Forget about the 2M resistors, and U2 totally to start. You show an input voltage on the (+) input of U1 as 0V (0 volts on input side of a 1K and a 20K to ground) and -9.5 volts on the (-) input of U1. That should drive the output of U1 to the positive rail. If it doesn't, then U1 is defective or you lost the +Vc supply to the op amp.
How do you model U1? Does it have polarity inversion when the negative input goes too negative? If so, have you tried forcing it into that state?
It does sometimes with some servo op-amps work according to the thread starter, which is why I thought of polarity inversion in U1 rather than a defective U1. When there is something wrong with U1's design that causes the output to go negative when the negative input is driven too low, you have a circuit with two valid bias points and depending on the power-up transients it ends up in either one or the other point. Clamping the servo output might then help, for example by making the negative servo supply rail less negative.
An unreliable connection of the main amp to the positive supply could indeed also be an explanation. It could randomly start and stop making contact when the thread starter puts in a new op-amp. That might also explain the issue of post #16.
Thank you very much. This helps a lot.
In my new preamp, with r1 r2 r3 r5 as 10kohm,
When I used ada4522, the opamp output pin is -22v and negative input of the amp is -9.5v. Output of the amp is -14v.
When I use no opamp at all, mean no servo, the output of the entire amp is the same -14v, the negative input is -7v.
This means the ada4522 is acting not as expect.
Thank you.
I thought that bigger resistor (2Mohm) loads less current and gives less noise.
The u1 is not an opamp but a solid state amp without dc offset trim.
I used to use dc servo only to trim the output dc to zero.
So it should be normal to have dc with itself.
Is it normal for U1 to have more than 9.5 volt of offset at its input?
Look at the voltages you annotated on your schematic for the fault condition and it becomes obvious that the servo integrator does exactly what it's supposed to do: it is a non-inverting integrator, its input is negative, hence its output goes to the negative rail. Whatever may be wrong with the circuit, it is not the servo as such. That leaves the main amplifier or the interfacing between the servo and the main amplifier.
Could you post the schematic of U1?
Look at the voltages you annotated on your schematic for the fault condition and it becomes obvious that the servo integrator does exactly what it's supposed to do: it is a non-inverting integrator, its input is negative, hence its output goes to the negative rail. Whatever may be wrong with the circuit, it is not the servo as such. That leaves the main amplifier or the interfacing between the servo and the main amplifier.
Could you post the schematic of U1?
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