On Sat, May 3, 2025 at 09:53 AM, thet wrote:
Yes when I first did this opamp tx circuit a couple of years ago, I had the experience that decoupling the transformer solved VG imbalance. I didn't need to refer it to actual ground IIRC.
In this case, initially that didn't work or wasn't enough. However when I get time I will try it again on this one.
Once the TX is decoupled, what difference do you think it makes referring it to actual ground rather than VG?
One would imagine the AC on the TX would average to 0v. I guess if that's not the case there might be an issue.
It's a nuisance with my pcbs though I'll have to cut traces and so on. Next revision I will leave the PCB agnostic as to TX connections.
?
I'm not sure there would be any significant difference in sound quality between Vgnd and Gnd as a reference for the transformer de-coupling?
I think it would simply mean that extra capacitance (the Vgnd de-coupling caps) are added into the signal path, so that the signal de-coupling is essentially still referenced to Gnd in the end.
I can't see that capacitance leakage current in those electrolytics would be a problem, with the Vgnd resistors at 47k.? Might be, if they were significantly higher values, as some circuits of this type seem to recommend.
I think your offset problems are occuring because any DC offset in the op-amp output is at a low impedance.
Introducing that offset into Vgnd? - via the DC feedback - is allowing Vgnd to shift, as part of the feedback loop.
As far as I can see, referencing the transformer de-coupling to Gnd removes all sources of any DC feedback in the 'loop' -- even any (pretty unlikely) effect from capacitance leakage.
