A really simple buffer with fets.

[ginetto61]

Dear Sirs,

I sincerely hope Mr. Echowars will excuse me but with the aim of building a very simple but elegant fet buffer I have stolen the following attached piece of schematic from one of his 3D about a Stax preamp
What intrigues me is the simplicity of this piece of circuit.
But now the doubts: can the Q208 and 209 couple work as a unity gain buffer ?
I would like to eliminate the output caps, deeming that the actual offset should be low.
In front of the pair are a coupling cap or biasing resistors needed ?
I have already a nice stepped attenuator to put in front of it.
Any suggestion would be very welcome and appreciated.
And of course thank you so much Mr. Echowars !

Kind regards,
ginetto

[gk7]

Yes it will work as a unity gain buffer and do so very well. Download "Jfets The New Frontier, Part 2" from Erno Borbelys website (Part 1 is recommended too of course) and have a look at Page 18, Figure 16B and Table 3/collumn 4.

[ginetto61]

Originally Posted by gk7

Yes it will work as a unity gain buffer and do so very well. Download "Jfets The New Frontier, Part 2" from Erno Borbelys website (Part 1 is recommended too of course) and have a look at Page 18, Figure 16B and Table 3/collumn 4.

thanks a lot for the very kind reply.
Moreover I wonder if any other passive component is needed between the pot and this stage, like a biasing resistor and/or a coupling cap.
Any help would be very much appreciated again.
kind regards,

ginetto

[gk7]

Sorry for the late reply I was on holidays. You would not need anything between the pot and the input of this stage, but there will be some offset at the output (depends how good the FETs are matched) and therefore an output coupling cap (can be omitted if the next stage has already an input cap) will be needed.

[R-K Rønningstad]

The first time I saw this solution was the input buffer for the negative input of the Mark Levinson ML-2.
See attachement

Rolv-Karsten

[ginetto61]

Originally Posted by gk7

Sorry for the late reply I was on holidays. You would not need anything between the pot and the input of this stage, but there will be some offset at the output (depends how good the FETs are matched) and therefore an output coupling cap (can be omitted if the next stage has already an input cap) will be needed.

Thank you so much !
Now this very simple schema is even more interesting
I will try it
thanks again and kind regards.
gb