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offcenter virtual ground, opamp mic with transformer

 

On my opamp mic with a transformer, my virtual ground is unbalanced, it is closer to actual ground than to V+

I thought it might be DC running through the transformer primary, I've tried with the transformer DC decoupled with a cap (on the virtual ground side), and I think that helped somewhat but I'm still getting an unbalanced virtual ground.

here's the schem:



(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.)

what am I missing here?

From memory the version without the transformer doesn't have this issue, but I don't see how the transformer could be causing it.

thanks in advance!
 

and here's the link direct to the schematic, which is what I meant to post:

On 30/04/2025 20:53, thet via groups.io wrote:
On my opamp mic with a transformer, my virtual ground is unbalanced, it is closer to actual ground than to V+

I thought it might be DC running through the transformer primary, I've tried with the transformer DC decoupled with a cap (on the virtual ground side), and I think that helped somewhat but I'm still getting an unbalanced virtual ground.

here's the schem:



(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.)

what am I missing here?

From memory the version without the transformer doesn't have this issue, but I don't see how the transformer could be causing it.

thanks in advance!
 

How far off-center is the virtual ground voltage?
?
I would try disconnecting stuff:
1) remove the opamp and measure voltages (all voltages)
2) restore the opamp, measure again?
3) disconnect the transformer, measure again,
4) disconnect the mic element, measure again
?
etc. The idea is to try to find what's creating the imbalance
 

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If anyone sees a theoretical issue with my schem I'd be grateful for any enlightenment.

If the schem is OK then I guess I'll find it through regular troubleshooting procedures.

Actually I'm getting readings all over the place.

Part of the problem seems to be that I'm testing the board out of context, as a bare board with a 100pF cap in place of the capsule.

Anything disturbing the input seems to result in massive changes to the virtual ground voltage. I'm not sure why this would so massively affect the DC operating point.

Removing the opamp is not an option as it's a surface mounted one and not socketed. The transformer isn't much easier.

I now think it probably isn't the transformer at issue.


On 30/04/2025 21:18, kennjava wrote:

How far off-center is the virtual ground voltage?
?
I would try disconnecting stuff:
1) remove the opamp and measure voltages (all voltages)
2) restore the opamp, measure again?
3) disconnect the transformer, measure again,
4) disconnect the mic element, measure again
?
etc. The idea is to try to find what's creating the imbalance
 

On Wed, Apr 30, 2025 at 09:53 PM, thet wrote:
On my opamp mic with a transformer, my virtual ground is unbalanced, it is closer to actual ground than to V+
How are you measuring it? What value are you measuring?
What voltage are you measuring on pins 2, 3 and 6 of the 1641?

I thought it might be DC running through the transformer primary,
Wrong assumption.

I've tried with the transformer DC decoupled with a cap (on the virtual ground side), and I think that helped somewhat but I'm still getting an unbalanced virtual ground.
That confirms it's not the origin of the problem.

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