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Re: offcenter virtual ground, opamp mic with transformer

 

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It's not a 15v zener any more I increased it to 27v to give me headroom for the opamp gain.

from my original post: "(possibly irrelevant aside - in my latest version I'm using a 7:1 transformer and appropriate value changes ie R3/R4 are 6.8k, D1 is 27V, R6 is 6.8k and R7 is 1k, but I have had this issue with a 1:1 transformer as well.)"

Agreed it shouldn't drop with a lower input resistor.

I cleaned the board better, paying special attention to the gap between pins 3 and 4 which are +In and V- and they are very close together on this smd opamp.

that helped! there must have been some flux still right under the edge of the opamp - the virtual ground is now at 13.5V with the 1G input resistor, so only a volt or so too high.

so if V- leaks to In+ how exactly does that make the virtual ground go high?

the opamp must(?) be amplifying DC from the leakage.

presumably it's worse because I have the opamp set up for gain of 7 rather than just unity?

On 01/05/2025 13:00, Jerry Lee Marcel via groups.io wrote:

On Thu, May 1, 2025 at 01:40 PM, thet wrote:
With a 100pF cap across the 1G resistor to simulate a capsule, I now have a very high virtual ground.

With a 10M resistor across the 1G resistor all the voltages come right.

Since I'm running the opamp with a gain of 7 and a 7:1 transformer I have my V+ at 24V
This is not normal, with a 15V zener the voltage should be less than 16V.
With just the 100pf cap I get a virtual ground of about 20V.
The VG voltage should be pretty close to half of V+.
Drop across R5 is just over 2V (~26-24v) so current draw is nominal.
OK
Pins 2 and 6 are approximately at virtual ground. ie 20v or so.
OK.
With the 10M resistor in place those pins drop to ~ 12v
This shouldn't happen.
If the board is leaky ie imperfect insulation across the input from dirt would it cause this?
It is a possibilit
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