from the forssell site and it sounded great and I was able to adjust dc offset to almost 0mv with the trimpot. I made a PCB and now in the same circuit I cannot trim the offset to less the +50 mV. It works and sounds OK but I can't seem to solve this. I have matched the Idss of the fets and even swapped out every Fet and resistor on the board. What am I missing?

Thanks, JEfff

 

[chrissugar]

It is not enough to sort the FETs by Idss.
You also need to find pairs of 2SK/2SJ for the output (Q5 and Q7 in schematic) that have similar, high Idss, and very close gate-source voltage (Vgs) to asure low DC offset.
If there is too big difference in Vgs, you can't correct the offset with the trimpot at the differential stage.

What circuit you used for testing the DC offset of the opamp.
Feedback usually lowers the DC offset.

chrissugar

 

[fum]

Doesn't Fred have a matching schemo on his site too:

http://forsselltech.com/schematics/JFET%20Jig1.pdf

Regards

ju

 

[theoverstayer]

Chrissugar-

I forget to mention that I did match Vgss as well for the output pair so it must be the circuit.

Thanks, Jeff

 

[Carl_Huff]

Sounds like a PCB layout problem. Assuming that your layout is correct, I'd be looking for 'hairline' shorts between traces with a magnifying lense.

 

[chrissugar]

In what configuration you used the opamp when you measured the DC offset?

chrissugar