if the virtual ground is high through the route you describe that
suggests that:
* isolating the transformer with a cap should help. It didn¡¯t
noticeably earlier but maybe now things are close and stable it
would.
* any leakage to the +ve non-inverting input must be coming from
the +ve rail not the negative since the VG is too high not too
low. This is despite the -V pin being much closer to the input.
Which way would the bias current alter the VG? Would isolating
the transformer help with that too?
If the route to unbalancing the VG is through the transformer
that would explain why I never had this problem with the
transformerless version, but not why it was unbalanced earlier
even with the transformer isolated.
I will try isolating the tx again and see what happens now the
board is cleaner
On 01/05/2025 18:33, Jerry Lee Marcel
via groups.io wrote:
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This is good.
Remember that, even with FET opamps, there is still some input
bias current, which may alter the ideal calculated value.
so if V- leaks to In+ how exactly does that make the virtual
ground go high?
Th evoltage at the non-inverting input is transmitted to the
output and to the xfmr's primary, which in turns is connected to
VG.
the opamp must(?) be amplifying DC from the leakage.
It sure does
presumably it's worse because I have the opamp set up for
gain of 7 rather than just unity?
Likely.
