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I worked it out by looking at the datasheet, inside the chip the audio out pin has a transistor to the + rail and a current mirror to ground, putting a 4k7 resistor from the audio output to ground now enables the chip to drive the pin from 1v to 4v with the tuning centred on signal at 2.5v, acceptable with a 0 - 6v supply and enables aboit +-100khz of control, I can probably even reduce that some.