I don't know enough about op-amps to skin this cat, so I was looking for ideas that might help from the gang.
The following circuit has a bad 781 kHz oscillation on the second stage of the chip. The first stage is oscillation free and clean.
The circuit is as follows:
[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/SolidStatePreamp.jpg[/IMGHTTPDEAD]
I have also tried removing resistors R116 and R117. I also tried that and shorting R118 to make a traditional inverted op-amp buffer.
However the second stage still oscillates and I do not know why.
The chip is bypassed by ceramic .1 uF caps (C111 & C112), which are jammed as close to the supply pins as I can physically get them.
The layout may be the cause, but I am just guessing. Board size is 2.5" high by .9" wide, so it is small. Here are the three layers. The ground plane is a single plane with the only external ground connection at the top of the board going to a star ground. The other two "ground" connections are part of a twisted pair that simply terminates as a dead end wire on the input and output to prevent ground loops.
The power plane is split so that it supplies + and - 15 VDC supply. C15 & C16 are 10 uF. C17, C18, C19, & C20 are by-pass caps (.1 uF and .01 uF for each rail).
The wing shaped polygons are connections to two 100K pots. This board sits just behind the pots.
[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pcb1.jpg[/IMGHTTPDEAD][IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pcb2.jpg[/IMGHTTPDEAD][IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pcb3.jpg[/IMGHTTPDEAD]
The following circuit has a bad 781 kHz oscillation on the second stage of the chip. The first stage is oscillation free and clean.
The circuit is as follows:
[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/SolidStatePreamp.jpg[/IMGHTTPDEAD]
I have also tried removing resistors R116 and R117. I also tried that and shorting R118 to make a traditional inverted op-amp buffer.
However the second stage still oscillates and I do not know why.
The chip is bypassed by ceramic .1 uF caps (C111 & C112), which are jammed as close to the supply pins as I can physically get them.
The layout may be the cause, but I am just guessing. Board size is 2.5" high by .9" wide, so it is small. Here are the three layers. The ground plane is a single plane with the only external ground connection at the top of the board going to a star ground. The other two "ground" connections are part of a twisted pair that simply terminates as a dead end wire on the input and output to prevent ground loops.
The power plane is split so that it supplies + and - 15 VDC supply. C15 & C16 are 10 uF. C17, C18, C19, & C20 are by-pass caps (.1 uF and .01 uF for each rail).
The wing shaped polygons are connections to two 100K pots. This board sits just behind the pots.
[IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pcb1.jpg[/IMGHTTPDEAD][IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pcb2.jpg[/IMGHTTPDEAD][IMGHTTPDEAD]http://www.mdbq.net/diyaudio/pcb3.jpg[/IMGHTTPDEAD]