isolating DC ground for -vdc rail...?

[PaulC]

Long time veiwer - first time caller...

I'm trying to find a  circuit to isolate the ground of a DC pwr supply so I can then center tap it for +-4.5vdc ground ref rails.  Can do it all day long with a batt/wall wort, but when using multi pwr supply units the pwr supply ground is the same as audio ground.  I don't need more than 9vdc, so I don't really want to go the charge pump way to get a -vdc rail.  Anybody know of anything?  Thanks, PaulC

[Mike Burgundy]

so basically you want opamps to think they have +/- 4.5VDC supply?
Easy: voltage devider, and DC-isolate signal lines from the opamp with caps.
Take a look at almost any schem using dual opamps (f'rinstance a tubescreamer) to get the idea.
You want to reference the + input to Vref. The opamp itself will the force the output to that level. This is done by hooking up a large-ish resistor to Vref.
So the signal chain would be:
signal in-isolation cap-R to Vref-+input opamp - ouput opamp (to feedbackloop to - input) - isolation cap-signal out.
You only need to isoltate an output if  that output goes to pots, or something directly coupled to mass (which would be strange indeed because you won't have any signal left). If it's directly coupled to a + input, don't isolate but use it to bias that next input at Vref...
If this isn't too clear, post back with a basic schem idea and we'll fill in the blanks
hih

[PaulC]

That's not quite what I was talking about.  I'm needing a true ground ref -vdc rail for things other than opamp supplies.  For instance running a source follower input stage with the source tied to -VDC and having the gate tied to ground.  sure you can do that with a split rail and tying the gate to the 1/2 B+ point, but I'm big into dumping caps where I can.  Also things like +- supplies for diode compliance circuits (much cooler than trying to do it with resistors).  Lots of little things would work better if it was a true ground ref rail.

[Mike Burgundy]

Active components don't give a hoot what you relate them to - they only see the differences between their three legs.
It's either/or in this case: either you supply the circuit with a rue dual supply, or you use a lot of extra caps.
You can float components in certain schematics, but it takes a lot of care and trouble to get this right without blocking caps if possible at all. If getting rid of caps is a real biggie, install just a little bit of extra circuitry and use a dual supply, OR redesign the circuit to have all stages on the same power-orientation.