need some help on fet stage coupling with opamp buffer

[dschwartz]

Hi all, i was playing around modifying the dr boogie ( as i ussually do).
I eliminated the last fet buffer stage and replaced it with an opamp buffer (tl062) coupled via 1uF cap and a 680K resistor to Vref (vref made with 2 10k res and 47uF cap) this way:

last fet gain stage--> 1Uf cap--> 680K R to 4.5V--> + input....     out and - input tied together..

the thing is it sounds like crap, like the bias point varies randomly, sounds OK,  and suddenly i have octave fuzzy sound, or even no sound, then back to OK....WTF!

i simulated the circuit on lt spice, and according to it, it should work OK....

can be this produced by the 1uF-680K filter letting pass really low frequencies, producing a bias shift?  

[aron]

If you measure the bias of the op amp, is it really shifting? Remember to save what you have now - take a picture or something. Someone will love what you are getting!

[dschwartz]

thanks aron..
well i have learned something new today...
putting a resistor between the fet stage and the buffer cured the problem.. looks like i was feeding too much current to the buffer..now it sounds great!