Mitsos,
From something Bo has posted:
About the cap, so if you only have a small cap in series with the signal patch, you get a high pass/lo-cut filter, togheter with the 1 Mohm resistor, (the op-amp +input ground reference) but with a resistor in paralell with this cap, you get a "treble boost filter", (same topology as the bright switch on a Fender amp :grin: ) and with a selected value it will compensate the treble loss you get in the 2,2 Mohm resistor (when it alone act as a pad-resistor) because the capacitance in the signal patch and op-amp input.
When I was working on my first VP312DI proto, I did not have a 18pf cap around. I remember a roll-off on the top end without this cap at all. I use a Cornell Dubilier mica cap that works pretty well. More or less back to "flat".
Sorry I can't be more help on your howling problem. Good luck with that.
Best, Jeff
From something Bo has posted:
About the cap, so if you only have a small cap in series with the signal patch, you get a high pass/lo-cut filter, togheter with the 1 Mohm resistor, (the op-amp +input ground reference) but with a resistor in paralell with this cap, you get a "treble boost filter", (same topology as the bright switch on a Fender amp :grin: ) and with a selected value it will compensate the treble loss you get in the 2,2 Mohm resistor (when it alone act as a pad-resistor) because the capacitance in the signal patch and op-amp input.
When I was working on my first VP312DI proto, I did not have a 18pf cap around. I remember a roll-off on the top end without this cap at all. I use a Cornell Dubilier mica cap that works pretty well. More or less back to "flat".
Sorry I can't be more help on your howling problem. Good luck with that.
Best, Jeff