Recently I've modified an MXR Blue Box to the point where it is no octave and is essentially just a normal fuzz. The schematic for this is attached however there is an oscillation problem with the circuit; originally I had a voltage divider for the virtual ground but even with trying the buffered virtual ground for the opamps there is still the oscillation noise. Any help would be appreciated.
(https://i.postimg.cc/zyhNhsPf/18-ACA0-C1-7-F14-4-C45-889-F-524442-C37-AEC.jpg) (https://postimg.cc/zyhNhsPf)
Quote from: RLawlor on July 19, 2019, 07:03:12 AM
originally I had a voltage divider for the virtual ground but even with trying the buffered virtual ground for the opamps there is still the oscillation noise.
Does now stand buffered Vbias..??
If yes, delete 100nF cap between 741 output & 4558 non-inverting input..
Proceed by placing a 100-150 pF in parallel with 27 + 1M pot..
Keep proceeding by placing a 47-100 pF cap between 2N3904 Collector & Base..
The Vref "must" have a bypass cap. Since you now have a buffered Vref, the cap goes on the voltage divider.
I am quite sure you mis-placed a resistor on the 2nd stage. That 56k should be in series with signal, not shunt. As you have it now, 2nd stage gain goes to INFINITY at high frequency. Which can't happen, since opamp gain falls at high frequency, so it does a 'wrong' thing (oscillates).
(https://i.postimg.cc/wRFM5zQ6/RLawlor-1.gif) (https://postimg.cc/wRFM5zQ6)