Pot across +12 to -12, bias off the wiper. Advice please

[Skruffyhound]

I'm using an LT1054 to create a negative rail. Is there any reason why I shouldn't use say a 1k pot/trimmer with 1 and 3 on the rails and the wiper adjusting the bias/Vref to 0V, much like the Shaka Tube.
Apparently it's bad for the 1054 to get it's "Vout pushed positive in respect to any of the other terminals",although I'm not entirely sure what that means practically.
Do I need some additional circuitry to make sure this ^^^doesn't happen?

[Skruffyhound]

I'm starting to think I do


If Beta is Ic /Ib and I need Iout too I'm going to need to know the load of all the circuits that will run off this supply,right?

There will be a Subcaster (0.3A heater) and a mic pre (one dual opamp NE5532 probably- max supply current 16mA,min 6mA) and a balanced line transmitter (one dual opamp TL072 1.2-1.4mA) some diodes (lets say 2 at 10mA each)

Can anyone lead me through the calculation so I can try to find a suitable R1

[armdnrdy]

Hey Aston,

It looks to me by the image you posted that you're referencing the Texas Instuments data sheet.

Take a look at the Linear Technology data sheet. It has more application info. Maybe it will help...

http://cds.linear.com/docs/Datasheet/1054lfg.pdf

[Skruffyhound]

Thanks Larry,
                    it is a slightly more helpful document with a vastly more funky layout
Still a bit stumped at the moment 

[armdnrdy]

I'm a bit unsure of what you're trying to do.

Do you need a bipolar supply or just a negative supply?

[Skruffyhound]

Sorry if I wasn't clear. I have a bipolar supply, +12V from a PSU(wallwart) and -12V from the LT1054.

This bipolar supply will power a mic preamp, a bipolar version of the Subcaster (6111 sub miniature tube overdrive) and a balanced line transmitter. A lot of it is on my breadboard now.

I am asking if I can use a pot to create bias/Vref by putting +12 into terminal 1 and -12 into terminal 3 so I can tune in the absolute center value for bias. + will probably be 11.75V and - maybe 11.60V so bias might be slightly positive with reference to ground. I saw this method used with Aron's Shaka tube project.

Hope that made more sense.

Alternatively, I could just use 0V ground and have done with it.

Either way I'm worried about Vout being dragged higher on the LT1054 at startup, because it will damage the chip.
It seems I have to implement the transistor but I am having difficulty working out the value of R1 in the app note above.

There's more of it here : http://www.diystompboxes.com/smfforum/index.php?topic=100442.0

[armdnrdy]

The negative voltage out of the LT1054 will always be a bit less than the supply voltage.

If an absolute Vref is required, maybe try matching the negative voltage by adjusting the positive voltage downstream from the LT1054.

[Skruffyhound]

Worth thinking about. Thanks