I have a volume control chip that needs a +-5vd. The Teensy needs 5vdc, could it use the +5v and common from a +-5vdc ac-dc converter? Or does it need its own power supply?
TI’s PGA2310 is a +/-15-V stereo audio volume control with low gain error (0.05). Find parameters, ordering and quality information
www.ti.com
Looks like it's meant to run with 3 power supplies: +5V, +15V, -15V. But specs say the +/-15V can be as low as +4.5V and -4.5V.
Datasheet says on page 5 the digital input is 2V for logic high and 0.8V for logic low. So at least for the Teensy to PGA2310 signals, direct connect out to be possible. PGA2310 does have 1 digital output which is expected to be 5V, so for that signal you'll need either a resistor divider or level shifting buffer to connect to Teensy.