Try to use a capacitor on the 555's output (and a resistor as a load), it will cut-off dc and you will have a wave that is symmetrical with reference to GND ..
I have made up the circuit in the diagram above. With the cap added to the output I get +1.3V and -2.0V. I've tried a few different values of cap and resistor load but no luck changing this.
Is the 50:50 mark space ratio the duty cycle? I.e. If I have the same on and off times for the output I will get the same positive and negative output, for example +0.5V and -0.5V ?
That is helpful thank you. I actually need a duty cycle of 50%. I just haven't adjusted the values for that, only frequency.
I can supply the full range of 5-15V. I want +2 to -2 output. The output will go to a potentiometer, essentially a volume control, and this to an amp.
Added after 3 minutes:
I need 100KHz, I'd actually prefer to find a duty cycle calculator and plug the numbers in to get the right cap and resistor values. I have been using this one.
Oh I thought the link from IanP would take me to the schematic diagram, not another page. I've just realised. I do have an output if I short out R3. I am currently using c=10nF, R1=680, R2= 47, R3=47
Thanks.
Added after 2 minutes:
Ultimately I want the 50% duty cycle so I can use a cap to get a +2 to -2V output that is uniform. Those values won't give the 100KHz I need, I've changed them while trying to understand how to get the duty cycle working at 50%. This is a work in progress