-vin at output

[janneman]

Originally posted by Adam M.
After taking the whole amp apart, I found the problem. I forgot to put a resistor from input to ground, and all is good now. I get the same effect if I just plug something into the input, but I was testing in steps.

For those who asked above, I was talking about the negative rail voltage showing up on the output.

As a theory question, why did not having a resistor from input to ground drive the output to -35Vdc?

Thanks.

There is DC current flowing into or out of the input (depending whether the input stage is PNP/NPN or equivalent FET). This current needs to go somewhere, either to the source (if it is DC coupled) or to ground via the resistor you didn't have. If the current can not go anywhere, the input drifts up or down (look at it as a cap that is being charged because the current has no way to go). Depending on the input stage it drifts to the pos or neg supply rail. Since the amp tries to replicate the input level at the output, your output also drifts to one of the supplies.

Jan Didden