> I've seen small cheap transformers like this operate at near saturation, 
> distorting the waveform. I had thought about including a switching power 
> supply but it violates the simplicity rule. I wonder if I could keep 
> current usage down and use a linear reg. direct from rectified AC. 
> Probably not though - I think I'm going to use an 18F or 30F device 
> which will use significant current (several mA perhaps).
> 
> Thanks for looking at it.
> -
> Martin

Using TR1 with the diodes attached will distort the waveform as you 
mentioned.  But you can do as Russell suggested and just include a 
separate transformer for the purpose of sampling the voltage.

You have an enormous range of gain on your amplifier circuits; is there 
a good reason for this?

C3 (and C4) are in the wrong place if you intend them for low pass 
filtering.  Think what happens at high frequency; the Xr goes down so 
the stage gain goes up, right through the roof.  It will be noisy at 
best and likely will oscillate.  If you are looking for low pass 
filtering, put the caps in parallel with the 10k feedback resistors.

Guessing here but are you trying to level shift the signal with your 
output amps?  Something else?  It "just don't look right".  You are 
feeding the outputs with a ground referenced AC signal but have a 
voltage divider that I can't quite figure out what you have in mind.

If you are trying to shift the output such that it is ready for unipolar 
a/d conversion, consider using a reference for the bottom side of the 
sensing transformers.  If the desired output is +2.5V plus/minus the 
signal, then put the bottom of the transformer at +2.5V.  Or otherwise, 
AC couple the signal maybe.

Maybe a little more explanation of just what you had in mind will help 
here.

Finally, don't hesitate to model this stuff in Linear Technologies 
"Switcher Cad" program.  Setup is about 5 minutes and then you can see 
if the output does whatever you think it should do.

Good luck!





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